The Carbonation Trap: Time Delays and Performance Uncertainty in Cement Climate Accounting

Abstract

Cement production accounts for 5%–8% of global CO2 emissions, prompting industry interest in carbonation—the natural reabsorption of atmospheric CO2 by concrete—as a climate mitigation strategy. Recent studies suggest carbonation could offset approximately 50% of process emissions, positioning concrete infrastructure as vast carbon reservoirs. However, systematic analysis reveals fundamental limitations challenging this assumption. Cement production generates concentrated CO2 pulses during manufacturing while carbonation proceeds slowly through diffusion-limited processes spanning decades, creating critical temporal asymmetry. When properly accounted for through time-adjusted climate assessments, this mismatch reduces claimed benefits by 30%–60% compared to conventional global warming potential calculations. Moreover, synthesis of published experimental data across 99 scenarios demonstrates that 52% exhibit less than 50% probability of achieving net emission reductions, with compressive strength penalties often requiring additional binder use that erodes nominal carbon gains. Critically, this perspective exposes three systematic failures in current climate accounting: ① temporal frameworks treating decades-delayed absorption as equivalent to immediate emission avoidance, ② selective reporting obscuring widespread performance failures, and ③ policy prioritization allocating resources to slow, uncertain processes while proven alternatives remain underutilized. By integrating sector-scale projections, lifecycle timing analyses, and comprehensive performance distributions under consistent boundaries, this cross-study synthesis reveals patterns invisible when research remains fragmented—establishing evidence-based hierarchies for near-term decarbonization. In contrast, proven alternatives demonstrate superior performance: supplementary cementitious materials offer 11%–34% emission reductions through direct clinker substitution, structural design optimization achieves 18.5% reductions without compromising safety, and service life extension strategies enable 75% total reduction potential by 2100—far exceeding carbonation-dependent pathways. Consequently, while carbonation remains chemically viable, its slow kinetics, performance uncertainty, and temporal misalignment with climate targets necessitate policy recalibration prioritizing transparent temporal accounting and proven alternatives over uncertain future absorption processes.