# Carbon mineralization from mafic, ultramafic rocks and alkaline industrial waste products

Pascale Benezeth, Giuseppe Saldi, Chiara Marieni, Barré Hugo, Simon

Blotevogel, Cédric Patapy, Castillo Alain

► To cite this version:

Pascale Benezeth, Giuseppe Saldi, Chiara Marieni, Barré Hugo, Simon Blotevogel, et al. Carbon mineralization from mafic, ultramafic rocks and alkaline industrial waste products. 3rd IAGC International Conference Water Rock Interaction-18 & Applied Isotope Geochemistry-15, Giovanni Battista de Giudici, Stefania De Pelo, Franco Frau, Jun 2025, Cagliari (Sardaigne), Italy. pp.105. hal-05336274

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# Carbon mineralization from mafic, ultramafic rocks and alkaline industrial waste products

Bénézeth, P.¹*, Saldi, G.², Marieni, C.³, Barré, H.⁴, Blotevogel, S.⁴,⁵, Patapy, C.⁴, and Castillo A.¹

1. GET-CNRS, University of Toulouse, IRD, OMP, 31400 Toulouse, France

2. Department of Physics and Geology, University of Perugia, 06123 Perugia, Italy

3. Carbfix ohf., Smáratorg 3, 201, Kópavogur, Iceland

4. LMDC, INSA/UPS Génie Civil, Toulouse, France

5. Ecocem Materials, 324061, Block F1, Eastpoint Business Park, Dublin 3, Ireland

* Corresponding author: pascale.benezeth@get.omp.eu

## ABSTRACT

While it is crucial to reduce CO₂ emissions to stabilize global temperatures, it might not be sufficient, and methods of further removing CO₂ from the atmosphere may be necessary to counterbalance residual emissions. This study explores the potential of CO₂ mineralization using mafic and ultramafic rocks (in-situ), as well as crystalline Ca/Mg-silicate wastes (EAF, ultramafic tailings, ...) (ex-situ). The findings contribute to the ongoing efforts, presenting viable pathways toward reducing CO₂ emissions.

## INTRODUCTION

Reaching net zero emissions by 2050 would require a decrease of ~1.4 GtCO₂ per year. This involves holistic solutions and a multifaceted engineering strategy that include substantial amounts of both Carbon Capture and Sequestration (CCS) and other Carbon Dioxide Removal (CDR) methods. Permanent mineralization of carbon in mafic and ultramafic geologic formations is one of a promising strategy for reducing anthropogenic CO₂, as demonstrated by two pioneering field projects (Carbfix and Wallula) (e.g., McGrail et al., 2017; Snæbjörnsdóttir et al., 2020). However, conditions and elementary processes that control the sequestration potential of mafic and ultramafic rocks depend on many factors (mineralogy, temperature, pH, pCO₂, fluid chemical composition) as well as thermodynamic, kinetic, hydrodynamic and mechanical constraints, which have been addressed by numerous experimental and numerical efforts (e.g., Kelemen et al., 2020; Raza et al., 2022; Bénézeth et al., 2022; Godard et al., 2022; Delerce et al., 2023; Nisbet et al., 2024; Saldi et al., 2025).

Moreover, in the context of the decarbonization of industry, innovative solutions and technological breakthroughs are also needed to reduce the CO₂ emissions, in particular for industries such as cement manufacturing, which represents ~8% of global CO₂ emissions (Andrew, 2019). Therefore, research is ongoing to find
alternatives to traditional Portland cement. Mineral carbonation of electric arc furnace (EAF) slags and other crystalline Ca/Mg-silicate waste-material streams could provide materials suitable to be used as additives in low-carbon cements (e.g., Kusin et al., 2020; Bonfante et al., 2024). However, for upscaling mineral carbonation processes to a common use in cementitious materials, further work is needed. In this paper, some examples of experimental results obtained during the carbonation of mafic, ultramafic rocks as well as alkaline industrial waste products are presented.

## METHODS

In order to evaluate the potential of various alkaline (EAF-slags) and mafic/ultramafic materials to mineralize CO₂, conventional batch- and mixed-flow reactor systems were used as a function of fluid temperature, pH and pCO₂ (e.g., Bénézeth et al., 2022; Saldi et al., 2025). The mineralogical composition of each minerals, rocks and EAF-slags used in these studies, as well as CO₂ uptake were determined before and after carbonation (XRD, SEM-EDX, Electron Microprobe and TEM, TGA) and the composition of the liquid phase was monitored during the reaction (ICP-OES, Alkalinity). Aqueous solution speciation calculations of the analyzed fluid samples were carried out using PHREEQC (Parkhurst and Appelo, 2013) with the Carbfix thermodynamic database (Voigt et al., 2018).

## RESULTS AND DISCUSSION

Table 1 below provides an example of typical carbonation experiments performed with epidote, epidote with a mineral mixture (Mix: antigorite, tremolite and clinochlore), serpentinized peridotites, and with EAF slags. With partially serpentinized peridotites the yield of the carbonation reaction was observed to increase from ~ 0 at 90 °C to a maximum of 31 mol % at 150 °C. Magnesites containing 3-9 wt % of Fe and silica polymorphs (SiO₂(am) and chalcedony) were the main reaction products (Fig. 1).

<table><caption>Table 1: details of batch experiments performed at 150°C</caption><thead><tr><th>Solid Sample</th><th>Aqueous fluid</th><th>pCO<sub>2</sub> bar</th><th>pH</th><th>Time days</th><th>Initial S/W (g/L)</th><th>Carbonation extent (mol %)</th></tr></thead><tbody><tr><td>Epidote</td><td>MQ water</td><td>20</td><td>5.2</td><td>17</td><td>14.8</td><td>&lt; 1%</td></tr><tr><td>Epidote-Mix</td><td>MQ water</td><td>20</td><td>4.8</td><td>26</td><td>13.9</td><td>&lt; 1%</td></tr><tr><td>Oman peridotite</td><td>NaCl 0.1M</td><td>15</td><td>5.4</td><td>31</td><td>11.9</td><td>31.4%</td></tr><tr><td>EAF slags</td><td>MQ water</td><td>15</td><td>6.7</td><td>14</td><td>11.8</td><td>18.6%</td></tr></tbody></table>

At T>150 °C an increasing fraction of secondary Mg-silicates formed, significantly reducing the extent of the carbonation reaction (Saldi et al., 2025). Because of their sluggish reactivity (Bénézeth et al., 2022), experiments conducted with epidote and the alteration minerals at 150 °C resulted in very low rates of carbonate conversion.
For the EAF-slags, XRD analysis (Fig. 2) shows clearly that the peak intensity for some of the Ca-silicates composing theses EAF slags (merwinite, bredigite, Larnite (β-C₂S) and wüstite) decreased to some extent, whereas new rather large peaks appeared in the range of different carbonates, namely calcite and ankerite as determined from Rietveld refinement.

<figure><img src="image_2.png" /><figcaption>Figure 1. SEM image of magnesite crystals formed on an olivine grain during the carbonation reaction of a partially serpentinized harzburgite (T= 150 °C, pCO₂ = 15 bar).</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Figure 2: XRD scans of the original EAF sample (red) and the EAF after carbonation (black).</figcaption></figure>

The intensity of the internal standard (CaF₂) remained constant so that changes in peak intensity can be mostly attributed to changes in the amount of the respective phase.

## CONCLUSIONS

The comparison of the extents of carbonation obtained from experiments with different minerals and rocks indicates that carbon mineralization in ultramafic rocks requires higher temperatures than in basalts to be more efficient (120-150 °C), mainly because of the slow rates of Mg-carbonate precipitation. On the other hand, the carbonation of basaltic rocks is more effective at lower temperatures, being limited by the formation of clay minerals and other secondary phases when T increases
above 100-130 °C. For EAF slags, carbonation shows to transform not only the Ca/Mg-silicates, but also some of the wüstite, suggesting that there is a further potential for CO₂ uptake of this high-volume by-product.

## ACKNOWLEDGEMENTS

This work has received funding from the EU for the GECO project (grant agreement No. 818169), from the French ANR-LISZT and ISIFOR C-Store projects.

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