Modeling of monolayer water adsorption on surface of cement minerals by molecule simulation and adsorption isotherm

Abstract

The performance of water on the pore surface governs the mechanical properties and durability of cementitious materials and structures. As a fundamental mechanism, the interaction between water and cementitious mineral micropore surfaces behaves differently under various circumstances, which can be investigated by molecular simulation methods in detail. Molecular simulations based on Grand Canonical Monte Carlo method were performed to obtain the water vapor adsorption isotherms on the surface of tricalcium silicate (C3S) and calcium hydroxide (CH), which are selected as typical cement mineral and hydration product respectively. The adsorption behavior of C3S and CH is compared by the adsorption energy and adsorption isotherm model. The condensation processes were explained by the Kelvin equation. The critical pore sizes of C3S and CH for condensation are 19 Å and 15 Å respectively. Beyond the critical pore size, water adsorption shows monolayer adsorption performance. The water adsorption process on the surface includes three stages: rapid adsorption, stable adsorption and slightly condensed adsorption, which are related to the surface interaction of minerals with water molecules. The Langmuir model is more suitable for the CH with homogeneous surface, while the Freundlich model is more suitable for the C3S model with nonhomogeneous surface. The adsorption heat of the first layer of water film and adsorption energy on the surface of CH are higher.