Effects of Relative Humidity on Dissolution and Carbonation Potential of Portlandite

The carbonation process of cementitious materials is gaining attention as a way to mitigate anthropogenic CO₂ emissions. Because the reactions occur as interfacial dissolution and precipitation within thin adsorbed water film on the substrate, understanding water adsorption and its influence on carbonation reactivity is essential. In this study, the water adsorption isotherms on portlandite surface were simulated by hybrid Grand Canonical Monte Carlo (GCMC)-Molecular Dynamics (MD) simulations and compared with the experiment. With the simulated water film under various relative humidity (RH), biased MD simulations were performed using a well-tempered metadynamics scheme to investigate how calcium dissolved from the portlandite surface. The results revealed that the dissolved Ca was likely to be trapped parallel to the substrate surface as inner-sphere adsorbed (adatom). The activation energies for detachment from the surface sites and for surface diffusion were calculated. In addition, the thermodynamical stability of the surface sites at various RH was evaluated by calculating the Gibbs energy for reaction (Δ_rG), showing a bilinear relationship with a decreasing trend until 40% RH and constant values above that, implying the threshold RH for Ca dissolution. Our simulations also revealed that the perpendicular movement of the dissolved Ca was restricted within where the H₂O layer was present, which may spatially limit subsequent nucleation and crystal growth of calcium carbonate and inhibit complete carbonation of the substrate. For comparison with these simulations, experiments were also performed to study the degree of carbonation (DoC) of portlandite at various RH. The experimental trend showed good consistency with simulations with respect to the reactivity threshold and the reaction-saturated RH.