Global hydration kinetics of tricalcium silicate cement - art. no. 016115

We reconsider a number of measurements for the overall hydration kinetics o f 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 unhydr ated silica mole numbers can be well characterized by two power laws in tim e, x/(1-x) similar to (t/t(x))(psi). For early times t<t(x) we find an "acc elerated" hydration (<psi>= 5/2) and for later times t>t(x) a parabolic beh avior ( 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 equat ion 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 as sociated 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 di ffusion limited precipitation, that the exponents psi= 5/2 and psi= 1/2 dir ectly indicate a preferentially platelike hydrate microstructure. This is e ssentially in agreement with experimental observations of cellular hydrate microstructures for this class of materials.