C-S-H decalcification in seawater: The view from the nanoscale

Decalcification of calcium silicate hydrates (C-S-H) is one of the most important issues for cement-based in-frastructures concerning long-term safety performance. While enhanced decalcification of C-S-H in saline solu-tion has been extensively characterized by experimental studies, the underlying microscale mechanisms are not well understood. Using molecular simulations with the metadynamics method, we compared the Ca dissolution free energy of amorphous C-S-H and C-S-H-like crystals in pure water and seawater. Previous experimental re-sults were well-explained by our calculations, and we revealed an important mechanism of accelerated C-S-H decalcification in seawater that has not been emphasized before, i.e., the "ion exchange" between surface Ca and electrical double layers in seawater significantly reduces the Ca dissolution free energy through excess entropy gains. This provides a new and sound understanding of decalcification in contrast to previous empirical ap-proaches stating that accelerated decalcification is mainly related to the pore solution's pH value and ionic solubility. It is also found that C-S-H with a larger Ca/Si ratio features lower dissolution free energy. Our finding enhances the assessment and prediction of cementitious materials' degradation and inspires relevant mitigation strategies. This work also enriches the understanding of mineral dissolution with a broader geoscience interest.