Alkali-Activated Mortars with Recycled Tyre Rubber Aggregates: A Preliminary Mechanical Study

This preliminary experimental study investigates mortars with partial replacement of natural sand by recycled tyre rubber (RTR) and alkali-activated fly ash as an alternative binder to Portland cement. An experimental programme was carried out on 14 mortar mixes, including cementitious mortars (CM) and alkali-activated mortars (AAM), with 5%, 10% and 20% RTR incorporation and with/without NaOH pre-treatment of rubber (saturated solution for 30 min). Fresh behaviour was assessed through flow and fresh density, while mechanical-related performance was evaluated at 28 and 56 days through compressive and flexural strength, modulus of elasticity and ultrasonic pulse velocity. Physical properties included open porosity, dry density, water absorption by immersion and capillarity. The alkali-activated reference mix achieved higher compressive strengths (46.1–53.0 MPa) than the cement reference (28.4–30.6 MPa) under the adopted mix-design and curing conditions. RTR incorporation reduced stiffness and strength in all specimens, with 20% RTR decreasing flexural strength by 26.6–51.4%, although losses were generally smaller in AAM than in CM. Under the treatment condition investigated, NaOH pre-treatment did not consistently improve mechanical-related performance, except for limited gains at 5% RTR. Overall, AAM with 5–10% RTR showed a better balance between density reduction and mechanical-related performance than CM.