Effect of Supersaturation on Competitive Nucleation of CaSO₄ Phases in a Concentrated CaCl₂ Solution

Nucleation is a combined result of thermodynamics and kinetics. Calcium sulfate dihydrate (DH), α-calcium sulfate hemihydrate (α-HH), and calcium sulfate anhydrite (AH) all have the potential to precipitate when the solution is supersaturated with them at the same time. The effect of supersaturation on CaSO 4 phase’s occurrence was investigated in a 3.00 m CaCl 2 solution at 91 °C to better understand the kinetics of competitive nucleation. In a wide range of supersaturations, DH and α-HH precipitated concomitantly without AH forming. The molar fraction of α-HH in the precipitate increased gradually from 47.23% to a summit of 98.91% when the supersaturation ( S HH ) varied from 1.90 to 3.76 and then followed a dramatic decrease to 18.77% with a further increase in S HH to 6.26. The predominant nucleation shifted from α-HH at the lower supersaturations to DH at the higher supersaturations. The crystallization rate evolution with supersaturation was responsible for the growth and decline of DH and α-HH in the precipitate.