Trace NaCl and Na₂EDTA Mediated Synthesis of α-Calcium Sulfate Hemihydrate in Glycerol–Water Solution

We report a new class of glycerol–water system to transform FGD gypsum into α-calcium sulfate hemihydrate (α-HH) mediated by trace NaCl and Na 2 EDTA to regulate the phase transformation rate as well as the crystal morphology to achieve crystals with high mechanical strength. NaCl plays a role in accelerating the nucleation and crystal growth process, whereas Na 2 EDTA regulates the morphology of α-HH from columnar to lamellar. The paste made from columnar α-HH with higher aspect ratio deserves higher dry bending/compressive strength of 13.6/37.6 MPa, which is comparable with those prepared in nitrate–water medium, chloride–water medium, or by autoclave method. This study provides an improved alternative of glycerol–water system to transform FGD gypsum into α-HH with controlled morphology, high mechanical strength, and less corrosion to the equipment. We also establish the relationship between the mechanical strength of paste and the aspect ratio of α-HH crystals.