Global hydration kinetics of tricalcium silicate cement

We reconsider a number of measurements for the overall hydration kinetics of tricalcium silicate pastes having an initial water to cement weight ratio close to 0.5. We find that the time dependent ratio of hydrated and unhydrated silica mole numbers can be well characterized by two power laws in time, x/(1-x) similar to (t/t(x))(psi). For early times t<t(x) we find an "accelerated" hydration (<psi>= 5/2) and for later times t>t(x) a parabolic behavior ( psi= 1/2). The crossover time is estimated as t(x)approximate to 16 h. We interpret these results in terms of a global second-order rate equation Indicating that (a) hydrates catalyze the hydration process for t<t(x), (b) they inhibit further hydration for t>t(x), and (c) the value of the associated rate constant is of magnitude 6 x 10(-7)-7x10(-6) 1 mol(-1) s(-1). We argue, by considering that the hydration process actually occurs via diffusion limited precipitation, that the exponents psi= 5/2 and psi= 1/2 directly indicate a preferentially platelike hydrate microstructure. This is essentially in agreement with experimental observations of cellular hydrate microstructures for this class of materials.