Dehydration Pathways of Gypsum and the Rehydration Mechanism of Soluble Anhydrite γ-CaSO(4)

[Image: see text] 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-CaSO(4) in a single-step process (CaSO(4)·2H(2)O → γ-CaSO(4)); with increasing water vapor pressure, gypsum undergoes the CaSO(4)·2H(2)O → γ-CaSO(4) → β-CaSO(4)·0.5H(2)O reaction path and as water vapor pressure increases further, the occurrence of a two-step conversion path CaSO(4)·2H(2)O → β-CaSO(4)·0.5H(2)O → γ-CaSO(4) 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-CaSO(4) rehydration forms CaSO(4)·0.67H(2)O in high relative humidity. Another important result revealed by the MC simulation is that γ-CaSO(4) 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.