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

Cement, a critical material for construction, is responsible for 7–8% of anthropogenic CO₂ emissions, with most impacts arising from the production of Portland cement clinker. Demand for cement has grown to roughly 4 Gt per year, and further growth is expected with global development and population trends. While substitution at the product level (e.g., replacing concrete with other materials) has limits at global scale, substituting clinker within cementitious binders using primary and secondary cementitious materials offers substantial mitigation potential. Despite many product-scale demonstrations of high clinker replacement, market-scale adoption is unclear, constrained by limited reporting on the potential supply of secondary CMs and reliance on a narrow set of materials (e.g., slag and fly ash). This study focuses on assessing global potentials for binder-level substitution using secondary CMs and the associated greenhouse gas mitigation opportunities.

Prior work identifies cement as a major, hard-to-abate emissions source and highlights binder substitution as a promising mitigation pathway. Standards allow substantial use of certain primary CMs (e.g., limestone, calcined clays), yet the supply and utilization of secondary CMs beyond common materials like granulated blast furnace slag and coal fly ash are underreported. Historical clinker-to-cement ratios have remained near ~0.75 globally, suggesting limited market-scale substitution progress. The literature also notes the potential of alkali-activated systems and geopolymers but emphasizes challenges related to activator availability, performance standardization, and technology readiness. These gaps motivate a systematic quantification of secondary CM supply and life-cycle emission reductions at national and global scales.

Scope and coverage: The study quantifies potential generation and utilization of key secondary cementitious materials across countries representing ~70% of global cement production (reference year 2018), and models greenhouse gas (GHG) reductions via substitution of clinker in cementitious binders. Data compilation and secondary CM generation: Secondary CMs considered include industrial by-products (coal fly ash, flue gas desulfurization gypsum, granulated blast furnace slag, silica fume, bauxite residue), agricultural by-product ashes, forestry by-product ashes, and end-of-life (EoL) binder recovered from construction and demolition waste. Generation estimates were derived by combining sectoral production statistics with by-product-to-main-product ratios: - Coal fly ash and FGD gypsum: Global electricity and coal combustion data from the International Energy Agency (IEA) were combined with U.S.-based by-product ratios to estimate generation; resulting global fly ash generation is ~0.35 Gt year⁻¹ since 2010. - Granulated blast furnace slag: Estimated using pig iron production data (USGS/worldsteel) and slag yields (25–30 mass% per unit pig iron for typical ore grades). - Silica fume: Estimated from silicon and ferrosilicon production (USGS), applying typical loss ratios to fume and conversion factors for electric arc furnaces; median product-to-by-product multipliers were used, treating values as upper-bound estimates. - Bauxite residue: Derived from alumina production statistics (USGS, International Aluminium Institute) using an average main product-to-by-product ratio of 1.19. - Agricultural by-product ashes: FAO crop production statistics combined with crop by-product ratios and assumed recovery/combustion for energy; values represent upper bounds due to competing uses of residues. - Forestry by-product ashes: FAO forestry statistics coupled with literature-based residue generation, assumed collection rates, and average ash contents to estimate potential ash generation. - End-of-life binder: Dynamic material flow analysis of in-use stocks and lifetimes across building and infrastructure sectors produced national EoL cement estimates, partitioned into mortar and concrete uses and converted to EoL binder using typical binder intensities. For 2018, ~81% of EoL binder was from concrete and ~19% from mortar. Life cycle assessment (LCA): A cradle-to-gate LCA quantified potential GHG reductions from replacing clinker with secondary CMs. The functional unit was 1.4 kg of cementitious binder (1.0 kg solids plus 0.4 kg mixing water) at the concrete batching plant, assuming functional equivalence of modeled binders. Inventory data were region- or country-specific (ecoinvent v3.6 and literature). System boundaries included raw material extraction and transport to cement plant, raw meal preparation and clinker pyroprocessing, and transport from cement plant to batching plant. Assumptions included 150 km transport from quarry to plant and 150 km from plant to batching plant by Euro 5 trucks (32 t payload). Upstream burdens for generating waste/by-products and their processing into secondary CMs were excluded; a sensitivity analysis applying fly-ash-like processing to all wastes showed minor contributions (up to ~0.01 kg CO₂-eq per functional unit). For high substitution scenarios, sodium silicate alkali activation was included (solid sodium silicate and water addition) based on literature mix designs, recognizing this may overestimate activator needs in blends with nonzero clinker. Impact assessment used ReCiPe 2016 midpoint and IPCC-based characterization factors. National and global results were scaled using 2018 cement production data (GNR, USGS).

- Potential supply: Up to ~3.5 Gt of secondary CMs could have been generated globally in 2018. The combined generation of coal fly ash and granulated blast furnace slag decreased in share from 25% of cement mass in 2002 (0.44 Gt) to 17% in 2018 (0.70 Gt), while total secondary CM generation remained high (~86% of cement production mass in 2018). - Clinker substitution potential: Mixing multiple secondary CMs could theoretically reduce the global clinker-to-cement mass ratio to ~0.14 (about a 61% reduction in clinker content), with standardized scenarios at 0.5 already feasible (e.g., CEM III, LC3). A theoretical global average substitution potential up to ~86.3 mass% is possible with alkali activation. - Emission reductions: Maximal substitution could have avoided up to ~1.3 Gt CO₂-eq in 2018, equivalent to ~44% of cement production emissions and ~2.8% of total anthropogenic GHG emissions. - Country-level potentials: Most large cement-producing nations can generate secondary CMs sufficient to enable ≥50% clinker substitution domestically. The United States, Germany, and South Korea could generate industrial by-products and EoL binder exceeding their 2018 cement production. China, the Philippines, and Egypt produced more cement than potential domestic secondary CM supply (ratios ~1.9×, 1.7×, and 3.3×, respectively), limiting substitution without imports. - Illustrative national GHG impacts: With maximal utilization, Brazil’s cement-related GHGs could be cut by ~84.5% (≈2.9% of national total); Turkey (~5.5%), South Korea (~5.4%), and China (~4.4%) also show large potential reductions due to the prominence of cement emissions in their national inventories. - Constraints and enablers: Alkali-activated binders can achieve very high substitution levels but require large-scale availability of activators (e.g., NaOH/Na₂SiO₃); global NaOH production (~80 Mt in 2013) is far below what would be required (~300 Mt/year) for maximal activation. Transport distance and local supply strongly influence practical substitution levels.

The analysis demonstrates that extensive substitution of clinker with regionally available secondary CMs can substantially reduce cement-sector GHG emissions, addressing a key mitigation challenge in heavy industry. While product-scale demonstrations show feasibility for high replacement, market-scale implementation requires systemic changes. Three major barriers were identified: (i) manufacturing shifts to process and utilize a broader portfolio of secondary CMs with variable and region-specific supply; (ii) policy and standards evolution toward performance-based specifications to accommodate diverse mixtures; and (iii) targeted R&D to raise technology readiness, especially for alkali-activated and multi-CM systems across different chemistries (high/low Ca, Fe, Mg). The study underscores the importance of regional optimization of CM mixes, improved collection and processing infrastructure (including EoL binder recovery), and complementary changes in construction practices. Off-site/digital construction could expand the range of acceptable binders by enabling tighter curing control, potentially overcoming performance constraints of highly substituted cements. International trade can balance regional supply-demand mismatches, enabling higher global substitution where local generation is insufficient (e.g., flows from European surplus to Turkey/Egypt; regional coordination around China’s large demand). Overall, integrating binder substitution with broader material efficiency and circularity strategies can deliver significant, system-level decarbonization benefits.

Secondary cementitious materials are abundant enough to enable deep reductions in clinker use and associated emissions. In 2018, up to ~3.5 Gt of secondary CMs could have supported theoretical global emission reductions of ~1.3 Gt CO₂-eq (≈44% of cement emissions; ≈2.8% of anthropogenic emissions), with potential average clinker-to-cement ratios approaching ~0.14 when combining multiple CMs and leveraging alkali activation. Many major producers can meet a large share of their needs domestically, though supply constraints in some countries necessitate trade. The study provides a quantitative framework and datasets to guide regional optimization of CM portfolios and informs LCA-based assessments of substitution strategies. Future work should prioritize: advancing performance-based standards; scaling collection and processing for diverse secondary CMs (including EoL binder); closing knowledge gaps on multi-CM alkali-activated systems across chemistries; assessing logistics and trade for global balancing; and evaluating the benefits of digital/off-site construction for enabling unconventional binders. These efforts, combined with material efficiency measures, can accelerate decarbonization of the cementitious materials cycle.

Results represent upper-bound potentials subject to several constraints and assumptions: (1) Secondary CM generation estimates often reflect maximum recoverable quantities (e.g., agricultural/forestry residues assume high collection and dedicated combustion), whereas real-world competing uses and logistics reduce availability. (2) End-of-life binder generation relies on regionalized stock and lifetime models with limited country-specific data and simplified carbonation treatment; national disaggregation carries uncertainty. (3) LCA assumes functional equivalence at a binder level (1.4 kg functional unit) across countries and high substitution mixes; actual performance equivalence may not hold universally, especially for unconventional CMs. (4) Upstream burdens of by-products/wastes and their processing were excluded; sensitivity analysis indicates minor effects but values remain optimistic. (5) High substitution scenarios require alkali activators at scales exceeding current global production, representing a major practical barrier. (6) Transport distances were simplified (two 150 km links) and may vary; locality strongly influences feasibility. (7) Market adoption depends on standards, manufacturing capacity, and R&D progress; thus, estimated substitution and GHG reduction potentials are theoretical upper limits rather than near-term projections.