Synthesizing amorphous single-phase of fly ash to understand its alkali activation effect

The heterogeneity of amorphous aluminosilicate phases leads to different chemical features among various fly ashes (FAs) in alkali-activated cement systems. Insufficient understanding of this heterogeneity and its underlying mechanisms limits the extensive utilization of FA. In this work, amorphous aluminosilicates in FAs were categorized into eight types via backscattered electron (BSE) imaging and energy dispersive spectroscopy (EDS) clustering techniques. An improved sol-gel method was innovatively introduced to synthesize single-phases with atomic stoichiometry and structure analogous to those of the categorized phases for systematic reactivity quantification. Phase-II and silicate, featuring high calcium content and elevated Si/Al molar ratios, exhibited optimal multi-scale activation effect, with their cumulative heat releases, 28-day reaction degrees, microhardness, and compressive strengths being 0.87–2.39, 2.43–4.78, 1.23–1.88, and 1.09–4.54 times higher than those of other phases, respectively. Alkali activation behavior demonstrated significant dependence on categorized phases. The proposed clustering method was validated, laying a foundation for phase-driven predictive modeling of FA-based alkali-activated materials. Coupling BSE-EDS clustering with sol-gel synthesis enables phase-separated investigation of amorphous aluminosilicates in solid wastes. This advancement provides critical scientific insights for elucidating reaction mechanisms and promoting high-value-added utilization of solid wastes.