Structural Insights into the Dehydration and Rehydration of Gypsum.

The formation and transformation of phases within the CaSO₄-H₂O system are crucial in natural and industrial processes. Despite intensive research, however, the mechanisms of gypsum's (CaSO₄·2H₂O) thermal dehydration and rehydration remain unclear. Here we investigated the crystallographic and textural relationship between parent and product phases during the dehydration and rehydration of gypsum single crystals using two-dimensional X-ray diffraction (2D-XRD), field emission scanning electron microscopy (FESEM), in situ and ex situ transmission electron microscopy (TEM), and selected area electron diffraction (SAED). Our findings reveal that the dehydration products, bassanite (CaSO₄·0.5H₂O) or soluble anhydrite (γ-CaSO₄), exhibit no clear structural relationship with the parent phase in the case of millimeter-sized gypsum crystals. Conversely, a topotactic replacement reaction is observed for micrometer-sized gypsum crystals. This disparity is due to localized nonoriented crystallization of α-hemihydrate in microhydrothermal cavities within large crystals resulting from local gypsum dehydration, whereas in smaller gypsum crystals unimpeded H₂O release results in oriented β-hemihydrate via a solid-state mechanism. Rehydration proceeds via a dissolution-precipitation mechanism, without structural continuity between parent and product phases. Importantly, gypsum relicts in partially dehydrated pseudomorphs serve as template for the epitaxial crystallization of gypsum during rehydration. These findings provide valuable insights for optimizing hemihydrate production and hydraulic setting, benefiting both construction and biomedical applications.