Influence of glass composition and alteration solution on leached silicate glass structure: A solid-state NMR investigation

A multinuclear solid-state NMR investigation of the structure of the amorphous alteration products (so called gels) that form during the aqueous alteration of silicate glasses is reported. The studied glass compositions are of increasing complexity, with addition of aluminum, calcium, and zirconium to a sodium borosilicate glass. Two series of gels were obtained, in acidic and in basic solutions, and were analyzed using H-1, Si-29, and Al-27 MAS NMR spectroscopy. Advanced NMR techniques have been employed such as H-1-Si-29 and H-1-Al-27 cross-polarization (CP) MAS NMR, H-1 double quantum (DQ) MAS NMR and Al-27 multiple quantum (MQ) MAS NMR. Under acidic conditions, Si-29 CP MAS NMR data show that the repolymerized silicate networks have similar configuration. Zirconium as a second nearest neighbor increases the Si-29 isotropic chemical shift. The gel porosity is influenced by the pristine glass composition, modifying the silicon-proton interactions. From H-1 DQ and H-1-Si-29 CP MAS NMR experiments, it was possible to discriminate between silanol groups (isolated or not) and physisorbed molecular water near Si (Q(2)), Si (Q(3)), and Si (Q(4)) sites, as well as to gain insight into the hydrogen-bonding interaction and the mobility of the proton species. These experiments were also carried out on heated samples (180 degrees C) to evidence hydrogen bonds between hydroxyl groups on molecular water. Alteration in basic media resulted in a gel structure that is more dependent on the initial glass composition. Al-27 MQMAS NMR data revealed an exchange of charge compensating cations of the [AlO4](-) groups during glass alteration. H-1-Al-27 CP MAS NMR data provide information about the proximities of these two nuclei and two aluminum environments have been distinguished. The availability of these new structural data should provide a better understanding of the impact of glass composition on the gel structure depending on the nature of the alteration solution. (c) 2006 Elsevier Inc. All rights reserved.