Comparative Life Cycle Assessment of Alternative Strategies to Decarbonize the Cement and Concrete Industry

Abstract

Cement and concrete manufacturers are exploring a broad range of strategies to reduce greenhouse gas (GHG) emissions while meeting the growing global demand for cement and concrete. Existing life cycle assessment (LCA) approaches often focus solely on conventional metrics like GHGs. Furthermore, the effectiveness of each strategy in mitigating environmental impacts within the cement and concrete sector varies substantially, and the adoption of these strategies is accompanied by various challenges, which are not accounted for in the current LCA studies. This thesis fills this gap by utilizing the LCA approach to evaluate and compare the environmental impacts and challenges associated with several decarbonization strategies, including alternative fuel (AF) use in cement kilns, replacing Portland cement (PC) with supplementary cementitious materials (SCMs) in concrete, carbon capture utilization and storage (CCUS), and cement electrolyzers (CE), in a single study. The first analysis in this thesis estimates GHG emissions and criteria air contaminants (CACs), while also considering the impact of biogenic carbon in biomass-based alternative fuels on the overall life cycle GHG emissions when replacing natural gas in a cement kiln through an LCA approach. The findings reveal that the substitution of natural gas with alternative fuels in a cement kiln could offer ~7 to 13% reduction in life cycle GHG emissions, depending on the treatment of biogenic carbon in biomass-based alternative fuels. The preliminary estimates suggest that the change to CACs will likely not be materially different from the current use of natural gas. The second analysis addresses the challenge of dust explosions when transitioning to alternative fuels in cement facilities. A dust hazard analysis is conducted to guide cement facilities in safely transitioning from fossil fuels to alternative fuels. The results emphasize the importance of conducting a professional dust hazard analysis before utilizing alternative fuels, as well as educating plant personnel on safety measures, prevention techniques, and the importance of regular inspection and maintenance drills to prevent dust explosions. Additionally, it recommends installing safety equipment to mitigate the catastrophic impacts of explosions. Since the potential for reducing GHG emissions and CACs from concrete using alternative fuels is lower compared to replacing PC with SCMs, the third analysis estimates GHG emissions and CACs from concrete mixes containing an SCM such as pumice. It also integrates the concrete performance data such as compressive strength and rapid chloride penetrability into the LCA model to consistently compare the LCA results of various concrete mixes for informed decision-making. The results demonstrate that replacing PC with ~20 wt% pumice maintains the compressive strength, results in low chloride ion penetrability, and reduces the GHG emissions and CACs by ~20% compared to 100% GU concrete, provided that both mixes meet the minimum performance requirements of the same application. This study concludes that GHG emissions from concrete mixes are only comparable per cubic meter if the mixes have comparable performance parameters, highlights the need to incorporate these parameters into functional units for accurate comparisons, and advises against comparing mixes with different performance parameters which makes them suitable for different applications, to avoid misleading results. The fourth and final analysis of this thesis utilizes the results from previous analyses and compares those strategies with two additional strategies: carbon capture utilization and storage, and cement electrolyzers, using a life cycle assessment approach. It also outlines the challenges that the cement and concrete manufacturers will face in deploying these strategies. The results of this chapter indicate that by utilizing cement produced through cement electrolyzers and CCUS technologies, concrete GHG emissions reductions could be as high as 75%, depending on the source of electricity, compared to current technology (cement obtained from a conventional cement kiln operating on 100% natural gas). However, it is important to note that the results presented for electrolyzer technologies are based on future best-case scenarios and not yet scaled, therefore, there is a higher level of uncertainty in these estimates. The LCA models presented in this thesis can guide cement and concrete manufacturers, stakeholders, policymakers, and regulatory bodies in understanding the emission reduction potential of different decarbonization strategies, both individually and in combination, and in evaluating the trade-offs between them.