Molecular and dissociative adsorption of a single water molecule on a β‐dicalcium silicate (100) surface explored by a DFT approach

The adsorption of water on a C 2 S surface initiates belite to hydrate. In the present work, the adsorption behavior of single water molecule on a β‐C 2 S (100) surface is explored using density functional theory ( DFT ) due to the lack of alternative approaches for direct observation. Four possible calcium atom sites on the β‐C 2 S (100) surface slab are considered in our calculations. The results show that water can adsorb on the 2 five‐coordinated calcium sites only via molecular adsorption with adsorption energies of 0.59 and 0.85 eV, respectively, but can dissociate on the other 2 six‐coordinated calcium sites with higher adsorption energies of 0.96 and 0.99 eV, respectively. The energy barriers to the dissociative adsorption of water at the Ca( III ) site(0.10 eV) is much lower than that at the Ca( IV ) site, indicating that water prefers to adsorb and dissociate on Ca( III ) sites. The dissociative adsorption of water causes more obvious surface calcium shifts and Si–O bond length increases than molecular adsorption. The dissociative adsorption of a water molecule changes the electron distribution, and the overlap between Ca 2 p and O 2 s orbitals leads to new Ca–O bond formations.