| The structure, acid durability, and surface layer formation of two series of fully polymerized aluminosilicate glasses with (CaO+Na2O):(Al 2O3) molar ratios of unity were examined. In particular, the effects of varying the network composition (Al2O3:SiO 2) on bulk glass structure, acid dissolution rate, and surface layer structure were determined. Surface layer formation and structure was investigated using a host of analytical techniques as a function of these compositional variables.; It is shown that although the (CaO+Na2O):(Al2O 3) molar ratio remains unity throughout the series, a fully polymerized structure (expected based on “traditional” glass structure models) could not be validated. Bulk glass structures were determined experimentally with ultra-violet Raman spectroscopy (UV-Raman), infrared absorption spectroscopy (IR-absorption), and 29Si magic angle spinning nuclear magnetic resonance (29Si MAS-NMR), and were modeled with molecular dynamic (MD) simulations. The relative concentration of network bonding structures including: non-bridging oxygen (NBO) species, Al-O-Al bonding, and oxygen triclusters, were determined. These characteristics were used to explain dissolution rate, modifier release rate, and surface layer formation of the glasses when exposed to acid. Dissolution rate decreases with increasing Si-O-Si concentration. Dissolution and surface layer formation were the result of breaking network forming bonds (Al-O-Si and Al-O-Al), release of aluminum, sodium, and calcium at equal rates, and rapid re-polymerization of insoluble silica. The surface layers did not form through diffusion-limited ion-exchange between proton-bearing solution species and modifier ions within the glass.; The transformed surface layers were found to be microporous, more structurally ordered than the bulk glass, and to contain primarily 3-membered and 4-membered silicate rings (with terminal silanols and trapped molecular water). The processes involved in the layer transformation were simulated using MD, and the resultant layer structures were consistent with the experimental 29Si MAS-NMR results; specifically localized condensation (relaxation) of insoluble silica as the reaction proceeds into the bulk glass. |
