The Effect of Natural Pozzolanic Coated Waste Tire Aggregates on the Mechanical, Transport and Durability Properties of Fiber-Reinforced and One-Part Hybrid Geopolymer Composites

This study examined the effects of coating waste tire aggregates (WTAs) with pumice, perlite, or diatomite, combined with polypropylene (PP) fiber addition, on the fresh, mechanical, transport, and durability properties of one-part hybrid geopolymer composites. Sixteen mixtures were produced using a Taguchi L16 design with a binder system of fly ash, CEM II/B-S cement, and sodium metasilicate powder. Coating type, WTA ratio, and PP fiber content were the key performance factors. Pumice coating performed best overall by improving the interfacial transition zone: 28-day compressive strength reached 15.5 MPa and flexural strength 1.60 MPa, while porosity and capillary water absorption decreased significantly. Among the studied WTA levels, 10% WTA yielded the most positive direct responses in compressive strength, flexural strength, toughness, and capillary water absorption, whereas higher contents weakened matrix continuity. The effect of PP fiber was response-dependent: 0.5% fiber maximized compressive strength and durability-related responses, while 2% fiber gave the greatest flexural strength and toughness; no single dosage was universally optimal. The pumice-coated series was also the most stable under high temperature, freeze–thaw, MgSO4, and H2SO4 exposure. Overall, waste tire aggregates can be technically incorporated into one-part hybrid geopolymer composites; a dedicated life-cycle assessment is nevertheless required to quantify the net environmental benefit.