Synergistic Impact of Bio-Based and Waste-Derived Contents on the Mechanical and Thermal Performance of Alkali-Activated Slag Mortars

The need to enhance both the mechanical and the thermal insulation performance of alkali-activated mortars has stimulated interest in natural fiber-based reinforcement strategies, particularly for sustainability-driven design and low-carbon construction. This study explores the combined effect of natural kapok fiber, petroleum coke (PC) and waste tire rubber powder (WTRP) on the mechanical, thermal, microstructural, mineralogical, and pore structure characteristics of alkali-activated slag-based mortars. While the beneficial effects of each of these bio-based and waste-derived materials have been widely reported, little attention has been paid to their combined use and the potential for them to work together in a way that is more effective than the sum of their parts. For this purpose, 20 different mortar mixtures were produced, incorporating kapok fibers at ratios of 0%, 0.5%, 0.75% and 1%. The beneficial effect of WTRP was investigated for the case of a 3% cement additive where natural sand was substituted by PC at ratios of 0%, 10% and 20%. The results indicated that the synergistic interaction enabled the development of a sustainable mortar with enhanced thermal insulation performance of up to 37% while maintaining flexural performance compared to the control case. Microstructural, mineralogical, pore structure and thermal analyses revealed that the bio-based and waste-derived contents significantly influenced the pore network, phase evolution and fiber–matrix interactions. Overall, the results highlight the potential of the proposed sustainable material system to produce mortars with enhanced thermal insulation, sufficient mechanical performance, and lowered environmental footprint.