Mechanical and microstructural assessment of sustainable geopolymer concrete incorporating waste glass powder and tire fiber aggregates

This study investigates the combined effects of waste glass powder (WGP) and waste tire fiber aggregate (TFA) on the mechanical and microstructural performance of fly ash (FA)–ground granulated blast furnace slag (GGBS)-based geopolymer concrete (GPC). Previous studies generally examined WGP and TFA separately, therefore, limited information is available regarding their combined influence on geopolymer concrete. The novelty of this research lies in evaluating the synergistic effects of WGP and TFA within a unified experimental framework to enhance sustainability and mechanical performance. In this research, the optimal WGP content was determined using slump and compressive strength tests at 3%, 6%, 9%, and 12% replacement levels. Subsequently, TFA contents of 5%, 10%, 20%, and 30% were incorporated into the optimum FA–GGBS–WGP blend as partial fine aggregate replacement. An alkaline activator consisting of Na 2 SiO 3 and NaOH at a 2:1 ratio with a 12 M concentration was used. The results showed that 9% WGP achieved optimal performance, increasing slump and improving 28-day compressive strength by 6% compared with the control mix. Incorporation of Low TFA contents (5–10%) enhanced both tensile and flexural strengths, with 10% TFA producing the highest improvements of 27% in tensile strength and 30% in flexural strength, due to improved crack-bridging behavior and fiber reinforcement. However, higher TFA contents (20–30%) substantially reduced workability and mechanical properties because of poor compaction and weak interfacial bonding. At 30% TFA, slump decreased by 48%, while compressive, tensile, and flexural strengths decreased by 58%, 31%, and 17%, respectively. SEM, XRD, and FTIR analyses provided evidence of N–A–S–H and C–A–S–H gel formation, which may have contributed to enhanced matrix integrity and microstructural development. Overall, the combined use of waste glass powder and tire fiber aggregate demonstrates strong potential for developing sustainable geopolymer concrete, where low TFA contents improve tensile and flexural performance, while also causing a moderate reduction in compressive strength.