Kinetics of Plaster Hydration and Structure of Gypsum: Experiments and Kinetic Monte Carlo Simulations with Added Gypsum Seeds

Experiments and simulation results are compared in order to make precise the conditions optimizing both the kinetics of plaster hydration and the structure of the resulting material. Calorimetry experiments performed for different amounts of added gypsum seeds are used to assign values to the simulation parameters controlling kinetics, e.g. time step, surface density of gypsum germs on plaster grains. Simulation provides a reliable quantity to follow the beginning of setting, the fraction of gypsum needles belonging to a percolating cluster, as proven by comparing its evolution with the one of the elastic modulus during shear experiments. The simulations reveal that the beginning of needle growth may be hindered by two many small plaster grains, whereas big plaster grains slow down the end of the reaction. We suggest to add gypsum germs with an m g / m p = 0.002 gypsum-to-plaster mass ratio in order to optimize homogeneity of the final material.