Water speciation in hydrous silicate and aluminosilicate glasses: Direct evidence from 29Si-1H and 27Al-1H double-resonance NMR

Through a combination of H-1 MAS NMR, H-1 -> Si-29 -> H-1 double cross-polarization (CP) MAS NMR and Al-27 -> H-1 CP MAS NMR, different OH species [SiOH, AlOH, and (Ca,Mg)OH (free OH)] have been unambiguously identified for hydrous Ca,Mg-(alumino)silicate glasses. This confirms my earlier speciation assignments made partially on the basis of H-1 chemical shift arguments. The dissoution mechanisms of water in both Al-free silicate and aluminosilicate glasses (quenched melts) are fundamentally similar. For relatively polymerized compositions, it involves dominantly the formation of TOH species (T: Si, Al) through the rupture of T-O-T linkages, in addition to molecular H2O; for more depolymerized compositions containing network-modifying cations of large field strength (e.g., Ca, Mg), free Oil species are also important.