A new calcium sulfate hemi-hydrate

Abstract

Calcium sulfate hydrates receive significant attention due to numerous large scale industrial applications. There has been a long debate on the possible existence of two gypsum hemi-hydrate polymorphs, denoted α- and β-CaSO₄·0.5H₂O. In this work, a new crystal structure of calcium sulfate hemi-hydrates is presented, denoted β-CaSO₄·0.5H₂O. The structure was solved using powder neutron diffraction data, the space group is P3₁ and the unit cell in a hexagonal setting a = 6.9268(1), c = 12.7565(3) Å. The structure has two calcium–oxygen coordination polyhedra: Ca1 is eight coordinated and has Ca–O bond lengths in the range 2.31(3) to 2.89(2) Å and Ca2 is nine coordinated and has one Ca–O_water bond length of 2.43(3) Å, and eight Ca–O bonds in the range 2.30(4) to 2.86(4) Å. Two sulfate ions have S–O bonds in the range 1.47(3) to 1.49(4) Å, and 1.47(3) to 1.50(3) Å, respectively. The water molecule forms a hydrogen bond of 2.55(4) Å to an oxygen atom in one of the sulfate ions. The structure of the hemi-hydrate β-CaSO₄·0.5H₂O has one-dimensional channels running parallel to the c-axis where the water molecules are located. This relates the structures of α- and β-CaSO₄·0.5H₂O and soluble anhydrite AIII-CaSO₄, which all have similar channel structures. The water molecules in the structure of β-CaSO₄·0.5H₂O are packed in the channels with a three fold (3₁) symmetry in a different way as compared to the pseudo hexagonal found in the structure of α-CaSO₄·0.5H₂O.