Stability and Structure of Hydrated Amorphous Calcium Carbonate

The
results of molecular dynamics simulations of hydrated amorphous
calcium carbonate (CaCO3·nH2O: ACC) are presented. ACC properties were investigated on atomistic,
supramolecular, and thermodynamic levels. The clustering of water
occluded in the ionic ACC framework was found to be well described
by percolation theory, and with a percolation transition for water
through ACC at a hydration level, n, of ca. 0.8.
Percolation in ACC systems is quantitatively similar to site percolation
on a simple cubic lattice where the percolation threshold is observed
at pc = 0.312. Predominantly
fourfold tetrahedral molecular coordination of water molecules in
the bulk liquid state is changed to sixfold connectivity in ACC. Kinetic
stability of ACC is enhanced by dehydration and reaches maximal values
when the water content is below the percolation threshold. The computed
free energy shows a region of thermodynamic stability of hydrated
ACC (1 < n < 6) with respect to calcite and
pure water. This region is bounded by two crystallohydrates, monohydrocalcite
(n = 1) and ikaite (n = 6), that
have lower free energies than ACC. During dehydration at n < 1 the thermodynamic stability of ACC decreases, which favors
the processes of nucleation and crystallization. On the other hand,
water mobility within ACC also decreases during dehydration, thus
making dehydration more difficult. So, the stability of hydrated ACC
is controlled by a balance of two opposing factors: kinetics and thermodynamics.