Anomalous Thermal Effects on Fracture in Anhydrous Amorphous Calcium Carbonate

Amorphous calcium carbonate (ACC) is a disordered phase of common CaCO3 polymorphs (calcite, aragonite, and vaterite) with critical roles in biomineralization and in calcium carbonate-based cements. Here, we present chemical mechanisms by which the structure of anhydrous ACC controls the resulting mechanical and fracture properties studied with classical molecular dynamics simulations. Anhydrous ACC structures were generated through the compression of Ca2+ and CO32- ions with progressively slower compression rates, which resulted in an increasing strength. Increases in the elastic constants were associated with decreasing Ca2+Ca2+ coordination numbers and smaller CaCaC and CaCaO bond angles. Fracture of anhydrous ACC was also studied under far-field loading conditions. Surprisingly, at lower temperatures (0.1 K), cracks were 50% longer than at higher temperatures (150 and 300 K). This temperature-dependency in crack growth arises from increased energy dissipation at higher temperatures, allowing for relaxation of the applied load without forming a new surface area from crack growth. Energy dissipation mechanisms include changes in the OCaO bond angles between 40 and 50 degrees and associated changes in the Ca2+ ion and CO32- ion coordination structure. These results on fracture in disordered calcium carbonate can be used to evaluate and predict the stability of systems that contain these solid phases.