Structural evolution of gypsum (CaSO₄·2H₂O) during thermal dehydration

Abstract

Herein, an in situ high-temperature synchrotron X-ray diffraction study of gypsum is performed in the temperature range of 30-200 °C to investigate the continuous structural change from gypsum to soluble anhydrite through hemihydrate. Thermogravimetric and differential thermal analysis curves reveal that dehydration occurs in two stages. The diffraction peaks of hemihydrate with the trigonal space group P3121 gradually become sharper above 90 °C, whereas those of gypsum become less intense and cannot be distinctly observed at 160 °C. The CaO8 dodecahedra and SO4 tetrahedra in gypsum expand negligibly with temperature. The site occupancy parameter of the water oxygen (Ow) atom in gypsum remains at approximately 1.0, within the experimental error. When water molecules are lost from gypsum, it immediately transforms into -hemihydrate, without maintaining its structure. The volumetric thermal expansion coefficient of gypsum is 1.31  10 -4 K -1 . The site occupancy of Ow in -hemihydrate continuously decreases from 0.8 and reaches approximately 0.5 at temperatures of 130-140 °C, where soluble anhydrite with a hexagonal space group P6222 begins to form. Therefore, hemihydrate dehydration can be translated by the chemical formula CaSO4•xH2O (0.5  x 