Crystallization Routes and Metastability of α-Calcium Sulfate Hemihydrate in Potassium Chloride Solutions under Atmospheric Pressure

The phase transitions of α-calcium sulfate hemihydrate (α-HH) in KCl solutions were studied at (80 to 105) °C under atmospheric pressure in an attempt to estimate the metastability of α-HH and to sketch out the routes for α-HH’s hydration, dehydration, or recrystallization. The crystal water content and morphology of the product during the reaction period were comprehensively examined. It was found that the metastability of α-HH heavily relied on KCl concentration and temperature. The phase transition of α-HH is successfully outlined as a function of KCl concentration and temperature. Calcium sulfate dihydrate (DH) is stable in region I and metastable in region III. Anhydrite (AH) is stable in regions III and IV and coexists with goergeyite. HH is metastable in only region II. The results show that α-HH tends to hydrate or dehydrate in KCl solution, but there is still a narrow metastable region for α-HH crystallization in which KCl concentration and temperature are strictly limited.