Formation and Transformation of Five Different Phases in the CaSO4−H2O System: Crystal Structure of the Subhydrate β-CaSO4·0.5H2O and Soluble Anhydrite CaSO4

At least five crystalline phases can be found in the CaSO4−H2O system, which are gypsum CaSO4·2H2O, the subhydrates α- and β-CaSO4·0.5H2O, and the soluble and insoluble anhydrite CaSO4. The formation of these five phases in the CaSO4−H2O system and their transformations were investigated by in situ time-resolved synchrotron radiation powder X-ray diffraction (SR-PXD) in this study. Furthermore, revised structural models for β-CaSO4·0.5H2O and soluble anhydrite CaSO4 are presented. The hydration of α-CaSO4·0.5H2O was studied at 25 °C and showed that the reaction with H2O started immediately after mixing the two reactants and that the formation of CaSO4·2H2O was coupled to the depletion of α-CaSO4·0.5H2O. The thermal decomposition of CaSO4·2H2O was investigated in the temperature range of 25−500 °C and showed the formation of α-CaSO4·0.5H2O followed by the formation of soluble anhydrite AIII-CaSO4, which was gradually converted to insoluble anhydrite AII-CaSO4. The thermal decomposition of α-CaSO4·0.5D2O was investigated in the temperature range of 25−500 °C and showed successive phase transformations to β-CaSO4·0.5D2O, soluble anhydrite AIII-CaSO4, and insoluble anhydrite AII-CaSO4. The two polymorphs of anhydrite coexist in the investigated temperature range of 200−500 °C. The hydrothermal decomposition of CaSO4·2H2O was investigated in the temperature range of 25−200 °C using a 1 M HNO3 or a 1 M LiCl solution, and in both experiments, CaSO4·2H2O was converted to α-CaSO4·0.5H2O and further to insoluble anhydrite AII-CaSO4. A structural model for β-CaSO4·0.5H2O is proposed on the basis of SR-PXD data and a trigonal unit cell (in hexagonal setting) a = 6.931 45(3), c = 12.736 17(4) Å, Z = 6, and space group P31. A structural model for soluble anhydrite AIII-CaSO4 is also proposed on the basis of powder neutron diffraction data, and a hexagonal unit cell parameters are a = 6.9687(1), c = 6.3004(1) Å, Z = 3, and space group P6222.