SI-29 MAS NMR SPECTROSCOPIC INVESTIGATION OF ALKALI-SILICA REACTION-PRODUCT GELS

Si-29 magic angle spinning NMR spectroscopy and H-1-Si-29 cross-polarization MAS results are reported for gels prepared by reactions of a Nevada opal, Beltane opal, and cristobalite with KOH in the presence and absence of Ca(OH)2. The extents of the reaction can be followed as functions of time. The opals react much more rapidly than cristobalite. In the presence of calcium, the predominantly Q4 sites of the starting materials change to predominantly Q2 and Q3 sites. Q1 sites are less abundant and in part-transient. Cross-polarization experiments confirm that all the reaction products are hydrous. Poor signal to noise ratios for the cross-polarization experiments at room temperture and increased signal/noise at -80-degrees-C suggest that most of the protons are loosely bound at room temperature and may occur predominantly in water molecules. The chemical shifts of the Q1 and Q2 sites observed for these products suggest local environments surprisingly similar to those of the calcium silicate hydrate gel produced in portland cement hydration. Gel products produced in the absence of calcium yield broad peaks corresponding to Q3, Q2, and Q1 sites, but the spectra vary in a non-monotonic way with reaction time.