Ash to asset: Evaluating the efficacy of reclaimed fly ash in geopolymer production

The availability of conventional fly ash (CFA) for producing building materials has decreased due to the closure of coal-fired power plants. Meanwhile, landfilled fly ash and the production of ordinary portland cement (OPC) present significant environmental and health threats. Thus, substitutes for CFA and OPC are necessary to meet the growing demand for sustainable and durable building materials. This study investigates the performance of ground granulated blast furnace slag (GGBFS)-based geopolymers that incorporate reclaimed fly ash (RFA) as a sustainable precursor material at replacement levels of 0, 20, 40, 60, 80, and 100 %. The performance of these geopolymers was thoroughly evaluated through physical, mechanical, durability, and microstructural analyses. The findings showed that RFA enhances the fluidity of geopolymers compared to GGBFS due to its spherical particle shape, similar to CFA. However, GGBFS-dominated specimens exhibited superior performance in strength and some durability properties compared to RFA-rich counterparts, likely due to the slower polymerization of RFA. Additionally, geopolymers with RFA demonstrated better resistance to alkali-silica reactivity, freeze-thaw cycles, and high-temperature exposure. Microstructural analysis revealed that the reactivity, amorphous phases, and bonding structure of RFA are like those of CFA. Although pure RFA may not be ideal for producing geopolymers, blending it with GGBFS meets performance standards and helps reduce the burden of landfilled fly ash. Notably, geopolymers with ≤60 % RFA were found to be particularly suitable, balancing performance and sustainability. Using RFA helps addresses the supply-demand gap of CFA, mitigating environmental degradation and reducing reliance on OPC.