Dehydration Pathways of Gypsum and the Rehydration Mechanism of Soluble Anhydrite γ-CaSO₄

Abstract

The dehydration products of gypsum under different temperature and water vapor pressure were investigated by thermodynamic theory. Additionally, the rehydration mechanism of soluble anhydrite was also studied by Monte Carlo (MC) simulations. The thermodynamic calculation results reveal that the dehydration mechanism of gypsum significantly depended on ambient temperature and water vapor pressure. In the high-temperature and low water vapor pressure region, gypsum dehydrates to form γ-CaSO₄ in a single-step process (CaSO₄·2H₂O → γ-CaSO₄); with increasing water vapor pressure, gypsum undergoes the CaSO₄·2H₂O → γ-CaSO₄ → β-CaSO₄·0.5H₂O reaction path and as water vapor pressure increases further, the occurrence of a two-step conversion path CaSO₄·2H₂O → β-CaSO₄·0.5H₂O → γ-CaSO₄ was observed. It was also found that gypsum is stable in the low-temperature and high water vapor pressure region and does not dehydrate to form any calcium sulfate hemihydrate. Finally, the rehydration mechanism of soluble anhydrite was studied by MC simulations. The simulation results are in agreement with the experimental data and support the finding that γ-CaSO₄ rehydration forms CaSO₄·0.67H₂O in high relative humidity. Another important result revealed by the MC simulation is that γ-CaSO₄ has an extraordinary ability to capture water molecules from an extremely dry atmosphere, which is very useful in some fields, such as in drying processes and even for extracting liquid water from extremely dry atmosphere.