  We have numerically investigated a reaction-diffusion model for the hydration
of calciumsulphate (gypsum). The simulations were conducted for two and three
dimensional systems. While the dissolution of anhydrous gypsum is considered
irreversible at a finite rate the precipitation/dissolution reaction for the
calciumdihydrate is considered reversible. The latter reaction is assumed to be
controlled by the dihydrate's equilibrium solubility {\em and} the abillity of
the system to react on supersaturation only at a certain velocity described by
the reaction rate constant of precipitation. For $d=2$ we find at early times
an accelerated hydration period followed by a maximum and a decreasing
hydration rate. For large times the ionic product of involved species assumes
closely the value of the di-hydrate equilibrium solubility. Calculated model
micro-structures exhibit typical features such as inner and outer hydrate
products, induction and dormant period as well as bridging. Furthermore we find
that the overall chemical reactivity as a function of initial anhydrous
(volume) concentration $p$ exhibits a maximum close to the percolation point of
the underlying lattice. Employing a rescaling procedure we find {\em two}
percolation thresholds in $d=2$, $p_c^{min}=0.44\pm 0.015$ and
$p_c^{max}=0.77\pm 0.02$, for the initial anhydrous gypsum concentration {\em
between} which percolating dihydrate structures can be attained. For $d=3$ we
find $p_c^{min}=0.10\pm 0.02$ and $p_c^{max}=0.95 \pm 0.02$.
