The climate benefits from cement carbonation are being overestimated

The cement industry's rapid decarbonization is crucial for meeting climate goals. Cement production, a significant source of CO₂ emissions (over 7% of global anthropogenic CO₂ emissions and 2–3% of global energy use), stems from fossil fuel combustion in high-temperature kilns and mineral-derived CO₂ emissions from limestone decarbonation (calcination). Strategies for CO₂ reduction include using novel materials and constituents, alternative fuels, and direct air capture. Carbon capture, utilization, and storage (CCUS) is often proposed but faces limitations due to high costs and infrastructure availability. Cement's ability to react with atmospheric CO₂ through carbonation—where CO₂ reacts with hydrated cement products to form carbonate minerals—has been suggested as a countermeasure to production emissions. Previous studies, using traditional global warming potentials (GWPs), have suggested substantial CO₂ uptake potential, but these studies overlook the long delay between initial emissions and CO₂ uptake, potentially distorting our understanding of climate benefits. This research aims to model global GHG emissions and carbon uptake from cement's full life cycle, using a time-dependent life cycle warming potential to accurately assess GHG mitigation strategies and avoid overestimation of carbonation benefits.

Prior research highlighted the significant CO₂ uptake potential of cement carbonation, claiming historic carbonation offset nearly half of all mineral-derived CO₂ emissions from cement production. Projections to 2100 estimated carbonation could re-absorb roughly 30% of global CO₂ emissions from cement production. However, these assessments used traditional GWP, assuming simultaneous fluxes, failing to reflect the actual decadal timescale of carbonation. They also ignored end-of-life (EoL) processing emissions. Initial modeling of individual concrete mixtures demonstrated that time dependencies could significantly alter the net climate impact of carbonation. For example, a case study in Quebec showed carbonation benefits, using dynamic warming potential models, to be 3–10% of net fluxes, much lower than traditional GWP estimates. This study addresses the gap by comparing dynamic methods, accounting for the global built concrete infrastructure, with traditional GWP.

This study uses a time-adjusted warming potential (TAWP), an alternative to GWP, to calculate the effect of timing on CO₂ emissions and removals throughout the cement life cycle. TAWP, similar to net present value calculations, represents the equivalent amount of CO₂ emitted or sequestered today, considering cumulative radiative forcing. The study models historic and future cement flows, considering end-use applications and environmental conditions (relative humidity, CO₂ concentration, concrete thickness, compressive strength, and SCM content). CO₂ uptake potential from high-surface area crushed concrete at EoL and demolition emissions are included. The model compares net CO₂ fluxes using traditional GWP and TAWP to understand overestimation of carbonation benefits. A sensitivity analysis assesses the impact of environmental factors and concrete composition on carbonation. Different SCM types and replacement levels are examined for their effect on carbonation, acknowledging potential trade-offs such as increased durability issues or reduced portlandite availability from pozzolanic reactions. Global and US cement production data, concrete end-use and longevity data, and a meta-analysis of carbonation data are used for regional and global-scale assessments.

The study finds that for 1 kg of cement, mineral- and energy-derived sources contribute -54% and 46% to net emissions at production, respectively. In a typical building (64-year lifespan), roughly 12% of calcination emissions are re-absorbed during use, and an additional 30% during demolition (assuming 1–40 mm particle size and 3.5 months exposure), but energy for crushing nearly negates this benefit. Demolition emissions, estimated using global average exposure time, account for 30–40% of carbon uptake reported in previous studies. For buried crushed concrete, additional uptake of up to 48% of calcination emissions is observed within 25 years. The dynamic effects of emissions and uptake on cumulative radiative forcing highlight the differences between GWP and TAWP methodologies. Traditional GWP overestimates carbonation benefits by over 100%. Global CO₂ uptake from 1950 to 2050 is -46 Gt (28% of total emissions), but accounting for timing reduces the global warming benefit by 67%. Demolition emissions over this period amount to roughly 2.5 Gt (86% of CO₂ uptake from demolished concrete). A US case study shows similar overestimation (53%). Sensitivity analyses show the time-dependent climate benefits of EoL strategies are roughly half the CO₂ savings (per kg of cement) using GWP. Crushing concrete to smaller particle sizes (1–10 mm and smaller) increases surface area, accelerating carbonation but also increases energy consumption and GHG emissions. The use of SCMs, while accelerating carbonation, primarily reduces radiative forcing due to production emissions reduction (34–78% of total TAWP reductions). A 25% or greater SCM replacement rate yields the largest TAWP reduction (24–53%). However, SCM supply may decrease with the transition to net-zero pathways in other industries. The study emphasizes the necessity of considering tradeoffs between increased carbonation and additional energy-related emissions and to consider that the majority of the climate benefit from SCM use comes from reducing production emissions.

The study's findings challenge the overestimation of carbonation's climate benefits, demonstrating the importance of incorporating time-dependent effects on cumulative radiative forcing. The substantial difference between GWP and TAWP highlights the need for more accurate accounting methods in decarbonization roadmaps. Strategies aimed at maximizing CO₂ uptake should consider the trade-off between increased carbonation and energy consumption, and their relative impacts on cumulative radiative forcing. Although EoL carbonation strategies are valuable, their effectiveness should be assessed within the context of overall lifecycle emissions, considering demolition emissions and production-related emissions reduction strategies. Future research could investigate the effects of varying concrete recycling rates, secondary use applications, and human health impacts from demolition processes.

This research demonstrates that the climate benefits of cement carbonation are significantly overestimated when using traditional GWP methods. The study highlights the crucial need for time-dependent assessments (like TAWP) in evaluating decarbonization strategies. While EoL carbonation offers some benefits, its potential is limited by demolition emissions and its overall contribution is significantly less than previously thought. Prioritizing reductions in production emissions through measures like increased SCM use is far more effective for mitigating the climate impact of cement production.

The model makes assumptions about average values for several parameters influencing carbonation rates, such as relative humidity, CO₂ concentration, and concrete thickness, which could introduce uncertainties. The model also simplifies the complexity of chemical reactions during carbonation and does not fully capture all potential interactions between different concrete components and environmental factors. Further, it does not account for potential changes in global CO₂ concentrations over time. The study assumes constant secondary life duration for concrete across different regions. More granular data on concrete recycling rates and secondary use applications could improve the accuracy of future models.