α or β?-hemihydrates transformed from dihydrate calcium sulfate in a salt-mediated glycerol–water solution

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α-and β-hemihydrate calcium sulfate (HH), the most important calcium sulfate phases, are usually achieved by autoclave hydrothermal reaction and calcination in air, respectively. Herein, we report an investigation on preparation of HH from dihydrate calcium sulfate(DH) in a salt-mediated glycerol-water solution. We found that both α-HH and β-HH in forms of apparently single crystals could be obtained at 110 ℃ only by tuning the concentrations of glycerol, Na2EDTA, and NaCl. TG-DSC, XRD, SEM, and Raman spectroscopy were used to identify and characterize the product crystals. The results indicate that the high relative supersaturation of HH could be the key factor determining the generation of β-HH crystal in solution. Glycerol and NaCl generally decrease the water activity, enhance the crystallization kinetics, and therefore allow the presence of a metastable HH phase (especially for compact α-HH bulk crystal). But too high concentration of glycerol (i.e., ≥70%) and NaCl (i.e., ≥0.2 M) would be favorable for β-HH crystallization with obviously defective crystal morphology due to the dramatically increased local relative supersaturation. Na2EDTA could retard the recrystallization but well regulate the morphology of HH bulk crystals.