Structural Investigations of Alkali Silicate Gels

Investigation of a series of synthetic alkali silicate gels and gels produced by the alkali silica reaction (ASR) in field concrete using ²⁹Si NMR spectroscopy, X-ray diffraction, and bulk chemical analysis shows that local structures of the synthetic and field gels are quite similar. The most abundant Si sites for the field and synthetic gels with similar compositions have Q³ polymerization, and the number of non-bridging oxygens per Si is similar for these samples. These samples also yield a basal X-ray diffraction peak near 8–12 Å, suggesting that the structure is dominated by sheet-like units, consistent with the dominant Q³ polymerization. Calculations based on the relative site abundances of the sites observed by ²⁹Si NMR and the bulk chemical compositions indicate that there is insufficient alkali to charge-balance all the non-bridging oxygens and that there is a significant concentration of Si–OH linkages. The results provide strong support for the basic structural concepts of the so-called kanemite model for ASR gel proposed by Wieker and coworkers, although the overall gel structure is likely to be more complex.