The simulation results are used here to investigate the influences of the simultaneous rate processes on the
net rate of hydration (i.e., the overall rate as would be measured by
isothermal calorimetry or chemical shrinkage measurements).
We also examine howmathematical models based on boundary nucleation
and growth (BNG) [5,6] are influenced by solution composition.
These latter models arewidely used to fit the kinetics of hydration [7–9]
and typically assume that the growth rate of the transformed (product)
phase is constant. This assumption has important consequences for the
results of those models. Here, however, we extend the standard BNG
model for cement hydration to incorporate time-dependence of the
driving force for (and, therefore, rate of) C–S–H growth indicated by
HydratiCA. The generalized BNGmodel still provides good fits to hydration
rate data, albeit with different values of the fitting parameters; this
indicates that the usual assumption of constant nucleation and growth
rates is likely to result in erroneous values and interpretation of fitting
parameters.
Finally, both HydratiCA and the generalized BNG model are used to
draw conclusions about microstructure development in hydrating C3S
pastes, and to assess the relative likelihood of several hypotheses
proposed in the literature to explain (1) the origin of the period of
slow reaction shortly after mixing C3S with water, (2) the observed
peak in hydration rate that separates the acceleration period from the
deceleration period and (3) the insensitivity of hydration kinetics to
water/cement mass ratio (w/c).