Hydration kinetics and microstructure evolution of NaCl-mixed tricalcium silicate pastes

The hydration kinetics of tricalcium silicate (C₃S) pastes mixed with different concentrations of NaCl solutions were investigated by performing isothermal calorimetry tests. At the same time, the Boundary Nucleation and Growth (BNG) model was used to analyze the calorimetry data. The model fitted well with the calorimetric results. C₃S pastes mixed with NaCl solutions followed a similar reaction path to those prepared with DI water, with an accelerated reaction rate as the NaCl concentration increased. The specific parameters, such as the induction and acceleration durations, were quantified, and the relationship was established. The BNG modelling results indicated that the increasing NaCl concentration significantly increased the nucleation rate of the hydration products while decreasing their growth rate slightly. That was supported by the SEM and STEM observations. The presence of NaCl modified the morphology of the early-age calcium silicate hydrates (C-S-H) to relatively shorter and thinner fibres, which had a more compact microstructure and higher micromechanical properties than those in pure C₃S pastes.