Early Stages of Crystal Growth In Supersaturated Solutions of Calcium Sulfate Hemihydrate Examined by Time-Resolved Cryogenic Transmission Electron Microscopy

ABSTRACT. The early stage of crystal growth from a supersaturated solution of calcium sulfate hemihydrate to form calcium sulfate dihydrate (gypsum) is studied using time-resolved cryogenic transmission electron microscopy (TR-cryo-TEM). We observe, for the first time, amorphous prenucleation clusters that evolve to larger amorphous particles and then to crystalline gypsum domains. The influence of low concentrations of the additives citric acid, potassium sulfate and magnesium sulfate on the evolution from the hemihydrate to gypsum is also investigated. The addition of citric acid significantly delays gypsum crystallization by adsorption and complexation of the carboxylic groups with calcium ions, effectively stabilizing the amorphous phase. The formation of gypsum is expedited when small quantities of potassium sulfate are added. Since the hydration of potassium sulfate is endothermic, addition of this salt accelerates the exothermic transformation of the calcium sulfate hemihydrate to gypsum.  In contrast, the hydration of magnesium sulfate is exothermic and addition of small quantities of this salt retards gypsum formation.  Results from this work are useful for modulating the rate of gypsum formation in chemical and construction applications.