Revisiting the roles of salinity, temperature and water activity in phase selection during calcium sulfate precipitation

Abstract

Calcium sulfate is an important mineral present in a wide variety of natural and engineered environments. Three phases (gypsum, bassanite, and anhydrite), differing in their degree of hydration, can occur in contact with water. Phase selection during crystallisation from aqueous solutions depends on numerous factors such as temperature and salinity, which influence the (relative) stabilities of the different crystal forms. In the present work, we have revisited the crucial parameters governing phase selection in the CaSO₄/H₂O/NaCl system. To that end, calcium sulfate was precipitated from aqueous solutions at various conditions. The obtained solids were characterised with respect to phase selection by IR spectroscopy, while elemental analyses were used to quantify possible incorporation of “spectator” ions like sodium and chloride into the calcium sulfate products. Our results confirm that the temperature needed to kinetically favour bassanite formation over the thermodynamic stable phase gypsum can strongly be reduced with increasing concentrations of added NaCl. In combination with solution speciation calculations, we conclude that initial phase selection is mainly driven by the relative levels of supersaturation of the involved phases, rather than only through changes in water activity or the incorporation of spectator ions.