Engineering and Environmental Assessment Framework for Calcined Clays as a Supplementary Cementitious Material

An average of 0.9 tonne of CO2 is released for each tonne of clinker produced worldwide. Around 0.5 tonne of CO2 originates from the decarbonation of limestone, and on aggregate are considered to be the largest non-combustion source of CO2 emissions from industrial production. The remaining CO2 releases are mainly due to the fuel combusted during manufacturing of clinker and in the corresponding supply chains. In light of the scale and urgency of climate change, development of environmentally less damaging cements as well as improvements in concrete production have been explored by many researchers. While upgrading current plants with new technologies is costly and has already been done in most countries, the use of supplementary cementitious materials (SCMs) has shown to be the most cost-effective approach to reducing emissions from cement manufacturing. However, slag and fly ash, the two most used SCMs of suitable quality for clinker replacement, are concentrated in regions where coal power plants and steel plants are located, and the transition to cleaner and more efficient technologies for energy generation and steel manufacturing are limiting their use as a long-term solution. For this reason, there is a vital need to find readily available alternative materials for widespread use as partial replacement of clinker./nIn this dissertation, the use of metakaolin (calcined clay) coupled with limestone as SCMs for partial replacement of clinker in cement is explored. Both their mechanical performance in mortar and concrete mixtures and their effects in reducing cement’s and concrete’s environmental burden are the main foci of this thesis. The importance of calculating the environmental impact of concrete binder made of cement, limestone, and metakaolin would allow us to understand their performance in reference to conventional concrete made of portland cement and commercially available Pozzolan-Portland-Cement concretes. In order to evaluate their environmental performance, Life-Cycle Assessment (LCA) of cements containing metakaolin is developed, along with a Life-Cycle Inventory (LCI) of each of its components. To this extent, a detailed description of an Excel-based tool, the MK-LCA Tool, is presented as a method to evaluate the cradle-to-gate environmental impacts of metakaolin by unit of mass. The environmental evaluation accounted not only for the direct emissions obtained from the production process, but also for indirect, supply-chain impacts of electricity generation and fuel pre-combustion. With an accurate evaluation of the emissions originating from manufacturing, the application of these materials in clinker replacement can be evaluated as a method to reduce cement’s, and in turn, concrete’s environmental footprint. Applications of the tool with a case study located in California showed a strong correlation between the global warming potential (GWP) of metakaolin’s production and the fuel used for its calcination, with values ranging from 340 kg CO2-eq/tonne to 46 kg CO2-eq/tonne when petroleum coke and waste wood were used, respectively. When included as SCM in ternary cement blend comprising clinker and limestone with a total substitution of 50% (i.e., 35% metakaolin and 15% limestone), significant reductions of greenhouse gas (GHG) emissions and life-cycle energy demand were achieved compared to portland cement (36% lower emissions and 19% lower energy demand when metakaolin was calcined using a dry rotary kiln, and 39% lower GHG emissions and 24% lower energy demand when using a flash calciner). When compared to commercially available blended cements, the ternary blends with metakaolin and limestone showed the lowest GWP, and energy demand was surpassed only by slag cement since the latter comprises an average replacement ratio of 85% of clinker by slag. However, all of the blended cements containing fly ash showed higher GHG intensities compared to the ternary blend with metakaolin and limestone cement (i.e., 16-42% increased GHG emissions). Additionally, data on the effect of the metakaolin-to-limestone ratio of ternary mortar blends on the mechanical performance and resistance to chloride penetration are also presented. It was observed that compressive strength is improved with the increase of limestone even for lower grades of calcined clay (i.e., low calcined kaolinite content). Furthermore, the transport properties are much improved with respect to those from portland cement concretes, which translates to more durable cement binders, demonstrating the ample applicability of these ternary cement blends. The development of ternary blends studied in this dissertation made with portland cement, calcined clay, and limestone at industrial scale will have the potential to contribute to 30-50% global reduction of GHG emissions from the cement industries with a focus on emerging economies that would benefit from low-carbon cements without the technical and economic challenges that limit the implementation of other highly technical solutions such as carbon capture and storage.

Advisor: Monteiro, Paulo J. M.
Horvath, Arpad