Leaching kinetics and reactivity evaluation of fly ash based on the synergistic effect of alkali and sulfate

Abstract

This work evaluates the reactivity of various minerals of fly ash based on the backscattered electron images analysis (BSE-IA) method and discusses the influencing factors and kinetics of Si and Al selective leaching of fly ash in alkali solution and alkali-sulfate composite solution, respectively. The relationships among the physicochemical properties, dissolution behavior, and reactivity of fly ash have been revealed. There are three typical phases in fly ash, namely the Si–Al phase, Ca–Si–Al phase, and Fe phase. The Ca–Si–Al phase is more soluble and preferentially participates in the reaction process than the other two phases. The dissolution process is greatly affected by temperature. Alkali and sulfate have a compound synergistic effect and SO42− further promotes the dissolution of Si and Al mainly by changing the gel property wrapped on the surface of fly ash particles. The leaching kinetics results indicate that the internal diffusion controls the reaction process. The research method and system in this work can be applied to other solid waste, providing reference significance for quickly exploring its reaction activity. Moreover, the synergistic effect of alkali and sulfate can be used to establish an alkali-sulfate composite cementitious system, guiding the application of fly ash as a supplementary cementitious material.