Stable Bassanite Bulk Phase Formed in Aqueous Solution under the Control of Polymer-Mediated Water Activity

Bassanite, as the metastable phase in the family of calcium sulfate minerals, plays an important role in natural and industrial processes, but a stable bulk bassanite can rarely precipitate in nature or be fabricated under ambient conditions. Here, we report the first example of stable bulk bassanite precipitated in solution, which fills in the gap of calcium sulfate phase diagram. By precisely controlling water activity with polymer (polyvinyl pyrrolidone), bulk bassanite is formed, instead of transforming into thermodynamically favorable gypsum. A new thermodynamic model of the non-classical crystallization process is described in terms of water-driven colloidal assembly, and molecular dynamics simulation further demonstrates the role of water molecules in interacting with the polymer ligands to initiate the assembly and growth of bassanite into bulk. These findings not only contribute to a deeper understanding of the role of water activity in the formation of stable bulk bassanite in extreme natural environments but also allow for the fabrication of bassanite materials for industrial applications at room temperature