How microstructure and pore moisture affect strength gain in portlandite-enriched composites that mineralize CO₂

Binders containing portlandite (Ca(OH)₂) can take up carbon dioxide (CO₂) from dilute flue gas streams (<15% CO₂, v/v), thereby forming carbonate compounds with binding attributes. While the carbonation of portlandite particulates is straightforward, it remains unclear how CO₂ transport into monoliths is affected by microstructure and pore moisture content. Therefore, this study elucidates the influences of pore saturation and CO₂ diffusivity on the carbonation kinetics and strength evolution of portlandite-enriched composites ("mortars"). To assess the influences of microstructure, composites hydrated to different extents and conditioned to different pore saturation levels (S_w) were exposed to dilute CO₂. First, reducing saturation increases the gas diffusivity and carbonation kinetics so long as saturation exceeds a critical value (S_w,c ≈ 0.10) independent of microstructural attributes. Second, careful analysis reveals that both traditional cement hydration and carbonation offer similar levels of strengthening, the magnitude of which can be estimated from the extent of each reaction. As a result, portlandite-enriched binders offer cementation performance that is similar to traditional materials while offering an embodied CO₂ footprint that is more than 50 % smaller. These insights are foundational to create new "low-CO₂" cementation agents via in situ CO₂ mineralization (utilization) using dilute CO₂ waste streams.