Keywords: water-to-cement ratio

Cement production consumes a significant amount of energy and releases CO₂ emissions, while concrete waste can potentially reabsorb CO₂. This study examined the effect of relative humidity (especially wet-dry cycles) on the carbonation of hardened cement pastes. Wet–dry cycles increased porosity and caused the decomposition of calcium silicate hydrate (C-S-H) through shrinkage and deformation during drying. As a result, the highest CO₂ absorption in the wet-dry cycle sample was twice that of the constant RH. Additionally, the amount of CO₂ captured during the 28-day wet-dry cycle accounted for about 17% of the annual CO₂ emissions from cement production.