Investigating the potential reactivity of fly ash for geopolymerization

This study investigated potential reactivity of fly ash for geopolymerization. A combined dissolution of fly ash in NaOH in conjunction with HCl extraction of fly ash-NaOH residue is proposed in this study to more accurately estimate the total reactive content and reactive Si/Al ratio of fly ash. Results show that the total reactive content of fly ash used in the current study is about 68.80%, similar to the amorphous content (66.28%) obtained using quantitative XRD and XRF. The values of reactive Si/Al ratios are between 2.65 and 2.98 depending on the duration of dissolution. The reactive Si/Al values differ significantly from the total Si/Al ratio (1.63) calculated using XRF results and also differ from the vitreous Si/Al ratio (3.75) calculated using combined XRF and XRD results. The results suggest that total or vitreous Si/Al ratio should not be used for mix proportioning of fly ash geopolymers as is generally used for metakaolin-based geopolymers. Instead, the reactive Si/Al ratio determined based on the current study should be used for fly ash geopolymer mix design.