Correlation between dissolubilities of Si, Al, and Fe from aluminosilicate precursor and strength of fly ash-based geopolymer

Fly ash (FA) is the most used aluminosilicate precursor to synthesize geopolymer. However, as a coal-combustion by-product, the inherent characteristics of FA can be significantly altered by multiple factors such as coal sources, combustion temperatures, particle size, and impurity level. Thus, unstable performances of FA-based geopolymer due to extremely varied reactivity of FA have been widely reported in the literature and are limiting the further valorization of FA. To offset the inconsistent characteristics of FA, this study provides a universal index to quantify and evaluate its reactivity to be used as aluminosilicate precursor feedstock. Alkali leaching tests were conducted on seven types of FA sourced from six different coal power plants in the Southeast Queensland region to determine the dissolubilities and dissolution efficiencies of reactive Si and Al within 2 h. The dissolubility and dissolution efficiency demonstrated decent correlations with the mechanical properties of the resultant FA-based geopolymer, especially 7-day strength. Moreover, bulk and reactive Si/Al molar ratios were derived to distinguish each type of FA further and verified by scanning electron microscopy / energy-dispersive X-ray spectroscopy. Lastly, a reactivity index considering the dissolution efficiency of Si, Al, and Fe and reactive Si/Al molar ratios was developed and correlated to the mechanical properties of FA-based geopolymer. Dissolubility at a leaching time of 30 min best reflected the reactivity of FA, having the highest correlation coefficient of 0.82.