Controlling the Selective Formation of Calcium Sulfate Polymorphs at Room Temperature

Calcium sulfate is a naturally abundant and technologically important mineral with a broad scope of applications. However, controlling CaSO4 polymorphism and, with it, its final material properties still represents a major challenge, and to date there is no universal method for the selective production of the different hydrated and anhydrous forms under mild conditions. Herein we report the first successful synthesis of pure anhydrite from solution at room temperature. We precipitated calcium sulfate in alcoholic media at low water contents. Moreover, by adjusting the amount of water in the syntheses, we can switch between the distinct polymorphs and fine‐tune the outcome of the reaction, yielding either any desired CaSO4 phase in pure state or binary mixtures with predefined compositions. This concept provides full control over phase selection in CaSO4 mineralization and may allow for the targeted fabrication of corresponding materials for use in various areas. Pick a polymorph: Precipitation of calcium sulfate from methanolic solutions containing very low amounts of water leads to the formation of phase‐pure anhydrite at ambient conditions. By changing the water content, all three polymorphs of CaSO4 (anhydrite, bassanite, or gypsum) can be obtained selectively, as well as binary mixtures of the different phases.