Sustainable enhancement of fly ash-based geopolymers: Impact of Alkali thermal activation and particle size on green production

Addressing the challenges posed by the slow reactivity of fly ash (FA) with alkali at ambient temperatures, this study delves into the enhancement of FA-based geopolymers through alkali thermal activation (ATA). The ATA process, conducted at 550°C for 1 h, significantly increases the availability of reactive silica and alumina, which are essential for geopolymerization. A critical focus of the research is the influence of FA particle size on ATA’s efficacy and the resultant mechanical properties of the geopolymers. Findings reveal that the ATA process facilitates the rapid dissolution of the vitreous phase in FA. This leads to a sequential release of silica and alumina, which is pivotal for the geopolymer’s matrix development. Notably, geopolymers synthesized from finely milled FA, post-ATA, demonstrate a marked increase in compressive strength, escalating from 30.51 MPa to an impressive 38.46 MPa. The study meticulously delineates geopolymerization into four distinct stages—initial dissolution, depolymerization, geopolycondensation and gelation, and final diffusion, with the initial dissolution and final diffusion stages being paramount in defining the reaction kinetics and the ultimate strength of the geopolymer. This enhanced understanding paves the way for optimizing FA utilization in geopolymers, promising a more sustainable and efficient pathway for producing construction materials with superior mechanical properties.