The effect of soluble phosphorus on CaSO4·2H2O (DH) dehydration to α-calcium sulfate hemihydrate gypsum (α-HH) in hydrothermal CaCl2 solutions and the crystal shape regulation were investigated. The action mechanism of soluble phosphorus was studied by means of growth of hemihydrate crystal. The system pH was the main factor to determine whether DH could be dehydrated to α-HH in 24% CaCl2 solution at 98℃. Acicular a-HH crystal could be obtained when pH was less than 2.0. High-strength α-HH crystal with aspect ratios from 0.5 to 3.0 could be observed when polycarboxylic crystal habit modifier NS was added. The corresponding compressive strength was more than 20 MPa. The presence of soluble phosphorus was favorable for α-HH formation by lowering system pH
increasing supersaturation degree and reducing interfacial energy. On the other hand
acid radical ions affected α-HH crystallization behavior and adsorption results of medium crystal agent NS.