Bulk structures of silica-rich calcium aluminosilicate (CAS) glasses along the molar CaO/Al2O3=1 join via molecular dynamics (MD) simulation

In order to understand how surface charge accumulation and other reactivity-related phenomena are controlled by the surface structure, it is necessary to have a detailed appreciation of the bulk glass structures. Here, we report a detailed MD study of the bulk structures of silica-rich calcium aluminosilicate (CAS) glasses along the tecto-silicate join. It is confirmed that the structures of these glasses contain small concentrations of over- and under-coordinated defect species, such as NBO, TBO and [5]-coordinated aluminum, Al-C5, in contrast to the simple idea that these fully charge-compensated compositions, with Ca/Al2O3 ratios of unity, should have fully polymerized structures. The Si,Al distribution was found to be largely random (i.e. Lowenstein's rule is not obeyed) and no significant segregation of Ca to Al was observed. Interactions between TBO and Al-C5 were manifest both in the presence of an additional small peak in the Al-O-Al BAD and in the formation of structural complexes involving small-membered rings, particularly 2-rings. These defect structural complexes contributed to the structural inhomogeneity of these glasses.