DRAFT FOR COMMENT
9‐29‐20

Authors
David Sandalow
Center on Global Energy Policy, Columbia University
Chair, ICEF Innovation Roadmap Project
Roger Aines
Lawrence Livermore National Laboratory
Julio Friedmann
Center on Global Energy Policy, Columbia University
Peter Kelemen
Department of Earth and Environmental Sciences, Columbia University
Colin McCormick
Walsh School of Foreign Service, Georgetown University
Ian Power
School of the Environment, Trent University

Briana Schmidt
Lawrence Livermore National Laboratory
Siobhan A. (Sasha) Wilson
Department of Earth & Atmospheric Sciences, University of Alberta

This roadmap was prepared to facilitate dialogue at the Eighth Innovation for Cool Earth Forum(October 2021), for final release at COP26 in November 2021.
We are deeply grateful to the Ministry of Economy, Trade and Industry (METI) and New Energy andIndustrial Technology Development Organization (NEDO), Japan for launching and supporting
the ICEF Innovation Roadmap Project of which this is a part.

CITATION
Cite report as: David Sandalow, Roger Aines, Julio Friedmann, Peter Kelemen, Colin McCormick,
Ian Power, Briana Schmidt, Siobhan (Sasha) Wilson, Carbon Mineralization Roadmap (ICEF Innovation
Roadmap Project, November 2021).

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Table of Contents
PREFACE ........................................................................................................................................................ 2
EXECUTIVE SUMMARY: ................................................................................................................................. 3
CHAPTER 1: ................................................................................................................................................... 8
INTRODUCTION TO CARBON MINERALIZATIONCHAPTER 2: ................................................................................................................................................. 15
SCIENCE PRIMERCHAPTER 3: ................................................................................................................................................. 20
UNDERGROUND INJECTION
CHAPTER 4: ................................................................................................................................................. 29
ENHANCED ROCK WEATHERING
CHAPTER 5: ................................................................................................................................................. 37
MINING WASTESCHAPTER 6: ................................................................................................................................................. 47
INDUSTRIAL WASTESCHAPTER 7: ................................................................................................................................................. 57
CEMENT AND CONCRETE
CHAPTER 8: ................................................................................................................................................. 65
CROSS‐CUTTING RESEARCH NEEDS
CHAPTER 9: ................................................................................................................................................. 72
POLICYCHAPTER 10: ............................................................................................................................................... 78
COMPARISON WITH DIRECT AIR CAPTURE
CHAPTER 11: ............................................................................................................................................... 83
FINDINGS AND RECOMMENDATIONS

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PREFACE
This ninth ICEF roadmap explores a topic that has received too little attention: the potential for
carbon mineralization to play an important role in helping fight climate change.
Carbon mineralization is a natural process in which carbon dioxide (CO2) becomes bound in rocks
as a solid mineral, permanently removing the CO2 from the atmosphere. This process could provide
the foundation for many activities that not only help fight climate change by removing additional
CO2 from the atmosphere, but create jobs and deliver local environmental benefits as well.
Resources for carbon mineralization are abundant and located in dozens of countries around theworld.
This roadmap builds on the body of literature produced annually in connection with the ICEF
conference. Previous roadmaps have addressed:








Biomass Carbon Removal and Storage (BiCRS) (2020)
Industrial Heat Decarbonization (2019)
Direct Air Capture (2018)
Carbon Dioxide Utilization (2017 and 2016)
Energy Storage (2017)
Zero Energy Buildings (2016)
Solar and Storage (2015)

This roadmap is a team effort. We are deeply grateful for the support provided by the ICEF
Secretariat, ICEF Steering Committee (including in particular its chair, Nobuo Tanaka), the New
Energy and Industrial Technology Development Organization (NEDO), experts at the Institute ofEnergy Economics‐Japan, and our design and copy edit team (including in particular Ms. Jeannette
Yusko and Dr. Kathryn Lindl).
The ICEF Innovation Roadmap Project aims to contribute to the global dialogue about solutions tothe challenge of climate change. We welcome your thoughts, reactions and suggestions.
David Sandalow
Chair, ICEF Innovation Roadmap Project
Inaugural Fellow, Center on Global Energy Policy,
Columbia University

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EXECUTIVE SUMMARY:
Chapter 1. INTRODUCTION TO CARBON MINERALIZATION
According to the Intergovernmental Panel on Climate Change (IPCC), significant volumes of carbon
dioxide (CO2) must be removed from the atmosphere for the world to achieve its climate goals.
One approach for CO2 removal that has received relatively little attention is carbon mineralization,a process in which CO2 becomes bound in rocks as a solid mineral. Carbon mineralization happens
naturally when certain rocks are exposed to CO2, permanently removing roughly 0.3 GtCO2 fromthe atmosphere each year.
There are two broad approaches to increasing the amount of CO2 removed from the atmospherevia carbon mineralization: injecting CO2‐rich fluids into rock formations deep underground (in situ
mineralization) and exposing crushed rocks on the Earth’s surface to CO2‐bearing gases (ex situ or
surficial mineralization).
As a strategy for CO2 removal and sequestration, carbon mineralization has many strengths. First,
mineralization resources are effectively unlimited and located in dozens of countries around theworld. Second, carbon mineralization offers one of the most permanent forms of CO2 removal andsequestration available, binding CO2 into solid rock. Third, the chemical reactions that mineralize
CO2 do not require energy inputs. Finally, the costs of carbon mineralization appear to bereasonable and could be reduced.
There are also several challenges with using carbon mineralization for CO2 removal andsequestration. First, most natural carbon mineralization happens very slowly. Second, thedistribution of optimal mineral resources is only coarsely understood. Third, while some productsof carbon mineralization have commercial value, those values are typically low. Finally, carbon
mineralization for climate mitigation is not yet practiced at large scale.
The authors estimate that, with strong and sustained policy support from governments around theworld, carbon mineralization processes could remove 1 GtCO2 from the atmosphere per year by2035 and 10 GtCO2 per year by 2050. More research is needed to test this hypothesis and define
conditions under which carbon mineralization could achieve this potential.

Chapter 2. SCIENCE PRIMER
Rocks are one of the Earth’s largest carbon reservoirs. CO2 is transferred from the atmosphere into
rocks in part through a process known as chemical weathering. (CO2 is also transferred from theatmosphere into rocks via photosynthesis, where carbon from marine organisms eventually forms
limestones and other carbon‐rich rocks.) Engineered carbon mineralization processes seek toreplicate and accelerate this natural process.
Several types of rocks are very well suited to carbon mineralization because they weather rapidly
and contain the right elements to form long‐lived carbonate minerals, namely magnesium (Mg)and calcium (Ca). Such rocks include ultramafic or mafic igneous rocks and the ultramafic
metamorphic rock serpentinite. These rocks are abundant at or near the Earth’s surface.

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Chapter 3. UNDERGROUND INJECTION

In situ mineralization is the process of injecting CO2 into subsurface mafic or ultramafic rocks. The
CO2 reacts with surrounding rocks to form solid, stable carbonates. Rocks suitable for in situ
mineralization are abundant and geographically widespread.
Two projects have demonstrated in situ mineralization to date: the Wallula pilot project inWashington State, US and the CarbFix project in Iceland. Both use CO2 captured from point
sources. Additional and larger‐scale field projects are the next logical step toward demonstratingthat the technology can scale to volumes that meaningfully contribute to climate mitigation.

Chapter 4. ENHANCED ROCK WEATHERING
Enhanced rock weathering (ERW) involves dispersing ultramafic or mafic rock powder over large
areas, such as agricultural fields. Calcium‐ and magnesium‐bearing minerals in the rock powder
then react with atmospheric CO2, trapping the CO2 in groundwater and solid carbonates.
ERW offers gigaton‐scale capacity to capture and permanently store CO2. The main advantages of
ERW are the simplicity of the approach, consistency with current mining and agricultural practices,and co‐benefits that may include improved soil fertility in some areas. The main challenges of ERW
are the lack of widely accepted methodologies for determining CO2 removal rates and concerns
about environmental risks from accumulation of metals, including nickel and chromium.

Chapter 5. MINING WASTES
Some waste rock from mining could be reacted with CO2 to form stable carbonates. These mining
wastes could provide hundreds of millions of tons per year of CO2 removal across dozens ofcountries. In addition, billions of tons of stockpiled mining wastes are already available as afeedstock for carbon mineralization. National and regional geological surveys are needed to map
these resources and assess their carbon mineralization potential and reactivity. Methods formonitoring and measuring CO2 uptake into mining wastes already exist, but a recognizedinternational standard for verifying CO2 sequestration of mining wastes is needed.
Carbon mineralization can also be used as an ore processing technology that improves recovery ofcritical metals for clean energy technologies. Demonstration projects are underway for accelerated
carbon mineralization in mining wastes, but more industry‐government‐university collaborations
on research and development (R&D) will be needed to bring this technology to scale.

Chapter 6. INDUSTRIAL WASTES
Some industrial wastes from iron and steel production, coal combustion and fertilizer production
could provide billions of tons of alkaline feedstock each year for carbon mineralization. In somecases, mineralizing these wastes would significantly reduce the cost and environmental impact of
their disposal. However, in industrialized countries a large portion of this waste is already used inways that have a climate benefit. Careful system analysis is needed to identify the climate impactsof alternative uses.
The largest potential for industrial waste as a carbon mineralization feedstock is in China, followedby India. Effective use of industrial wastes for mineralization will require additional applied

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research and demonstration because wastes are highly varied and mineralization processes have
not yet been developed at scale.

Chapter 7. CEMENT AND CONCRETE
Billions of tons of cement and concrete are produced annually, contributing approximately 8% of
global emissions. Carbon mineralization could reduce these emissions, through three primarypathways: mixing carbonation, carbon curing and synthetic aggregates. These three pathways haveeach seen early‐stage commercial deployments, mostly at a small scale
In general, these mineralization techniques are compatible with other methods for reducing
cement and concrete emissions, including the use of supplementary cementitious materials todisplace clinker, the use of electric or hydrogen‐fired kilns to reduce thermal emissions, and theuse of point‐source carbon capture. In principle, the use of mineralization in combination withthese other mitigation methods could lead to fully carbon‐negative concrete under certain
circumstances, an important prospect given the enormous size of the global cement and concrete
industry.

Chapter 8. CROSS‐CUTTING RESEARCH NEEDS
R&D needs in carbon mineralization focus on increasing the speed with which the reactions occur,
accessing a significant volume of reactive material (surficial or subsurface), disposing of carbonate
material produced via surficial methods, measuring the total carbon impact of these approaches
and improving the economics of the processes. Significant cross‐cutting issues are water use andsafety from potentially toxic and hazardous components present in both minerals and industrial
wastes.
R&D on these topics should be supplemented by work on system integration, energy efficiency andcost reduction. Large‐scale demonstrations are vital for addressing these needs across all forms ofcarbon mineralization.

Chapter 9. POLICY
Policymakers have paid scant attention to carbon mineralization as a strategy for fighting climatechange. For carbon mineralization to remove gigatons of CO2 from the atmosphere each year, thatmust change.
Supportive policies will be essential. That could include funding in government R&D budgets,
preferences in government procurement and incentives in tax regimes. Recognition of carbon
mineralization as a compliance option in emissions trading programs is one of the most important
potential tools. International cooperation—including programs to share information and jointly
conduct demonstration projects—could accelerate progress.
Regulatory issues will also require considerable attention. Carbon mineralization projects may besubject to regulations on diverse topics, including groundwater protection, hazardous waste
management, foreign ownership and financial transparency. Governments could support pilot anddemonstration projects to help assess regulatory issues. Relevant ministries and departments
could examine the overall regulatory environment for carbon mineralization projects. Policy
frameworks that engage key stakeholders will be essential.

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Chapter 10. COMPARISONS WITH DIRECT AIR CAPTURE
The term “direct air capture” (DAC) is generally used to refer to engineered methods that remove
CO2 from air with synthetic sorbents or solvents. As a strategy for removing CO2 from theatmosphere, carbon mineralization shares many characteristics with DAC. Both are early stage andcurrently expensive. Both have clear pathways to reduce costs, can be deployed in many locations
around the world and show considerable promise in helping achieve net‐zero emissions.
The two strategies also have important differences. Most carbon mineralization processes requireless energy than DAC. Some carbon mineralization processes offer ancillary benefits not available
from DAC. However, measurement and verification of CO2 removal is more difficult for mostcarbon mineralization processes than for DAC. Since several DAC demonstration projects are
currently operating or under construction, while just a few carbon mineralization methods havebeen verified at a pilot scale, more scientific research is required for full deployment of carbon
mineralization than for DAC.

Chapter 11: FINDING AND RECOMMENDATIONSFINDINGSFINDING 1: Carbon mineralization has the potential to permanently remove and sequester many
gigatons of CO2 from the atmosphere each year.
FINDING 2: As a strategy for carbon removal and sequestration, carbon mineralization has many
strengths.
FINDING 3: As a strategy for carbon removal and sequestration, carbon mineralization also has
several challenges.
FINDING 4: Carbon mineralization is not one pathway—it is multiple pathways.
FINDING 5: Carbon mineralization receives little recognition or support in climate change policies
around the world.
FINDING 6: Current scientific knowledge and technical experience are sufficient to support carbon
mineralization projects at the pilot and demonstration scale today.
FINDING 7: The key technical challenges in carbon mineralization are (1) speeding up thechemical reaction between atmospheric CO2 and minerals, (2) maximizing the CO2
content of mineralized materials and (3) minimizing the space required for permanentstorage.
FINDING 8: Measurement and verification tools today are sufficient for some carbon
mineralization approaches (including ex situ conversion of mine tailings) andinsufficient for others (including enhanced rock weathering).
FINDING 9: Field pilots and demonstration projects could significantly improve understanding of
the potential impacts and costs of carbon mineralization processes, while improving
monitoring and verification methods.

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RECOMMENDATIONSRECOMMENDATION 1: Policy makers should add carbon mineralization to the portfolio of climatechange mitigation options.
RECOMMENDATION 2: Governments and companies should invest in R&D on carbon mineralization.
RECOMMENDATION 3: Mining companies and manufacturers should seek opportunities for carbon
mineralization in their exploration and production activities.
RECOMMENDATION 4: Governments and companies with net‐zero commitments should consider
carbon mineralization as part of their portfolio of options.

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CHAPTER 1:
INTRODUCTION TO CARBON MINERALIZATIONThe concentration of heat‐trapping gases in the atmosphere is now higher than at any time inhuman history and continuing to climb.
Dozens of countries, representing more than 70% of global emissions, have now made net‐zeropledges. Yet the path to achieving these net‐zero pledges is uncertain at best. While steep declines
in the cost of renewable power and energy storage offer hope for decarbonizing the global power
sector, pathways for decarbonizing other sectors, including industry, aviation and agriculture, are
far less clear.1
And time is short. The heat waves, floods, droughts and wildfires in the past year highlight theenormous and growing risks of climate change.2 To prevent even more dangerous impacts,
emissions must fall quickly in the decades ahead.
However falling emissions will not be enough. For the world to achieve its climate goals, significant
volumes of CO2 must also be removed from the atmosphere. As the Intergovernmental Panel on
Climate Change (IPCC) found in its 1.5 C report, “All pathways that limit global warming to 1.5 Cwith limited or no overshoot project the use of carbon dioxide removal (CDR) on the order of 100–
1000 GtCO2 over the 21st century.”3 Approaches such as direct air capture (DAC) of CO2, biomass
carbon removal and storage (BiCRS), and afforestation will be essential.4

Carbon Mineralization: The Basics
One approach for CO2 removal that has received relatively little attention is carbon mineralization,a process in which CO2 becomes bound in rocks as a solid mineral. Carbon mineralization happens
naturally when certain rocks are exposed to CO2, which then binds with calcium (Ca), magnesium(Mg) or other elements in these rocks to form “carbonate minerals.” The CO2 in the carbonate
minerals is trapped in a solid, stable and non‐toxic form, in which it is permanently removed fromthe atmosphere.
Carbon mineralization is a natural process that removes roughly 0.3 Gt CO2 from the atmosphereeach year.5 The main goal of the carbon mineralization processes discussed in this report is toincrease this amount. Two approaches are possible:In the first, CO2‐rich fluids are injected underground into rockformations. This approach is known as in situ mineralizationIn the second, CO2‐bearing gases are exposed to crushed rocks,
such as mine tailings or industrial wastes, on the Earth’s
surface. This is known as ex situ or surficial mineralization

In both approaches, CO2 is permanently incorporated intominerals. There are differences between the two approaches,
including in cost, ease of verification and operational needs.

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Strengths
The strengths of carbon mineralization as a climate mitigation strategy are enormous.
First, the potential capacity of the mineralization resource is effectively unlimited. Estimates of thegross CO2 removal capacity of carbon mineralization range from many trillions to quadrillions oftons of CO2.6,7 In addition, mineral resources that can be used for CO2 removal are available insubstantial quantities in many countries around the world (Figure 1).8 The sheer abundance andwide dispersion of carbon mineralization resources means that many nations and companies could
use carbon mineralization as a CO2 reduction and removal strategy at scale.

Figure 1.1 Locations and distribution of carbon mineralization resources. Source: CDR Primer, Chapter 3, 2021.9

In addition, some industrial wastes provide CO2 mineralization opportunities.9 Such wastes include
steel slag, fly ash and phosphogypsum. These wastes are often located at industrial facilities and/or
near CO2 sources. In some cases, carbon mineralization of geological resources or industrial wastes
can yield building materials with economic value, such as concrete aggregate.
Second, carbon mineralization offers the most permanent form of CO2 removal and sequestration
available, binding CO2 into solid rock. Although some other carbon removal strategies (such asreforestation) can provide many important benefits, including biodiversity protection, such

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strategies often risk release of CO2 back into the atmosphere on short timescales (such as throughforest fires). This risk is essentially absent with carbon mineralization.
Third, the chemical reactions that mineralize CO2 do not require energy inputs. In fact,
spontaneous carbon mineralization reactions generate heat. As a result, carbon mineralization
strategies generally do not compete for energy resources and often have minimal operating
expenses (although special‐purpose reactor vessels, heat and other reagents may be needed toaccelerate key reactions in the near term as the relevant technologies mature and scale).
Finally, the costs of carbon mineralization appear to be reasonable and could be reduced.
Estimates today range widely—from $10–1000/tCO2 removed—as a function of resource quality
and process characterization.10 There are many options to reduce costs through operational
improvements, novel engineering and better chemistry. New projects (like CarbFix11) and newcompanies (like Heirloom Carbon Technologies12) are providing more information on real costs andopportunities to improve them.
As a result of these features, carbon mineralization has the potential to remove CO2 from theatmosphere at a scale of gigatons per year. A number of peer‐reviewed studies have found that
~1–10 GtCO2 per year could be removed from the atmosphere globally from ex situ mineralizationof mine tailings and industrial wastes alone.10‐18 In most of these studies, the timeframe in which
technology could be deployed to achieve these removals is unclear.

Challenges
For carbon mineralization to reach its full potential as a climate change mitigation strategy, several
important challenges must be overcome.
First, most natural carbon mineralization happens very slowly, consuming available magnesium
and/or calcium at rates of a few percent per year or less. This process can be accelerated via
grinding or “reaction‐driven cracking” to increase the surface area of minerals; drilling toaccelerate subsurface CO2 delivery; or the addition of heat, acid or other reagents. However, thesesteps can add cost as well as produce CO2 emissions. Reducing these costs and CO2 emissions is a
key focus of ongoing carbon mineralization research.
Second, the distribution of mineral resources appropriate for carbon mineralization remains only
coarsely understood. Although the location of rock types most favorable for carbon mineralization(including ultramafic rocks and basalt) is broadly known, the ability to use these geologic resources
for mineralization of CO2 at scale requires more granular information on the concentrations,compositions and volumes of minerals at specific locations. This will require substantial new
geologic resource assessment and characterization.
Third, while some products of carbon mineralization have commercial value, those values are
typically low, creating little incentive to invest or change existing practices. For this reason, policy
support is needed to accomplish the widespread adoption and implementation of carbon
mineralization.
Finally, carbon mineralization for climate mitigation is not yet practiced at large scale, and newchallenges will likely be identified in the future. Fortunately, many related practices, such as

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mining, rock grinding and wellbore injection, are in widespread use around the world, suggesting
the foundations for large‐scale carbon mineralization already exist in many places.

How Many Gigatons of Carbon Removal by When?
One important question is the timeframe in which carbon mineralization could deliver significant
rates of carbon removal. Some prior research has considered carbon mineralization’s potential for
carbon removal at scale, with leading estimates in the range of 1–10 GtCO2 per year.13‐21 However
the literature is mostly silent on the length of time required to deploy the infrastructure needed
for these removals. An important reason for this silence is that the factors determining how quickly
carbon mineralization could be scaled up largely depend on policy decisions, including the level of
incentives and willingness of governments to streamline regulatory barriers.
We considered how quickly carbon mineralization could remove CO2 from the atmosphere if
governments were strongly committed to supporting it as a climate change mitigation strategy. In
that scenario, companies that decided to pursue large‐scale carbon mineralization operations
would need to (1) buy or lease processing equipment, (2) hire a trained workforce, (3) obtain landand (4) secure a source of both mineral feedstock and CO2. For ex situ mineralization methods,
companies would also need to secure offtake or disposal options for mineralized products.
The processing equipment required for most mineralization methods is very similar or identical toequipment already in widespread use in the mining, oil and gas, and agriculture industries. An
extensive manufacturing base for this equipment already exists in many countries, and very little
specialized or non‐commodity equipment would be needed. This suggests that obtaining therequired equipment for mineralization would not be a significant bottleneck to scale‐up.
Similarly, the skills required for jobs in carbon mineralization are similar to the skills required forjobs in the mining and oil and gas sectors. As a result, a well‐trained labor pool for carbon
mineralization projects already exists in many places. Carbon mineralization projects could help
protect jobs in communities in which employment opportunities in the mining and oil and gas
sectors are shrinking.
Obtaining land and mineral feedstocks could represent a larger challenge for companies seeking toscale carbon mineralization. While in situ and some ex situ mineralization methods are not highlyland‐intensive, some surficial methods require large tracts of land (although not for exclusive use).
In some cases, looping reagents (CaO and/or MgO derived from rocks) through multiple cycles of
CO2 removal from air and/or stacking material for ambient weathering could reduce land use
requirements significantly. Even so, identifying appropriate
sites and obtaining rights to this land could be time‐consuming.
Obtaining community acceptance, particularly for methods thatuse significant land area, would be imperative. In general,carbon mineralization methods that use degraded or unused
land (potentially former mine sites or other industrial land)
would be likely to encounter fewer challenges to rapid scale‐up. Mineral feedstocks for ex situ methods could be sourced
from mining or industrial wastes or from virgin rock. Access to

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these resources would depend on regulatory permission, which could emerge as a principal
bottleneck to rapid scale up.
Access to CO2 could prove challenging for rapid scale‐up of some methods. Mineralization methods
requiring pure CO2 would need to either capture and transport it from point sources or use
methods such as DAC to remove it directly from the air. Successful scale‐up of mineralization
would require a careful matching between CO2 sources and mineralization feedstock, and either
could serve as the limiting factor or bottleneck under different circumstances. In situ methods thatuse atmospheric CO2 dissolved in surface waters could face challenges related to securing
sufficient water. Ex situ methods that produce valorized products, such as aggregates, would needto secure buyers, but these markets are large and unlikely to saturate quickly.
Taking all these factors into account, we believe there are no technical barriers to scaling carbon
mineralization methods to capture and/or store billions of tons of CO2 per year. Most proposed
methods use existing technology that is already in widespread use with known costs. However, forcarbon mineralization to reach its full potential as a climate mitigation strategy, strong andsustained policy support will be needed. While emphasizing the significant uncertainty about thepotential pace of scale‐up, we estimate that, with strong and sustained policy support fromgovernments around the world, carbon mineralization processes have the potential to remove 1
GtCO2 from the atmosphere per year by 2035 and 10 GtCO2 per year by 2050. These estimates are
based on the authors’ expert judgment. More research is needed to test this hypothesis and define
conditions under which carbon mineralization processes could achieve this potential.

Path Forward
Interest and investment in carbon mineralization is growing.22‐24 In largepart this is due to increasing recognition that an enormous amount of
CO2 must be removed from the atmosphere to achieve global climate
goals, such as those set forth in the Paris Agreement. As a result, newcompanies pursuing CO2 removal via carbon mineralization have comeon the scene, and investment in this approach for CO2 removal has
increased.11,12,25 One technology company, Stripe, has purchased verysmall volumes of CO2 removal through carbon mineralization.
Yet the use of carbon mineralization as a climate change strategy is tiny in relation to its potential.
One reason is a chronic underfunding of research and development (R&D) related to carbon
mineralization.26 Another is the absence of carbon markets in many jurisdictions, meaning that the
carbon removal service of carbon mineralization often has little commercial value, making it
difficult for projects and new companies to gain investors. Beyond the lack of R&D funding andcarbon markets, policy makers and businesses generally lack understanding of carbon
mineralization or its potential to contribute to achieving net‐zero emissions.
Ultimately, policy decisions will be crucial in determining the rate of scale‐up for carbon
mineralization. It is worth noting that the world has observed an extraordinary example oftechnology scale‐up over the past two years in another field—public health. Given the enormouspressures to act in response to the COVID‐19 pandemic, governments mobilized resources andstreamlined regulations in order to scale production of multiple vaccines—some of which were

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based on unproven technologies—from zero to billions of doses in less than 24 months. This
example illustrates how quickly new technologies can be deployed if political will is strong.
Although there are important differences between carbon mineralization and vaccines, theexample is instructive. The urgency of the climate crisis should inspire policy makers to proceedwith carbon mineralization and other low‐carbon technologies on an urgent basis.
In that spirit, this ICEF Roadmap provides background on carbon mineralization’s potential forclimate change mitigation and recommends steps to help achieve that potential. The roadmap
discusses scientific and technological aspects of carbon mineralization for CO2 removal andstorage, as well as system‐level characterization and optimization challenges. It explores different
approaches, feedstocks and business models and compares carbon mineralization as a climatechange mitigation strategy with DAC. The roadmap provides a set of cross‐cutting research
priorities and policy options for scaling up carbon mineralization. The final chapter offers findings
and recommendations.

REFERENCES
1

The International Energy Agency (IEA). Net Zero by 2050, <https://www.iea.org/reports/net‐zero‐by‐2050>(IEA, Paris; 2021).

2

Christopher Flavelle, Anne Barnard, Brad Plumer & Michael Kimmelman. Overlapping Disasters Expose Harsh
Climate Reality: The U.S. Is Not Ready, <https://www.nytimes.com/2021/09/02/climate/new‐york‐rain‐
floods‐climate‐change.html> (NY Times; 2021).

3

Intergovernmental Panel on Climate Change (IPCC). Global Warming of 1.5 °C: Headline Statements from theSummary for Policymakers, <https://www.ipcc.ch/site/assets/uploads/sites/2/2019/06/SR15_Headline‐
statements.pdf> (2019).

4David Sandalow, Julio Friedmann, Colin McCormick & Sean McCoy. ICEF Roadmap 2018: Direct Air Capture of
Carbon Dioxide, <https://www.icef.go.jp/pdf/2018/roadmap/ICEF2018_DAC_Roadmap_20181210.pdf>(Innovation for Cool Earth Forum; 2018).

5

P. Ciais et al. Carbon and Other Biogeochemical Cycles (in Climate Change 2013: The Physical Science Basis.
Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate
Change). ((eds. T.F. Stocker, D. Qin, G.‐K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex,
P.M. Midgley) Cambridge University Press; Cambridge, United Kingdom and New York, NY, USA, 2013).

6

National Academies of Sciences (Engineering and Medicine) (NASEM). Negative Emissions Technologies andReliable Sequestration: A Research Agenda. (Washington, DC: The National Academies Press, 2019;https://doi.org/10.17226/25259).

7

Energy Futures Initiative. Rock Solid: Harnessing Mineralization for Large‐Scale CarbonManagement.,<https://static1.squarespace.com/static/58ec123cb3db2bd94e057628/t/6011a6b8633ac2604c92a022/1611
769538335/Rock+Solid.pdf.pdf> (December 2020).

8

J Wilcox, B Kolosz & J Freeman. CDR Primer, <https://cdrprimer.org > (2021).

9

Heping Xie et al. Scientific and Engineering Progress in CO2 Mineralization Using Industrial Waste andNatural Minerals. Engineering 1, 150‐157, doi:https://doi.org/10.15302/J‐ENG‐2015017 (2015).

10

Engineering National Academies of Sciences & Medicine. Negative Emissions Technologies and Reliable
Sequestration: A Research Agenda. (The National Academies Press, 2019).

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11

CarbFix. <https://www.carbfix.com> (Accessed September 2021).

12

Heirloom Carbon Technologies. <https://www.heirloomcarbon.com/> (Accessed September 2021).

13

David J. Beerling et al. Potential for large‐scale CO2 removal via enhanced rock weathering with croplands.
Nature 583, 242‐248, doi:10.1038/s41586‐020‐2448‐9 (https://www.nature.com/articles/s41586‐020‐2448‐
9) (2020).

14Peter Kelemen, Sally M. Benson, Hélène Pilorgé, Peter Psarras & Jennifer Wilcox. An Overview of the Status
and Challenges of CO2 Storage in Minerals and Geological Formations. Frontiers in Climate 1,
doi:10.3389/fclim.2019.00009 (2019).

15

Noah McQueen, Peter Kelemen, Greg Dipple, Phil Renforth & Jennifer Wilcox. Ambient weathering ofmagnesium oxide for CO2 removal from air. Nat. Commun. 11, 3299, doi:10.1038/s41467‐020‐16510‐3(2020).

16

Corey Myers & Takao Nakagaki. Direct mineralization of atmospheric CO2 using natural rocks in Japan.
Environ. Res. Lett. 15, 124018, doi:10.1088/1748‐9326/abc217 (2020).

17

Corey A. Myers, Takao Nakagaki & Kosei Akutsu. Quantification of the CO2 mineralization potential ofironmaking and steelmaking slags under direct gas‐solid reactions in flue gas. Int. J. Greenh. Gas Control 87,
100‐111, doi:https://doi.org/10.1016/j.ijggc.2019.05.021 (2019).

18

Hesam Ostovari, André Sternberg & André Bardow. Rock ‘n’ use of CO2: carbon footprint of carbon capture
and utilization by mineralization. Sustainable Energy & Fuels 4, 4482‐4496, doi:10.1039/D0SE00190B (2020).

19

Shu‐Yuan Pan et al. CO2 mineralization and utilization by alkaline solid wastes for potential carbon
reduction. Nature Sustainability 3, 399‐405, doi:10.1038/s41893‐020‐0486‐9 (2020).

20

Phil Renforth. The negative emission potential of alkaline materials. Nat. Commun. 10, 1401,
doi:10.1038/s41467‐019‐09475‐5 (2019).

21

Caleb M. Woodall, Noah McQueen, Hélène Pilorgé & Jennifer Wilcox. Utilization of mineral carbonation
products: current state and potential. Greenhouse Gases: Science and Technology 9, 1096‐1113,
doi:https://doi.org/10.1002/ghg.1940 (2019).

22

Devin Coldewey. 44.01 secures $5M to turn billions of tons of carbon dioxide to stone,<https://techcrunch.com/2021/08/10/44‐01‐secures‐5m‐to‐turn‐billions‐of‐tons‐of‐carbon‐dioxide‐to‐stone/> (TechCrunch; August 2021).

23

Douglas Fox. Rare Mantle Rocks in Oman Could Sequester Massive Amounts of CO2. Scientific American 325,
1, 44‐53, doi:10.1038/scientificamerican0721‐44 (https://www.scientificamerican.com/article/rare‐mantle‐
rocks‐in‐oman‐could‐sequester‐massive‐amounts‐of‐co2/) (July 2021).

24

James Temple. A startup using minerals to draw down CO2 has scored funding—and its first buyer,<https://www.technologyreview.com/2021/05/26/1025402/heirloom‐stripe‐breakthrough‐energy‐
lowercarbon‐carbon‐removal/> (MIT Technology Review; May 2021).

25

44.01. <https://4401.earth/> (Accessed September 2021).

26

Varun Sivaram, Colin Cunliff, David Hart, Julio Friedmann & David Sandalow. Energizing America: A Roadmap
to Launch a National Energy Innovation Mission. (Columbia School of International and Public Affairs –Center on Global Energy Policy; 2020).

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CHAPTER 2:SCIENCE PRIMER
Rocks are one of the Earth’s largest carbon reservoirs. Rocks in the Earth’s crust contain about
100,000 trillion tons of carbon (~370,000 trillion tCO2), mostly in the form of carbonate minerals.1
Crustal rocks incorporate carbon in part through a process known as chemical weathering. CO2 in
the atmosphere dissolves into rain and surface water—similar to the way CO2 is dissolved incarbonated beverages. The dissolved CO2 in water forms a weak acid called carbonic acid (H2CO3).
This acid then dissolves minerals in rocks, releasing ions including calcium (Ca), magnesium (Mg)and sodium (Na). Some of these ions react locally with dissolved CO2 to form carbonate minerals,
while others are carried by rivers to the ocean where they precipitate in carbonate minerals—suchas calcite (CaCO3), dolomite (CaMg(CO3)2), magnesite (MgCO3) and natron (mixtures of hydrated
sodium carbonates)—that accumulate on the ocean floor to form sedimentary rocks, such as
limestone and evaporites.
(Crustal rocks also contain carbon removed from the air via photosynthesis. See, for example,
limestones, black shales, and oil and gas reservoirs.)
Through chemical weathering, CO2 is transferred from the atmosphere to carbonate minerals,
where the CO2 is stored over long timescales. CO2 is released from carbonate rocks—also over long

Modified from source:
http://butane.chem.uiuc.edu/pshap
ley/Environmental/L29/2.html

Figure 2.1. Slow Carbon Cycle.

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timescales—when those rocks are carried underground into the Earth’s mantle in subduction
zones. Heat and pressure in the mantle cause carbonate minerals to react, releasing CO2 from theminerals. Some of this CO2 eventually re‐enters the atmosphere through volcanic eruptions.
The natural process of chemical weathering is part of the so‐called “slow carbon cycle,” which isestimated to draw down about 0.3 billion tons of atmospheric CO2 per year.2 Over long timescales
this drawdown is approximately balanced by the amount of CO2 released from volcanoes, such thatthe average amount of CO2 in the atmosphere stays roughly constant. In contrast, human activities
release more than 40 billion tCO2 every year. Carbon mineralization practices seek to speed up thenatural process of chemical weathering to draw down more atmospheric CO2.

Figure 2.2. Following carbon molecules through the mineralization process.

Three primary factors can limit the natural rate of chemical weathering: mineral dissolution, CO2
supply and carbonate formation.3,4 Each of these factors are in turn impacted by a number of otherfactors.


Mineral dissolution rates depend on average temperature, humidity, precipitation, the surface
area of the mineral exposed to the atmosphere and other factors.

Even though human activities have greatly increased atmospheric concentration of CO2, thatconcentration is still relatively low overall (0.04%), which limits the amount of CO2 available forchemical weathering processes. In the subsurface, dissolved carbon contents in circulating
fluids depend on pressure and temperature. Carbon delivery to sites of mineral dissolution andprecipitation depends on factors including fluid flow rates, which in turn are controlled bypermeability, pressure gradients, fluid viscosity and—on a small scale—diffusivity.

The rate of carbonate mineral precipitation depends on factors including pH, temperature andavailability of cations, such as magnesium (Mg) and calcium (Ca).

Engineered carbon mineralization practices aim to optimize or enhance one or more of these rate‐
limiting factors. As discussed in detail in subsequent chapters, interventions range from simple andrelatively inexpensive, such as finely grinding rocks to increase surface area, to much more
complex and energy and cost intensive, such as heating the minerals to high temperatures toincrease their reactivity and exposing them to higher concentrations of CO2.

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Box 2-1
ULTRAMAFIC AND MAFIC ROCKS
Rocks can be classified into four categories based on their chemical composition: ultramafic, mafic,
intermediate and felsic. These categories form a spectrum, with ultramafic rocks at one end and felsic
rocks at the other end. Ultramafic rocks have the highest proportions of magnesium (Mg) and iron (Fe) andlowest proportion of silica (SiO2). Mafic rocks are next on the spectrum, with less Mg and Fe and more SiO2
than ultramafic rocks, as well as more calcium (Ca).
The differences between rocks in each category are caused by geologic processes such as melting, eruption
and crystallization. Ultramafic and mafic rocks are most suitable for carbon mineralization because they
contain the highest proportions of Mg and Ca, which form stable carbonate minerals, such as magnesite(MgCO3) and calcite (CaCO3).
The most abundant ultramafic rocks on Earth are “peridotites,” which are the dominant rock in Earth’s
upper mantle. They are most common in Oman, the United Arab Emirates, New Caledonia, Papua New
Guinea and Albania. There are also substantial peridotite massifs in Japan and the western United States.
The Earth’s oceanic crust is composed of a remarkably uniform 7‐km‐thick layer of mafic rocks. Rocks
compositionally similar to the oceanic crust are common on land in many regions, including Japan, Hawaii,
the northwest United States and Iceland.

Rocks most suitable for carbon mineralization are those that undergo rapid chemical weatheringand release cations that readily form stable carbonate minerals, such as magnesium and calcium.
(Carbonates containing other cations, such as sodium (Na) and potassium (K), can also form
through chemical weathering, however such carbonates dissolve readily and are therefore notlong‐term sinks for CO2.) Suitable rocks include two categories of igneous rocks known as mafic andultramafic rocks, which include dunite, peridotite and basalt, and the metamorphosed version of
the ultramafic rocks known as serpentinites. These rocks are composed of magnesium‐ and iron‐rich and calcium‐bearing silicate minerals, including olivine, pyroxenes and serpentine, and may

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also contain other reactive minerals, such as the hydroxide mineral brucite. The mineralwollastonite also reacts rapidly and is found in either rocks where silica and limestone aremetamorphosed at high temperatures or in rocks known as carbonatites, which form frommagmas rich in carbon. However, the global reserves of wollastonite are ~100 million tons,
whereas there are hundreds of trillions of tons of peridotite, basalt and other mafic rocks within a
few kilometers of the Earth’s surface. Other rocks, minerals and alkaline industrial wastes may also
be suitable, as discussed in more detail in subsequent chapters.
Ultramafic rocks suitable for carbon mineralization consist mainly of those from the Earth’s mantle
that are now exposed at the surface. As shown in Chapter 1, Figure 1, these rocks occur along
current and former tectonic plate boundaries and areas of tectonic uplift and rift zones. Mafic
rocks suitable for carbon mineralization consist primarily of basaltic lavas and their subsurface
equivalents, gabbros. These rocks form the 7‐km‐thick igneous crust underlying the ocean basins
and are abundant on land in flood basalt provinces (e.g., the Columbia River basalts), volcanic
ocean islands (also known as “hotspots”, e.g., Hawaii, Iceland), volcanic arcs above subduction
zones (e.g., most of Japan, the Cascade volcanoes in the United States), and continental rift zones(e.g., the Rio Grande rift).
The various suitable rocks and their constituent minerals react at different rates. Brucite,wollastonite, fibrous serpentine minerals (e.g., chrysotile asbestos), olivine and basaltic glass areamong the fastest reacting, while pyroxenes and feldspars are slower to react.3 As noted above,
the reaction rate depends on factors including temperature, partial pressure of CO2 and pH. For agiven combination of minerals and fluid compositions, the amount of time to achieve complete
carbon mineralization can vary from hours to years, depending on the initial surface area and grain
size.3
Some reactive rocks and minerals are abundant, whereas others are relatively scarce. Brucite
typically makes up only a small fraction of ultramafic rocks—at most 10 percent by weight but
typically much less. Wollastonite is also not geologically abundant, occurring in significant
quantities in only a few locations globally. Conversely, olivine and serpentine are abundant,
particularly in ultramafic rocks, such as peridotite. Mafic rocks are somewhat slower to react thanultramafic rocks because they contain higher proportions of slower reacting minerals. Also, they
have a smaller CO2 storage capacity than ultramafic rocks. However, mafic rocks are also about anorder of magnitude more abundant at the Earth’s surface than ultramafic rocks and so, overall,
represent a larger possible CO2 sink.
The total amount of CO2 that can be removed using engineered carbon mineralization depends onwhich minerals are dissolving and which minerals are forming and, particularly, on the amount ofmagnesium and calcium ions contained in the dissolving minerals, as well as the ratio of CO2
magnesium or calcium incorporated in the carbonate minerals that form. On average,
approximately 0.4 tCO2 can be sequestered for every ton of reactive rock.4 More specifically, theamount of CO2 that can be removed at any particular site or using any particular rock depends onsite‐specific mineralogy and geochemistry. Approximately 0.55 tCO2 can be sequestered for every
ton of typical ultramafic rock (mantle peridotite), and about one third of that amount can besequestered for every ton of mafic rock.

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Carbon mineralization can be engineered to achieve (1) CO2 removal from air (CDR) or (2)permanent, inert, non‐toxic storage. These goals are generally accomplished via one of three broad
methodologies, as will be discussed in more depth in the chapters that follow:
1. In situ: CO2‐bearing aqueous fluids or H2O‐bearing dense CO2 fluids under pressure (“dampsupercritical CO2,” sc CO2) are circulated underground through reactive rocks to achieve
subsurface carbon mineralization. Carbon‐depleted aqueous fluids can then be returned to thesurface, either to draw down CO2 from air or simply to be recycled in order to reduce water
withdrawals and consumption.
2. Surficial: Captured or ambient CO2 is reacted with crushed rock. This approach can be
performed where the rocks occur (e.g., in mine tailings and alkaline industrial waste sites) or inmore distal settings (e.g., as amendments to agricultural soil or as dispersed material along
beaches).
3. Ex situ: Reactive rocks are transported to a source of CO2, industrial facility or other location,
where they are crushed and ground and then reacted with captured or ambient CO2, often
under high temperature and/or pressure conditions.
In addition to CO2 removal from air and storage of CO2 captured elsewhere, a set of hybrid
methods can be envisaged. These methods start with gas enriched in CO2 (flue gas or CO2‐rich gas
produced via partial DAC). CO2 from the enriched gas is then dissolved to form CO2‐enriched waterat depth. Then, the CO2‐enriched water is circulated through reactive materials. Because CO2 ismore soluble in water than other gases that are abundant in air (e.g., oxygen, nitrogen) andbecause, in turn, carbon mineralization further “purifies” CO2 in carbonate minerals thereby
removing it from aqueous fluids, such hybrid methods achieve both CO2 removal from air andpermanent solid storage. This is discussed more fully in Chapter 3.

REFERENCES
1

John M Hayes & Jacob R Waldbauer. The carbon cycle and associated redox processes through time.
Philosophical Transactions of the Royal Society B: Biological Sciences 361, 931‐950,
doi:10.1098/rstb.2006.1840 (https://royalsocietypublishing.org/doi/abs/10.1098/rstb.2006.1840) (2006).

2

P. Ciais et al. Carbon and Other Biogeochemical Cycles (in Climate Change 2013: The Physical Science Basis.
Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate
Change). ((eds. T.F. Stocker, D. Qin, G.‐K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex,
P.M. Midgley) Cambridge University Press; Cambridge, United Kingdom and New York, NY, USA, 2013).

3

National Academies of Sciences (Engineering and Medicine) (NASEM). Negative Emissions Technologies andReliable Sequestration: A Research Agenda. (Washington, DC: The National Academies Press, 2019;https://doi.org/10.17226/25259).

4

Ian M. Power et al. Carbon Mineralization: From Natural Analogues to Engineered Systems. Reviews in
Mineralogy and Geochemistry 77, 305‐360, doi:https://doi.org/10.2138/rmg.2013.77.9 (2013).

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CHAPTER 3:
UNDERGROUND INJECTIONINTRODUCTION
When carbon dioxide (CO2) is injected underground into certain types of rocks, the CO2 reacts withthose rocks to form solid, stable, non‐toxic minerals that permanently remove the CO2 from theatmosphere. This is known as in situ mineralization.
In situ mineralization requires the presence of subsurface rocks rich in magnesium and/or calcium,commonly termed “mafic” and “ultramafic” rocks. These are found in many places around theworld. (See text box and map in Chapter 2.) The CO2 is injected underground in fluids (either
aqueous fluids containing dissolved CO2 or CO2 fluids at high pressure).

TYPES OF IN SITU MINERALIZATION
In situ mineralization can be divided into three broad categories: (1) CO2 removal from air withmineral storage (CO2 removal), (2) CO2 capture from point sources with mineral storage (CO2
storage) and (3) hybrid approaches. The key distinguishing feature of these different categories iswhether mineralization itself is removing CO2 from the atmosphere (removal and hybrid
approaches) or whether CO2 is being removed or captured using other technologies (storage).

Figure 3.1. In Situ Mineralization.

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1. CO2 removal from air with mineral storage (CO2 removal)
Water at the Earth’s surface naturally contains dissolved CO2 derived from the atmosphere.
When this water circulates underground through ultramafic rocks, such as “peridotite,” theCO2 in the water reacts with minerals in the rock to form new carbonate minerals. When
returned to the surface, the CO2‐depleted water then absorbs more atmospheric CO2, and theprocess is repeated. This natural process can be replicated and accelerated to increase therate of atmospheric carbon removal by drilling a series of wells to increase the rate of water
circulation and CO2 removal.1 Because the amount of CO2 removed is greater than any
emissions associated with the process, negative emissions can be achieved.
Production and re‐use of carbon‐depleted water can reduce water withdrawal andconsumption. That said, this method is water‐intensive and might best be pursued wherereactive rocks are present along coastlines or beneath the seafloor. Also, because theconcentration of dissolved CO2 in surface waters is low, compression or pumping of surface
water is expensive per ton of CO2 removed. Thus, thermal convection and natural
groundwater flow are small but important drivers for CO2 transport, limiting the flux of fluid
down boreholes and the amount of CO2 removed from air per borehole per year.
2. CO2 captured from point sources with mineral storage (CO2 storage)
CO2 captured from point sources—such as coal or natural‐gas power plants or industrial
facilities that produce process emissions, such as cement and steel plants—can also be stored
through in situ mineralization. Captured CO2 is injected—as a CO2‐rich aqueous fluid or a dense
CO2 fluid at high pressure—into subsurface mafic or ultramafic rocks where it reacts to form
carbonate minerals. In this case, in situ mineralization is a form of geologic carbon storage, not anegative emissions technology in and of itself, and the associated emissions reductions are
considered avoided emissions rather than negative emissions.
To date, two different methods have been used for mineral storage of CO2 captured frompoint sources: injection of dense, supercritical CO2 fluid and injection of CO2 dissolved inwater.


2a. Supercritical CO2: Using this method, captured CO2 is compressed, reaching a density
similar to that of water, and then injected underground. For mineralization to occur, either asmall amount of water must be dissolved into the CO2 fluid or the formation into which the CO2
is injected must contain water. Supercritical CO2 is more buoyant than water at practicallyaccessible depths, so it is crucial that the storage formation is capped by an impermeable
caprock formation that traps the CO2 underground until it mineralizes, known as “structural
trapping.”

2b. Dissolved CO2: Using this method, captured CO2 is dissolved in water and then injectedunderground. The dissolved CO2 is no longer buoyant, providing additional storage security—
known as “solution trapping”—prior to mineralization. This method allows storage informations that do not have an impermeable caprock. The amount of water required todissolve a given amount of CO2 depends on factors including temperature and pressure. For
example, at the CarbFix Phase I project in Iceland, compressed CO2 was bubbled into
descending water in a borehole at a depth of ~ 345 m, corresponding to a water pressure ofNovember 2021

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about 35 atmospheres. At this pressure, approximately 25 tons of water were used to dissolve
1 tCO2. Because the water at this site flows freely downward, only the CO2 requiredcompression in this method (to about 27 bars at the surface), reducing costs substantially. As in
the CO2 removal methods discussed above, the water used for solution trapping can bebrought back to the surface and re‐used.
3. Hybrid approaches
In situ mineralization can also be paired with other carbon dioxide removal (CDR)
technologies, such as direct air capture (DAC) or biomass carbon removal and storage (BiCRS),
to generate negative emissions. The use of nearly pure CO2 gas or CO2 fluid requires priorproduction of purified CO2 via DAC or capture from point sources. However, the purity of CO2‐
bearing gas produced by DAC methods can vary. From a theoretical perspective, the higher
the purity of CO2, the more energy is required for DAC. Thus, for example, enrichment of air to
10 mol% CO2 uses about one third of the energy required to enrich air to 95 mol% CO2.2
As a result, hybrid methods are possible. These methods start with CO2‐enriched gas (flue gas
or CO2‐rich gas produced via partial DAC). CO2 from the enriched gas is then dissolved to form
CO2‐enriched water at depth. Then, the CO2‐enriched water is circulated through reactive
materials. Because CO2 is more soluble in water than other gases that are abundant in air (e.g.,oxygen, nitrogen) and because carbon mineralization further “purifies” CO2 in carbonate
minerals thereby removing it from aqueous fluids, such hybrid methods achieve CO2 capture(from air or flue gas), as well as permanent solid storage. Such hybrid methods can achieve
greater CO2 removal per borehole than CO2 removal alone.1 Storing concentrated CO2
captured from DAC or BiCRS in mafic or ultramafic rocks could also expand the geographic
range for these CDR technologies to places where sedimentary geologic storage is notavailable.

LOCATION OF APPROPRIATE ROCKS AND NATURAL ANALOGS
Ultramafic rocks suitable for CO2 removal from air with mineral storage consist mainly of mantle
peridotite, magnesium‐rich rocks from the Earth’s interior that are exposed at the Earth’s surface
by uplift and erosion. As shown in the map in Figure 1.1, these rocks occur along current andformer tectonic plate boundaries and areas of uplift or extension. Tens to hundreds of trillions oftons of peridotite are located within 3 km of the Earth’s surface, with several particularly promisinglocations, including Oman, the United Arab Emirates, Papua New Guinea, New Caledonia, the east
coast of the Adriatic Sea and the western Mediterranean.3
Mafic rocks suitable for CO2 storage consist primarily of basalts formed through volcanism
associated with tectonic plate boundaries (convergent and divergent) and mantle hot spots. The
majority of the ocean floor is made of basalt, and enormous volcanic deposits known as “floodbasalts” occur on many continents, including at the Columbia River Basalt Group in the UnitedStates, the Deccan Traps in India and the Siberian Traps in Russia.4 Volcanic oceanic islands, such as
Iceland and Hawaii, are also primarily composed of basalt.
Natural carbonation of ultramafic rocks occurs in various locations around the world. The best‐studied example is the Samail ophiolite in Oman.5 The Samail ophiolite is a block of oceanic crust

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and underlying mantle peridotite that is now exposed at the Earth’s surface due to tectonic
processes, faulting and erosion. The mantle peridotite has been partially altered throughinteraction with heat and water to serpentinite, and both the peridotite and the serpentinite show
abundant evidence of carbon mineralization. Approximately 104–105 tons of atmospheric CO2 areremoved from air and sequestered each year through carbon mineralization of mantle peridotite in
Oman.6 This result indicates the potential for mineralization to draw down significant quantities ofatmospheric CO2. 7,8

Box 3-1
IN SITU MINERALIZATIONOPPORTUNITIES IN JAPANJapan is ideally suited for in situ (subsurface) carbon mineralization, both for CO2 removal from air andfor CO2 storage. The many active volcanoes on and near the Japanese coast (including in shallow
submarine locations) form young, glassy basalts, which are ideal reactants for CO2. Many locations
along the Japanese coast offer excellent opportunities for the “solution trapping” method of storing
CO2 via mineralization. These sites are visible at a regional scale as coastal “non‐alkaline mafic volcanic
rocks" in the geological maps compiled by Myers and Nakagaki, 2020.7
South‐central Hokkaido, near the southern tip of Cape Erimo‐Misaki, alsopresents significant opportunities for storing CO2 using carbon mineralization.8
This site is ideal for storing CO2 by injecting water into peridotite to form solidcarbonates. If the overlying crustal rocks have low permeability, this site could
also be suitable site for injection of supercritical CO2 into peridotite.
Because this area is remote, direct air capture (DAC) at a nearby site
might be the most suitable source of CO2‐enriched gas in this region.

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CURRENT PROJECTSTo date, two projects have demonstrated in situ mineralization of CO2 in the field: the Wallula pilot
project in Washington State, US and the CarbFix pilot (I) and industrial (II) projects in Iceland.
1. Wallula
The Wallula project in Washington State was conducted by the US Department of Energy’s
Pacific Northwest National Laboratory (PNNL) to test the feasibility of permanently and safely
sequestering CO2 in basalt formations. The project injected approximately 1000 metric tons ofdamp supercritical CO2 into two permeable horizons in the Columbia River Basalt Group at
830–890‐m depth over 3 weeks in 2013. In the two years following injection, analysis ofperiodic fluid samples, geophysical data and sidewall cores all showed that new carbonate
minerals had formed as a result of injecting CO2. Unmineralized CO2 remained beneath thecaprock at the top of the injection zone, indicating that not all of the CO2 had mineralized.9,10
Instead, geophysical data suggest that approximately 60% of the injected CO2 wassequestered via mineralization within two years. The fate of the remaining CO2 is unclear, but
no leakage has been detected. Carbonate minerals that formed as a result of the experimentoccupy only ~ 4% of the available reservoir pore space, so the remaining storage potential isvery large.11
2. CarbFix
The CarbFix project was conceived as a method to safely and permanently capture CO2 andH2S from emissions sources and store it as rock in the subsurface by imitating and accelerating
the natural process of carbon mineralization. The CarbFix I pilot project injected 175 tons of
CO2 from January to March 2012 and 73 tons of 75% CO2‐25% H2S gas mixture (non‐condensable gases from the Hellisheiði geothermal plant) from June to August 2012. Gases
were injected into descending water in a borehole as described above, forming CO2‐richaqueous fluids for solution trapping. Tracer studies showed that 95% of injected CO2 wasmineralized in <2 years.12,13 These encouraging results led to upscaling. Larger‐scale
operations began in 2014, simultaneously capturing CO2 and H2S from two of six high‐pressure
turbines at the Hellisheiði geothermal plant, with plans to eventually capture virtually all
emissions. To date a total of approximately 70,000 metric tons of CO2 have been injected aspart of the CarbFix industrial project, and the project is still operational. In this later, larger‐
scale process, more than 60% of injected CO2 is mineralized.14
CarbFix has also partnered with the Swiss DAC company, Climeworks. Together, they built asmall‐scale pilot DAC facility called Arctic Fox as part of CarbFix II. Following on the success ofthat project, the two companies built a combined DAC–carbon mineralization project called
Orca, which commenced operations in 2021, with a capacity to remove and store 4000 tCO2
per year.15

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UNCERTAINTIES AND BARRIERS TO SCALE-UP
The Wallula and CarbFix projects demonstrated the viability of secure CO2 storage and rapid
mineralization in basalt. Additional and larger‐scale field projects are the next logical step toquantify storage capability and capacity in other regions and to demonstrate that the technology
can accommodate storage volumes that meaningfully contribute to climate mitigation. Such pilot
projects should build on the lessons learned from existing projects. Key research questions for insitu mineralization include the topics discussed below.
1. Monitoring
Unlike surficial and ex situ mineralization, subsurface carbon mineralization processes cannot
be observed directly, and collection of physical samples of mineralization products is more
complicated and costly. Reliable, rapid and cost‐effective indirect methods for characterizing
the rate and extent of carbon mineralization are needed. A variety of measurement,
monitoring and modeling techniques have been deployed at Wallula and CarbFix to assessmineralization reactions and efficiency.9‐14,16‐19 Additional field projects can help constrain
which techniques will be most successful for large‐scale deployment and can contribute todevelopment of strategies to optimize mineralization efficiency under real‐world conditions.
2. Negative and positive feedbacks
Passivation of the mineralization reaction over time is a potential issue for in situ
mineralization projects. The three primary causes of this passivation would be (1) nearlycomplete consumption of rapidly reacting minerals, leaving only less reactive minerals behind;(2) coating of reactive surfaces with non‐reactive minerals and (3) clogging of pore space by
newly precipitated minerals. Temperature and volume changes occurring during
mineralization could lead to thermal‐ and/or mineralization reaction–driven microfracturing of
the host rock, which could help prevent loss of reactivity by opening cracks that expose freshrock surfaces and help maintain permeability.20‐22 Dissolution of minerals in low‐pH, CO2‐richaqueous fluids near the site of injection could also enhance permeability, forming highporosity channels of focused fluid flow via the “reactive infiltration instability”).23 Additional
research is needed to understand the controls on the negative‐feedback passivation andclogging and positive‐feedback cracking and channeling mechanisms.5 Co‐injection of different
fluids at adjacent boreholes could potentially be used to localize mineral precipitation,
reaction‐driven cracking and formation of dissolution channels.24
3. Environmental and human health and safety impacts
Two primary environmental and human health and safety concerns associated with
underground storage of CO2 and other fluids include the following:a. Potential impacts to drinking water aquifers. Supercritical CO2 is buoyant in the subsurface
and can migrate vertically if an open pathway is present, such as a transmissive fault, a newly
formed hydraulic fracture, an acid‐induced dissolution channel (also known as a “wormhole”)
or an improperly constructed, maintained or abandoned wellbore. Such leakage could

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potentially allow injected CO2‐containing solutes derived from the host rock to impact
shallow groundwater or surface water.
b. Injection‐induced earthquakes. Scientist have known for decades that injecting fluids
underground can cause earthquakes by creating increased pore fluid pressure and/orchanges in rock volume that allow faults to slip. As of 2013, no induced earthquakes large
enough to be felt at the surface have been documented as resulting from CO2 storage.25
Both issues have been well‐studied in sedimentary rocks through decades of experience with
underground injection. Best practices and lessons learned from sedimentary settings can beapplied to CO2 storage in basalts and other mafic and ultramafic rocks. Although no impacts
from CO2 storage have been documented to date, the risks scale as the number and size of
projects increase. Methods and practices to monitor for such impacts can be integrated into insitu field projects to help constrain the specific risks of carbon mineralization and to develop
methods to mitigate these risks.
4. Reaction kinetics, rock mechanics and site identification
Additional basic research on topics such as reaction kinetics and rock mechanics of ultramafic
rocks combined with small‐scale field trials (similar in scale to Wallula or CarbFix I) to
characterize the geology, rock properties and mineralization rates of various potential carbon‐
removal sites are crucial to scaling CO2 removal from air with mineral storage.1,5

REFERENCES
1

Peter B. Kelemen et al. Engineered carbon mineralization in ultramafic rocks for CO2 removal from air:
Review and new insights. Chem. Geol. 550, 119628, doi:https://doi.org/10.1016/j.chemgeo.2020.119628(2020).

2

Jennifer Wilcox, Peter C. Psarras & Simona Liguori. Assessment of reasonable opportunities for direct air
capture. Environ. Res. Lett. 12, 065001, doi:10.1088/1748‐9326/aa6de5 (2017).

3Peter B. Kelemen et al. Rates and Mechanisms of Mineral Carbonation in Peridotite: Natural Processes andRecipes for Enhanced, in situ CO2 Capture and Storage. Annu. Rev. Earth Planet. Sci. 39, 545‐576,
doi:10.1146/annurev‐earth‐092010‐152509 (https://www.annualreviews.org/doi/abs/10.1146/annurev‐
earth‐092010‐152509) (2011).

4

B. Peter McGrail et al. Potential for carbon dioxide sequestration in flood basalts. Journal of GeophysicalResearch: Solid Earth 111, doi:https://doi.org/10.1029/2005JB004169 (2006).

5

National Academies of Sciences (Engineering and Medicine) (NASEM). Negative Emissions Technologies andReliable Sequestration: A Research Agenda. (Washington, DC: The National Academies Press, 2019;https://doi.org/10.17226/25259).

6

Peter B. Kelemen & Jürg Matter. In situ carbonation of peridotite for CO(2) storage. Proc Natl Acad Sci U S A
105, 17295‐17300, doi:10.1073/pnas.0805794105 (2008).

7

Corey Myers & Takao Nakagaki. Direct mineralization of atmospheric CO2 using natural rocks in Japan.
Environ. Res. Lett. 15, 124018, doi:10.1088/1748‐9326/abc217 (2020).

November 2021

26

8

Hiroshi Yamamoto et al. Subhorizontal tectonic framework of the Horoman peridotite complex andenveloping crustal rocks, south‐central Hokkaido, Japan. Island Arc 19, 458‐469,
doi:https://doi.org/10.1111/j.1440‐1738.2010.00714.x (2010).

9

B. P. McGrail et al. Wallula Basalt Pilot Demonstration Project: Post‐injection Results and Conclusions.
Energy Procedia 114, 5783‐5790, doi:https://doi.org/10.1016/j.egypro.2017.03.1716 (2017).

10

B. P. McGrail, F. A. Spane, E. C. Sullivan, D. H. Bacon & G. Hund. The Wallula basalt sequestration pilot
project. Energy Procedia 4, 5653‐5660, doi:https://doi.org/10.1016/j.egypro.2011.02.557 (2011).

11

Signe K. White et al. Quantification of CO2 Mineralization at the Wallula Basalt Pilot Project. Environ. Sci.
Technol. 54, 14609‐14616, doi:10.1021/acs.est.0c05142 (2020).

12

Juerg M. Matter et al. Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide
emissions. Science 352, 1312‐1314, doi:10.1126/science.aad8132 (2016).

13

Sandra Ó Snæbjörnsdóttir et al. The chemistry and saturation states of subsurface fluids during the in situ
mineralisation of CO2 and H2S at the CarbFix site in SW‐Iceland. Int. J. Greenh. Gas Control 58, 87‐102,
doi:https://doi.org/10.1016/j.ijggc.2017.01.007 (2017).

14

Ingvi Gunnarsson et al. The rapid and cost‐effective capture and subsurface mineral storage of carbon andsulfur at the CarbFix2 site. Int. J. Greenh. Gas Control 79, 117‐126,
doi:https://doi.org/10.1016/j.ijggc.2018.08.014 (2018).

15

CarbFix. Our Story, <https://www.carbfix.com/our‐story> (Accessed September 2021).

16

Edda S. P. Aradó r, Eric Sonnenthal, GrÌmur Bjˆrnsson & Hannes JÛnsson. Mul dimensional reac ve
transport modeling of CO2 mineral sequestration in basalts at the Hellisheidi geothermal field, Iceland. Int. J.
Greenh. Gas Control 9, 24‐40 (2012).

17

Eric H. Oelkers et al. Using stable Mg isotope signatures to assess the fate of magnesium during the in situ
mineralisation of CO2 and H2S at the CarbFix site in SW‐Iceland. Geochimica et Cosmochimica Acta 245, 542‐
555, doi:https://doi.org/10.1016/j.gca.2018.11.011 (2019).

18

Sandra Ó Snæbjörnsdóttir, Sigurdur R. Gislason, Iwona M. Galeczka & Eric H. Oelkers. Reaction pathmodelling of in‐situ mineralisation of CO2 at the CarbFix site at Hellisheidi, SW‐Iceland. Geochimica et
Cosmochimica Acta 220, 348‐366, doi:https://doi.org/10.1016/j.gca.2017.09.053 (2018).

19

Philip A. E. Pogge von Strandmann et al. Rapid CO2 mineralisation into calcite at the CarbFix storage sitequantified using calcium isotopes. Nat. Commun. 10, 1983, doi:10.1038/s41467‐019‐10003‐8 (2019).

20

John F. Rudge, Peter B. Kelemen & Marc Spiegelman. A simple model of reaction‐induced cracking applied toserpentinization and carbonation of peridotite. Earth and Planetary Science Letters 291, 215‐227,
doi:https://doi.org/10.1016/j.epsl.2010.01.016 (2010).

21Peter B. Kelemen & Greg Hirth. Reaction‐driven cracking during retrograde metamorphism: Olivine
hydration and carbonation. Earth and Planetary Science Letters 345‐348, 81‐89,
doi:https://doi.org/10.1016/j.epsl.2012.06.018 (2012).

22

Owen Evans, Marc Spiegelman & Peter B. Kelemen. Phase‐Field Modeling of Reaction‐Driven Cracking:Determining Conditions for Extensive Olivine Serpentinization. Journal of Geophysical Research: Solid Earth
125, e2019JB018614, doi:https://doi.org/10.1029/2019JB018614 (2020).

November 2021

27

23

E. Aharonov, M. Spiegelman and P.B. Kelemen. Three‐dimensional flow and reaction in porous media:
Implications for the Earth's mantle and for sedimentary basins. Journal of Geophysical Research 102 (B7),14821‐14833 (1997;
http://science.whoi.edu/labs/mclean210/kelemen/pages/Aharonov%20et%20al%20JGR%2097.pdf).

24Peter B. Kelemen, Heather Savage, Theodore A. Koczynski & Columbia University. Methods and systems forcausing reaction driven cracking in subsurface rock formations. US Patents 20150129209A1 & 9657559B2.

25

National Research Council. Induced Seismicity Potential in Energy Technologies. (The National Academies
Press, 2013; https://doi.org/10.17226/13355).

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CHAPTER 4:
ENHANCED ROCK WEATHERING
Introduction and fundamentals“Weathering” is the breakdown or dissolution of rocks at the Earth’s surface and has been an
important regulator of atmospheric carbon dioxide (CO2) over geologic time.1,2 This weathering
process involves the reaction of CO2 in rainwater with rocks, dissolving out calcium andmagnesium. These elements and dissolved CO2 are transported in surface and ground waters to
the oceans, where “carbonate minerals” may form, trapping the CO2 in a solid and permanently
removing it from the atmosphere.
This form of carbon mineralization occurs at vast spatial and temporal scales. The current level of
CO2 removal through natural silicate weathering is approximately 0.3 Gt of atmospheric CO2 peryear.1,3
Enhanced rock weathering (ERW) aims to dramatically accelerate this natural CO2 removal process.
This approach involves applying rock powder with the ability to react with CO2 over large land
areas, along coasts, or in the oceans.4 ERW raises the alkalinity of soils and waters, thereby drawing
down CO2 from the atmosphere and storing it as a dissolved phase in water and solid carbonate
mineral.3,5,6
ERW can have positiveenvironmental
impacts beyond removing CO2
from the atmosphere, including
neutralizing acidification. CO2
emissions cause acidification in a
number of ecosystems, including
the oceans, since CO2 dissolvedin water produces weak carbonicacid (H2CO3). Spreading alkaline
rock powder neutralizes thisacidity, similar to the use of lime
for neutralizing acid rain causedby sulfur dioxide (SO2) emissions.
Suitable rock types for ERW
include mafic and ultramafic
rocks such as basalt, dunite andperidotite. These rocks containalkaline minerals such as olivine,a magnesium silicate.7‐12 The key
selection criterion is that the rock Figure 4.1. Enhanced Rock Weathering.

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must contain an abundance of alkaline Mg‐ and Ca‐bearing
minerals that readily weather at the Earth’s surface, thus
providing relatively fast CO2 removal rates. Many other rocks
and minerals can be utilized for ERW and carbon mineralization,
including serpentine, wollastonite, brucite and kimberlite.13‐16
Ultramafic mine wastes have been shown to sequester
atmospheric CO2 through surface weathering17 and may besuitable for ERW. Furthermore, numerous industrial wastes,
including blast furnace and steel slag, cement kiln dust, andwaste concrete, are reactive with CO2 and could be used as an
ERW feedstock instead of natural rock.18 In addition to properties of the rock powder (e.g.,mineralogy and grain size), a dispersal site’s climate and environmental conditions (e.g., soil
properties) will significantly influence the rates of CO2 removal. Proposed sites for ERW include
arable lands,7 oceans,11 coastal areas19 and degraded lands, such as forests that have been
impacted by acid rain.20
For the abundant silicate minerals that offer the greatest capacities for CO2 sequestration, slow
dissolution rates are expected to be the main rate‐limiting factor of ERW and carbon
mineralization.21 The dissolution rates of minerals can vary by orders of magnitude. Selecting rockwith reactive minerals is therefore paramount. Finer grain sizes with higher surface areas can help
minimize this limitation.22
ERW is an attractive option for carbon removal for several reasons. First, ERW is a low‐tech processthat mimics natural carbon mineralization. Second, ERW offers essentially unlimited CO2
sequestration capacity. Third, ERW utilizes existing technology in industries such as mining andagriculture. Finally, ERW does not compete with other land uses (in contrast with some othernegative emissions technologies).
ERW has several parallels with the direct air capture (DAC) technologies that Climeworks andCarbon Engineering are developing.23 These technologies use alkaline sorbents that absorb CO2
from the atmosphere. Alkaline powdered rock also “absorbs” CO2 through weathering andcarbonation. Although alkaline powdered rocks are less reactive than DAC sorbents, these rocks
are abundant on Earth and inexpensive to quarry and grind without special engineering or
technologies. Another important distinction between these approaches is that ERW accomplishes
both CO2 capture and storage, whereas DAC technologies only capture CO2, requiring subsequentstorage or use via other processes.

Capacities and co-benefits
ERW using natural bedrock has an essentially unlimited capacity to sequester CO2. For instance, thepotential for CO2 removal using dunite and basalt powders applied over agricultural lands could be
95 and 4.9 GtCO2 per year, respectively,22 assuming use of all the vast agricultural lands in warm
(5.1 × 106 km2) and temperate (2.8 × 106 km2) regions. Consistent with this approximation, anotherestimate suggests that 0.5–2.0 GtCO2 per year could be removed from the atmosphere if dispersed
basalt powder were applied to 10–50% of global cropland.24 Similarly, dispersal of basalt dust over
all of the Earth’s hinterland (55 million km2) could remove 2.5 GtCO2 per year over 50 years,

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including the estimated contribution of additional biomass accumulation.25 In coastal
environments, a one‐time application of 26 Gt of olivine powder (12 km3) along beaches is
estimated to have the potential to remove up to 0.3 GtCO2 per year over 30 years.26 As such, thepotential for sequestering CO2 through ERW is immense if scaled through expanded mining andwide dispersal over large areas.
ERW has several co‐benefits that may be initial drivers for its deployment. Applying powdered rockreleases nutrients (such as potassium and phosphorous), stabilizes organic matter and improveswater retention, all of which enhance soil fertility25 and buffer soil acidity. Thus, ERW may benefit
areas historically impacted by acid rain. For example, when applied to an impacted forested
watershed, wollastonite powder neutralized acidity, replenished nutrients and led to sequestrationof 8.5–11.5 tCO2 per hectare.20 For these reasons, agricultural fields are a prime dispersal site,
given their vast land areas and the widely used technology for applying fertilizers. Moreover,
applying rock powder to agricultural fields is an existing practice. For example, pulverized
limestone is used to reduce soil acidity. In addition to improved crop yields, increasing soil alkalinity
also prevents nitrous oxide (N2O) and methane (CH4) loss, further reducing greenhouse gas (GHG)
emissions.24,27 Countries with relatively warm climates and productive agriculture industries, suchas the United States, Brazil, China and India, have the greatest potential to capitalize on ERW in
agricultural soils.24

Current status of field pilots, research needs and costs
Several field pilots have or will soon be initiated to evaluate ERW efficacy, CO2 removal rates andpotential environmental co‐benefits and impacts. Carbon Drawdown28 aims to test the integrationof ERW with other negative emissions technologies, using mixtures of basalt and olivine combinedwith biochar in agricultural fields in Germany and Greece. Carbon Removed29 and greenSand aim toadvance the use of olivine for several ERW applications. Project Vesta30 examines the application ofolivine to coastal areas, thereby trapping CO2 as a soluble phase and directly addressing ocean
acidification. OceanNETs31 aims to advance ocean alkalinization and other negative emissions
technologies, including blue carbon management. In yet another approach, the Future Forest
Company32 is carrying out large‐scale ERW trials in forested areas to better define co‐benefits,potential negative impacts and costs. The Leverhulme Centre for Climate Change Mitigation33 haslaunched several field trials to answer questions related to weathering of basalt dust in agriculturalsoils and how this may increase food and bioenergy crop production.24 In addition, Carleton
College initiated an agricultural pilot to address the lack of data on ERW at the field scale. Similarly,
other field trials have investigated the use of wollastonite powder on agricultural fields for its dual
purposes of enhancing soil fertility and CO2 removal.13 The
findings of these larger and long‐term pilots will enable
researchers to address uncertainties and research needs foradvancing the technology readiness level (TRL) of ERW.
Chief among these needs are methodologies for determining
CO2 removal rates, which depend on rock properties (e.g.,mineralogy), dosages (mass applied per area), andenvironmental conditions of the dispersal site. Although ERW is
technologically simple compared to DAC, carbon accounting isNovember 2021

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more challenging with ERW due to its slow CO2 removal rates and storage of CO2 as either asoluble or solid phase. Furthermore, measuring spatially variable increases in soil carbon content
against a spatially and temporally variable background provides additional challenges, as does the
behavior of open‐system reservoirs (e.g., removal of carbon in crops, crop residues and weeds andremoval of dissolved CO2 from soils in groundwater and surface runoff). Improving and developing
assessment tools to verify carbon sequestration by ERW will require specific monitoring programs
to track carbon or proxies to monitor ERW. Furthermore, verification tools are needed to accountfor the carbon stored as a soluble phase and exported into rivers and oceans, which is likely to bechallenging to track.
Assessing environmental risk to soils, waters, air and ecosystems is important in deploying ERW.
Although alkaline rocks contain nutrients, they may also contain metals that can be toxic if
accumulated in soils and converted into bioactive compounds over time. Nickel and chromium areof the most concern. For example, accumulation of nickel in soils, leached from olivine‐richfeedstock, may be detrimental to local ecosystems; although the levels observed so far are unlikely
to cause harmful effects.26,34 The amount of metal‐loading will depend on the rock powder, dosage
and application frequency. Dust may also be an issue to air quality during or after applying rockpowder. Again, field pilots are necessary to address these concerns and uncertainties and to gain
public acceptance.
The response and role of microbial communities in soils where rock powder is being applied also
require further exploration. Soil microbes may accelerate mineral weathering by producing acids
and chelating agents and by facilitating chemical reactions.35 Soil microbes may also slow mineraldissolution rates by passivating surfaces.10 In addition, plant processes may affect mineralweathering. All these issues will be specific to the soil environment of the application site and,thus, may be highly variable. Further research is also needed on the interaction between inorganicand organic carbon cycling in soils and on the potential impact of ERW on other factors affecting
soil carbon storage, such as crop rotation, low or no‐till practices, and retention of crop residues.
As part of field pilots, mine‐to‐field life‐cycle analyses will improve estimates of costs andadditional GHG emissions, particularly those from mining and transporting rock. For croplandareas, assessments to date suggest that ERW may cost approximately 60 $/tCO2 for dunite and 200$/tCO2 for basalt for cropland areas.22 Most costs are incurred through mining and crushing,followed by those for transport, estimated at 0.05 $/km/t of rock, and distribution, estimated at
12–14 $/t of rock. Dispersal of basalt dust in areas besides croplands will likely be more expensive
due to increased application costs. Estimates range from 100–500 $/tCO2.1 Mining rock in large
masses (megatonnes per year per site) and minimizing transportation distances between sourcesand dispersal sites are essential to lowering costs.

Moving toward implementation
Widespread deployment of ERW will require an enormous increase in mining and dispersal overvast areas, as well as accurately measuring and verifying CO2 storage. In addition to acting as asupplier of feedstock, the mining industry can assist in identifying and evaluating suitable rocks for
ERW and ultimately mining these rocks to sell as a product. An ideal scenario would establish a
local or regional network of ERW sites near mines or quarries with suitable rock.

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Testing protocols for evaluating the suitability and reactivity of powdered rocks for ERW must also
be available.15 A process of approving the safe and effective use of powdered rocks is also
required. Ensuring and demonstrating little to no environmental impact is critical to increasing
public acceptance of the technology.36 Regulations for application will also be needed. These willvary depending on the current land use of a given site (e.g., agricultural or disturbed lands) andmay relate to existing environmental protection laws and restrictions for using solid wastes.37
Furthermore, ERW and carbon mineralization are not included in many existing carbon accounting
schemes (e.g., California LCFS, 45Q). Doing so would raise the profile of ERW as an economically
viable negative emissions technology.
Addressing scientific uncertainties through field trials, evaluating practical issues including costs
through life cycle analyses, and engaging with industry and government are crucial to advancing
ERW as a viable negative emissions technology that can achieve meaningful CO2 removal at a
global scale.

Photograph is from an application of metabasalt powder (~2240 kg/hectare) to a test site of canola stubble near
Wilcox, Saskatchewan, Canada (Courtesy of Ryan MT Brophy, V6 Agronomy Inc.).

REFERENCES
1

Nils Moosdorf, Phil Renforth & Jens Hartmann. Carbon Dioxide Efficiency of Terrestrial Enhanced
Weathering. Environ. Sci. Technol. 48, 4809‐4816, doi:https://doi.org/10.1021/es4052022 (2014).

2

Donald E. Penman, Jeremy K. Caves Rugenstein, Daniel E. Ibarra & Matthew J. Winnick. Silicate weathering
as a feedback and forcing in Earth's climate and carbon cycle. Earth‐Sci. Rev. 209, 103298,
doi:https://doi.org/10.1016/j.earscirev.2020.103298 (2020).

3

Jens Hartmann et al. Enhanced chemical weathering as a geoengineering strategy to reduce atmosphericcarbon dioxide, supply nutrients, and mitigate ocean acidification. Rev. Geophys. 51, 113‐149,
doi:https://doi.org/10.1002/rog.20004 (2013).

4

The Intergovernmental Panel on Climate Change (IPCC). Global Warming of 1.5°C ‐ An IPCC Special Report.
(eds. V. Masson‐Delmotte, P. Zhai, H.‐O. Pörtner, D. Roberts, J. Skea, P. R. Shukla, et al., 2018(https://www.ipcc.ch/sr15/)).

November 2021

33

5

H. D. Holland, B. Lazar & M. McCaffrey. Evolution of the atmosphere and oceans. Nature 320, 27‐33,
doi:10.1038/320027a0 (https://www.nature.com/articles/320027a0) (1986).

6

Lee R. Kump, Susan L. Brantley & Michael A. Arthur. Chemical Weathering, Atmospheric CO2, and Climate.
Annu. Rev. Earth Planet. Sci. 28, 611‐667, doi:10.1146/annurev.earth.28.1.611(https://www.annualreviews.org/doi/abs/10.1146/annurev.earth.28.1.611) (2000).

7David J. Beerling et al. Farming with crops and rocks to address global climate, food and soil security. Nat.
Plants 4, 138‐147, doi:10.1038/s41477‐018‐0108‐y (2018).

8Peter B. Kelemen et al. Rates and Mechanisms of Mineral Carbonation in Peridotite: Natural Processes andRecipes for Enhanced, in situ CO2 Capture and Storage. Annu. Rev. Earth Planet. Sci. 39, 545‐576,
doi:10.1146/annurev‐earth‐092010‐152509 (https://www.annualreviews.org/doi/abs/10.1146/annurev‐
earth‐092010‐152509) (2011).

9

Filip J. R. Meysman & Francesc Montserrat. Negative CO2 emissions via enhanced silicate weathering incoastal environments. Biol. Lett. 13, 20160905, doi:https://doi.org/10.1098/rsbl.2016.0905 (2017).

10Eric H. Oelkers, Julien Declercq, Giuseppe D. Saldi, Sigurdur R. Gislason & Jacques Schott. Olivine dissolution
rates: A critical review. Chem. Geol. 500, 1‐19, doi:https://doi.org/10.1016/j.chemgeo.2018.10.008 (2018).

11

Ioannis Rigopoulos et al. Carbon sequestration via enhanced weathering of peridotites and basalts inseawater. Appl. Geochem. 91, 197‐207, doi:https://doi.org/10.1016/j.apgeochem.2017.11.001 (2018).

12

Lyla L. Taylor et al. Enhanced weathering strategies for stabilizing climate and averting ocean acidification.
Nat. Clim. Chang. 6, 402‐406, doi:https://doi.org/10.1038/nclimate2882 (2016).

13

Fatima Haque, Rafael M. Santos & Yi Wai Chiang. CO2 sequestration by wollastonite‐amended agriculturalsoils – An Ontario field study. Int. J. Greenh. Gas Control 97, 103017,
doi:https://doi.org/10.1016/j.ijggc.2020.103017 (2020).

14

Anna L. Harrison, Ian M. Power & Gregory M. Dipple. Accelerated Carbonation of Brucite in Mine Tailings for
Carbon Sequestration. Environ. Sci. Technol. 47, 126‐134, doi:10.1021/es3012854(https://pubs.acs.org/doi/10.1021/es3012854) (2013).

15

Carlos Paulo et al. Evaluating feedstocks for carbon dioxide removal by enhanced rock weathering and CO2
mineralization. Appl. Geochem. 129, 104955, doi:https://doi.org/10.1016/j.apgeochem.2021.104955 (2021).

16

James G. M. Thom, Gregory M. Dipple, Ian M. Power & Anna L. Harrison. Chrysotile dissolution rates:
Implications for carbon sequestration. Appl. Geochem. 35, 244‐254,
doi:https://doi.org/10.1016/j.apgeochem.2013.04.016 (2013).

17

Siobhan A. Wilson et al. Offsetting of CO2 emissions by air capture in mine tailings at the Mount Keith Nickel
Mine, Western Australia: Rates, controls and prospects for carbon neutral mining. Int. J. Greenh. Gas Control
25, 121‐140, doi:https://doi.org/10.1016/j.ijggc.2014.04.002 (2014).

18

Phil Renforth. The negative emission potential of alkaline materials. Nat. Commun. 10, 1401,
doi:10.1038/s41467‐019‐09475‐5 (2019).

19

Suzanne J. T. Hangx & Christopher J. Spiers. Coastal spreading of olivine to control atmospheric CO2
concentrations: A critical analysis of viability. Int. J. Greenh. Gas Control 3, 757‐767,
doi:https://doi.org/10.1016/j.ijggc.2009.07.001 (2009).

November 2021

34

20

L. L. Taylor et al. Increased carbon capture by a silicate‐treated forested watershed affected by aciddeposition. Biogeosciences 18, 169‐188, doi:10.5194/bg‐18‐169‐2021(https://bg.copernicus.org/articles/18/169/2021/bg‐18‐169‐2021.html) (2021).

21

Ian M. Power et al. Carbon Mineralization: From Natural Analogues to Engineered Systems. Reviews in
Mineralogy and Geochemistry 77, 305‐360, doi:https://doi.org/10.2138/rmg.2013.77.9 (2013).

22

Jessica Strefler, Thorben Amann, Nico Bauer, Elmar Kriegler & Jens Hartmann. Potential and costs of carbon
dioxide removal by enhanced weathering of rocks. Environ. Res. Lett. 13, 034010, doi:10.1088/1748‐
9326/aaa9c4 (https://iopscience.iop.org/article/10.1088/1748‐9326/aaa9c4) (2018).

23

David W. Keith, Geoffrey Holmes, David St. Angelo & Kenton Heidel. A Process for Capturing CO2 from theAtmosphere. Joule 2, 1573‐1594, doi:https://doi.org/10.1016/j.joule.2018.05.006 (2018).

24

David J. Beerling et al. Potential for large‐scale CO2 removal via enhanced rock weathering with croplands.
Nature 583, 242‐248, doi:10.1038/s41586‐020‐2448‐9 (https://www.nature.com/articles/s41586‐020‐2448‐
9) (2020).

25

Daniel S. Goll et al. Potential CO2 removal from enhanced weathering by ecosystem responses to powderedrock. Nature Geoscience 14, 545‐549, doi:10.1038/s41561‐021‐00798‐x (2021).

26

Francesc Montserrat et al. Olivine Dissolution in Seawater: Implications for CO2 Sequestration throughEnhanced Weathering in Coastal Environments. Environ. Sci. Technol. 51, 3960‐3972,
doi:https://doi.org/10.1021/acs.est.6b05942 (2017).

27

Ilsa B. Kantola, Michael D. Masters, David J. Beerling, Stephen P. Long & Evan H. DeLucia. Potential of global
croplands and bioenergy crops for climate change mitigation through deployment for enhanced weathering.
Biol. Lett. 13, 20160714, doi:https://doi.org/10.1098/rsbl.2016.0714 (2017).

28

Carbon Drawdown Initiative. <www.carbon‐drawdown.de/home‐en> (Accessed September 2021).

29

Carbon Removed. <www.carbonremoved.com> (Accessed September 2021).

30

Project Vesta. <www.projectvesta.org> (Accessed 2021).

31

OceanNETs. <www.oceannets.eu> (Accessed September 2021).

32

The Future Forest Company. <www.thefutureforestcomany.com> (Accessed September 2021).

33

The Leverhulme Centre for Climate Change Mitigation. <https://lc3m.org/> (Accessed September 2021).

34

Lennart T. Bach, Sophie J. Gill, Rosalind E. M. Rickaby, Sarah Gore & Phil Renforth. CO2 Removal With
Enhanced Weathering and Ocean Alkalinity Enhancement: Potential Risks and Co‐benefits for Marine Pelagic
Ecosystems. Frontiers in Climate 1, doi:10.3389/fclim.2019.00007(https://www.frontiersin.org/articles/10.3389/fclim.2019.00007/full) (2019).

35

Igor Daniel Alves Ribeiro et al. Use of Mineral Weathering Bacteria to Enhance Nutrient Availability in Crops:
A Review. Front. Plant Sci. 11, doi:https://www.frontiersin.org/articles/10.3389/fpls.2020.590774/full(2020).

36

Elspeth Spence, Emily Cox & Nick Pidgeon. Exploring cross‐national public support for the use of enhanced
weathering as a land‐based carbon dioxide removal strategy. Clim. Change 165, 23,
doi:https://doi.org/10.1007/s10584‐021‐03050‐y (2021).

November 2021

35

37

Romany M. Webb. The Law of Enhanced Weathering for Carbon Dioxide Removal. (Sabin Center for Climate
Change Law, Columbia Law School, 2020;
https://climate.law.columbia.edu/sites/default/files/content/Webb%20‐%20The%20Law%20of%20Enhanced%20Weathering%20for%20CO2%20Removal%20‐%20Sept.%202020.pdf).

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CHAPTER 5:
MINING WASTESMining wastes are an important potential feedstock for carbon mineralization. Carbonation ofmining wastes could remove hundreds of megatons of CO2 from the atmosphere each year in thedecades ahead, as the mining industry grows. Reaching that potential would require investments inresource assessment and mapping; protocols for measurement, monitoring and verification;industry‐government‐university collaborations on research and development (R&D); and favorable
policies. This chapter provides background on the global mining industry and examines thepotential for mining wastes to play a role in climate mitigation.

Mining sector backgroundAn estimated 99 Gt of rock were mined for metals and minerals in 2016 (Table 1). Of this, 8.9 Gt offinely pulverized mineral wastes (“tailings”) were disposed of onsite in waste storage facilities.1 This
amounts to 38 million Olympic swimming pools‐worth of tailings produced in 2016 alone. Of thesemine tailings, on the order of 420 Mt in a given year (~5% of total global production) are fromultramafic ore deposits and are likely to be suitable for carbon mineralization.2
Table 1. Estimated annual production of mine tailings with examples of the scale of stockpiled tailings.1‐4
All rock mined annually

99 Gt/yr

Waste rockOre
Tailings (less reactive to CO2)
Tailings (potentially highly reactive to CO2)

80 Gt/yr
10.2 Gt/yr
8.48 Gt/yr
0.42 Gt/yr

Known stockpiles of chrysotile tailings>2 Gt in North America alone

Expected tailings produced from one large nickel mine over 0.3–1.2 Gt
a 30-year lifetime

At the scale of current production, the maximum carbonation potential of these 420 Mt of tailingsis ~175 MtCO2 per year, assuming all magnesium (Mg) and calcium (Ca) in the tailings are fully
carbonated. Global annual production of ultramafic tailings is projected to at least double by 2100
owing to increasing demand for raw materials to fuel economic growth and the energy transition.
Due to a projected increase in demand for metals, ultramafic mine tailings could contribute toremoval of up to ~300 MtCO2 from the atmosphere each year by the end of this century.5
Carbon mineralization in mining is attractive because it is one of the few industries that extracts
and moves materials on the Gt/year‐scale. Most tailings are finely pulverized, making themattractive for carbon mineralization since little or no additional energy is required for grinding.
Ultramafic mine tailings produce neutral to alkaline mine waters, which are very effective at
dissolving CO2 from air.

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Furthermore, decarbonizing the mining sector may require
negative emissions technologies due to the role of fossil energy
in fueling heavy‐duty vehicles, such as mine trucks, and ingenerating electric power at remote locations without
significant, year‐round renewable resources. Carbon
mineralization can play an important role in helping mining
companies reach net‐zero emissions.

CO2 removal with mine tailings
Most ultramafic tailings are produced by nickel, cobalt andplatinum group element mining, with lesser amounts comingfrom production of chromium, diamonds and talc. Historically,
Heavily carbonated nickel minechrysotile asbestos (“white asbestos”) production contributed
6
tailings
from the Mount Keith~20–80 Mt of ultramafic tailings per year. Although asbestos
mine, Western Australia.4
mining has stopped in most jurisdictions, some governments
are now left with the liability of stockpiled chrysotile minetailings. For instance, an estimated 5–8 Mt and more than 2 Gt of chrysotile asbestos mine waste
remains unremediated in the United States and Canada, respectively (reviewed by Power et al.,2013).2 Furthermore, a large modern nickel mine can produce on the order of 10–40 Mt of tailings
per year over an operational lifetime of several decades.3,4 Therefore, many more Gt of reactive
material have certainly been produced within the past few centuries, although the exact scale ofthis stockpiled resource and its reactivity to CO2 remain uncertain.
Many published estimates of the CO2 sequestration potential of alkaline wastes, including minetailings, assume that essentially every atom of magnesium and calcium can be used to bind CO2,
turning it into a solid carbonate mineral. However, the efficiency of tailings carbonation is strongly
dependent on rates of mineral dissolution and CO2 supply. Every sample of mine tailings is a
mixture of several distinct magnesium‐ and calcium‐bearing materials, each with its own
dissolution rate and reactivity to CO2, and this holds true for all alkaline wastes. Ultramafic minetailings can have four distinct styles of reactivity with CO2, some of which bind CO2 very quickly andwith little engineering and others that are slower and require a more engineered approach toaccelerate reaction.
CO2 can be mineralized in ultramafic mine tailings by four different processes:
1. Fast carbonation of the magnesium hydroxide mineral, brucite, which typically occurs at an
average abundance of less than 5% by weight in serpentinite mine tailings.
2. Fast absorption of CO2 by hydrotalcite minerals, which are found at up to ~10% abundance by
weight in some, but not all, nickel mine tailings.
3. Fast cation exchange reactions of swelling clays, which can be used to bind CO2 but must be
driven by addition of salt treatments to tailings.7
4. Relatively slow dissolution of calcium and magnesium silicate minerals (olivine, serpentines andpyroxenes), which can be accelerated by adding strong acids (e.g., hydrochloric, sulfuric) during
processing or tailings management.

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Both brucite and hydrotalcites have been shown to remove CO2 from air in mine tailings storage
facilities without any intervention. In one reported case at the Mount Keith nickel mine in Australia,
these minerals are removing between 39,800 tCO2 and 79,800 tCO2 per year from air, offsetting
11–22% of the mine’s annual greenhouse gas (GHG) emissions.4,8 This annual rate is on the orderof the amount of CO2 sequestered by the Carbfix project between 2014 and 2021.9 The measured
sequestration rate at Mount Keith could be doubled simply by depositing the tailings in thinnerflows to allow the remaining, unreacted brucite to bind CO2 from the atmosphere.
Owing to the differential in reactivity among minerals in ultramafic mine tailings and to theextraordinarily fast reaction of brucite with CO2, it has become conventional to think about thepercentage of emissions offsets that can be achieved by a single mine or mining company using
brucite carbonation alone. Every ore deposit or mine has its own “grade” of brucite that would
make mining CO2 neutral. For instance, the cut‐off brucite grade for opening a carbon‐neutral mine
could be calculated using the tCO2e emissions per year for mine operations and the mass of tailings
produced annually (Box 5.1). Nevertheless, in typical ultramafic tailings, >90% of the total carbon
mineralization potential comes from silicate minerals, and deposits do not always contain brucite.
Thus, the slow reactivity of abundant magnesium and calcium silicate minerals in ultramafic minetailings must be accelerated to turn mining into a negative emissions industry.

Location of resource and reserves
The amount of ultramafic rock at and near Earth’s surface is vast. The mining concepts of“resource” and “reserve” provide a measure of what rocks are well understood in terms of their
quality, quantity and economic viability of extraction (resource) and what rocks are known to holda resource and could feasibly be mined using existing technology (reserve). The amount of theresource is generally much greater than that of the better‐characterized reserve.
All ultramafic rocks contain nickel, but they do not always contain other desirable metal resources.
The scale of ultramafic rock that could potentially be mined
economically in part for carbon mineralization can therefore be
estimated from Earth’s known nickel resources and reserves.
The world’s largest producers of nickel include Indonesia, thePhilippines, Russia, New Caledonia, Australia, Canada, China andBrazil (Figure 5‐1). Globally, 2.5 Mt of nickel were produced fromserpentinites and nickel laterites (highly weathered serpentinites)in 2020,10 up from about 1 Mt in 1990. Known reserves amount to
94 Mt of nickel with an estimated global resource of 300 Mt ofnickel in land‐based deposits averaging ≳0.5% nickel by weight.
This means that the known and accessible amount of serpentinites
and laterites with a nickel grade of ≳0.5% by weight is likely on theorder of 60 Gt of rock. However, lower‐grade nickel resources are
becoming economical to mine.11

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Box 5-1
CALCULATING THE BRUCITE GRADEREQUIRED FOR CARBON NEUTRALITY
Gradebrucite, the average wt% abundance of brucite required to completely offset the CO2 emissions froman operating mine, can be calculated using Eq. 1:
𝐺𝑟𝑎𝑑𝑒

𝑚
𝑚

∙

𝑀
𝑀

∙ 100 wt%

Eq. 1

Where mCO2 and mtailings are the masses of annual CO2 emissions and annual tailings production from a mine,
respectively. Mbrucite and MCO2 are the molar masses of brucite and CO2. This equation assumes that amagnesium carbonate mineral, such as nesquehonite, that contains a 1:1 molar ratio of magnesium to CO2
is produced by brucite carbonation. Eq. 1 can be rewritten for other product minerals, such ashydromagnesite, that contain a lower proportion of CO2 relative to magnesium.
This equation can be simplified as Eq. 2 for routine use by inputting the constant values of the molar
masses:𝐺𝑟𝑎𝑑𝑒𝑚𝑚∙ 1.325 ∙ 100 wt%

Eq. 2

Example: Using published values from the Mount Keith nickel mine in Australia4,11 yields an average carbon
neutral brucite grade of 4.5 wt% (Eq. 3).
𝐺𝑟𝑎𝑑𝑒370,000 t∙ 1.325 ∙ 100 wt%
11,000,000 t

4.5 wt% Eq. 3

For instance, the giant Decar and Dumont nickel projects in Canada are under development tomine nickel at grades between 0.1 and 0.3 wt%.12,13 The Dumont Project in Québec will produce
more than 2 Mt of nickel and an estimated 1.18 Gt of ultramafic mine tailings.13,14 Consequently,
assuming nickel demand continues to grow, future estimates of resources and reserves for nickel
and ultramafic rock must take into account such low‐grade, high‐tonnage deposits.
Although platinum group element deposits commonly contain some olivine‐ and serpentine‐richultramafic rock, the bulk of the ore mined from these settings is relatively unreactive pyroxenites
and mafic gabbros.15 As such, targeted processing of serpentinite units for carbon mineralization
would need to be considered at platinum group element mines using the sort of carbonation
potential maps that are being developed by Vanderzee et al. (2019).16 Locally, other ultramafic‐
hosted resources may be more abundant than nickel and platinum group elements. For instance,

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Figure 5.1. Major world producers of nickel with their percentage production.

Botswana and South Africa could implement carbon mineralization in their world‐class diamondmines.
There is potential to build capacity for carbon mineralization across the mining sector, which
operates in dozens of countries around the world. Surveys are needed to assess carbonation
grades, reactivity styles, and the resource and reserve of minerals that bind CO2. Evaluations of theresource and reserve of critical metals found in ultramafic rock are also needed. Mapping of theseresources should include tailings from former mine sites, ore and tailings from operating mines,
and prospective ore from greenfield mineral exploration. Tailings from nickel, cobalt, chromium,diamond and chrysotile asbestos mines should be prioritized as these are likely to be the mostreactive. Tailings produced by platinum group element mines would be the next priority. However,understanding the deportment and mineralogy of ultramafic rocks within these ore deposits may
be more useful for future mine planning (after Vanderzee et al., 201916). Countries with rich
deposits of these metals and minerals and strong industry expertise, including but not limited toAustralia, Botswana, Brazil, Canada, China, Russia, South Africa and the United States, could takethe lead in mapping their resources and reserves for carbon mineralization in both tailings and ore
deposits (e.g., Figure 5‐217,18). Much of this work is likely to fall under the auspices of national andregional geological surveys and could be done in partnership with the mining and university sectors
to build capacity.

Current status of carbon mineralization in the mining industry
Increasingly, major mining companies are making the commitment to reach net‐zero operational
GHG emissions (Scopes 1 and 2) within the coming decades. For instance, Anglo American19 hascommitted to achieving carbon‐neutral mining by 2040, and both BHP20 and Rio Tinto21 have

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Figure 5‐2. Example country maps for South Africa and Canada highlighting geographic locations of nickel, cobalt, diamondand platinum group element resources hosted by ultramafic rock suitable for carbon mineralization (reproduced from The
Mining Association of Canada, 202017; South African Council for Geoscience, 200218).

committed to reaching net‐zero emissions by 2050. De Beers,22 the world’s largest diamond miner,
has committed to reaching net‐zero operations nine years from now, in 2030. Carbon
mineralization is one pathway that is being considered by major and junior mining companies toreach their climate ambitions.
Carbon mineralization in the mining industry is currently at the stage of small field demonstrations
on the 1‐m3 to 200‐m3 scale. The earliest reports of field results have come from the following
mines and projects:
1. The closed Thetford mines (Québec, Canada) where field trials were done to monitor
passive, unaccelerated CO2 removal from air into chrysotile‐bearing serpentinite tailings.23
2. The pre‐mining Dumont project (Québec, Canada) where pilots were carried out to monitor
passive, unaccelerated CO2 removal from the atmosphere into nickel‐bearing serpentinite
tailings.13,24
3. The closed Woodsreef mine (New South Wales, Australia) where field trials for accelerated
tailings dissolution and microbial carbonation were done in nickel‐ and chrysotile‐bearing
serpentinite tailings.25,26
4. Since the mid‐ to late 2010s, field experiments for accelerated carbon mineralization inmine tailings have scaled up in Canada and South Africa as part of De Beers’ Project
CarbonVault.27,28
Perhaps the most important outcome of past experiments is the observation that more field trials
are needed to refine monitoring techniques, optimize acceleration strategies and account for

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variability in tailings composition, local environmental conditions and mining practices. Other key
findings from these first field trials confirm that mineral dissolution rates and slow transport of CO2
into deeper parts of mine tailings are the key constraints on carbon mineralization rates. These
trials show that accelerated carbon mineralization requires interventions such as (1) using acids toenhance the dissolution rates of silicate minerals and (2) using calcifying microorganisms or gas
streams containing higher concentrations of CO2 to supply more CO2 into tailings.
At present, field‐based acceleration of carbon mineralization stands at a technology readiness level(TRL) of 6 or 7, depending on the acceleration pathway. Physical, chemical and biologicalapproaches to enhancing the reactivity of ultramafic mine tailings to CO2 have been tested underoperationally relevant conditions in laboratories and the field. These pathways include injection ofconcentrated CO2 into tailings, the use of bioreactors employing benign microorganisms toaccelerate tailings dissolution and carbonation, and a hybrid approach to tailings carbonation thatleverages enhanced rock weathering in soils.

Critical metal recovery and supply security
Carbon neutrality cannot be achieved without increasing mining globally because metals are
essential components of the clean energy technologies that must be deployed at vastly increased
scale for the world to achieve carbon neutrality.29 These metals include nickel, platinum, palladiumand others that are typically mined from ultramafic rocks.
Carbon mineralization can be used for more efficient production of critical metals for clean energy
technologies. Several recent studies have shown that employing carbon mineralization as an ore‐processing technology improves recovery of nickel. For instance, Khan et al. (2021)30 describe amodel system that employs high‐pressure and ‐temperature carbonation prior to nickel sulfide
concentration using froth flotation. Their laboratory experiments show that both the rate andefficiency of nickel recovery improve for carbonated ore compared to uncarbonated ore. Wang etal. (2021)31 have developed a method that simultaneously carbonates nickel ore while converting
nickel‐bearing silicates—which were previously impossible to process for nickel—into nickel
sulfides that can be recovered easily using existing technology. Hamilton et al. (2020, 2021)25,32
have demonstrated an acid heap‐leaching technique that concentrates nickel, cobalt and othercritical metals for recovery using conventional technologies while producing a separate, high‐magnesium leachate that is ideal for carbonation. The anticipated doubling in the scale of mining
from ultramafic ore deposits by the year 21005 offers an opportunity to expand the scale of carbon
mineralization while also reframing it as a more effective and efficient approach to ore processing.

Scaling up
For carbonation of mine tailings to play a meaningful role in climate change mitigation, several
steps will be required.
First, industry‐government‐university collaborations on R&D will be needed to foster technology
transfer since much of the science behind mine tailings carbonation currently comes out of
university laboratories.
Second, tools for accurate measurement and monitoring of carbon mineralization in mine tailings
will need to be refined, tested at scale and deployed. Procedures for measuring and monitoring

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CO2 uptake into mine tailings are
now almost ready for use incommercial laboratories. Ideally,
carbon mineralization potential ofmine tailings will soon be measured
just as routinely as their acid‐generating or acid‐neutralization
potential are measured today.
Resources should be invested intraining the skilled analytical
workforce needed to manage thismeasurement and monitoring.

The Mount Keith nickel mine in Western Australia.
(photo courtesy of BHP)

Third, a recognized international standard for verifying CO2 sequestration in mine tailings will beneeded. Some carbon registries have developed preliminary protocols for verifying CO2 removal
through mineralization. These protocols will need to be refined for use in the mining industry andgain widespread acceptance. Development work is also needed to certify international standards
that can be used by commercial labs for quality assurance and quality control. Bodies such as theInternational Standards Organization and the US National Institute of Standards and Technology
could provide standard methods and standard reference materials for this purpose.
Fourth, common tools for technoeconomic analysis (TEA) and life‐cycle assessment (LCA) will beneeded. These tools would support decision making at pre‐commercial stages and de‐risk
investment by mining companies that wish to implement carbon mineralization at their operations.
Finally, supportive policies will be required. Governments should examine their regulatory
frameworks for mining operations, seeking ways to provide incentives for carbon mineralization, as
well as streamlining permitting processes while protecting public health and safety.

REFERENCES
1

Elaine Baker, Michael Davies, Andy Fourie, Gavin Mudd & Kristina Thygesen. “Mine tailings facilities:
overview and industry trends.” in Towards Zero Harm – A Compendium of Papers Prepared for the Global
Tailings Review. (August 2020 (https://globaltailingsreview.org/ch‐ii‐mine‐tailings‐facilities_overview‐and‐industry‐trends/)).

2

Ian M. Power et al. Carbon Mineralization: From Natural Analogues to Engineered Systems. Reviews in
Mineralogy and Geochemistry 77, 305‐360, doi:https://doi.org/10.2138/rmg.2013.77.9 (2013).

3

Ian M. Power, Gregory M. Dipple, Peter M. D. Bradshaw & Anna L. Harrison. Prospects for CO2
mineralization and enhanced weathering of ultramafic mine tailings from the Baptiste nickel deposit in
British Columbia, Canada. Int. J. Greenh. Gas Control 94, 102895,
doi:https://doi.org/10.1016/j.ijggc.2019.102895 (2020).

4

Siobhan A. Wilson et al. Offsetting of CO2 emissions by air capture in mine tailings at the Mount Keith Nickel
Mine, Western Australia: Rates, controls and prospects for carbon neutral mining. Int. J. Greenh. Gas Control
25, 121‐140, doi:https://doi.org/10.1016/j.ijggc.2014.04.002 (2014).

November 2021

44

5Phil Renforth. The negative emission potential of alkaline materials. Nat. Commun. 10, 1401,
doi:10.1038/s41467‐019‐09475‐5 (2019).

6

Erin R. Bobicki, Qingxia Liu, Zhenghe Xu & Hongbo Zeng. Carbon capture and storage using alkaline industrial
wastes. Progress in Energy and Combustion Science 38, 302‐320,
doi:https://doi.org/10.1016/j.pecs.2011.11.002 (2012).

7

Zeyen et al. (2020).

8

Stuart J. Mills et al. The crystal structure of stichtite, re‐examination of barbertonite, and the nature ofpolytypism in MgCr hydrotalcites. American Mineralogist 96, 179‐187, doi:doi:10.2138/am.2011.3531(2011).

9

CarbFix. <https://www.carbfix.com> (Accessed September 2021).

10

USGS: National Minerals Information Center. Nickel Statistics and Information,<https://www.usgs.gov/centers/nmic/nickel‐statistics‐and‐information> (Accessed September 2021).

11

B.A. Grguric. Minerals of the MKD5 nickel deposit, Mount Keith, Western Australia. Australian Journal of
Mineralogy 9, 55‐71 (2003).

12

Ron Britten. Regional Metallogeny and Genesis of a New Deposit Type—Disseminated Awaruite (Ni3Fe)
Mineralization Hosted in the Cache Creek Terrane☼. Economic Geology 112, 517‐550,
doi:10.2113/econgeo.112.3.517 (2017).

13

A. Gras et al. Isotopic evidence of passive mineral carbonation in mine wastes from the Dumont Nickel
Project (Abitibi, Quebec). Int. J. Greenh. Gas Control 60, 10‐23,
doi:https://doi.org/10.1016/j.ijggc.2017.03.002 (2017).

14

L.P. Staples, Bowen, J.M., Bernier, S.B., Warren, D.A., Scott, C.C., Duncan, J.F., Murphy, B.A., Bertrand, V.J.,
Scott, K.C., Latulippe, S. . Technical report on the Dumont Ni project. Launay and Trécesson Townships,
Québec, Canada. (2013 (https://www.miningdataonline.com/reports/Dumont_Feasibility_07252013.pdf)).

15

N. A. Meyer et al. Mineral carbonation of PGM mine tailings for CO2 storage in South Africa: A case study.
Minerals Engineering 59, 45‐51, doi:https://doi.org/10.1016/j.mineng.2013.10.014 (2014).

16

S.S.S. Vanderzee, G.M. Dipple & P.M.D. Bradshaw. Targeting Highly Reactive Labile Magnesium in Ultramafic
Tailings for Greenhouse‐Gas Offsets and Potential Tailings Stabilization at the Baptiste Deposit, Central
British Columbia (NTS 093K/13, 14). In Geoscience BC Summary of Activities 2018: Minerals and Mining,Geoscience BC, Report, 109–118 (2019(http://cdn.geosciencebc.com/pdf/SummaryofActivities2018/MM/Schol_SoA2018_MM_Vanderzee.pdf)).

17

Mining Association of Canada. Facts and Figures 2020, <https://mining.ca/documents/facts‐figures‐2020/>(2020).

18

South African Council for Geosciences. Mineral Locality Maps of South Africa,<https://www.geoscience.org.za/index.php/publication/downloadable‐material > (2002).

19

AngloAmerican. Climate Change & Mining,<https://www.angloamerican.com/sustainability/environment/climate‐change> (Accessed September2021).

20

BHP Billiton. Climate Change, <https://www.bhp.com/sustainability/climate‐change> (Accessed September2021).

November 2021

45

21

Rio Tinto. Climate Change, <https://www.riotinto.com/en/sustainability/climate‐change> (Accessed
September 2021).

22

De Beers Group. Protecting the Natural World: Our 2030 Goals Explained,<https://www.debeersgroup.com/sustainability‐and‐ethics/building‐forever‐2030‐goals/protecting‐the‐natural‐world‐goals> (Accessed September 2021).

23

Karl Lechat, Jean‐Michel Lemieux, John Molson, Georges Beaudoin & Réjean Hébert. Field evidence of CO2
sequestration by mineral carbonation in ultramafic milling wastes, Thetford Mines, Canada. Int. J. Greenh.
Gas Control 47, 110‐121, doi:https://doi.org/10.1016/j.ijggc.2016.01.036 (2016).

24

A. Gras, G. Beaudoin, J. Molson & B. Plante. Atmospheric carbon sequestration in ultramafic mining residues
and impacts on leachate water chemistry at the Dumont Nickel Project, Quebec, Canada. Chem. Geol. 546,
119661, doi:https://doi.org/10.1016/j.chemgeo.2020.119661 (2020).

25

J. L. Hamilton et al. Carbon accounting of mined landscapes, and deployment of a geochemical treatment
system for enhanced weathering at Woodsreef Chrysotile Mine, NSW, Australia. Journal of Geochemical
Exploration 220, 106655, doi:https://doi.org/10.1016/j.gexplo.2020.106655 (2021).

26

Jenine McCutcheon, Connor C. Turvey, Siobhan A. Wilson, Jessica L. Hamilton & Gordon Southam.
Experimental Deployment of Microbial Mineral Carbonation at an Asbestos Mine: Potential Applications to
Carbon Storage and Tailings Stabilization. Minerals 7, 191 (2017).

27

Evelyn M. Mervine et al. Potential for offsetting diamond mine carbon emissions through mineralcarbonation of processed kimberlite: an assessment of De Beers mine sites in South Africa and Canada.
Mineralogy and Petrology 112, 755‐765, doi:10.1007/s00710‐018‐0589‐4 (2018).

28

Robert F. Service. The Carbon Vault: Industrial waste can turn planet‐warming carbon dioxide into stone.
Science, doi:10.1126/science.abe6031 (https://www.science.org/content/article/industrial‐waste‐can‐turn‐planet‐warming‐carbon‐dioxide‐stone) (2020).

29

Olivier Vidal, Fatma Rostom, Cyril François & Gael Giraud. Global Trends in Metal Consumption and Supply:
The Raw Material–Energy Nexus. Elements 13, 319‐324, doi:10.2138/gselements.13.5.319 (2017).

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CHAPTER 6:
INDUSTRIAL WASTES
Overview
Several industrial processes generate large volumes of waste suitable for carbon mineralization.
Process changes and innovation in the management of these wastes could help permanently store
large amounts of CO2, reduce local environmental hazards and cut disposal costs—a “triple win.”
Industries with significant potential for carbon mineralization include iron and steel production,coal power generation, solid waste combustion and phosphoric acid production, which collectively
generate billions of tons of waste per year globally. These and other industries have large legacy
stockpiles of waste, collectively totaling billions more tons globally. These industrial wastes containsignificant amounts of calcium (Ca), magnesium (Mg) and other minerals that can react with CO2 toform stable mineral carbonates. The wastes are often located near point sources of CO2 emissions.
Industrial wastes have the potential to remove hundreds of millions of tons of CO2 globally eachyear through carbon mineralization, with significant ancillary benefits. This chapter examines thepotential use of industrial wastes as feedstocks for carbon mineralization.

Iron and steel slag
The iron and steel industry generates a large amount of calcium‐ and magnesium‐rich solid waste,
generically known as slag. This material comes from the ore and from decomposition of carbonate
minerals (such as limestone) that are added to furnaces during iron and steel production for the
purpose of removing impurities, particularly silicon and aluminum. Although precise data are notavailable, the global production of slag in 2019 was approximately 320–384 million tons from ironproduction and 190–280 million tons from steel production.1 Globally, this slag has the potential tomineralize up to 268 MtCO2 per year, based on current iron and steel production. Legacy slag (thatis, slag previously produced and stored as waste) could provide an additional cumulative total of
8.2 GtCO2 of carbon mineralization.2
Slag can be mineralized in three ways.


The first method is direct gas‐solid mineralization, in which gaseous CO2 is reacted directly withsolid slag. This method is limited by slow reaction rates, due in part to passivation: as the outersurface of bulk slag reacts with CO2, it can form solid carbonate layers that are impermeable toCO2, preventing further reaction with slag material deeper within the bulk.

The second method is direct aqueous mineralization, in which CO2 is first dissolved inpressurized water, which is then exposed to slag. This method tends to be faster than direct
gas‐solid mineralization, but the dissolution rate of calcium and magnesium can limit theoverall reaction. More research is needed on understanding the optimal combination ofpressure, temperature, slag grain size and other factors to increase the speed of this process.

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The third method is indirect mineralization, in which calcium and magnesium are first extracted
from slag by acid leaching, followed by dissolution of pressurized CO2 in the leachate andprecipitation of carbonate. This approach is more complex, but it allows the conditions ofleaching, CO2 dissolution and carbon mineralization to be separately optimized.2,3

The actual extent of carbon
mineralization achieved by thesemethods varies significantly withdifferent slag sources and processing
steps; values ranging from 0.1–0.4
tCO2 per ton of slag have recently
been reported, with reaction
timescales of several hours.4,5 While
the overall storage capacity is large(CaO + MgO concentrations can reachtens of wt%), much slag resembles
feldspar‐rich mafic rocks in which theabundance of slowly reacting alumino‐
silicate and pyroxene minerals canFigure 6‐1. Iron slag being poured from slag pot.
lead to slow CO2 uptake. Processing
steps can partly overcome this slow reaction rate but negatively impact the overall carbon storage
potential on a life‐cycle basis.
Because these methods are all currently envisaged as using purified, pressurized CO2, they are alldesigned for CO2 storage, rather than removal from air. However, hybrid methods for combined
capture and storage—for example, using flue gas—could be considered.
Despite its large potential, the use of slag for mineralization faces several important challenges.
The first challenge is that a large fraction of slag produced in OECD countries is already utilized forother purposes. In particular, during iron‐making in blast furnaces (BF), approximately 0.25–0.50
tons of slag are generated per ton of pig iron produced. When this BF slag is rapidly water‐cooled(quenched), it forms a glassy, granulated material (known as ground granulated BF slag or GGBS)
that can be crushed or milled and used as a supplementary cementitious material (SCM) to partly
replace clinker produced by cement manufacture. This utilization of slag avoids emissions fromclinker production, corresponding to approximately 0.58 tCO2 per ton of slag, which is higher thanthe emissions removal potential of slag mineralization, which is 0.24–0.52 tCO2 per ton of slag.
Therefore, in general this slag should not be redirected from its current use.1,6,7
By contrast, slag produced in basic oxygen furnaces (BOF) or electric arc furnaces (EAF) during
steel‐making is not as appropriate for displacing cement. This slag is also of limited use for related
applications, such as replacing aggregate in concrete, because it contains large amounts of freelime (CaO), leading to swelling upon reaction with air to form Ca(OH)2 and CaCO3 and subsequentdamage in civil engineering applications. (Accelerated hydration through steam treatment can
mitigate this issue in some cases.8) Because of its relatively low silicon content and high ironcontent, steel slag rarely vitrifies and instead forms a crystalline, gravel‐sized material that is quite
hard due to the presence of high amounts of iron oxide (Fe2O3).9

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Steel slag is of interest for mineralization because of its high calcium content, which is commonlyover 33 wt%.10 Similar to BF slag, over 85% of steel slag in Japan, Europe and the United States isused for purposes such as blast furnace re‐use and agriculture.11 However, the life‐cycle CO2
emissions impacts of these uses are not as well understood as those of GGBS and may not be as
beneficial from a CO2 perspective as mineralization.
In contrast to OECD economies, utilization rates of slag in China and India—the world’s first andsecond largest steel producers—are much lower, at approximately 30%.12 In China, US‐ and China‐
based Greenore is developing technology to treat steel slag with CO2, producing carbonates andother products. In India, steel slag disposal is likely to become an ever more acute problem as thecountry seeks to triple its steel production by 2030.13 While carbon mineralization is one option,
numerous other uses with higher inherent economic value are also under active consideration.14
Absent an effective carbon price or focused government policy, carbon mineralization alone
appears unlikely to emerge as a primary utilization pathway. An important area for new research istherefore to develop processes that mineralize steel slag and produce valorized products.15
The second major challenge in using slag for mineralization is the fact that it is highly
heterogenous, with slag from different iron and steel facilities having extremely different
characteristics. Slag’s chemical content varies due to different ores and different amounts ofrecycled steel content, as well as different carbonate additives. Slag’s mineral content and particle
size vary due to different cooling and heat recovery techniques. Both forms of heterogeneity canhave large impacts on carbonation reaction rates and on the total fraction of slag that reacts with
CO2. In general, when direct gas‐solid mineralization processes are used, fully crystalline slags
undergo much more complete mineralization than poorly crystalline and glassy slags with the same
composition. Additional grinding of poorly crystalline and glassy slags can compensate for this, but
the required energy input generally leads to greater overall emissions.7
In addition to ongoing slag production, legacy slag represents an important potential mineralization
resource that faces its own set of challenges. China alone has roughly 2 billion tons of legacy steelslag waste; although this waste is not well characterized.16 In fact, few countries havecomprehensive mapping or characterization of legacy iron and steel slag. A rare exception is theUnited Kingdom, whose long history of industrialization has resulted in an estimated 190 million
tons of legacy slag.17 Almost all this material likely remains uncarbonated, although a substantial
amount (approximately 38%) is under urban or suburban developments, making recovery
challenging.18 A smaller but still significant additional fraction is in ecologically sensitive areas, also
rendering recovery problematic.

Recommendations:

Expand research on the optimum use of slag from a system‐level life‐cycle standpoint, including
tradeoffs with other slag uses.

Expand applied research on the optimum methods for reacting CO2 with slag to achieve fast
reaction rates and high conversion of calcium and magnesium to carbonates.

Expand applied research on improved methods for cooling slag to simultaneously optimize heatrecovery and preparation for mineralization.

November 2021

49

Expand applied research on efficient chemical, mineralogical and morphologicalcharacterization of steel slag.



Expand efforts to improve understanding of legacy steel slag waste, including the quantities,
characteristics and locations of slag heaps.

Increase policy support for use of iron and steel slag as carbon mineralization feedstock,
including launching a series of industrial pilot projects.

Fly ashFly ash is a byproduct of burning
coal and other fuels such as
municipal solid waste. It takes theform of a fine dust that is mixed withthe combustion flue gas and, toprotect air quality, is usually
removed before it reaches thesmokestack by mechanical orelectrostatic precipitators. Fly ash
consists of a mixture of small(approximately 10‐micron‐diameter)glassy beads rich in silica (SiO2),
alumina (Al2O3), iron oxide (Fe2O3)
Figure 6‐2. Fly ash pond/landfill.
and lime (CaO) as the main
components. The exact compositionof the fly ash depends strongly on the fuel that was burned and the combustion conditions. In
general, lignite and sub‐bituminous coal produce fly ash with lime content over 20%, while
bituminous coal and anthracite produce fly ash with less than 10% lime.19
One of the major advantages of using fly ash for carbon mineralization is that little to no
pretreatment, such as grinding, is required since the fly ash is already fine‐grained with highsurface area. However, the amount of carbon uptake by weight is limited because of its relatively
low content of CaO + MgO compared to other industrial wastes. Fly ash with high lime content is
the most desirable for mineralization and has been demonstrated to react with CO2 to form
cemented solids that can be used in concrete, with a CO2 uptake of 9 wt%.20 Ongoing research
focuses on process improvements for increasing the rate and extent of CO2 uptake.21‐25 Low‐lime
fly ash shows lower potential for mineralization but continues to receive some researchattention.26 In general, the optimum reaction conditions for efficient carbonation, including
reaction temperature and pressure, CO2 partial pressure, and solid‐to‐liquid ratio, remain thesubject of active research.19 So far, most studies have used purified CO2 as a reactant, thus
investigating the potential for storage, but not removal from air. However, the fine‐grained natureand high lime content of fly ash suggest that this material could also be used for CO2 removal fromambient air. However, some fraction of the lime content may be embedded in glassy spheres andtherefore unreactive, reducing the overall mineralization potential. In this case, some additional
pretreatment may be advantageous.

November 2021

50

Fly ash is the primary solid waste stream from coal‐fired power plants, with 250–300 kg produced
during the combustion of one ton of coal.19,27,28 As a result, approximately 1 billion tons areproduced worldwide each year.29 Globally, a large fraction of this waste is dumped in ash ponds orlandfills for disposal, despite the potential environmental hazards from leaching of heavy metals. In
some countries, significant amounts of fly ash are instead utilized for beneficial purposes, including
as an SCM, partly replacing cement, although it cannot be added to cement in as high a proportion
as blast furnace slag.30 Fly ash is also used in agriculture as a soil amendment, in mine reclamationas backfill material, and in the production of construction materials, such as bricks and ceramics.
Rates of re‐use are extremely high in Japan and Europe (over 90%), with lower rates in the UnitedStates, China and India (ranging from 50–80%).
Because of high re‐use rates and declining coal‐fired power generation in many OECD countries,
the largest opportunities for fly ash mineralization are in China and India. China alone produced
540 million tons of coal fly ash in 2019.28 One challenge for increasing coal fly ash utilization in
China is a regional imbalance in supply and demand. Coal‐fired generation in the west and north ofthe country is far from fly ash demand in the urbanized southeastern coast, and the low value‐to‐weight ratio makes fly ash uneconomical to transport long distances. Declining rates of
construction may also soon lead to reduced demand for fly ash in concrete and paving, highlighting
the need to identify new utilization opportunities. Mineralization has received some researchattention in this context. However, many potentially high‐value uses other than mineralization arebeing actively researched, including as a source of aluminum (displacing bauxite) and in thetreatment of wastewater.31 India’s production of coal fly ash has been steadily increasing, with 226
million tons produced in 2019–2020.32 Utilization rates have also been steadily increasing, led byconstruction materials (cement/bricks/ceramics), land reclamation, ash dyke raising and roads.
Mineralization has received relatively little attention as an opportunity for further increasing fly ash
utilization.
The issue of multiple existing and proposed uses of fly ash means that careful system analysis isneeded to identify the optimal use of this material. However, in some cases there are usefulsynergies between mineralization and valorization of certain fly ash types for additional use. For
example, fly ash from fluidized bed boilers often contains a relatively high fraction of free lime,
exceeding the maximum standard for use in cement production. This free lime can be reacted with
CO2 in the presence of water, leading to mineralization and reduction of the free lime content to
the point that the treated fly ash can be used for cement applications.33 Further research on thesesynergistic approaches to mineralization and valorization is needed.
Legacy coal fly ash also represents a major environmental burden. It contains heavy metals,
including arsenic and lead, which can contaminate soil and groundwater; wind can also blow ash
particles into the air, leading to poor air quality and human exposure to these toxins when
inhaled.34 Mineralization would largely eliminate these problems by fixing heavy metals in solidcarbonates. Russia is estimated to have over 1.5 billion tons of legacy coal fly ash as of 2017,35 andChina has over 3 billion tons as of 2020.31 This material is stored in a range of forms, including in
ash ponds and landfills. While these locations are generally better mapped than legacy iron andsteel slag, they are rarely fully characterized and require better resource assessment as a potential
mineralization feedstock.

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51

Recommendations:

Expand applied research on process development and intensification for mineralization of fly
ash, particularly for synergistic mineralization and valorization of resulting products.

Expand systems and life‐cycle assessment (LCA) research on identifying the optimum use of fly
ash, particularly in the context of existing and emerging uses other than mineralization.



Expand efforts to improve the understanding of legacy fly ash waste, including the quantities,
characteristics and locations of ash ponds and other storage sites.



Increase policy support for the use of fly ash as carbon mineralization feedstock, including
launching a series of industrial pilot projects.

Phosphogypsum
The production of phosphoric acid (primarily for fertilizer applications) from phosphorous ore
produces phosphogypsum waste. Approximately 5 tons of waste are produced for each ton ofphosphoric acid. Phosphogypsum primarily consists of the mineral gypsum (calcium sulfate
dihydrate—CaSO4 • 2H2O). While conventional gypsum is used as a construction material,
phosphogypsum commonly contains trace amounts of radioactive uranium, thorium and radiumand is generally not approved for industrial re‐use. As a result, large amounts are stockpiled
globally with significant associated environmental hazards.
Globally, approximately 200 million tons of phosphogypsum are produced annually, with autilization rate of only 10–15%, primarily in agriculture and construction materials.36 China alone
produces roughly 75 million tons per year, with legacy stocks of approximately 600 million tons.28
Russia has legacy stocks of approximately 500 million tons.36 In the United States, over 1 billion
tons of legacy stocks are present, concentrated in Florida.37 Until 2020, when the use ofphosphogypsum was approved for road construction,38 its use was largely prohibited in the UnitedStates due to its radioactive content.
Because of its high calcium content, phosphogypsum has high potential for carbon mineralization.
Relatively little pre‐treatment (grinding) is required due to its small particle size. The most studied
phosphogypsum mineralization process is based on CO2 capture using purified CO2 as a reactant.
Treatment with chilled aqueous ammonia is followed by reaction with solid phosphogypsum toproduce calcium carbonate (CaCO3) and ammonium sulfate ((NH4)2SO4).39‐41 Research has also
focused on alternative pathways with enhanced product purity, including ones that react
phosphogypsum with waste from aluminum anodizing.42‐45 In general, these processes achievealmost complete (>95%) carbonation of calcium ions with fast reaction rates. In many cases, theproduced calcium carbonate is pure enough for industrial re‐use, leading to significant revenue

November 202152

streams. Notably, CO2 storage via
carbonation of phosphogypsum
using purified CO2, captured
elsewhere, can reduce theenvironmental burden ofphosphogypsum disposal, andtreatment of legacy
phosphogypsum could help avoid
threats to groundwater fromstorage stacks.46 An important
aspect of the use ofphosphogypsum mineralization is
the fate of the ammonium sulfate
Figure 6‐3. Phosphogypsum stack.
that is produced from the sulfur
initially bound in the gypsum
reactant. This material can be used as a fertilizer but would saturate relevant markets if it were
produced at large scale, limiting the suitability of phosphogypsum mineralization for CO2 removalfrom air.
Key areas for further research include process intensification and improved routes to high‐purity
calcium carbonate. Much of the research on phosphogypsum mineralization is also relevant tobyproduct gypsum from flue gas desulfurization at coal‐fired power plants (whose annual
production rate is about half that of phosphogypsum), and improved insights could be gained fromfurther integrating research on these related materials.47,48 Also, despite the promise of carbon
mineralization using industrial gypsum for CO2 storage, comprehensive LCAs and cost analyses arelargely unavailable and should be the focus of significant research.

Recommendations:

Pursue cost and LCA research on phosphogypsum mineralization.



Expand applied research on process intensification for phosphogypsum mineralization, as well
as mineralization of other industrial gypsum sources.



Increase policy support for the use of phosphogypsum as carbon mineralization feedstock,
including launching a series of industrial pilot projects.

REFERENCES
1

Kenneth C. Curry (USGS). U.S. Geological Survey, Mineral Commodity Summaries: Iron and Steel Slag,<https://pubs.usgs.gov/periodicals/mcs2020/mcs2020‐iron‐steel‐slag.pdf> (2020).

2

Corey A. Myers, Takao Nakagaki & Kosei Akutsu. Quantification of the CO2 mineralization potential ofironmaking and steelmaking slags under direct gas‐solid reactions in flue gas. Int. J. Greenh. Gas Control 87,
100‐111, doi:https://doi.org/10.1016/j.ijggc.2019.05.021 (2019).

3

Qing Zhao et al. Co‐treatment of Waste From Steelmaking Processes: Steel Slag‐Based Carbon Capture andStorage by Mineralization. Frontiers in Chemistry 8, doi:10.3389/fchem.2020.571504 (2020).

November 2021

53

4

R. Baciocchi et al. Thin‐film versus slurry‐phase carbonation of steel slag: CO2 uptake and effects onmineralogy. Journal of Hazardous Materials 283, 302‐313,
doi:https://doi.org/10.1016/j.jhazmat.2014.09.016 (2015).

5

P. Nielsen, M. A. Boone, L. Horckmans, R. Snellings & M. Quaghebeur. Accelerated carbonation of steel slagmonoliths at low CO2 pressure – microstructure and strength development. Journal of CO2 Utilization 36,
124‐134, doi:https://doi.org/10.1016/j.jcou.2019.10.022 (2020).

6

Shubhi Dubey, Ashita Singh & Suresh Singh Kushwah. Utilization of iron and steel slag in building
construction. AIP Conference Proceedings 2158, 020032, doi:10.1063/1.5127156 (2019).

7

Corey Adam Myers & Takao Nakagaki. Effect of Solidification and Cooling Methods on the Efficacy of Slag as
a Feedstock for CO2 Mineralization. ISIJ International 58, 211‐219, doi:10.2355/isijinternational.ISIJINT‐2017‐478 (2018).

8

Praveen Kumar, D Satish Kumar, K Marutiram & SMR Prasad. Pilot‐scale steam aging of steel slags. Waste
Management & Research 35, 602‐609, doi:10.1177/0734242x17694247 (2017).

9

Irem Zeynep Yildirim & Monica Prezzi. Chemical, Mineralogical, and Morphological Properties of Steel Slag.
Advances in Civil Engineering 2011, 463638, doi:10.1155/2011/463638 (2011).

10

Krishna R. Reddy, Archana Gopakumar & Jyoti K. Chetri. Critical review of applications of iron and steel slags
for carbon sequestration and environmental remediation. Reviews in Environmental Science and
Bio/Technology 18, 127‐152, doi:10.1007/s11157‐018‐09490‐w (2019).

11

Raghavendra Ragipani, Sankar Bhattacharya & Akkihebbal K. Suresh. A review on steel slag valorisation via
mineral carbonation. Reaction Chemistry & Engineering 6, 1152‐1178, doi:10.1039/D1RE00035G (2021).

12

Jianlong Guo, Yanping Bao & Min Wang. Steel slag in China: Treatment, recycling, and management. Waste
Management 78, 318‐330, doi:https://doi.org/10.1016/j.wasman.2018.04.045 (2018).

13

India Brand Equity Foundation (IBEF). Iron & Steel Industry in India,<https://www.ibef.org/industry/steel.aspx> (Accessed September 2021).

14

Somnath Kumar. Recent trends in slag management & utilization in the steel industry. 1, 94‐102 (2019).

15

Shu‐Yuan Pan et al. CO2 Mineralization and Utilization using Steel Slag for Establishing a Waste‐to‐Resource
Supply Chain. Scientific Reports 7, 17227, doi:10.1038/s41598‐017‐17648‐9 (2017).

16

LV Jingwen 36Kr (Nikkei Asia). Chinese startup tackles 2bn tons of slag with new technology,<https://asia.nikkei.com/Business/36Kr‐KrASIA/Chinese‐startup‐tackles‐2bn‐tons‐of‐slag‐with‐new‐technology> (2020).

17

Alex L. Riley et al. Legacy iron and steel wastes in the UK: Extent, resource potential, and management
futures. Journal of Geochemical Exploration 219, 106630, doi:https://doi.org/10.1016/j.gexplo.2020.106630(2020).

18

Huw Pullin et al. Atmospheric Carbon Capture Performance of Legacy Iron and Steel Waste. Environ. Sci.
Technol. 53, 9502‐9511, doi:10.1021/acs.est.9b01265 (2019).

19

Ukwattage NL and Ranjith PG. Accelerated Carbonation of Coal Combustion Fly Ash for Atmospheric Carbon
dioxide Sequestration and Soil Amendment: An Overview. Journal of Pollution Effects & Control 6: 210,
doi:10.4172/2375‐4397.1000210 (https://www.longdom.org/open‐access/accelerated‐carbonation‐of‐coal‐
combustion‐fly‐ash‐for‐atmosphericcarbon‐dioxide‐sequestration‐and‐soil‐amendment‐an‐overview‐2375‐
4397‐1000210.pdf) (2018).

November 2021

54

20

Zhenhua Wei et al. Clinkering‐free cementation by fly ash carbonation. Journal of CO2 Utilization 23, 117‐
127, doi:https://doi.org/10.1016/j.jcou.2017.11.005 (2018).

21

Agnieszka Ćwik, Ignasi Casanova, Kwon Rausis, Nikolaos Koukouzas & Katarzyna Zarębska. Carbonation ofhigh‐calcium fly ashes and its potential for carbon dioxide removal in coal fired power plants. Journal of
Cleaner Production 202, 1026‐1034, doi:https://doi.org/10.1016/j.jclepro.2018.08.234 (2018).

22

Long Ji et al. CO2 sequestration by direct mineralisation using fly ash from Chinese Shenfu coal. Fuel
Processing Technology 156, 429‐437, doi:https://doi.org/10.1016/j.fuproc.2016.10.004 (2017).

23

Jung Hyun Kim & Woo Teck Kwon. Semi‐Dry Carbonation Process Using Fly Ash from Solid Refused FuelPower Plant. Sustainability 11, 908 (2019).

24

Katja Ohenoja, Jouni Rissanen, Paivo Kinnunen & Mirja Illikainen. Direct carbonation of peat‐wood fly ash forcarbon capture and utilization in construction application. Journal of CO2 Utilization 40, 101203,
doi:https://doi.org/10.1016/j.jcou.2020.101203 (2020).

25

Jakub Sobala et al. The Mineral Carbonation Process of High Calcium Fly Ash under Elevated Carbon Dioxide
Pressure and Temperature. Scholars International Journal of Chemistry and Material Sciences 3(6), 71‐77,
doi:10.36348/sijcms.2020.v03i06.003 (https://saudijournals.com/media/articles/SIJCMS_36_71‐77_c.pdf)(2020).

26

Alicja Uliasz‐Bocheńczyk & Eugeniusz Mokrzycki. The potential of FBC fly ashes to reduce CO2 emissions.
Scientific Reports 10, 9469, doi:10.1038/s41598‐020‐66297‐y (2020).

27

Shengxiang Jin et al. Comparison and Summary of Relevant Standards for Comprehensive Utilization of Fly
Ash at Home and Abroad. IOP Conference Series: Earth and Environmental Science 621, 012006,
doi:10.1088/1755‐1315/621/1/012006 (2021).

28

Junlin Meng, Wenjie Liao & Guoquan Zhang. Emerging CO2‐Mineralization Technologies for Co‐Utilization ofIndustrial Solid Waste and Carbon Resources in China. Minerals 11, 274 (2021).

29

Shanjida Sultana et al. “Coal Fly Ash Utilisation and Environmental Impact” in Clean Coal Technologies:
Beneficiation, Utilization, Transport Phenomena and Prospective (eds Rajesh Kumar Jyothi & Pankaj Kumar
Parhi) 381‐402 (Springer International Publishing, 2021).

30David J. M. Flower & Jay G. Sanjayan. Green house gas emissions due to concrete manufacture. The
International Journal of Life Cycle Assessment 12, 282, doi:10.1065/lca2007.05.327 (2007).

31

Yang Luo et al. Utilization of coal fly ash in China: a mini‐review on challenges and future directions.
Environmental Science and Pollution Research 28, 18727‐18740, doi:10.1007/s11356‐020‐08864‐4 (2021).

32

Government of India: Ministry of Power. Report On Fly Ash Generation at Coal/Lignite‐Based Thermal Power
Stations and its Utilization in the Country for the Year 2019 ‐20, <https://cea.nic.in/wp‐
content/uploads/tcd/2021/01/flyash_2019‐20.pdf> (2020).

33

Marian Jacek Łączny, Sebastian Iwaszenko & Adam Smoliński. Process Kinetics of the Carbonation of Fly
Ashes: A Research Study. Materials 14, 253 (2021).

34

Kristina M. Zierold & Chisom Odoh. A review on fly ash from coal‐fired power plants: chemical composition,
regulations, and health evidence. Reviews on Environmental Health 35, 401‐418, doi:10.1515/reveh‐2019‐0039 (2020).

November 2021

55

35

Oksana Marinina, Marina Nevskaya, Izabela Jonek‐Kowalska, Radosław Wolniak & Mikhail Marinin. Recyclingof Coal Fly Ash as an Example of an Efficient Circular Economy: A Stakeholder Approach. Energies 14, 3597
(2021).

36

Yelizaveta Chernysh, Olena Yakhnenko, Viktoriia Chubur & Hynek Roubík. Phosphogypsum Recycling: A
Review of Environmental Issues, Current Trends, and Prospects. Applied Sciences 11, 1575 (2021).

37

H. Liang, P. Zhang, Z. Jin & D. DePaoli. Rare earths recovery and gypsum upgrade from Florida
phosphogypsum. Minerals & Metallurgical Processing 34(4), 201‐206 (2017).

38

United States Environmental Protection Agency (US EPA). EPA Approves Use of Phosphogypsum in Road
Construction, <https://www.epa.gov/newsreleases/epa‐approves‐use‐phosphogypsum‐road‐construction>(2020).

39

M. Contreras et al. Fractionation and fluxes of metals and radionuclides during the recycling process ofphosphogypsum wastes applied to mineral CO2 sequestration. Waste Management 45, 412‐419,
doi:https://doi.org/10.1016/j.wasman.2015.06.046 (2015).

40

Hannu‐Petteri Mattila & Ron Zevenhoven. Mineral Carbonation of Phosphogypsum Waste for Production of
Useful Carbonate and Sulfate Salts. Frontiers in Energy Research 3, doi:10.3389/fenrg.2015.00048 (2015).

41

Xolani Msila, David Gordon Billing & Werner Barnard. Capture and storage of CO2 into waste
phosphogypsum: the modified Merseburg process. Clean Technologies and Environmental Policy 18, 2709‐
2715, doi:10.1007/s10098‐016‐1157‐4 (2016).

42

Qiuju Chen, Wenjin Ding, Hongjuan Sun, Tongjiang Peng & Guohua Ma. Indirect mineral carbonation ofphosphogypsum for CO2 sequestration. Energy 206, 118148,
doi:https://doi.org/10.1016/j.energy.2020.118148 (2020).

43

Wenjin Ding, Qiuju Chen, Hongjuan Sun & Tongjiang Peng. Modified mineral carbonation of phosphogypsum
for CO2 sequestration. Journal of CO2 Utilization 34, 507‐515,
doi:https://doi.org/10.1016/j.jcou.2019.08.002 (2019).

44

I. Romero‐Hermida et al. New method for carbon dioxide mineralization based on phosphogypsum andaluminium‐rich industrial wastes resulting in valuable carbonated by‐products. Journal of CO2 Utilization 18,
15‐22, doi:https://doi.org/10.1016/j.jcou.2017.01.002 (2017).

45

Wei Zhang et al. An efficient methodology to use hydrolysate of phosphogypsum decomposition products
for CO2 mineral sequestration and calcium carbonate production. Journal of Cleaner Production 259,
120826, doi:https://doi.org/10.1016/j.jclepro.2020.120826 (2020).

46

Daljit Sandhu et al. Fate and transport of radioactive gypsum stack water entering the Floridan aquifer due
to a sinkhole collapse. Scientific Reports 8, 11439, doi:10.1038/s41598‐018‐29541‐0(https://www.nature.com/articles/s41598‐018‐29541‐0) (2018).

47

Nadeesha H. Koralegedara, Patricio X. Pinto, Dionysios D. Dionysiou & Souhail R. Al‐Abed. Recent advances influe gas desulfurization gypsum processes and applications – A review. Journal of Environmental
Management 251, 109572, doi:https://doi.org/10.1016/j.jenvman.2019.109572 (2019).

48

Bo Wang, Zihe Pan, Huaigang Cheng, Zhien Zhang & Fangqin Cheng. A review of carbon dioxidesequestration by mineral carbonation of industrial byproduct gypsum. Journal of Cleaner Production 302,
126930, doi:https://doi.org/10.1016/j.jclepro.2021.126930 (2021).

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CHAPTER 7:
CEMENT AND CONCRETE
The cement and concrete industry is a significant source of greenhouse gas (GHG) emissions—roughly 8% of the global total.1 Several carbon mineralization processes could significantly reducethese emissions. This chapter provides an overview of the cement and concrete industry anddiscusses three processes with the potential to reduce its emissions—mixing carbonation,
carbonation curing and production of synthetic aggregates.

Overview of the cement and concrete industry
In 2019, approximately 4.1 Gt of cement was produced globally, making cement one of the few
products that is manufactured at the gigaton scale.2 Despite this enormous scale, cement is neverconsumed directly; instead, it is mixed with water and aggregate materials (sand, gravel and stone)
to produce concrete, which is then used in construction. The total amount of concrete consumed
globally is therefore much larger, approximately 30 Gt in 2019.
Cement and its precursor, clinker, tend to be produced in proximity to sources of limestone, which
is used as feedstock.3 Much of cement production is consolidated among a small number of large
multinational producers with annual sales over $10 billion.4 By contrast, concrete production is
overwhelmingly local and carried out at much smaller scales close to areas where construction
occurs because (1) the low value‐to‐weight ratio of finished concrete makes longer distanceBox 7.1
CEMENT AND CONCRETE
Concrete that is used to build roads, buildings and bridges is a mixture of cement (also known asbinder), water and solid aggregate materials, such as gravel, sand and crushed stone (also known
as filler).
Typical concrete includes 10–15% cement, 15–20% water, and 60–75% aggregate by volume.
Conventional cement, called Ordinary Portland Cement (OPC), is manufactured by quarrying andcrushing limestone and clay and then heating it in a rotating kiln at temperatures
around 1400C. This process drives off CO2 and other gases (process emissions),
resulting in small nodules of a material known as clinker. After cooling,clinker is ground to a fine powder and mixed with additives to produce
cement. One way to lower CO2 emissions from cement production is by
substituting some clinker with supplementary cementitious materials(SCMs) rich in CaO, such as blast furnace slag and fly ash, which iscommon practice in regions where these materials are widely available.

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transportation uneconomical and (2) concrete permanently hardens only a few hours after
production. As an illustration of this industry structure, the United States has only approximately
100 cement plants, while concrete plants number in the thousands.5 Global demand for cement
and concrete is projected to grow by 12–23% by 2050 compared to 2014, with much of the growthin Asia.6

Emissions associated with cement and concrete production
The production of cement has estimated life‐cycle emissions of 0.6–0.8 tCO2e per ton of cement,
due primarily to process emissions.7 The life‐cycle emissions of concrete are approximately 0.15–
0.23 tCO2e per ton of concrete.8 These emissions are lower on a weight basis because cement is asmall fraction of the overall weight of concrete, and the emissions associated with the otheringredients of concrete (water and aggregate) are relatively small. Many methods have been
proposed, tested and (in some cases) implemented for reducing the emissions of concrete,
including use of SCMs (as discussed above), electric or hydrogen‐fired kilns, point‐source carbon
capture during cement manufacturing and carbon mineralization.9,10 If these methods are all
implemented simultaneously, finished concrete has the theoretical potential to be carbon‐
negative, a prospect that is tremendously important given the enormous global consumption of
concrete.
The following discussion focuses on carbon mineralization methods for cement and concrete. In
general, these methods are compatible with the other cement and concrete emissions–reducing
strategies noted in the previous paragraph. Research on carbon mineralization for cement andconcrete should therefore not be pursued in isolation or at the expense of other pathways toward
low‐carbon or carbon‐negative concrete but rather as part of a holistic effort.
A wide array of mineralization/carbonation methods for cement and concrete have been proposedand explored. The most mature of these are mixing carbonation (injecting purified CO2 duringconcrete mixing), carbonation curing (replacing water/steam with purified CO2 during curing), andthe production of synthetic aggregate. Each of these methods has advantages and disadvantages,
as will be discussed below. Because the first two processes are often viewed as adding carbon to
concretes, they are commonly referred to as “carbonation” rather than “carbon mineralization.”
Mixing carbonation is primarily used for ready‐mix concrete, which is usually delivered toconstruction sites by mixing (barrel) trucks. This process consists of injecting a small dose of pure(gaseous) CO2 into the concrete mix during batching, which leads to the formation of nano‐scalesolid calcium carbonate (CaCO3) particles distributed evenly throughout the mix, which storescarbon for the lifetime of the concrete, approximately 50–100 years.
While some early work on mixing carbonation
aimed to maximize the amount of CO2 injected
into the mix, large amounts of CO2 can havenegative impacts on the performance of theconcrete. However more recent work has
shown that a relatively small dose of CO2(~0.15% of the weight of cement or ~0.02% of
the weight of concrete) leads to improvements

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in performance, the most important of which is an increase in the compressive strength of thefinished concrete.11 Because concrete strength is primarily determined by the amount of cement in
the mix (more cement leads to higher strength), this increase in strength enables a corresponding
reduction in cement content to achieve the same overall performance. This results in economic
savings and emissions reductions because cement is both the most expensive and the mostemissions‐intensive component of concrete.12 Mixing carbonation also has favorable impacts onother concrete properties, including decreasing set time and increasing durability.
Mixing carbonation has gained significant traction in the market, with Canada‐based CarbonCure
offering commercial products with widespread uptake. A key commercial advantage of thetechnology is that it requires only minor process changes to conventional concrete production,with low‐cost additional equipment, making it relatively easy to adopt by most concrete producers.
Carbonation curing is primarily applicable to precast and masonry concrete products, which areconventionally cured using steam in closed reactors shortly after casting. Carbonation curing partly
or completely replaces this steam with pure CO2 gas.13 During conventional curing of ordinary
portland cement (OPC), reactions between water and cement produce calcium‐silicate‐hydrate(C‐S‐H) crystals—whose interlocking structure provides strength—and calcium hydroxide. In
carbonation curing, C‐S‐H is still produced but, instead of calcium hydroxide, the reaction leads toparticles of solid calcium carbonate (CaCO3), sequestering carbon in solid form within the concrete
in a manner similar to mixing carbonation.
One advantage of using CO2 to cure concrete rather than (or in combination with) water is thatdifferent, lower‐emissions formulations of cement can be used. US‐based Solidia Technologies hascommercialized this approach by producing a cement with significantly reduced limestone content,
lowering the emissions intensity by 30% compared with OPC.14,15 This cement will not set (harden)
with conventional water‐based curing; instead, it requires a mixture of CO2 and steam. During
curing, up to 30 wt% of the gaseous CO2 is converted into solid calcium carbonate, sequestering it
for the lifetime of the concrete.
Other efforts to commercialize carbonation curing for precast concrete products include CO2‐
SUICOM developed by Kajima Corporation, the Chugoku Electric Power Company, Denka Company
and Landes Corporation of Japan16,17; Canada‐based CarbiCrete18,19; US‐based CarbonBuilt(formerly CO2Concrete)20; Saudi Aramco/Korea Advanced Institute of Science and Technology21,22;
and Belgium‐based Orbix.23,24
Mixing carbonation and carbonation curing share several similarities. Importantly, unlike otherforms of carbon mineralization, they do not require additional sources of calcium or magnesium
because calcium is a fundamental constituent of cement. This greatly simplifies logistics and cost.
Further, the only novel materials‐handling challenge is providing purified CO2, and these processes
are often discussed as examples of “carbon capture, utilization and storage” (CCUS) because they
store CO2 in a useful product. Much of the attention these technologies receive is in this context.
These CCUS concretes can help expand the market for captured CO2, giving it economic value,
which in turn supports implementation of CO2 capture methods.
From this perspective, carbonation curing appears to be a more attractive CCUS option than mixing
carbonation because it consumes a much larger amount of CO2 per unit mass of concrete: amounts

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ranging from 20–50 kgCO2 per m3 of concrete (on the order of 0.1–0.5 wt%) have been reported in
the literature.25 An additional benefit is the fact that steam for conventional curing is energy‐
intensive to produce, and displacing steam with CO2 can reduce emissions. However, someapproaches to carbonation curing tend to reduce the compressive strength of the resulting
concrete, meaning that more binder (OPC) is required to achieve the same compressive strength.
The emissions associated with this additional binder can change the life‐cycle emissions impacts ofcarbonation significantly, in some cases eliminating all emissions benefits entirely.25 As a result, theattractiveness of both mixing carbonation and carbonation curing as avenues to expanded CO2
markets must be balanced with an understanding of net CO2 emissions impacts using careful life‐
cycle assessment (LCA).
The primary research and development (R&D) priority for mixing carbonation is to increase the CO2
loading in concrete, while maintaining the compressive strength gain and other favorable
properties displayed by current low‐loading mixes. The primary R&D priority for carbonation curing
is to develop curing protocols that will lead to increased compressive strength, allowing a reduced
use of OPC.26
A second, related R&D priority is to better understand the impact of carbonation curing on the
durability of concrete, particularly how it impacts corrosion of steel. While carbonation can
increase corrosion under some circumstances, rendering concrete incompatible with steel
reinforcement, this may be largely mitigated through improved process design. More research isneeded to clarify this issue.26,27
For both mixing carbonation and carbonation curing, other R&D priorities include (1) improving theelectrical energy efficiency of the process, such as the use of waste heat for drying, in order toreduce the associated emissions from consumed electricity and (2) gaining better understanding of
how different supplementary cementitious materials (SCMs), such as blast furnace slag and fly ash,
impact carbonation.28
Mixing carbonation and carbonation curing generally use purified CO2 that is captured elsewhere.
When the source of that CO2 is biogenic, such as point‐source capture from an ethanol production
facility, mixing carbonation forms part of an overall method that removes CO2 from theatmosphere and stores it in a long‐duration form. This is also the case if the source of CO2 is fromdirect air capture (DAC). Alternatively, if the source of CO2 is from fossil‐fuel combustion (such as
point‐source capture from flue gas at a coal‐fired power plant) then mixing carbonation forms partof an overall method for carbon capture and storage. Mixing carbonation and carbonation curing
are unlikely to be used for the direct removal of CO2 from air without some form of initial capture
and purification.
Carbon mineralization can also be used to produce synthetic aggregate. This process involves
reacting CO2 with alkaline feedstock containing calcium and/or magnesium, including recycledconcrete and a variety of industrial wastes (see Chapter 6). This produces calcium and magnesium
carbonates, which can then be used in concrete to replace mined aggregates, such as sand, gravel
and crushed stone. While this substitution does not displace a large amount of emissions (theaverage carbon intensity of conventional aggregates is approximately 8 kgCO2e per ton, far lower
than cement),29 it can serve as an economically valuable form of CO2 sequestration. Several
companies have commercialized variations of this process. UK‐based O.C.O. Technologies (formerly

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Carbon8 Aggregates) provides synthetic aggregates produced from air pollution control (APC)
residues and industrial CO2.30 US‐based Blue Planet provides synthetic aggregates produced fromalkaline industrial waste and dilute CO2 in flue gas.3
Because the global annual aggregates market is approximately 50 Gt, the potential scale of impact
from synthetic aggregates is significant.30 Pressures from limited landfill space and high tipping fees
have already led to approximately 10% of aggregates used in Europe coming from recycled or
manufactured materials.3 However, aggregates are a low‐value product by weight, with crushedstone valued at approximately $12 per ton in the United States.31 This suggests that synthetic
aggregate will have difficulty competing with conventional aggregate on cost. While production ofsynthetic aggregate may allow more careful control over aggregate size, resulting in a moreconsistent product than virgin‐mined aggregate, this improved feature alone is unlikely to
overcome cost differentials.
A further issue for synthetic aggregate is its mechanical performance when used in concrete.
Conventional aggregate is classified as fine or coarse, depending on whether particles are smalleror larger than 4.75 mm (No. 4 sieve). It is also classified by shape, such as rounded, angular or
elongated. Both the size and shape of aggregates impact many important properties of concrete,
including cement and water requirements in the mix, pumpability, workability and durability.
Conventional aggregate is typically washed, crushed and screened to obtain desirable size andshape distributions. Synthetic aggregate will need to broadly conform to these size and shaperequirements and be of sufficient mechanical strength to avoid crumbling or breaking during
handling (high friability).
A closely related form of mineralization is the treatment of recycled concrete aggregates (fromdemolition waste) with CO2 to improve their strength, a process known as carbon conditioning.
This process allows these materials to be used as aggregate in new concrete mixes, providing goodmechanical properties.32,33 One approach to this process involves producing calcium bicarbonate
solution via reaction of concrete demolition waste, water and captured CO2 and then exposing
crushed demolition waste aggregate particles to the solution. This leads to the precipitation of
CaCO3 crystals, which bond to recycled aggregate and provide it sufficient mechanical strength toallow its re‐use (calcium carbonate circulation system for construction, C4S).34 Similar challenges
regarding size, shape and friability apply to these materials, as well as to synthetic aggregates.
R&D priorities for synthetic aggregate production include optimizing (ideally reducing or
eliminating) transportation of feedstock materials (such as industrial waste) and purified CO2. To
this end, increased efforts are needed to co‐locate synthetic aggregate production with sources of
CO2 and industrial facilities producing alkali waste35 (recent announcements from O.C.O.
Technologies about aggregate manufacturing at a refinery are encouraging36). For aggregate
production, a better understanding of the tradeoffs between using relatively pure CO2 anduntreated flue gas is also needed, given the improved mineralization efficiency with pure CO2 but
the associated energy and emissions penalty from CO2 capture. For widespread market uptake,synthetic aggregate must undergo mechanical performance testing with R&D focused on
mitigating any significant gaps in comparison with conventional aggregate; the same is true forcarbon‐conditioned recycled concrete demolition waste.

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Similar to mixing carbonation and carbonation curing, the production of synthetic aggregates has
primarily been proposed to use purified CO2 that was captured elsewhere, although some methods
also envision using flue gas directly (i.e., without first separating and purifying CO2). When thesource of that CO2 is biogenic, such as point‐source capture from an ethanol production facility or
the use of flue gas from combustion of biomass, production of synthetic aggregate throughmineralization forms part of an overall method that removes CO2 from the atmosphere and storesit in a long‐duration form. This would also be the case if the source of CO2 were from DAC.
Alternatively, if the source of CO2 is from fossil‐fuel combustion (such as point‐source capture fromflue gas at a coal‐fired power plant) then the production of synthetic aggregate throughmineralization forms part of an overall method for carbon capture and storage. The production ofsynthetic aggregates is unlikely to be used for the direct removal of CO2 from air without someform of initial capture and purification.

REFERENCES
1

Johanna Lehne and Feliz Preston (Chatham House). Making Concrete Change: Innovation in Low‐carbon
Cement and Concrete (Chatham House Report), <https://www.chathamhouse.org/2018/06/making‐concrete‐change‐innovation‐low‐carbon‐cement‐and‐concrete> (2018).

2

The International Energy Agency (IEA). Cement, <https://www.iea.org/reports/cement> (IEA, Paris; 2020).

3

National Academies of Sciences Engineering and & Medicine (NASEM). Gaseous Carbon Waste Streams
Utilization: Status and Research Needs. (The National Academies Press, 2019(https://www.nap.edu/catalog/25232/gaseous‐carbon‐waste‐streams‐utilization‐status‐and‐research‐needs)).

4David Perilli (Global Cement). 2020 roundup for the cement multinationals,<https://www.globalcement.com/news/item/12085‐2020‐roundup‐for‐the‐cement‐multinationals> (2021).

5

IBISWorld. Industry Statistics ‐ United States: Ready‐Mix Concrete Manufacturing in the US ‐ Number ofBusinesses 2005–2027, <https://www.ibisworld.com/industry‐statistics/number‐of‐businesses/ready‐mix‐
concrete‐manufacturing‐united‐states/> (Accessed September 2021).

6

The International Energy Agency (IEA). Technology Roadmap ‐ Low‐Carbon Transition in the Cement Industry(Technology Report), <https://www.iea.org/reports/technology‐roadmap‐low‐carbon‐transition‐in‐the‐cement‐industry> (IEA, Paris; 2018).

7

Jane Anderson & Alice Moncaster. Embodied carbon of concrete in buildings, Part 1: analysis of published
EPD. Buildings and Cities 1(1), 198‐217, doi:http://doi.org/10.5334/bc.59 (2020).

8

Hao Huang et al. Life‐cycle assessment of emerging CO2 mineral carbonation‐cured concrete blocks:
Comparative analysis of CO2 reduction potential and optimization of environmental impacts. Journal of
Cleaner Production 241, 118359, doi:https://doi.org/10.1016/j.jclepro.2019.118359 (2019).

9

Jacob Schneider. Decarbonizing construction through carbonation. Proceedings of the National Academy of
Sciences 117, 12515‐12517, doi:10.1073/pnas.1913867116 (https://www.pnas.org/content/117/23/12515)(2020).

10

Caleb M. Woodall, Noah McQueen, Hélène Pilorgé & Jennifer Wilcox. Utilization of mineral carbonation
products: current state and potential. Greenhouse Gases: Science and Technology 9, 1096‐1113,
doi:https://doi.org/10.1002/ghg.1940 (2019).

November 2021

62

11

Sean Monkman, Mark MacDonald, R. Doug Hooton & Paul Sandberg. Properties and durability of concrete
produced using CO2 as an accelerating admixture. Cement and Concrete Composites 74, 218‐224,
doi:https://doi.org/10.1016/j.cemconcomp.2016.10.007 (2016).

12

Sean Monkman & Mark MacDonald. On carbon dioxide utilization as a means to improve the sustainabilityof ready‐mixed concrete. Journal of Cleaner Production 167, 365‐375,
doi:https://doi.org/10.1016/j.jclepro.2017.08.194 (2017).

13

Duo Zhang, Zaid Ghouleh & Yixin Shao. Review on carbonation curing of cement‐based materials. Journal of
CO2 Utilization 21, 119‐131, doi:https://doi.org/10.1016/j.jcou.2017.07.003 (2017).

14

Vincent Meyer, Nick de Cristofaro, Jason Bryant & Sada Sahu. Solidia Cement an Example of Carbon Captureand Utilization. Key Engineering Materials 761, 197‐203,
doi:https://doi.org/10.4028/www.scientific.net/KEM.761.197 (2018).

15

Vincent Meyer, Sada Sahu & Andrew Dunster. in 1st International Conference on Innovation in Low‐Carbon
Cement & Concrete Technology (London, United Kingdom, 2019;https://assets.ctfassets.net/jv4d7wct8mc0/4g12cODXSpblK6Vja1FUvx/f0eb602207fd202925a35b137e818fd
e/2019_ILCCC_‐_Properties_of_Solidia_Cement_and_Concrete_copy.pdf).

16

Challenge Zero: Innovation Challenges – Mitsubishi Corporation. Carbon negative concrete "CO2 SUICOM",<https://www.challenge‐zero.jp/en/casestudy/616> (Accessed September 2021).

17

K. Yoshioka et al. New Ecological Concrete that Reduces CO2 Emissions Below Zero Level ∼ New Method for
CO2 Capture and Storage ∼. Energy Procedia 37, 6018‐6025,
doi:https://doi.org/10.1016/j.egypro.2013.06.530 (2013).

18

CarbiCrete. Technology, <https://carbicrete.com/technology/> (Accessed September 2021).

19

Mehrdad Mahoutian, Omar Chaallal & Yixin Shao. Pilot production of steel slag masonry blocks. Canadian
Journal of Civil Engineering 45, 537‐546, doi:10.1139/cjce‐2017‐0603 (2018).

20

CarbonBuilt. <https://www.carbonbuilt.com/> (Accessed September 2021).

21

Aramco. Managing our footprint: Carbon curing, <https://www.aramco.com/en/sustainability/climate‐change/managing‐our‐footprint/carbon‐curing> (Accessed September 2021).

22

Joon Ho Seo et al. CO₂ Uptake of Carbonation‐Cured Cement Blended with Ground Volcanic Ash. Materials(Basel) 11, 2187, doi:10.3390/ma11112187 (https://www.mdpi.com/1996‐1944/11/11/2187) (2018).

23

Andrea Di Maria, Ruben Snellings, Luc Alaerts, Mieke Quaghebeur & Karel Van Acker. Environmental
assessment of CO2 mineralisation for sustainable construction materials. Int. J. Greenh. Gas Control 93,
102882, doi:https://doi.org/10.1016/j.ijggc.2019.102882 (2020).

24

Hadi Kazemi Kamyab, Peter Nielsen, Peter Van Mierloo & Liesbeth Horckmans. Carbstone Pavers: A
Sustainable Solution for the Urban Environment. Applied Sciences 11, 6418 (2021).

25

Dwarakanath Ravikumar et al. Carbon dioxide utilization in concrete curing or mixing might not produce a
net climate benefit. Nat. Commun. 12, 855, doi:10.1038/s41467‐021‐21148‐w(https://www.nature.com/articles/s41467‐021‐21148‐w) (2021).

26

Zhen Li, Zhen He & Xiaorun Chen. The Performance of Carbonation‐Cured Concrete. Materials 12, 3729
(2019; https://www.mdpi.com/1996‐1944/12/22/3729).

November 2021

63

27

Duo Zhang & Yixin Shao. Enhancing Chloride Corrosion Resistance of Precast Reinforced Concrete by
Carbonation Curing. ACI Materials Journal 116, doi:10.14359/51714461 (2019).

28

Stefanie von Greve‐Dierfeld et al. Understanding the carbonation of concrete with supplementary
cementitious materials: a critical review by RILEM TC 281‐CCC. Materials and Structures 53, 136,
doi:10.1617/s11527‐020‐01558‐w (2020).

29

Atac Bascetin, Deniz Adiguzel & Serkan Tuylu. The investigation of Co2 emissions for different rock units inthe production of aggregate. Environmental Earth Sciences 76, 279, doi:10.1007/s12665‐017‐6602‐0(https://link.springer.com/article/10.1007%2Fs12665‐017‐6602‐0) (2017).

30

Colin D. Hills, Nimisha Tripathi & Paula J. Carey. Mineralization Technology for Carbon Capture, Utilization,and Storage. Frontiers in Energy Research 8, doi:10.3389/fenrg.2020.00142(https://www.frontiersin.org/articles/10.3389/fenrg.2020.00142/full) (2020).

31

Jason Christopher Willett (USGS). U.S. Geological Survey, Mineral Commodity Summaries: Stone (Crushed),<https://pubs.usgs.gov/periodicals/mcs2021/mcs2021‐stone‐crushed.pdf> (2021).

32

Vivian W. Y. Tam, Anthony Butera, Khoa N. Le & Wengui Li. Utilising CO2 technologies for recycled aggregate
concrete: A critical review. Construction and Building Materials 250, 118903,
doi:https://doi.org/10.1016/j.conbuildmat.2020.118903 (2020).

33

Dongxing Xuan, Baojian Zhan & Chi Sun Poon. Assessment of mechanical properties of concrete
incorporating carbonated recycled concrete aggregates. Cement and Concrete Composites 65, 67‐74,
doi:https://doi.org/10.1016/j.cemconcomp.2015.10.018 (2016).

34

Noguchi Takafumi (Nedo). Moonshot Research and Development Program (Project Overview) – C4S Researchand Development Project – C4S: Calcium Carbonate Circulation System for Construction,<https://www.nedo.go.jp/content/100923495.pdf> (Accessed September 2021).

35

Renato Baciocchi & Giulia Costa. CO2 Utilization and Long‐Term Storage in Useful Mineral Products by
Carbonation of Alkaline Feedstocks. Frontiers in Energy Research 9, doi:10.3389/fenrg.2021.592600(https://www.frontiersin.org/articles/10.3389/fenrg.2021.592600/full) (2021).

36

Anthony Wright (Gasworld). O.C.O Technology partnership paves way for Europe’s first carbon negative
aggregate plant, <https://www.gasworld.com/oco‐technology‐partnership‐paves‐way‐for‐europes‐first‐
carbon‐negative‐aggregate‐plant/2021500.article> (2021).

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CHAPTER 8:
CROSS‐CUTTING RESEARCH NEEDS
Research and development (R&D) needs in carbon mineralization focus on (1) increasing the speedwith which the reactions occur, (2) accessing a significant volume of reactive material (surficial or
subsurface), (3) disposing of carbonate material produced via surficial methods, (4) measuring thetotal carbon impact of these approaches and (5) improving the economics of the processes.
Significant cross‐cutting issues are water use and safety from potentially toxic and hazardous
components present in both minerals and industrial wastes.
Carbon mineralization is often viewed as a lab‐ and bench‐scale chemical engineering research
activity, but much of the associated laboratory and theoretical work has been sufficiently resolved
to move forward. Large‐scale demonstrations are vital for addressing systems integration, energy
efficiency and cost reduction across all forms of carbon mineralization.
Priority areas for cross‐cutting research are discussed below.

1. Accelerating mineralization reaction rates
Carbon mineralization fundamentally depends on combining calcium and magnesium with CO2.
However, the rate of that reaction is dependent on the structure and chemistry of the initialmaterial and the contact mechanism with CO2. Major topics include:The effect of size, shape and crystallinity on dissolution of minerals and industrial wastes(including, e.g., steel slag).

The reaction rates of mixed materials when one mineral like brucite (Mg(OH)2) dominates
short‐term kinetics but the majority of the material is composed of minerals with longer
reaction time.

The reaction mechanisms for mineralization (e.g., which mechanism controls the reaction:mineral dissolution or CO2 transfer into a liquid layer?).



The impact of grinding, heating or acid pretreatments on complex mixtures.

The impact of different transport fluids—such as pure CO2 gas, flue gas, surface waters, CO2‐
rich aqueous solutions under pressure and supercritical CO2 under pressure—on mineralization
rates and outcomes.

2. Hazardous material removal and handling
Many mineral and industrial waste mixtures contain hazardous materials, including asbestos andthe heavy metals chromium (Cr) and nickel (Ni). Removing these materials may be possible but, if
not, the methods for handling them safely must be established. Major topics include:

The fate of hazardous components during mineralization reactions.

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 Asbestos fibers with their high surface area per unit mass may rapidly and preferentiallyform carbonate minerals, mitigating a significant health hazard.
 Accelerated carbon mineralization, particularly using acidic solutions, may release dissolved
Ni, Cr and other potentially toxic elements and compounds.


Methods for pretreatment or removal of hazardous materials prior to utilization.
 Removing asbestos by rapidly reacting it with CO2.
 Pretreating minerals and industrial wastes to extract and use heavy metal components.



The development of protocols for safe handling of these materials.

3. Underground injection of CO2 in basalts and ultramafic rocks
The rate and extent of capture and long‐term storage of CO2 via carbon mineralization arechallenging to measure and predict. Current knowledge is based on laboratory experiments and alimited number of field experiments with sparse sampling. Tracking the fate of CO2 transported
through reactive rocks in the subsurface will require research into the rate of undergroundreactions and the limiting processes that control how much CO2 a rock can absorb. Major topics
include:Controlled laboratory experiments on the fate of CO2 in basalts and ultramafic rocks.

Instrumented field tests to investigate the fate of injected CO2, particularly in basalts andperidotites over 10–100 m distances, permitting a complete mass balance on the inputs,
outputs and exchanges that may occur depending on the methods used for CO2 transport (seebelow).

Observing and controlling outcomes of long‐term, subsurface carbon mineralization in termsof “cracking versus clogging,” formation of dissolution channels and/or permeability barriers,
evolution of porosity and reactive surface area, changes in rock volume and associateddeformation, and displacement of pore water.



Practical implementation of specific methods for transporting CO2 through reactive materials
for subsurface mineralization. Ideas include injecting supercritical CO2 (but this option requires
an impermeable caprock), bubbling CO2 into water in boreholes and/or pore space at depth toproduce CO2‐rich aqueous fluids, or alternating injection of water and supercritical CO2 inproportions such that the CO2 dissolves in water at depth. Many variations and combinations
are possible and testing them will require field‐scale demonstration, not laboratory‐scale
research.



System integration of CO2 sources, heat, water exchange and heat management. Advances inthis area could dramatically improve the energy, material and water efficiency of theseprocesses.

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4. Surface reaction of CO2 with mafic and ultramafic tailings and alkaline
industrial wastes
Many schemes for utilizing calcium and magnesium in rocks and industrial wastes have the same
basic issues: reactivity, contact mechanism with CO2 and permanence of products. Research efforts
that identify the basic reaction mechanisms and the physical properties that affect thosemechanisms will be beneficial across a wide range of processes. Major topics include:

Understanding the reactivity of calcium‐ and magnesium‐bearing minerals and wastes. Primaryconcerns are determing the impact of particle or crystallite size on reaction rate, separating theeffects of multi‐component systems on overall rates, and understanding the impacts of reacted
coatings that can impede the progress of mineralization.



Determining the rate of CO2 uptake from air. In both quarried and waste materials, the rate of
CO2 uptake directly from air is affected by many minor processes, which can be hard to
separate. Studies are needed to understand the controlling factors for direct atmosphericreaction with carbonation substrates.

Measuring and improving physical properties of beneficial products. Conversion of wastes tobeneficial materials is impacted by the physical properties of the resultant product. The key
properties include strength, solubility in water, hardness and impurities.



Exploring additional treatment methods. Methods of treating industrial wastes upon
production may improve their use in carbon mineralization by increasing surface area orreactivity.



Extending and improving lifecycle emissions and cost analyses. These analyses are needed todetermine the best uses of industrial slags and wastes, especially vis a vis their alternative use
in cements. Metrics for relative benefits should be developed and standardized.



Determining strategies for storage and disposal of carbonated products not suitable for
utilization. Some abundant geological materials are likely to undergo only partial mineralization
over short timescales. Finding safe locations to sequester large amounts of this material—
which may be an order of magnitude larger by mass than the CO2 removed—is an important
challenge to address. The oceans may provide one potential location.

5. Mineral additives to soils
Many of the proposed methods for enhanced weathering utilize powdered rock material spread on
cultivated land. Basaltic lava is the rock type most commonly considered for use. The benefits areoften envisioned in terms of crop yield and overall CO2 flux, but these are extraordinarily complex
topics in an active agricultural ecosystem. Additional research into interactions between added
material and the soil system is needed to understand both the benefits and possible problems withthis approach. Major topics include:

Understanding calcium and magnesium impacts on soil health. Calcium is routinely added tosome kinds of depleted soil in more readily available forms (e.g., limestone) today. Research

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into the mechanisms by which mafic rocks provide cations is needed. The impact of magnesium
is important since it can be a problematic cation in agriculture.


Understanding life‐cycle benefits of soil mineral addition. The overall life‐cycle benefits of soil
addition need to be evaluated in the context of baseline conditions and alternative approaches.
For instance, for the money invested, is adding powdered basalt the best way to achieve bothclimate and agronomic benefits?



Identifying optimal locations. The locations where mineral additions to soil may be mostbeneficial need to be evaluated in terms of the two topics above.



Identifying potential hazards. Accumulation of minor elements that occur in basalt or
peridotite, such as Ni and Cr, over time may produce hazardous concentrations in soils andcrops.



Understanding impacts on microbes. Microbes are a critical part of the active soil ecosystem.
Studies of the impact of minerals on microbes, and of microbes on minerals, are needed tounderstand ecosystem effects.

6. Monitoring CO2 uptake in surface systems
Measuring CO2 uptake is fairly easy in systems that create a solid product or produce purified CO2gas, but in systems at the surface the problem can be daunting due to the flux of CO2 in and out of
the system. For example, carbon can be lost from agricultural soil due to (1) the removal of crops,
crop residues and weeds and (2) the flow of bicarbonate ions dissolved in groundwater and surface
runoff into rivers and the oceans. Moreover, the effects of carbon mineralization need to bequantified against a noisy background of natural and agriculturally induced spatial and temporalvariability in the carbon contents of soil and groundwater. Before conducting mineralization
experiments, studies of surface systems are needed to understand background concentrations,variability and fluxes. Then a full accounting of the inputs and outputs of carbon during and after
experiments is required. Major topics include:Measurement of groundwater flux of carbon, primarily in the form of bicarbonate.

Measurement of reduced carbon converted to CO2 and back, particularly by microbes.



Large area atmospheric CO2 flux measurements coupled with detailed on‐groundmeasurements of CO2 flux.

The development of statistically robust sub‐sampling methods that permit accurate estimationof integrated fluxes based on measurement of very small signals per unit area, potentiallyvarying over a huge area, against a noisy background of natural processes.
 Development of sampling methods could begin with comprehensive surveys at a very finescale, but such surveys would not be sustainable for gigaton‐scale deployment overdecades.

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68 The goal should be not only to assess net carbon budgets, but also to develop sub‐sampling
methods that return accurate flux estimates with greatly reduced sampling density andfrequency.
 This challenge is similar to that posed by evaluation of net carbon fluxes produced by“sustainable agriculture” methods (such as no‐till, low‐till, crop rotation and deep roots)
and by addition of alkalinity to the oceans. Focusing on this generalized goal could be used
to establish an overarching research program of its own.

7. Novel processes
Research is needed into new methods for reacting CO2 with calcium and magnesium and into new
products or value‐added processes that could improve the economics of carbon mineralization.
Major topics include:

Novel reactors for surface treatment of rocks and wastes with concentrated CO2, CO2‐bearing
fluids or air.



Improved methods for integration of mineralization with existing industrial process.

Improved methods for integration of mineralization with inexpensive, partial direct air capture(DAC) to produce hybrid methods.

8. Measurement and accounting
Accurate measurement is important for any commodity traded in markets. For carbon
mineralization to participate in carbon markets, accurate measurement of the carbon removed bymineralization will be essential. To facilitate large‐scale financial support in carbon markets, carbon
mineralization requires high‐fidelity accounting approaches for carbon removal and storage
quantities as well as durations. Major topics include:

Development of standard measurement methods for quantifying CO2 removed from theatmosphere by all major mineralization pathways. These should include appropriate deductionsof estimated counter‐factual CO2 removal through passive (untreated) means.

Development of standard forecasting methods for predicting the storage duration ofmineralized CO2 in surface and subsurface contexts and in valorized products.

9. Resource mapping
For carbon mineralization to reach its full potential as a climate change strategy, one of the mosturgent needs is to identify the locations and quality of mineralization resources around the world.
This includes:


Producing maps of ultramafic rocks (e.g., mantle peridotite) that include their mineralogy,
especially brucite and asbestos content.

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69Producing maps of mafic rocks (e.g., basaltic lavas but also mafic intrusions) that include theirmineral and glass content, as a precursor to development of estimates of their potential rates
of CO2 uptake through mineralization.

Conducting hydrological surveys of volcanic provinces and peridotite localities in the US.
Understanding the groundwater hydrology of these areas is crucial to discerning which of them
may be practical sites for carbon mineralization, with low impact and co‐benefits for local
communities (see Water considerations).

Producing maps showing the location of legacy industrial wastes and mine tailings and theircondition and usability.

Producing maps of available mineral resources near agricultural areas with soils that would
benefit from additional calcium and/or magnesium.



Expanding analysis of the impact of varying carbon prices on the economic feasibility of mining
different mineral deposits, taking into consideration revenues from carbon‐removal services
through mineralization.

10.

Water considerations

Many carbon mineralization methods produce, consume or cycle significant amounts of water. This
could potentially result in impacts on water quality, process scale constraints due to water
availability or other impacts. The role of water in carbon mineralization therefore requires more
research attention to ensure that overall impacts and constraints are not problematic. Major topics
include:

Improving the understanding of the hydrological cycle in areas of high geological potential for
carbon mineralization, notably volcanic provinces and peridotite localities where surface water
may be limited.



Developing advanced methods for injecting CO2‐bearing water using brackish water orseawater, minimizing demands on fresh water.



Developing improved methods for treating and managing produced water from injection of CO2
into the subsurface.



Developing improved methods for modeling, monitoring and preventing potential groundwatercontamination from large‐scale subsurface injection.



Developing improved methods for monitoring carbon flux in agricultural runoff as part ofsurficial enhanced weathering processes.

11.

Social science

More work is needed to understand how carbon mineralization projects will affect a range ofstakeholders, including the communities in which these activities take place. More work is alsoneeded to help ensure that carbon mineralization can best deliver meaningful benefits to thecommunities where such activities occur. Priority topics for research include:

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Improving the understanding of agronomic and ecological effects of surficial carbon
mineralization approaches.

Improving the understanding of environmental and economic impacts of quarrying natural
materials for carbon mineralization.

Improving the understanding of the overall impacts of using ongoing or legacy industrial wastes
for carbon mineralization at gigaton scale.

Improving the understanding of the impact of large‐scale fluid injection and subsurface
mineralization, in terms of deformation, micro‐seismicity and hydrology.



Expanding assessments of the potential impact of large‐scale mineralization approaches onland use and land use change, particularly for surficial enhanced weathering processes.



Identifying employment needs and opportunities in different regions from all major
mineralization processes, including specific job skills/categories in quarrying, bulk transport,
chemical engineering, rock mechanics, reservoir engineering, monitoring and verification.



Identifying and estimating overall potential economic benefits to communities from large‐scale
mineralization projects.

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CHAPTER 9:
POLICY
Policymakers have paid scant attention to carbon mineralization as a strategy for fighting climatechange. Of the more than 190 national climate action plans submitted to the UN Framework
Convention on Climate Change (known as “Nationally Determined Contributions”), only one(Iceland’s) mentions carbon mineralization.1,2 Very few climate change laws or regulations do so.
Few, if any, laws or regulations that govern mining or underground injection have been adapted toreflect the potential for carbon mineralization to help meet climate change goals.
For carbon mineralization to remove CO2 from the atmosphere at a scale of gigatons per year,
supportive policies will be essential. Leading options for such policies include funding ingovernment research and development (R&D) budgets, preferences in government procurement,
recognition in emissions trading programs and incentives in tax regimes. Permitting processes formining activities may need adjustment. International cooperation—sharing information and bestpractices—could accelerate progress.
Policy frameworks that engage all key stakeholders will be essential. Some carbon mineralization
projects could create good high‐paying jobs in mining communities and employ engineers currently
employed in oil and gas extraction. Other projects could contribute to agricultural productivity(see Chapter 4). At the same time, some projects create risks of surface‐water contamination andecosystem disturbance if poorly managed. Building public support by maximizing the benefits andminimizing the risks of carbon mineralization processes will be important to their successful
implementation at scale.
This chapter discusses policy options for scaling up carbon mineralization as a strategy forremoving CO2 from the atmosphere.

1. GOVERNMENT SUPPORT FOR R&D
National governments spend at least $18 billion annually
on R&D for climate mitigation. These programs haveplayed important roles in developing countlesstechnologies in recent decades.3
As set forth in Chapter 8, R&D on a range of topics isimportant for carbon mineralization to reach its fullpotential for removing CO2 from the atmosphere. Priority
topics include the following:

Reactivity of minerals and industrial wastes



New methods for reacting CO2 with calcium and magnesium

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Fate of CO2 in basalts and ultramafic rocksImpacts of calcium and magnesium additives on soil health

Tools for measuring and monitoring CO2 removal from air with carbon mineralization

Much of this research involves basic science or pre‐commercial topics that private companies havelittle, if any, incentive to fund. As a result, government funding for research on these topics will beessential. National governments, including those in the United States, Japan, the European Union,
Russia and China, have historically provided most of the support for basic science and pre‐commercial research.
In June 2021, ministers from more than 20 countries announced Mission Innovation 2.0, pledging“a decade of innovation to catalyze increased investment in clean energy research, development
and demonstrations to deliver affordable clean energy solutions by 2030.”3 Member governments
include Japan, the United States, China, the United Kingdom, Germany and Saudi Arabia. The
resulting increase in government R&D budgets in these countries in the years ahead offers an
opportunity to increase R&D funding for carbon mineralization.

2. GOVERNMENT PROCUREMENT
In many countries, government procurement makes up more than 10% of GDP.4 This spending
provides an important opportunity to jumpstart markets for new products with public benefits,
such as CO2 removal from air. First, government purchase contracts can provide manufacturers
with an assured market, which can be especially important in securing debt capital. Second,
government purchases can help establish standard technical specifications for new products, which
can help catalyze efficient supply chains.
As an example, governments are major purchasers of concrete, which can be made from cement
and aggregates that include mineralized carbon. Establishing strong preferences for products madewith mineralized carbon in government procurement could help jumpstart markets and promote
carbon mineralization as a climate solution.

3. EMISSIONS TRADING PROGRAMS
Recognizing carbon mineralization as a compliance option in emissions trading programs is one of
the most important tools for realizing its full potential for climate change mitigation.
Under emissions trading programs, the right to emit requires a permit. Governments give or sell
these permits to emitters, who may then trade the permits among themselves. Under many
emissions trading programs, governments gradually reduce the number of permits (often calledemissions allowances), thereby reducing total pollution. Emissions trading programs for CO2 are
now in place in the European Union, China, California, the northeast United States and Canada,
among other places.
At present, no emissions trading programs recognize carbon mineralization as a compliance option.
Such programs could do so by authorizing facilities that mineralize carbon to sell allowances equal
to the CO2 removed from the atmosphere, measured on a full life‐cycle basis. Indeed such

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allowances might merit a premium of some kind, in light of the permanence of CO2 removal withcarbon mineralization (as compared, for example, to CO2 removal with trees, which can burn). Thisapproach would provide facilities that remove CO2 from air via carbon mineralization with afinancial reward for doing so.
Such an approach would require widely accepted protocols for monitoring, reporting and verifying
CO2 removal from air via carbon mineralization. Such protocols are currently lacking. Governments
could accelerate development of such protocols by contributing funding to standards‐settingorganizations, such as the International Standards Organization (ISO) and ASTM International, as
well as support for pilot projects.

4. LOW-CARBON PRODUCT STANDARDS
A low‐carbon product standard sets a limit on the product’s life‐cycle emissions. Low‐carbon fuelstandards—the leading example of such an approach—have been adopted in California, Oregon,
British Columbia and the European Union.
Low‐carbon product standards could promote carbon mineralization in two ways.
First, low‐carbon standards could be applied to products that can include mineralized carbon, suchas cement and concrete (see Chapter 7). Firms that make such products would be required tosteadily reduce life‐cycle CO2 emissions or to purchase credits from firms that have done so. Such arequirement could significantly increase demand for products that include mineralized carbon.
Second, carbon mineralization could be recognized as a compliance option for low‐carbon
standards for other products, such as fuels. CO2 removed through carbon mineralization could be
used as an offset to satisfy part of a firm’s obligation to reduce the carbon footprint of a product.
Limitations would likely be needed to ensure that this compliance option does not undercut the
principal objective of the standard, which is to steadily reduce the carbon content of the productitself. As with emissions trading programs, widely‐accepted protocols for monitoring reporting andverifying life‐cycle CO2 removals from carbon mineralization would be essential.

5. FINANCIAL INCENTIVES
Many carbon mineralization projects will be large and capital‐intensive. Some may be funded inpart by private capital, with investors seeking returns from the sale of products that incorporate
mineralized carbon or carbon credits earned from mineralization. However, first‐of‐a‐kind facilities
are unlikely to be able to attract private capital in amounts sufficient for initial deployment at scale.
(This is the classic second “valley of death” for energy technologies.)
Governments play a central role in supporting deployment of large, capital‐intensive projects withclimate change benefits. Government support can take several forms, including the following:a. Tax Incentives. Tax incentives can play an important role in spurring deployment of climate
mitigation technologies. In Norway, for example, generous tax incentives have helped plug‐inelectric vehicles capture more than 50% of new car sales in recent years. In the United States,
federal tax incentives have played an important role in promoting deployment of solar and wind
power.

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The Section 45Q Carbon Capture Tax Credit, enacted in the United States in 2018, provides atax credit for each ton of CO2 sequestered. Section 45Q provides tax credits of $50 per tCO2
sequestered in geologic formations and $35 per tCO2 used in products, such as fuels or cement.
As of this writing, the US Congress is considering several proposals to increase the value of the
45Q credit.
Tax incentives have many possible structures. They include the following:

Investment tax credits. Governments could provide businesses a tax credit for a percentage ofthe capital costs incurred in mineralizing carbon. (This credit would be similar to the US federal
government’s investment tax credit for solar power, which has historically provided a tax creditof 30% of the cost of any solar installation in the United States.)Production tax credits. Governments could provide a tax credit for any products manufactured
at facilities that mineralize carbon. (This credit would be somewhat similar to the US federal
government’s production credit for wind power, which provides a tax credit based on the kWhof wind power sold at a facility.) Because some companies do not have tax liabilities,
governments can provide refundable tax credits or cash payments in lieu of tax credits underthese programs.



Waiver of sales, value‐added taxes or import taxes. Governments could waive taxes that would
otherwise be imposed on products manufactured with mineralized carbon. (Such waivers
would be similar to Norway’s incentives for electric vehicles, which include waivers of importand sales taxes.)b. Grants. Grants are among the most direct ways to provide financial support for the low‐carbon
transition. Grant programs are widespread in many countries, often to assist in deploying first‐
of‐a‐kind or early‐stage technologies. Governments could provide grants to help defray thecapital costs associated with facilities that mineralize carbon.
c. Loan guarantees. Cutting the cost of debt capital can help make a project financially viable.
Government loan guarantee programs seek to do this by reducing risk to lenders, resulting inlower borrowing costs. The US Department of Energy’s loan guarantee programs helped launch
the utility‐scale solar industry in the United States, among other successes. Loan guarantees forthe capital expenditures required for carbon mineralization facilities could significantly speed
deployment.

6. MANDATESPerhaps the simplest way for governments to provide incentives for carbon mineralization is torequire it. For example, government mandates could require mining companies at sites withpotential for carbon mineralization to take steps to realize the potential for carbon capture andstorage at those sites.
Government mandates can be effective in helping reduce emissions and in building markets forclean energy products. In the United States, many state governments require utilities to purchase aminimum percentage of their power from renewable sources. In India, a similar requirement isNovember 2021

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imposed by the Ministry of New and Renewable Energy. These requirements have been importantto the early growth of wind and solar power in both countries.
Other experiences with government mandates suggest caution, however. The United States federal
government has required the use of cellulosic ethanol in fuel supplies for more than a decade.
Nevertheless, the cellulosic ethanol industry remains in its infancy. Waivers to that requirement
have been granted on a regular basis. Technology‐forcing requirements—in which governments
require private actors to meet standards that are not yet technically achievable—have beensuccessful in some instances but not in others.

7. ENVIRONMENTAL, SOCIAL AND GOVERNANCE ISSUES
Carbon mineralization projects can have far‐reaching impacts beyond removing CO2 from theatmosphere. Potential environmental impacts range from positive (such as soil quality
enhancement) to negative (such as groundwater contamination and ecosystem disturbance).
Social impacts could also be positive (such as job creation or preservation) or negative (such as
disrupting communities or appropriating project benefits without compensating local stakeholders).
In part as a result, carbon mineralization projects may be subject to extensive regulatory oversightin some countries. Ex situ projects may be subject to mining sector regulations and permit
requirements on diverse topics ranging from groundwater protection to hazardous waste
management to foreign ownership to financial transparency.5,6 (The Extractive Industries
Transparency Initiative (EITI), for example, promotes “the disclosure of information along theextractive industry value chain from the point of extraction, to how revenues make their way
through the government, and how they benefit the public.”7 More than 55 countries have pledged
to meet EITI’s standards.) In situ projects may be subject to regulations on underground injection(likely designed for unrelated purposes).
As governments consider strategies for scaling up carbon mineralization for CO2 removal from air,regulatory issues will require considerable attention. Governments could support pilot anddemonstration projects to help assess a range of regulatory issues. Governments could also ask
relevant ministries and departments to examine the overall regulatory environment for carbon
mineralization projects and recommend whether modifications may be appropriate. Expedited
permitting could be considered for projects with significant potential to remove carbon from theatmosphere and create jobs (while being careful to avoid projects that could pollute local
environments or impair carbon mineralization projects’ social license to operate).
Engaging communities most directly affected by carbon mineralization projects will be essential.
Without support from such communities, carbon mineralization projects are unlikely to reach thescale needed for meaningful climate change impacts. Ensuring that carbon mineralization projects
deliver both local and global benefits will be important to their long‐term success.

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8. INTERNATIONAL COOPERATION
Opportunities for CO2 removal through carbon mineralization are widely dispersed around theglobe (see Chapters 1 and 2). International cooperation in mapping mineral resources, sharing bestpractices and developing MRV protocols could help accelerate carbon mineralization as a climate
mitigation strategy.
A number of international organizations could launch or deepen work on this topic. One potential
venue is the Clean Energy Ministerial, which hosts initiatives sponsored by interested governments.
Countries with significant carbon mineralization potential—including the United States, Japan,
Canada, India, the United Arab Emirates and Saudi Arabia—could launch a Clean Energy Ministerial
initiative to work together on this topic. Another possible venue is the UN Framework Convention
on Climate Change, which could task a subsidiary body to gather information on this topic from
Member States, including information on mineral resources that are especially well‐suited tocarbon mineralization. Finally, the Intergovernmental Panel on Climate Change (IPCC) could expand
coverage of carbon mineralization in future assessments.

REFERENCES
1

United Nations Framework Convention on Climate Change (UNFCCC). Nationally Determined Contributions(NDCs): The Paris Agreement and NDCs, <https://unfccc.int/process‐and‐meetings/the‐paris‐
agreement/nationally‐determined‐contributions‐ndcs/nationally‐determined‐contributions‐ndcs> (Accessed
September 2021).

2

United Nations Framework Convention on Climate Change (UNFCCC). NDC Registry (Interim): The Latest
Submissions, <https://www4.unfccc.int/sites/ndcstaging/Pages/LatestSubmissions.aspx> (Accessed
September 2021).

3

Mission Innovation. Mission Innovation launches a decade of clean energy innovation to accelerate achieving
the Paris Agreement Goals, <http://mission‐innovation.net/2021/06/02/decade‐clean‐energy‐innovation‐
mi‐6/> (2021).

4

Esteban Ortiz‐Ospina and Max Roser. "Government Spending" (Published online at OurWorldInData.org),<https://ourworldindata.org/government‐spending > (2016; Online resource; accessed September 2021).

5

Investment Climate Advisory Services – World Bank Group. Sector Licensing Studies – Mining Sector,<https://openknowledge.worldbank.org/bitstream/handle/10986/27787/587890WP0Secto1BOX353819B00
1PUBLIC1.pdf?sequence=1&isAllowed=y> (Accessed September 2021).

6

Baker McKenzie. Global Mining Guide, <https://resourcehub.bakermckenzie.com/en/resources/global‐
mining‐guide> (Accessed September 2021).

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CHAPTER 10:
COMPARISON WITH DIRECT AIR CAPTURE
As a strategy for removing CO2 from the atmosphere, carbon mineralization shares many
characteristics with direct air capture (DAC). Both are early stage and expensive. Both have clear
pathways to reduce costs, can be deployed in many locations around the world and show
considerable promise in helping to achieve net‐zero emissions.
The two strategies also have important differences. Carbon mineralization processes typicallyrequire far less energy than DAC. Some carbon mineralization processes offer ancillary benefits notavailable from DAC. Carbon mineralization processes inherently provide a mechanism for long‐term storage of CO2, which DAC processes do not. However, measurement and verification of CO2
removal is more difficult for some carbon mineralization methods than for DAC. Also, morescientific research is required for full deployment of carbon mineralization than for DAC.
The following table summarizes key similarities and differences.
Table 10‐1. Comparisons between direct air capture (DAC) and carbon mineralization.
Direct Air Capture

Carbon Mineralization

Process type

Purely engineered process

Accelerated natural process

Potential for abatement

Effectively limitless (10–20 trillion
tCO2 storage in conventional
reservoirs)

Effectively limitless (>10,000 trillion tCO2
storage potential)

Potential to achieve gigaton-scalecarbon removal before 2040

High

High

Geographic range

Very broad

Very broad

Cost today

Above $200/ton

Above $200/ton

Likelihood of costs below $150/ton
before 2030

High

High

Requirement for energy inputs

Very high (~250 MW/Mton) – must
be near-zero carbon footprint

Zero to medium – must be near-zerocarbon footprint

Land-use requirement for process

Small (0.1 km2/Mt)

Varies widely (1–400 km2/Mt)

Land-use requirement for low-carbon
energyVaries widely (0.1–100 km2/Mt)

Varies widely

Uses Earth-abundant materials

In some cases

Yes

Ancillary benefits beyond climate
mitigation

Limited – some fresh-waterproduction

Substantial in some approaches, including
remediation of hazardous materials,
building material production and more

Potential additional environmental
concerns

LimitedMediumTechnical Readiness Level

TRL 8–9 (established companies)
TRL 5 (emerging companies)
TRL 1–2 (new approaches)

TRL 5 (emerging companies)
TRL 1–2 (new approaches)

Need for additional scientific research

LimitedSubstantial

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A few similarities and differences are especially noteworthy:

Cost: For both strategies, costs today are fairly high in practice—above $200 per tCO2.
Although published estimates for potential costs are much lower (e.g., <$150 per ton for DAC,<$100 per ton for carbon mineralization), these lower costs have not been achieved in realprojects or active markets. In both cases, the thermodynamic limits permit very low costs(~$18 per ton for DAC, $10 per ton for carbon mineralization), and there is enormous potential
to reduce costs significantly and quickly for both strategies through innovation and design.

Technical readiness: Both strategies are relatively immature. Neither has mature technicalpathways or contracts to deliver CO2 removal greater than 10,000 tons per year. DAC issomewhat more mature: three companies have commissioned projects and have commercial
contracts. In contrast, while a few carbon mineralization companies have announced
substantial pilot projects, they are still quite early in technical development.



Geographic range: Both strategies can take advantage of different geological and energy
resources around the world. They can operate under different temperature and humidity
conditions, store CO2 in varying geological sites and have access to different low‐carbon energy
resources. While the specific conditions will affect cost and performance, a mixture of pilots
and demonstrations around the world could provide key information to companies,
governments, investors and innovators regarding performance under a range of conditions.

All three of these points suggest that both carbon mineralization and DAC are good candidates for
an innovation agenda. Specifically, research should focus on cost reduction and a mixture of pilot
and demonstration projects. This would grow companies, foster competition and validate designs
and business models. Ideally, nations and projects around the world should share findings andresults of innovation with the goal of accelerating learning and time to market.
A comparison of carbon mineralization and DAC suggests several important research directions:

Energy Consumption: This is the most dramatic difference between the two technologies.
Carbon mineralization provides much of the required energy from the chemistry of the rocks orslag, while DAC requires energy input entirely from an external source. The amount of energy
DAC requires varies depending on the technology, but NASEM (2019)1 estimated about 250
MW per 1 million tons of CO2 removed per year (with a range of 100–500 MW). Thus,
mineralization offers a pathway to carbon removal that requires less clean energy
development.
 The major R&D activity in this area is to further refine the energy needs of the twopractices, particularly around supply chains.



Accounting: The clear accounting associated with DAC coupled with geologic storage is a strong
advantage for DAC. The amount of CO2 removed and stored is easily measured by conventional
chemical monitoring systems, and geologic storage has been extensively evaluated for thecontrol and monitoring of leaks from deep storage. Thus, errors in the accounting of DAC
should be negligible.

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Accounting for mineralization is significantly more difficult when the CO2 comes fromatmospheric uptake. The formation of solid carbonates is equivalent to geologic storage in its
simplicity and accuracy of measurement, but in most systems open to the air, a significant
amount of the captured CO2 may leave the location in crops, crop residues, weeds and as
bicarbonate ions dissolved in water. The bicarbonate ions are actually good for capture
efficiency since two bicarbonate ions are required for charge balance of each dissolved ion of
calcium or magnesium. However, monitoring the concentration and flux of bicarbonate ingroundwater and surface runoff is a key research challenge, especially for agricultural
applications and ocean‐based systems.
The major R&D activity in this category is to improve the monitoring and life‐cycle assessment(LCA) for mineralization approaches.
Land Use: Availability of land area for CO2‐removal (CDR) technologies is likely to be a major
limit on their deployment. Here there is no obvious winner because the two approaches use
land in very different ways. DAC’s direct footprint is small, almost negligible. LLNL (2019)2
estimated the land area footprints for solvent‐ and sorbent‐mediated DAC applied in California
set forth in the table below.

Table 10‐2. Land area footprints for direct air capture and energy joint facilities capturing 1 million tons of CO2 per year.
In the case of the solvents, only solar and wind are powering the electric component of the energy, with natural gas
powering the thermal component.1‐4
Natural Gas

Solar

Wind

Solv.
LimitedSorb. Effectively
limitless(>10,000 trillion
tCO2 storage
potential)

Solv. (NG)Substantial

Sorb.

Solv. (NG)

0.26

0.43

21

5 (0.37)217

51 (0.37)

Direct Air Capture
Footprint [km2]

2

7

2

7

2

7

Total Land Area
Footprint [km2]

2.3

7.4

23

12.4

250

58.4

Sorb. Effectively
limitless (10–20 trillion
tCO2 storage inconventional
reservoirs)
Energy Footprint [km2]

However, if solar power is used to drive DAC systems, the land footprint is significant: 12–23
km2 per million tons of CO2 removal capacity per year. For example, removing 1 GtCO2 from airwith a DAC system covering 23 km2 would require 23,000 km2 for photovoltaic power
generation.
There are many ways to consider the land area required for mineralization. Kelemen et al.
(2020)5 made a simple estimate for the long‐term weathering of exposed ultramafic rock,capturing and storing about 2.5 kgCO2 per m2 of exposed fine‐grained material. Thus, removing
1 GtCO2 from air by this simple process would require 400,000 km2 of surface. A much more

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intensive process using stacked material in greenhouses is envisioned by Myers and Nakagagi(2020).6 They estimate land requirements of about 1000 km2 to remove 1 GtCO2 from air per
year.
The land area requirements for mineralization are thus dependent on the method used and its
efficiency, unlike current knowledge of land use for DAC, which appears more certain. In any
case, both require significant land area. Of immediate interest is whether the land required inboth case could be put to dual use. The comparative value of the land for other purposes will
also be a significant factor.
The major R&D activity in this topic is to evaluate the true land‐use requirements for various
carbon mineralization approaches and to compare the availability of land for both DAC andcarbon mineralization to determine how much land could realistically be committed for those
purposes.
The foregoing suggests that DAC faces several challenges that carbon mineralization does not.


Carbon mineralization processes generally use standard equipment widely available around theworld. DAC processes, in contrast, often require specialized equipment that is not widelycommoditized and lacks a broad manufacturing base. Obtaining equipment and securing
critical materials could be a significant bottleneck to scaling up DAC.7



Carbon mineralization processes generally use far less energy than DAC, imposing fewer
constraints with respect to the availability of low‐carbon energy

For both carbon mineralization and DAC, the required skill sets for workers are likely to be similar
to those required for current jobs in oil and gas, making labor unlikely to be a major bottleneck.
Indeed, the potential to create good‐paying jobs for oil and gas workers may help create politicalsupport for both carbon mineralization and DAC, accelerating their scale‐up.
Carbon mineralization and DAC could both provide substantial CO2 removal and climate mitigation
once scaled. The two strategies can also complement each other and provide important synergies.
This is already underway at the Climeworks‐CarbFix project,8,9 in which carbon mineralization and
DAC work in tandem. Similar positive synergies are provided in the Heirloom process10 (wherecarbon mineralization is the mechanism for DAC) and the 44.01 project11 (which combines carbon
mineralization and DAC in the deserts of northwestern Oman).

REFERENCES
1

National Academies of Sciences (Engineering and Medicine) (NASEM). Negative Emissions Technologies andReliable Sequestration: A Research Agenda. (Washington, DC: The National Academies Press, 2019;https://doi.org/10.17226/25259).

2

Joshuah K. Stolaroff Sarah E. Baker, George Peridas, Simon H. Pang, Hannah et al. Getting to Neutral: Options
for Negative Carbon Emissions in California. (2020 (Lawrence Livermore National Laboratory, LLNL‐TR‐
796100); https://www‐gs.llnl.gov/content/assets/docs/energy/Getting_to_Neutral.pdf).

3

J Wilcox, B Kolosz & J Freeman. CDR Primer, <https://cdrprimer.org > (2021).

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4Energy Futures Initiative (EFI). Clearning the Air: A Federal R&D Initiative and Management Plan for Carbon
Dioxide Removal Technologies,<https://static1.squarespace.com/static/58ec123cb3db2bd94e057628/t/5d899dcd22a4747095bc04d5/156
9299950841/EFI+Clearing+the+Air+Summary.pdf> (2019).

5Peter B. Kelemen et al. Engineered carbon mineralization in ultramafic rocks for CO2 removal from air:
Review and new insights. Chem. Geol. 550, 119628, doi:https://doi.org/10.1016/j.chemgeo.2020.119628(2020).

6

Corey Myers & Takao Nakagaki. Direct mineralization of atmospheric CO2 using natural rocks in Japan.
Environ. Res. Lett. 15, 124018, doi:10.1088/1748‐9326/abc217 (2020).

7

Noah McQueen et al. A review of direct air capture (DAC): scaling up commercial technologies andinnovating for the future. Progress in Energy 3, 032001, doi:10.1088/2516‐1083/abf1ce (2021).

8

Climeworks. <https://climeworks.com> (Accessed September 2021).

9

CarbFix. <https://www.carbfix.com> (Accessed September 2021).

10

Heirloom Carbon Technologies. <https://www.heirloomcarbon.com/> (Accessed September 2021).

11

44.01. <https://4401.earth/> (Accessed September 2021).

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CHAPTER 11:
FINDINGS AND RECOMMENDATIONSFINDINGSFINDING 1: Carbon mineralization has the potential to permanently remove and sequester
many gigatons of CO2 from the atmosphere each year. Carbon mineralization can remove CO2
directly from the atmosphere and permanently store that CO2 by incorporating it into rocks. Carbon
mineralization can also permanently store CO2 captured from point sources, such as coal power plants
and industrial facilities, again by incorporating that CO2 into rocks. With strong and sustained policy
support from governments around the world, carbon mineralization processes have the potential toremove 1 GtCO2 from the atmosphere per year by 2035 and 10 GtCO2 per year by 2050. More researchis needed to define conditions under which carbon mineralization processes could achieve thispotential.
FINDING 2: As a strategy for carbon removal and sequestration, carbon mineralization has
many strengths. First, rock types suitable for carbon mineralization are found in dozens of countries.
Second, chemical reactions that mineralize CO2 do not require energy inputs. Third, once CO2 isincorporated into rocks (becoming part of a carbonate mineral), that CO2 is sequestered for millennia.
The combination of widely available resources, favorable thermodynamics and permanent CO2 storage
makes carbon mineralization an important potential strategy for meeting climate change goals.
FINDING 3: As a strategy for carbon removal and sequestration, carbon mineralization also has
several challenges. First, most natural carbon mineralization happens very slowly. Second, thedistribution of optimal mineral resources is only coarsely understood. Third, while some products ofcarbon mineralization have commercial value, those values are typically low. Finally, carbon
mineralization for climate mitigation is not yet practiced at large scale.
FINDING 4: Carbon mineralization is not one pathway—it is multiple pathways. All carbon
mineralization approaches are based on reacting CO2 with minerals in some form. However, the sourceof CO2 and feedstock, the nature of the mineralization processes and the fate of the resulting
carbonate minerals all vary dramatically from one pathway to another. CO2 may come directly from theair or from point‐source carbon capture systems. Feedstock materials may be found in geologic
formations, mine tailings or industrial wastes. The mineralization process may involve transporting CO2
to minerals rich in magnesium, calcium or similar elements (e.g., with pipelines to quarries) or exposing
fine‐grained materials to weathering (e.g., with powdered rock applied to farm fields). In some cases,
the carbonate minerals produced have economic value and can be used in construction or agriculture,
while in other cases the minerals remain sequestered in the subsurface.
FINDING 5: Carbon mineralization receives little recognition or support in climate change
policies around the world. Only one nation (Iceland) mentions carbon mineralization in its Nationally
Determined Contribution under the Paris Agreement. Investment in research and development (R&D)
on carbon mineralization is minimal. Carbon markets, including the European Emissions Trading System

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83and the California Low‐Carbon Fuel Standard, do not provide credit for carbon mineralization. Few if
any jurisdictions offer tax incentives or subsidies for carbon mineralization.
FINDING 6: Current scientific knowledge and technical experience are sufficient to support
carbon mineralization projects at the pilot and demonstration scale today. The large and readily
accessible literature on mineral structure, reaction kinetics and thermodynamics provides a strong
foundation for such projects. Conventional mining operations can help in assessing the engineering
requirements and costs. Methods for monitoring and verifying carbon removal and storage areavailable for many carbon mineralization strategies. This body of knowledge and experience can help
carbon mineralization play an important role in meeting climate change goals on time.
FINDING 7: The key technical challenges in carbon mineralization are speeding up the chemical

reaction between atmospheric CO2 and minerals, maximizing the CO2 content of mineralizedmaterials and minimizing the space required for permanent storage. Although some rocks andminerals react with CO2 quickly, most do not under normal circumstances. Achieving economically
viable mineralization techniques requires processes to accelerate these reactions, which can include
grinding, heating and chemical treatment. Maximizing the CO2 content of mineralized materials can
reduce the amount of rock or waste material required, the cost of obtaining and grinding that material,
and the volume of carbonated material that must be stored permanently. For surficial carbon
mineralization, low‐cost, low‐impact space for permanent storage is essential.
FINDING 8: Measurement and verification tools today are sufficient for carbon mineralization

approaches (including ex situ conversion of main tailings) and insufficient for others (including
enhanced rock weathering). Measurement and verification methodologies for ex situ conversion ofmine tailings and alkali wastes provide sufficient clarity for attribution and permitting today. Those forin situ CO2 injection may suffice but require validation. Those for enhanced weathering of minerals
added to fields, beaches, the oceans or sedimentary systems currently lack the precision and accuracy
needed to certify and permit CO2 reduction or removal. Refinement of measurement and verification
tools for carbon mineralization processes should be a priority.
FINDING 9: Field pilots and demonstration projects could significantly improve understandingof the potential impacts and costs of carbon mineralization processes, while improving
monitoring and verification methods. The core questions regarding scale‐up and deployment ofcarbon mineralization are operational and must be addressed in field pilots and demonstration
projects. (Lab and benchtop research are insufficient.) Data and experience from field pilots anddemonstration projects will provide important insights regarding the cost, efficacy and scale of CO2
removal and storage with carbon mineralization processes. Some field pilots and demonstration
projects could be launched quickly (e.g., in operating mines under public‐private partnershipagreements). Others would require more time in preparation. Field pilots and demonstration projects
are necessary precursors to investment, commercialization and scale up.

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RECOMMENDATIONS
Recommendation 1: Policy makers should add carbon mineralization to the portfolio of climatechange mitigation options. Governments should consider a range of options, including tax incentives,procurement programs that favor products of carbon mineralization, and recognition of carbon
mineralization as a compliance option in emissions trading programs. Governments should examine theregulatory framework for carbon mineralization activities, exploring ways to expedite permitting while
protecting public health and safety. Many nations should include carbon mineralization in their
Nationally Determined Contributions under the Paris Agreement.
Recommendation 2: Governments and companies should invest in R&D on carbon mineralization.
This should include basic and applied science, engineering tests and development, novel reactor design,novel approach development, and dedicated field pilots. Priority topics should include (1) assessment ofgeological resources and industrial wastes, with great detail and high fidelity, (2) development andrefining of measurement and verification protocols and (3) analysis of potential environmental demands(e.g., water requirements) and consequences (e.g., heavy metal contamination). Most importantly, R&D
programs should include field pilots and demonstrations, using public‐private partnerships whereappropriate.
Recommendation 3: Mining companies and manufacturers should seek opportunities for carbon
mineralization in their exploration and production activities. Companies that extract mineral resourcesfrom mafic and ultramafic rock bodies (e.g., nickel, chromium and diamond mines) should consider
assessing the carbon mineralization resources in tailings and host rock and modifying the treatment anddisposal of tailings. This assessment may entail new assays within existing operations and expanded
assays during exploration that identify potential lodes and minerals suited to rapid mineralization (e.g.,rocks rich in brucite and/or olivine). Companies that produce alkaline metal wastes (e.g., primary ironproduction and fly ash) should consider processes and approaches that can reduce or balance theiremissions. Both classes of companies should develop methodologies to validate and certify CO2 captureand storage, as well as contracting CO2 removal–specific products and services.
Recommendation 4: Governments and companies with net‐zero commitments should consider
carbon mineralization as part of their portfolio of options. As national and subnational governments
consider ways to meet net‐zero targets in the years ahead, carbon mineralization provides an attractive
option for offsetting emissions that are difficult or impossible to eliminate. For companies intending to
reduce Scope 1 emissions through point‐source carbon capture and storage, mineralization may offer
advantages over injection into saline aquifers. For companies intending to procure CO2 removal services
to offset remaining emissions, carbon mineralization using CO2 captured from the atmosphere orbiogenic sources may offer reliable, additional, long‐duration CO2 removal.

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Carbon Mineralization Roadmap

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Carbon Mineralization Roadmap

Carbon Mineralization Roadmap

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DISCLOSURE STATEMENT
Names of companies and descriptions of their activities are provided at several places in this Roadmap
to better inform the reader. The co‐authors have no affiliation with any of the companies mentioned in
the Roadmap, except that (1) Peter Kelemen is an advisor to and shareholder in Heirloom Carbon
Technologies and 44.01 and (2) Ian Power and Sasha Wilson are currently receiving research funding fromDe Beers and Rio Tinto.
DISCLAIMER
Roger Aines and Briana Schmidt contributed to the technical evaluations but not the policy
recommendations in this document.
CITATIONDavid Sandalow, Roger Aines, Julio Friedmann, Peter Kelemen, Colin McCormick, Ian Power,
Briana Schmidt, Siobhan (Sasha) Wilson, Carbon Mineralization Roadmap (ICEF Innovation Roadmap
Project, November 2021).

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