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

Abstract

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. ■ INTRODUCTIONCrystallization of different polymorphs or phases is determined by the interplay between thermodynamics and kinetics. Gibbs free energy is minimized during crystallization until the system reaches a thermodynamic equilibrium state. The kinetics of crystallization influences what polymorphs are observed over time as the system reaches equilibrium. When multiple phases crystallize simultaneously, the kinetics of competitive nucleation plays a vital role in the formation of stable or metastable phases.Calcium sulfate dihydrate (DH), α-calcium sulfate hemihydrate (α-HH), and calcium sulfate anhydrite (AH) are the three phases most commonly observed during crystallization from electrolyte solutions. The thermodynamic stability of the crystal phases is defined by the solubility in specific crystallization media, and phase-transition diagrams have been established through solubility determination. 1,2 Water-mediated transformation can take place among different phases based on a dissolution−crystallization mechanism. 3,4 In electrolyte solutions at elevated temperature, DH can transform into α-HH, 5,6 which is a kind of important cementitious material and has been widely applied in the molding, binding, and construction industry for its superior workability and high strength, 3 and as bone cements owing to its excellent biocompatibility and osteoconductivity. 7 In solutions that are supersaturated with respect to DH, α-HH, and AH, competitive nucleation is supposed to occur. DH is the only phase precipitated from concentrated NaCl solutions (up to 6 m) at elevated temperatures up to 90°C, 8 which is attributed to its faster nucleation rate than that of thermodynamically stable form AH. Our previous studies 9,10 found that α-HH was the only crystal phase produced during spontaneous precipitation of calcium sulfate from concentrated CaCl 2 solutions at elevated temperature, implying that α-HH has the largest nucleation rate under those conditions.According to the Ostwald's rule of stages, the l...

Effect of CuCl2 on hydrothermal crystallization of calcium sulfate whiskers prepared from FGD gypsum

“…The mass loss of the hydrothermal products grown in H 2 SO 4 ‐CuCl 2 (15 g/kg gypsum)‐H 2 O was 6.3 wt.% at 200 °C, closely corresponding to the theoretical content of crystal water in HH. The data generated in this study is comparable to those available in the literature . The result also implies that the crystallization of CSWs can be improved in H 2 SO 4 ‐H 2 O when the purified FGD gypsum is used as raw material with CuCl 2 as the crystal modifier.…”

“…Calcium sulfate dihydrate (DH), calcium sulfate hemihydrate (HH), and calcium sulfate anhydrite (AH) are three calcium sulfate phases that are most often observed. During the simultaneous crystallization of multiple phases, the competitive nucleation among these phases would occur . Therefore, crystallization of CSWs grown in the H 2 SO 4 ‐H 2 O system, when CuCl 2 is used as the crystal modifier, is a competitive nucleation process, which makes investigation on the phase structures of hydrothermal products necessary.…”

Microscopic Studies of Calcium Sulfate Crystallization and Transformation at Aqueous–Organic Interfaces

“…It is well known that primary anhydrite crystal growth is extremely difficult to achieve at moderate temperatures, due to the high kinetic barrier for its formation. 15,20,22,24,25,27 Therefore, secondary anhydrite nucleation via the formation of gypsum and/or anhydrite seeds is often realized, 17,18,26 and anhydrite crystallization has been witnessed to occur simultaneously with crystalline gypsum needle dissolution. 17 It is important to point out that, in our system, gypsum needles fully convert to anhydrite.…”

“…For aqueous systems, direct dehydration from gypsum to anhydrite is most often realized, 18,24,26 whereas for solid-state transitions occurring above 100 °C, stepwise dehydration is favored. [50][51][52] For solid-state transitions, the direct removal of all the water molecules from gypsum to form insoluble anhydrite would be unlikely, due to large differences in the two crystal structures.…”

“…15,18,[20][21][22][23][24][25] There is much debate as to the transition temperature. Previous reported values under atmospheric conditions generally fall within the broad range of 42 -60 °C, 15,26 although in practice, it is extremely difficult to synthesize anhydrite at temperatures lower than 70 °C.…”