Cement substitution with secondary materials can reduce annual global CO₂ emissions by up to 1.3 gigatons

Cement, a crucial material for construction, is a major source of CO₂ emissions, contributing 7-8% of anthropogenic emissions globally. The production of cement, concrete, and mortar also consumes significant energy and resources. While various mitigation strategies exist, substituting Portland cement clinker with other cementitious materials (CMs) is particularly promising due to the high environmental impact of clinker production. This study focuses on CM substitution in cementitious binders, as complete substitution with non-cementitious materials is largely infeasible in the near future. The study specifically aims to address the unclear potential of clinker substitution at the market scale (national and global) by systematically reviewing the potential global supply of secondary CMs and their associated GHG emission reduction benefits. The research covers countries responsible for approximately 70% of global cement production and key secondary CMs with high adoption potential.

Existing literature highlights the environmental impact of cement production, including significant CO2 emissions, energy consumption, and particulate matter generation. Studies have explored various mitigation strategies, emphasizing the potential of using secondary cementitious materials to replace Portland cement clinker. While some studies demonstrate the feasibility of complete clinker replacement at the product scale, the potential at the market scale (national and global level) remains unclear. A lack of comprehensive data on the potential supply of secondary CMs hinders the development and adoption of technologies that enable large-scale clinker substitution. The current clinker-to-cement mass ratio has remained relatively stable, suggesting a significant untapped potential for utilizing a wider range of secondary CMs beyond the commonly used granulated blast furnace slag and coal fly ash.

The study systematically and quantitatively reviews the potential global supply of secondary CMs and their greenhouse gas (GHG) emission reduction benefits. The analysis covers countries responsible for approximately 70% of global cement production and focuses on several key secondary CMs with high potential for large-scale adoption. The researchers modeled potential generation rates of various secondary CMs, including coal fly ash, flue gas desulphurization gypsum, granulated blast furnace slag, silica fume, bauxite residue, agricultural by-product ashes, forestry by-product ashes, and end-of-life binder. Data sources include the International Energy Agency (IEA), the United States Geological Survey (USGS), the Food and Agriculture Organization (FAO), and other relevant databases and publications. The methodology involved applying by-product-to-main product ratios to production statistics to estimate secondary CM generation. Life cycle assessment (LCA) was conducted to quantify potential GHG emission reductions associated with clinker substitution by secondary CMs. The LCA used a cradle-to-gate scope and considered country-specific inventory data. The researchers modeled potential cementitious binders with maximal secondary CM substitution using sodium silicate activated materials, acknowledging that this might overestimate GHG emissions in some cases. A functional unit of 1.4 kg cementitious binder was used to ensure consistent comparison across different mixes. The ReCiPe 2016 midpoint impact assessment method was employed to estimate GHG emissions.

Cement production more than doubled from 2002 to 2018, primarily due to growth in China. The total generation of secondary CMs increased but at a slower rate than cement production. The study found that the potential global generation of secondary CMs in 2018 was 3.5 Gt. Maximal clinker substitution could have avoided up to 1.3 Gt of CO₂-eq emissions, representing a 44% reduction in cement production emissions and a 2.8% reduction in global anthropogenic GHG emissions. Nearly all of the highest cement-producing nations can locally generate and use secondary CMs to substitute up to 50% of Portland cement clinker, with some countries possessing the potential for 100% substitution. However, achieving high substitution rates often requires alkali-activation technology, which presents a barrier due to the limited production of alkaline activators. Transportation costs also limit the use of secondary CMs, with preferred sourcing distances generally under a few hundred kilometers. The study reveals significant variations in the potential for clinker substitution across countries. While some nations can generate sufficient secondary CMs to fully replace clinker, others, like China, require imports to achieve significant substitution. The GHG emission reduction potential through CM substitution is substantial; for instance, Brazil could have reduced its cement production emissions by 84.5%, equating to 2.9% of its national GHG emissions. Globally, the potential CO2-eq emission reduction from maximal secondary CM utilization was estimated at 4.4% (1.3 Gt).

The findings demonstrate a significant potential for reducing CO₂ emissions from cement production through increased utilization of secondary CMs. The theoretical possibility of a 44% reduction in cement production emissions globally highlights the substantial untapped potential of these materials. However, realizing this potential requires overcoming several challenges. These challenges include the need for a shift in manufacturing practices to accommodate a wider range of secondary CMs and increase processing capacity; policy interventions such as the standardization of secondary CMs and CM-containing materials; and continued investment in materials research and development, particularly in understanding the properties of alkali-activated materials with multiple secondary CMs. The study underscores the importance of regionally optimized CM mixes, international cooperation (e.g., trading secondary CMs), and improving material efficiency in the construction sector. The study also indicates the potential role of international trade in supplementing domestic secondary CM supplies.

This study reveals the substantial potential for reducing greenhouse gas emissions from cement production by maximizing the utilization of secondary cementitious materials. The findings highlight the need for a systemic approach, encompassing manufacturing shifts, policy interventions, and further research to overcome the barriers to widespread clinker substitution. Future research should focus on optimizing regional CM mixes, exploring the potential of alkali-activated materials, and investigating the systemic impacts of digital and off-site construction.

The LCA results are based on certain assumptions, such as the use of sodium silicate activated materials for maximum secondary CM substitution, which may overestimate GHG emissions in some scenarios. The study also excludes upstream impacts from processing wastes/by-products into secondary CMs, although a sensitivity analysis suggests a minor effect on the overall results. The analysis focuses on a limited number of countries representing approximately 70% of global cement production. Data availability for certain secondary CMs might also affect the accuracy of generation rate estimations.