Intrinsic Acidity of Surface Sites in Calcium Silicate Hydrates and Its Implication to Their Electrokinetic Properties

Calcium Silicate Hydrates (C-S-H) are the major hydration products of portland cement paste. The accurate description of acid base reactions at the surface of C-S-H particles is essential for both understanding the ion sorption equilibrium in cement and prediction of mechanical properties of the hardened cement paste. Ab initio molecular dynamics simulations at the density functional level of theory were applied to calculate intrinsic acidity constants (pK(a)'s) of the relevant SiOH and CaOH2 groups on the C-S-H surfaces using a thermodynamic integration technique. Ion sorption equilibrium in C-S-H was modeled applying ab initio calculated pK(a)'s in titrating Grand Canonical Monte Carlo simulations using a coarse-grained model for C-S-H/solution interface in the framework of the Primitive Model for electrolytes. The modeling results were compared with available data from electrophoretic measurements. The model predictions were found to satisfactorily reproduce available experimental data.