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

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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 β-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 β-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 β-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, β-hemihydrate dehydration can be translated by the chemical formula CaSO₄·xH₂O (0.5 ≤ x ≤ 0.8). The volumetric thermal expansion coefficient of β-hemihydrate, determined at temperatures between 90 and 140 °C is 1.54 × 10^–4 K^–1. β-Hemihydrate coexists with soluble anhydrite above 140 °C; however, the amount of β-hemihydrate decreases with temperature. In β-hemihydrate, water molecules are continuously released from the CaO₉ tetradecahedra, thereby resulting in its contraction. Consequently, the structural change to a smaller CaO₈ dodecahedron triggers its transformation into soluble anhydrite without the collapse of its one-dimensional linear chains. With further heating, β-hemihydrate completely transforms into soluble anhydrite at 170 °C. The volumetric thermal expansion coefficient of soluble anhydrite determined in the temperature range of 170–200 °C is 1.69 × 10^–5 K^–1, which is an order of magnitude smaller than the values of gypsum and β-hemihydrate.