Stability of Layered Sodium Disilicate during Hydration Process As Studied by Multinuclear Solid State NMR Spectroscopy

Solid-state 1H, 23Na, and 29Si NMR spectroscopies, combined with X-ray powder diffraction (XRD) technique, were used to study the hydration process of a layered sodium disilicate SKS-5 (having a crystal structure of α-Na2Si2O5). NMR and XRD experiments suggest that SKS-5 is very sensitive to absorbed water molecules and the crystal structure of SKS-5 converts gradually from α-Na2Si2O5 to a new phase during the hydration process. Although the new phase contains about 9% Na2Si4O9, its crystal structure is still unclear. The hydration of SKS-5 begins with the formation of hydrogen-bonded water (with the silicate framework), accompanied by a partial hydrolysis of the Si−O−Si linkages in the framework that generates isolated and hydrogen-bonded silanols (with proton bonded to oxygen atom in the adjacent layer). With an increase of the hydration degree, the above process proceeds and the concentrations of hydrogen-bonded water and hydrogen-bonded silanol increase significantly. When the hydration time is extended to 6 months, the conversion from SKS-5 to the new phase is accomplished and a new kind of hydrogen-bonded silanols (with the proton bonded to nonbridging oxygen at the same silicon atom) appears. The hydrogen-bonding network formed during the hydration process may probably strengthen the stability of the new phase. The silicon sites in the new phase are confirmed to be mainly Q2 [(SiO)2Si(OH)O-Na+] and Q3 [(SiO)3SiO-Na+] groups. The coordination of sodium ions is also significantly altered after the hydration. The results of 23Na NMR experiments agree well with the data from 1H and 29Si NMR experiments, and five sodium sites with different local environments can be resolved by the 23Na MAS and two-dimensional 23Na multiple quantum MAS spectra in different hydration periods. Probably, the hydrolysis of the SKS-5 framework and the variation of the sodium coordination lead to the structural conversion during the hydration process.