The kinetics of ettringite formation

Conversion of hemihydrate to gypsum, and ettringite formation in the system C₃A---CaSO₄·1/2H₂O---H₂O three hydration temperatures using x-ray diffraction. Analysis of the kinetics of gypsum formation suggests that the rate limiting step changes from an initial dependence on the hemihydrate surface area to a dependence on the gypsum growth rate. The formation of ettringite was found to be controlled by diffusion. In accord with the needle-like morphology that ettringite exhibits, a nucleation and growth model predicted one-dimensional growth. An apparent activation energy calculated for ettringite formation is consistent with a diffusionally controlled process.