Hydration Kinetics and Microstructure Development of Normal and CaCl 2 -Accelerated Tricalcium Silicate Pastes

Microstructure development and the kinetics of hydration of pure tricalcium silicate (C 3 S) and CaCl 2 -accelerated C 3 S 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 CaCl 2 significantly increases the rate of nucleation of hydration product on the surface of the C 3 S 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 CaCl 2 . However, in the CaCl 2 -accelerated paste the surface fractal scattering associated with the deposition of hydration product onto the initially smooth surfaces of the C 3 S particles rapidly declines in intensity and essentially disappears by the age of 1 day, while in the pure C 3 S 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 C 3 S paste the C−S−H hydration product forms initially with a low packing density and then densifies at later times.