Hydration Kinetics and Microstructure Development of Normal and CaCl2-Accelerated Tricalcium Silicate Pastes

Microstructure development and the kinetics of hydration of pure tricalcium silicate (C3S) and CaCl2-accelerated C3S pastes were investigated by performing isothermal calorimetry and in situ small-angle neutron scattering (SANS) measurements on parallel specimens during the first few days of hydration, as well as on 28-day old specimens hydrated under the same curing conditions (water/cement ratio = 0.5, 20 °C). Calorimetry experiments were also performed over a range of hydration temperatures from 10 to 40 °C. The calorimetry data were analyzed by applying a previously described boundary nucleation and growth model. The model indicates that CaCl2 significantly increases the rate of nucleation of hydration product on the surface of the C3S particles but has relatively little effect on the product growth rate. The SANS measurements indicate that the composition and density of the calcium−silicate−hydrate (C−S−H) nanoparticles is unchanged by the addition of CaCl2. However, in the CaCl2-accelerated paste the surface fractal scattering associated with the deposition of hydration product onto the initially smooth surfaces of the C3S particles rapidly declines in intensity and essentially disappears by the age of 1 day, while in the pure C3S paste the surface fractal scattering remains prominent throughout the hydration process. The key observations from both the calorimetry and SANS analysis can be explained if in a pure C3S paste the C−S−H hydration product forms initially with a low packing density and then densifies at later times.