Analysis of Active Ion-Leaching Behavior and the Reaction Mechanism During Alkali Activation of Low-Calcium Fly Ash

The dissolution and release of active ions, such as Si4+, Al3+ and Ca2+, from fly ash directly affect the rate and extent of reaction product formation, which in turn affect the physical and mechanical properties of fly ash filling materials. In this study, low-calcium fly ash was soaked and activated in NaOH solutions with different concentrations (approximating the optimum dose range) for different lengths of time. The amounts of active ions leached and the changes in the mineral composition, chemical functional groups and surface morphology were tested and analyzed via ICP-OES, XRD, FTIR and SEM/EDS techniques. Based on these analyses, the reaction mechanism of alkali activation of low-calcium fly ash was further investigated. The results showed that the NaOH activation effect can significantly increase the amount of active ions leached from low-calcium fly ash. Notably, the amount of Si4+ and Al3+ leached clearly increased with increases in both NaOH concentration and soaking time. The plausible reaction mechanism is discussed in detail, which is that the alkali activator principally affected the surface of the vitreous particles of low-calcium fly ash and induced differing surface modifications in the dissolution stage, depolymerization stage, polycondensation and polymer gel stage and diffusion stage. It was observed that the progress of the reaction is controlled by dissolution in the early stages, whereas activation is governed by diffusion when the surfaces of the fly ash particles are covered by precipitates.