Mechanical properties and structural behavior of optimized fly ash-based geopolymer concrete: Experimental and numerical investigation

This study examines the mechanical properties and flexural behavior of fly ash–based geopolymer concrete (GPC) for sustainable structural applications. A two-factor Central Composite Rotatable Design (CCRD) was employed to optimize NaOH molarity and the activator-to-fly ash ratio, yielding a reliable regression model for compressive strength (R² = 0.94). Heat-cured GPC (60 °C for 24 h) achieved 28-day compressive strengths of 40–60 MPa with elastic moduli of 27–32 GPa. The elastic modulus increased with compressive strength and was slightly lower than OPC predictions, while the flexural tensile strength was 7–27% higher than that of OPC at equivalent compressive strength. Nine under-reinforced GPC beams tested under four-point bending exhibited typical three-stage flexural behavior and ductile failure after steel yielding, with ultimate load increasing by up to 56% as the reinforcement ratio increased. Finite element simulations using the Concrete Damaged Plasticity model accurately reproduced the experimental response, with deviations within 5–10%. The results demonstrate that fly ash–based geopolymer concrete can provide structural performance comparable to OPC while offering environmental benefits.