Compositional shifts in fly ash over a decade and its effect on geopolymer properties

Geopolymers are emerging sustainable alternatives to conventional cements in concrete production, aiming to reduce reliance on virgin resources and mitigate high embodied greenhouse-gas-emissions. However, as feedstock quality changes with time and composition (especially for by-product feedstocks) so too must the processes for quality assurance and quality control (QAQC). Optimisation of these processes are required for consistent production and are essential to commercialisation. This study investigates the compositional change of Western Australian fly ashes in recent years and its impact on compressive strength. Furthermore, this study highlights a bimodal compressive strength profile for fly ash-based geopolymers, contrasting with many previous unimodal findings. It suggests significant shifts in synthesis behaviour with varying molar Si/Al, precluding reproducibility until it is better understood and potentially increasing the embodied cost, energy and carbon footprint of geopolymer systems as practitioners attempt to synthesise them at common compressive strength baselines. Bulk chemistry and phase composition of fly ash samples from 2007-2023 reveal significant compositional shifts, including a 7.5(2) wt.% higher mullite content and varied Al 4+2x Si 2-2x O 10-x stoichiometries, 12.0(5) wt.% lower amorphous fractions and altered amorphous Si/Al ratios (e.g. 2.5(2) vs. 1.7(2) Si/Al) in the modern/recent fly ash. This study underscores the role of compositional drift and importance in understanding its impacts on geopolymer systems. It provides a foundation for the standardisation, QAQC and commercialisation of fly-ash based geopolymer technologies and consistent material characterisations. • Compositional changes in recently produced fly ashes impacts its geopolymer reactivity • Temporal variations in fly ash highlight extrapolation difficulty for geopolymer QAQC • Recently produced Collie fly ashes have a higher molar Si/Al and lower amorphous fraction • Geopolymer strength profile shows bimodal behaviour & contrasts previous research • Intra-source fly ash comparisons highlighted for QAQC & geopolymer standardisation