The reaction of water with tricalcium silicate (Ca3SiO5, C3S) is the most crucial reaction which results in strength development in cementitious materials.In spite of several decades of study, details of such reactions,are not yet fully understood.In this work, first-principles calculations are applied to develop a fundamental understanding of how molecular water reacts with bare surfaces of TI-Ca3SiOs, i.e., at low surface coverage.As a first step the surface energetics of low index orientations are calculated and used in construction of the Wulff (equilibrium) shape.