Evaluation of Waste Tire Rubber as an Alternative Aggregate in Geopolymer Mortars

This study evaluates the potential of using Granulated Waste Tire Rubber (GWTR) as an alternative raw material in geopolymer mortars an eco-friendly, low-carbon alternative to traditional cement-based systems. The research investigates the synergistic effect of industrial by-products, such as slag (from ferrochrome plants) and fly ash (from thermal power plants), combined with varying proportions of GWTR (1/4, 1/3, and 1/2 by volume). A total of 22 mixtures were prepared using diverse binder pastes, including pure cement, slag-based, and fly ash-based geopolymer systems, alongside their cement-substituted derivatives. The mechanical and physical performances were assessed through compressive strength, flexural strength, and Ultrasonic Pulse Velocity (UPV) tests at 3, 7, 28, and 180 days, complemented by SEM microstructural analyses. The findings indicate that while GWTR significantly reduces the mechanical properties of pure cement matrices, this negative impact is substantially mitigated in geopolymer mortars supplemented with 5–10% cement. Mixtures containing 1/4 GWTR with 90–95% slag or fly ash (M6, M7, M15, M16) yielded the most successful results in terms of both strength and sustainability, specifically, mixtures M7 and M16 because the hybrid binder synergy effectively compensated for the rubber-induced porosity, ensuring a denser matrix and structural-grade compressive strength alongside high sustainability. Significant decreases in performance were observed at higher GWTR ratios, particularly at the 1/2 level. Overall, the study demonstrates that integrating GWTR into optimized geopolymer systems offers a viable pathway for the valorization of environmental waste and minimizing the ecological footprint of the construction industry.