Stability of layered sodium disilicate during hydration process as studied by multinuclear solid state NMR spectroscopy

Solid-state (1)'H, Na-23, and Si-29 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 alpha-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 alpha-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 tune 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 Q(2) [(SiO)(2)Si(OH)O-Na-] and Q(3) [(SiO)(3)SiO-Na+] groups. The coordination of sodium ions is also significantly altered after the hydration. The results of Na-23 NMR experiments agree well with the data from H-1 and Si-29 NMR experiments, and five sodium sites with different local environments can be resolved by the Na-23 MAS and two-dimensional Na-23 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.