The transformation of a fluid slurry of cement particles into an elastic
solid (a process known as “setting” of cement paste) involves the dissolution
of the anhydrous cement and precipitation of the calciumsilicate
hydrate binder phase (called C–S–H3) onto the surfaces of the cement
grains; setting corresponds to percolation of the overlapping product
layers.
Dissolution of a solid, such as tricalciumsilicate inwater, is a net process
consisting of one or more reaction paths leading to a final product
of dissociated ions in solution. Each reaction path comprises multiple
elementary reaction steps at the solid–liquid surface, although for
tricalcium silicate the reaction paths and their elementary steps are,
generally, poorly understood. Nevertheless, the overall driving force
for dissolution is the free energy difference ΔΦd,net between the
products of the final step (i.e., the dissociated ions in solution) and the
reactants of the first step (i.e., the components in the solid); the net
process will occur only if ΔΦd,net b 0, which means that the solution is
undersaturated with respect to the dissolving solid. Likewise, growth
of a solid such as C–S–H is a net process that likely has one ratecontrolling
step among all the elementary steps leading from the reactant
ions in solution to the final solid product, and that net process
can occur only if ΔΦd,net b 0,meaning that the solution is supersaturated
with respect to the growing solid.