The Characterization, Stability, and Reactivity of Synthetic Calcium Silicate Surfaces from First Principles

Calcium silicate compounds belong to a complicated class of silicates. Among their many industrial applications, calcium silicates are heavily used as a building material as they constitute the main ingredient in today's cement clinker. We report here an extensive surface analysis of synthetic calcium silicate phases (tricalcium silicate, C3S, and dicalcium silicate, C2S) using first-principles computational methods. We calculate surface energies (gamma) for all lower-index orientations and determine the most stable surfaces as well as the equilibrium Wulff structures. We analyze the variation of gamma with surface coordination number and find an interesting and unexpected trend where loss of coordination of ionic Ca and O atoms can lower gamma. The stability of surface orientations is examined as a function of oxygen partial pressure. Finally, we compute the energy required to remove Ca from different surfaces and find that it is inversely proportional to gamma, supporting the energetic preference of extracting atoms from higher energy surfaces. Knowledge of the atomic structure and properties of calcium silicate surfaces is important for understanding and controlling the hydration of such systems.