Atomistic modelling of the hydration of CaSO4

Atomistic modelling techniques, using empirical potentials, have been used to simulate a range of structures formed by the hydration of γ-CaSO₄ and described as CaSO₄·nH₂O (0<n<1). The hemihydrate phase (n=0.5) is of commercial importance and has been subjected to much experimental study. These simulation studies demonstrate significant water-matrix interactions that influence the crystallography of the hydrated phase. The existence of two types of hydration site has been predicted, including one within the Ca²⁺coordination sphere. Close correlation between water molecule bonding energy, Ca²⁺-O_w bond length and unit-cell volume have been established. This shows that as the number of water molecules within the unit cell increases, the bonding energy increases and the unit cell contracts. However around n=0.5, this process reaches a turning point with the incorporation of further waters resulting in reduced binding energy and an expanding unit cell.