Novel attempts for the synthesis of calcium sulfate hydrates in calcium chloride solutions under atmospheric conditions

The medical grade calcium sulfate is widely used in clinical applications for treating bone defects. A high-purity and predictable calcium sulfate (CS) synthesis process is desirable in the medical industry. The objective of this study was to develop a one-pot method for the direct preparation of calcium sulfate hemihydrates (CSHs; such as the α- and β-forms) in a CaCl₂ solution. CS was synthesized by mixing K₂SO ₄ and Ca(NO₃)₂·4H₂O under various CaCl_2(aq) concentrations and reaction temperatures under atmospheric pressure. The calcium sulfate dihydrate (CSD) was found to be an intermediate phase that converts to CSH during the synthesis process, and α-CSH was gradually transformed from β-CSH over time. Moreover, the kinetic of CSD conversion to CSH was strongly accelerated by increasing the CaCl₂ concentration. As the reaction temperature was fixed in 90 °C, the form of the CS reactant with an increase in the CaCl₂ concentration was in the following sequence: CSD → CSD + β-CSH → CSD + β-CSH + α-CSH → α-CSH. In this study, the synthesis processing window of the CS reactant was established according the test results, and it is worth noting that all phases of CS hydrate could be synthesized with this system and well predicted by the constructed processing window.