Mechanical Response and Interfacial Deterioration of Red Mud-Based Synthetic Sand Mortar Under Freeze–Thaw Cycles

Freeze–thaw resistance is important for the use of red mud-based synthetic sand in cold regions. Mortars containing synthetic, standard, and fluvial sand were compared at a nominal water-to-cement ratio of 0.50 and cement-to-sand mass ratio of 1:3. Strengths were measured at 3, 7, 14, and 28 d. After 28 d of curing, specimens underwent 25 freeze–thaw cycles, each comprising 12 h at −20 °C and 12 h in water at 20 ± 1 °C. Digital image correlation (DIC) during loading at 0.2 kN/s and scanning electron microscopy (SEM) assessed strain localization and interfacial morphology. At 28 d, compressive and flexural strengths were 45.3 ± 0.6 and 9.9 ± 0.6 MPa for synthetic-sand mortar, 44.0 ± 0.6 and 8.9 ± 0.8 MPa for standard-sand mortar, and 42.5 ± 1.0 and 7.3 ± 0.8 MPa for fluvial-sand mortar, respectively. After cycling, synthetic sand mortar exhibited mass, compressive strength, and flexural strength losses of 0.2%, 6.1%, and 8.6%, respectively, lower than those of both reference mortars. DIC showed increased strain localization, while SEM suggested less interfacial deterioration in synthetic sand mortar. Its rough surface and possible mechanical interlocking may contribute to the observed resistance under these test conditions.