Formation of Hydroxyl Groups at Calcium-Silicate-Hydrate (C-S-H): Coexistence of Ca-OH and Si-OH on Wollastonite(001)

Total-energy calculations based on density-functional theory are combined with ab initio thermodynamics to better understand the pH-value-dependent water-wollastonite(001) (CaSiO3) interaction. The truncation of wollastonite(001) is found to lead to nearly negligible ionic relaxation with respect to the bulk geometry. The thermodynamic ground state for low water coverage gives rise to a molecular adsorption energy of about 2 eV and features coexisting Si-OH and Ca-OH groups at the wollastonite(001) surface. The adsorption energy per molecule decreases to 1.4 eV with increasing the coverage to one monolayer. For water coverages in excess of one monolayer, adsorption energies close to the value characteristic for water adsorption on ice are obtained. More favorable than stoichiometric interfaces, however, are water wollastonite(001) interfaces that are enriched in Si-OH sites: Even at basic pH values, a metal-proton exchange reaction at the water wollastonite(001) interface lowers the total energy.