Coverage-dependent adsorption of H2O on dicalcium silicate (100) surface: A DFT study

Dicalcium silicate (C2S) is one of the dominating components in Portland cement and is of particular importance for the production of low energy cement. Understanding the interaction between water molecules and C2S low-index surfaces is a key step for revealing the hydration mechanism of C2S. Herein, first-principle density functional theory (DFT) calculations were used to analyse the adsorption of water molecules on (3-C2S (100) surface with different coverage. Considering the reactive Ca sites on beta-C2S (100) surface, four degrees of water coverage (theta = 1/4, 1/2, 3/4, and 1) were investigated. Adsorption configuration, adsorption energy, electron transfer, partial density of states, and charge density were analysed. Results indicate that the adsorption energy was in the range of -0.70 eV to -0.99 eV, -1.40 eV to -1.93 eV, -2.13 eV to -2.43 eV, and -3.00 eV for 1-4 water molecules adsorption, respectively. The multi-water molecules adsorption was generally anti-cooperative when theta <= 1 and the adsorption energy was mainly originated from the surface-water electronic interaction. Molecular adsorption was observed and the adsorption configuration was influenced by the Ca sites. The more the adsorbed water molecules, the more obvious the outward movement and the deeper the adsorption influence into the solid. Electrons were transferred from the surface to water molecules after adsorption. Water molecules became more stable upon adsorption and strong overlapping between water O and surface Ca orbitals was observed.

Automated summary

This study used first-principle density functional theory (DFT) calculations to analyze the adsorption of water molecules on C2S surfaces. The results showed that the adsorption energy increased with the number of adsorbed water molecules, and multi-water molecules adsorption was generally anti-cooperative. Electrons were transferred from the surface to water molecules after adsorption, and water molecules became more stable upon adsorption.