Strength and Microstructural Characteristics of Activated Fly Ash–Cement Paste

Incorporating high volumes of fly ash (FA) in filling materials reduces costs and carbon emissions, but low early strength limits its use. This study investigates the effects of sodium sulfate decahydrate (Na2SO4·10H2O) and calcium hydroxide (Ca(OH)2) as activators at concentrations of 0.5%, 1.0%, 1.5%, and 2.0% on the mechanical properties and microstructure of tailings–cement–fly ash composites. Compressive strength testing, X-ray diffraction (XRD), and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) were used to evaluate performance at different curing stages. Results indicate that all activators enhance early strength, with 2.0% Ca(OH)2 yielding the greatest improvement. Microstructural analysis showed that activators boost quartz reactivity and create denser structures. Na2SO4 promotes ettringite and gypsum formation, while Ca(OH)2 increases alkalinity, enhancing gel formation from FA. These findings clarify how activators improve the performance of activated fly ash–cement paste (AFCP).