Mix design optimization via response surface methodology and flexural performance of beams in solid alkali-activated fly ash-based concrete

The use of industrial solid waste in concrete represents a promising green technology that reduces cement’s environmental impact and promotes resource recycling. However, the structural application of high-volume fly ash concrete has been hindered by an insufficient understanding of mix design interactions and the flexural performance of structural members. This study employed response surface methodology (RSM) to optimize the mix design of solid alkali-activated fly ash-based concrete (AFC), using granulated blast furnace slag (GBFS), silica fume (SF), and NaOH as factors, with workability and mechanical property as the target responses. The optimized mixture was further subjected to microstructural characterization and beam flexural testing. The results indicate that the optimized mix (11.7% GBFS, 6.9% SF, 1.67% NaOH) exhibited good workability and mechanical property, all with prediction errors below 5%. The interaction between GBFS and NaOH was the most influential factor. Microstructural analysis revealed the formation of N-A-S-H and C-(A)-S-H gels, enhancing density through micro-filling and improving the interfacial transition zone. Flexural tests on reinforced AFC beams (AFCBs) revealed typical flexural failure modes. AFCB exhibited 5%–8% higher load capacity but greater deformation than ordinary Portland cement beams due to a lower elastic modulus. Increasing the strength grade had limited effect on flexural capacity. Furthermore, cracking progressed more slowly and controllably in AFCBs and a maximum crack width of 0.4 mm is proposed. Finally, existing design equations for conventional reinforced concrete were demonstrated to remain applicable to AFCBs. This study demonstrates that AFCB exhibits excellent structural performance, supporting the broader application of RSM-optimized AFC in sustainable construction.