Effects of Particle Size Distribution on the Kinetics of Hydration of Tricalcium Silicate

An analysis of the validity of approximating selected particle size distributions by small numbers of size classes was carried out. It was determined that the selection of a small number of particle size classes is not adequate to describe a particle size distribution. One consequence of this is that the particle size distribution, if not properly accounted for, can mask the kinetics of a hydration process. The hydration rates of two finnesses of tricalcium silicate, each with known particle size distributions, were measured by isothermal calorimetry for a period of 28 d. These data were integrated, normalized, and represented as α-time curves as a basis for comparison with kinetic models of hydration. Agreement with kinetic models was found to be strongly influenced by the effect of particle size distribution. However, the rate-limiting mechanisms appear to be independent of particle size distribution No single kinetic model was adequate to describe C₃S hydration over the first 28 d. A kinetic model that assumes initial surface-area control with diffusion control dominating subsequently provided an excellent fit to the experimental data.