Modeling Gypsum Crystallization on a Submicrometric Scale

Gypsum needle crystallization from a suspension of calcium sulfate hemihydrate grains in an aqueous solution is studied on a mesoscopic scale. We build a simulation model, which assumes that gypsum formation is limited by heterogeneous nucleation and precipitation with an initial autocatalytic surface-controlled needle growth and a final diffusion-controlled growth. The model introduces a minimal number of parameters whose effect on growth dynamics and gypsum morphology is analyzed. We find that the increase of nucleus number per hemihydrate grain decreases induction time and needle length and increases needle entanglement, i.e., improves the mechanical properties of the material. The simulation results correctly reproduce experiments with and without treatment of the reactants. The simulation model can be used to predict the behavior of the reactive system in conditions that are not compatible with experimental observations.