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The nanostructural evolution during formation of geopolymers and its correlation with setting have not been well understood. In this study, penetration resistance and ultrasonic wave reflection tests were conducted to measure setting, and solid‐state 27Al NMR and liquid‐state 29Si NMR were used to examine nanostructural changes in a metakaolin geopolymer as a function of time. Aluminum was released rapidly during the first 10 hour after mixing and immediately condensed with silicate species in solution to form larger sized aluminosilicate oligomers, which then condensed to form large structural units. Our evidence suggests these units form near metakaolin particle surface. Smaller sized silicate ions in the sol phase then attach to these units to form a gel with a more interconnected network structure. The initial stage of this attaching process was seen to be associated with set, which in this mixture occurred at 15 hour.

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.

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Geopolymer setting is seen to be substantially accelerated by addition of calcium and the objective of this study was to determine the mechanism for this effect by examining metakaolin geopolymers with and without calcium. Solid‐state 27Al NMR tests were used to examine the dissolution extent both qualitatively and quantitatively. Solid‐state 29Si NMR tests were conducted to determine the amount and structure of each phase. Prior to the quantitative tests, chemical extractions were used to facilitate assignment of peaks in each spectrum. On addition of calcium, it was found that both the rate and the extent of metakaolin dissolution were enhanced. Accelerating dissolution increases the Al concentration in solution, thus reducing Si/Al available for geopolymer gel formation and further accelerating the gel formation to cause faster setting. Although C‐A‐S‐H was observed in the calcium mix, no evidence indicated that it is directly involved in setting.

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