The adsorption of water on a C2S surface initiates belite to hydrate. In the present work, the adsorption behavior of single water molecule on a -C2S (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 -C2S (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 0.85eV, respectively, but can dissociate on the other 2 six-coordinated calcium sites with higher adsorption energies of 0.96 and 0.99eV, respectively. The energy barriers to the dissociative adsorption of water at the Ca(III) site(0.10eV) 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 2p and O 2s orbitals leads to new Ca-O bond formations.