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 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.