Study on the compressive strength and microstructure of fly ash-based geopolymer recycled aggregate concrete

Development of compressive strength and microstructure of fly ash-based geopolymer concrete at different curing stages. Orthogonal tests were employed to investigate the effects of NaOH solution concentration (8/9/10 mol/L), pre-treatment methods for recycled aggregate (water-washed, soaked in NaOH or Na2SiO3 solution), and the replacement rate of recycled aggregate (0/50%/100%) on concrete compressive strength at various curing stages, and to identify the optimal mix ratio. Additionally, microscopic tests were conducted on concrete samples to analyse changes in microstructure, including mineralogical composition, evolution of characteristic functional groups, and the degree of hydration of fly ash. The results showed that the optimum compressive strength was obtained when the concentration was taken as 10 mol/L, the pre-treatment method was NaOH solution infiltration, and the substitution rate was 50%, and the optimum level was A3B2C2. The bonding mechanisms primarily arise from the physicochemical interactions between the geopolymer binder and aggregates. An appropriate concentration of alkaline activators facilitates the complete dissolution of precursor materials, resulting in a sufficient quantity of Si-O-Al bonds. Moreover, alkali residue from the NaOH solution pre-treatment of recycled aggregates can partially offset alkali deficiency that may occur in the later stages of the reaction, thereby promoting the continued hydration of fly ash.