Characterisation of class F fly ash geopolymer pastes immersed in acid and alkaline solutions

► As prepared fly ash based geopolymer is weak in strong acid and alkali solutions. ► Acid and alkaline resistance of fly ash based geopolymers were improved by preliminary calcination. ► Preliminary calcination caused a partial surface crystallisation of geopolymer constituents. ► Solubilities of geopolymer cations in acid and alkali were reduced up to 16-fold by calcination. Acid and alkaline resistance of class F fly ash based geopolymer pastes has been investigated. As prepared geopolymers showed high solubility in both strong alkali and acid solutions. Calcination of the fly ash based geopolymers at 600 °C resulted in a decrease of amorphous component from 63.4 to 61.6 wt.%. However, the solubility of the Al, Si and Fe ions in 14 M NaOH and 18% HCl after 5 days immersion decreased from 1.3 to 16-fold in comparison to as prepared geopolymer samples. Calcination of the geopolymers also resulted in a 30% reduction in compressive strength. Acid and alkali resistance of the geopolymers investigated strongly depends on mineralogical composition change of the calcined geopolymer. Partial crystallisation of non-reacted fly ash particles in the geopolymer decreases its solubility in acid and alkali solutions.