Calcium sulfate polymorph evolution dominated by competitive nucleation in gypsum metastable zone

Calcium sulfate polymorph evolution during spontaneous precipitation was probed in 1.75 m CaCl2 solutions at 85.0 °C. Metastable calcium sulfate dihydrate (DH) and unstable α-hemihydrate (α-HH) precipitated concomitantly with the dominant phase presenting an order of DH → α-HH → DH upon supersaturation. Competitive nucleation assisted with heterogeneous nucleation of relatively less stable phase at lower supersaturations accounts for such a polymorph evolution. Lower interfacial energy and higher supersaturation result in the DH formation, while heterogeneous nucleation triggers the α-HH formation. Increment in CaCl2 concentration favors α-HH precipitation by decreasing the water activity and narrowing the supersaturation gap between DH and α-HH. This work presents a facile rule manipulating the polymorph evolution and provides a control strategy for selective synthesis of α-HH in industry.