Thermodynamic Preparation Window of Alpha Calcium Sulfate Hemihydrate from Calcium Sulfate Dihydrate in Non-Electrolyte Glycerol–Water Solution under Mild Conditions

Through vapor pressure measurements, water activities of the glycerol–water solution were first examined under temperatures of 30–85 °C and glycerol concentrations of 0–50 mol %. The water activity is the only determinant of phase-transition point among CaSO4·2H2O (DH), α-CaSO4·0.5H2O (α-HH), and CaSO4 (AH) in glycerol–water solution. Accordingly, a phase-transition diagram was constructed as a function of temperature (0–140 °C) and glycerol concentration (0–50 mol %), which provided a thermodynamic preparation window of α-HH from DH. Controlling glycerol concentration and temperature within the window, direct transition of DH to α-HH can be regulated. However, the window is apt to shrink as the operating time is shortened, indicating that its available profile is directed by transition kinetics of DH to α-HH besides transition thermodynamics. Overall, the glycerol–water solution could act as a novel category of nonelectrolyte medium for α-HH preparation from DH.