Mechanisms of change in accelerated carbonation progress in cement paste under different relative humidity conditions

This study investigated the evolution of water content and carbonation in OPC cement paste under low, intermediate, and high relative humidity (RH) conditions, emphasizing the role of water in accelerated carbonation and microstructural modifications. Under high RH, the carbonation suppressed after 28 days owing to calcium ion accumulation at the surface, precipitating as calcite. This calcite formation leads to a dense calcium carbonate layer, reducing the gas–liquid interfacial area for CO₂ dissolution and blocking further water evaporation and carbonation process. Low RH conditions cause the agglomeration of calcium silicate aluminate hydrate (C–(A)–S–H), increasing empty space in the microstructure. This facilitates CO₂ penetration but limits CH carbonation due to insufficient water availability. These findings highlight the significance of water distribution and microstructural evolution in CO₂ sequestration, revealing that RH critically influences the progress of carbonation by altering the pore structure and subsequent water availability in cementitious materials.