The Charge-Balancing Role of Calcium and Alkali Ions in Per-Alkaline Aluminosilicate Glasses.

The structural arrangement of alkali-modified calcium aluminosilicate glasses has implications for important properties of these glasses in a wide range of industrial applications. The roles of sodium and potassium and their competition with calcium as network modifiers in peralkaline aluminosilicate glasses have been investigated by Al and Si MAS NMR spectroscopy. The Si MAS NMR spectra are simulated using two models for distributing Al in the silicate glass network. One model assumes a hierarchical, quasi-heterogeneous aluminosilicate network, whereas the other is based on differences in relative lattice energies between Si-O-Si, Al-O-Al, and Si-O-Al linkages. A systematic divergence between these simulations and the experimental Si NMR spectra is observed as a function of the sodium content exceeding that required for stoichiometric charge-balancing of the negatively charged AlO tetrahedra. Similar correlations between simulations and experimental Si NMR spectra cannot be made for the excess calcium content. Moreover, systematic variations in the Al isotropic chemical shifts and the second-order quadrupole effect parameters, derived from the Al MAS NMR spectra, are reported as a function of the SiO content. These observations strongly suggest that alkali ions preferentially charge-balance AlO as compared to alkaline earth (calcium) ions. In contrast, calcium dominates over the alkali ions in the formation of nonbridging oxygens associated with the SiO tetrahedra.