Effect of Hybrid Surface Treatment of Tire Rubber Aggregates on the microstructure and mechanical properties of Standardized Mortars

The incorporation of tire rubber aggregates in cement-based mortars reduces the consumption of natural sand but typically leads to substantial strength losses because of the weak and porous interfacial transition zone (ITZ) formed around untreated rubber. This study develops a hybrid mineral-epoxy surface treatment designed to overcome this limitation by forming a continuous composite shell composed of sand, cement, silica fume and epoxy resin. Microstructural analyses show that the treatment yields a uniformly coated rubber surface, a denser particle morphology and a markedly refined ITZ: the interfacial layer around untreated rubber is wide and highly porous (≈14–15 µm), whereas the hybrid coating reduces it to a compact band of about 1–2 µm. These refinements translate into consistent mechanical gains across all curing ages, with flexural strength increasing by 11–27% and compressive strength by 23–76%, depending on substitution level and age. In parallel with these mechanical and microstructural improvements, the method offers a favourable cost-performance balance, particularly thanks to the possibility of reducing resin consumption through optimisation of the adhesive film. The hybrid coating concept significantly strengthens the feasibility of producing high-performance and economically viable rubberized mortars and, ultimately, rubberized concretes. • A novel one-step hybrid surface treatment for tire rubber aggregates is developed. • The coating combines a thin epoxy adhesive film with a mineral shell composed of cement, silica fume and fine sand. • SEM–EDS analyses confirm a densified, continuous and chemically integrated interfacial transition zone (ITZ). • Hybrid-treated mortars exhibit compressive strength recoveries up to 76% and flexural strength improvements up to 27%, depending on curing age and rubber content. • A clear microstructure–mechanical performance correlation is established, linking ITZ refinement to strength recovery. • The treatment provides a significantly improved performance-to-cost balance, with stable mechanical gains across all substitution levels. • The approach is scalable and suitable for extension to concrete.