Effect of particle size on the hydration kinetics and microstructural development of tricalcium silicate

Impure tricalcium silicate (Alite, C3S) is the most important and abundant phase of Portland cement. It reacts quickly with water to give calcium hydroxide (CH) and calcium silicate hydrate (C-S-H) which is the most important phase for strength development. It is well known that the C3S particle size distribution plays an important role on the hydration kinetics, microstructural development and final properties of hydrated cement pastes. The general aim of the present investigation was to gain insight on the effect of particle size on the hydration kinetics and microstructural development of alite pastes. In this study, alite was synthesized and separated into different gradations. The hydration kinetics was investigated by means of isothermal calorimetry and chemical shrinkage. The microstructural developments of the different gradations of alite were studied and compared using different techniques such as: SEM, TEM, XRD, He pycnometry, N2 adsorption, etc. The hydration kinetics during the different stages of the hydration was observed to be dependent on the initial particle size of the grains and also related with the defect concentration on the surface of the grains. Higher rates of reaction were observed for powders with higher specific surface area. A linear relationship between the rate constant and the specific surface area was observed. Further, it was observed that the hydration kinetics of multi-sized system during the nucleation and growth process can be modeled as the linear addition of individual particles. The microstructural development of C3S pastes at later ages was found to be poorly affected by the initial particle size; the nature of the hydration products is the same. The only differences concern the capillary porosity and the C-S-H density. Slightly higher values of capillary porosity and C-S-H density were measured for smaller particles. The most likely explanation can be related with a different initial arrangement of the grains.