Fiber‐reinforced fly ash geopolymers: Effect of fiber type and curing duration on mechanical strength, weight loss, and microstructure

Abstract This study investigates the effect of curing duration on the mechanical properties and weight loss behavior of fly ash‐based geopolymer composites (GPC) reinforced with 1% Carbon (C), 1% Kevlar (K) fibers, and 1% hybrid (C + K) fibers. The GPCs were synthesized using a 10 M NaOH and Na 2 SiO 3 alkaline activator at a fixed Na 2 SiO 3 /NaOH ratio of 3. Curing periods of 3, 7, 14, and 28 days were evaluated. The results showed that for compressive strength, the hybrid composite (C + K) reached a maximum strength of 18.22 MPa at 14 days, outperforming 1% C (14.75 MPa at 7 days) but lower than 1% K (21.39 MPa at 7 days). Unreinforced geopolymer (UGP) showed a continuous development in strength, reaching 14.75 MPa at 28 days. Both 1% C and 1% K peaked on the 7th day, with subsequent strength decline likely due to fiber degradation or interfacial microcracking. The hybrid composite showed peak strength at 14 days, followed by a decrease. Flexural strength exhibited a similar trend, with the hybrid composite showing 3.05 MPa at 14 days, lower than 1% C (3.62 MPa at 7 days) and 1% Kevlar (4.27 MPa at 7 days). Weight loss analysis showed that 1% C experienced the highest loss (4.66%) on the 7th day, while UGP exhibited the most (8.31%) by day 28. The hybrid sample had the lowest loss (7.35%). SEM analysis revealed enhanced fiber‐matrix bonding, particularly with Kevlar. These findings support the use of fiber‐reinforced GPCs as a sustainable alternative to conventional materials for construction applications.