Dilatometry of geopolymers as a means of selecting desirable fly ash sources

The use of dilatometry as a screening tool for the analysis of fly ash reactivity in geopolymers is presented and validated by testing six fly ash sources across a range of mix designs. In particular, a prediction of geopolymer mechanical performance can be obtained from the temperature at which the high-temperature expansion peak attributed to the release of strongly chemically-bound water is observed; high-strength samples generally display this expansion at a higher temperature than low-strength samples. Samples made either with a poorly reactive fly ash or an excessive SiO2 content in the activating solution also expand by up to 10–15% below 200 °C, which indicates a low extent of crosslinking in the silicate gel phase. The second derivative of the dilatometric data can be helpful in identifying the onset temperature of the high-temperature expansion peak, as it is sometimes overlaid on a shrinkage phenomenon attributed to viscous sintering.

► Dilatometry shows high-temperature expansion in geopolymer gels.
► The onset temperature of expansion correlates with the strength of the geopolymer.
► Dilatometry can identify excess Si content in activating solutions, which may not be evident in strength data.