The hydration of calcium silicate phases dominates the application of cement clinkers and has important practical implications. The reaction mechanisms between water and calcium silicate phases are complex and remain poorly understood at the atomic level. Herein, the single water adsorption on all low-index surfaces of β-C2S and M3-C3S was investigated using DFT-D calculations. The surface energy was calculated and the influence of surface cleavage discussed. Both dissociative and molecular adsorptions were investigated. Molecular adsorption was energetically favoured on β-C2S surfaces while dissociative adsorption was energetically favoured on M3-C3S surfaces. A Wulff construction was used to describe the equilibrium morphology with and without an adsorbed water molecule for the β-C2S phase. Water adsorption promoted the solid dissolution by weakening the bond among surface atoms. Electron transfer was observed mainly from the surface atoms to water atoms during the adsorption. These findings provide a novel insight into the adsorption of a water molecule on different calcium silicate surfaces using the same level of theory, which includes a representation of van der Waals forces, therefore, laying the foundation for better understanding the hydration mechanism.