  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, $x/(1-x)\sim
(t/t_x)^\psi$. For early times $t < t_x$ we find an `accelerated' hydration
($\psi = 5/2$) and for later times $t > t_x$ a `deaccelerated' behavior ($\psi
= 1/2$). The crossover time is estimated as $t_x \approx 16 hours$. We
interpret these results in terms of a global second order rate equation
indicating that (a) hydrates catalyse the hydration process for $t<t_x$, (b)
they inhibit further hydration for $t > t_x$ and (c) the value of the
associated second order rate constant is of magnitude 6x10^{-7} - 7x10^{-6}
liter mol^{-1} s^{-1}. We argue, by considering the hydration process actually
being furnished as a diffusion limited precipitation that the exponents $\psi =
5/2$ and $\psi = 1/2$ directly indicate a preferentially `plate' like hydrate
microstructure. This is essentially in agreement with experimental observations
of cellular hydrate microstructures for this class of materials.

Materials Science;Chemical Physics