Experimental Study on the Physical Properties and Freeze–Thaw Resistance of Rice Straw–Mortar Composites

With the increasing demand for the resource utilization of agricultural waste and the development of sustainable building materials, rice straw–mortar composites have attracted considerable attention owing to their renewability and environmental friendliness. In this study, a three-factor and three-level regression analysis experiment was carried out using Design-Expert software to obtain the optimal physical properties and mix ratio of rice straw–mortar composite materials. With an optimal mix ratio of 15% gypsum, 20 mm straw length, and 15% straw content, the composite achieved a compressive strength of 3.06 MPa, a dry density of 1.340 g·cm−3, and a water absorption of 20.8%. A freeze–thaw cycling test of 15 cycles from −20 °C to 20 °C resulted in a strength loss of 32.2–47.2%, a mass loss of 2.1–4.5%, and water absorption of 21.8–37.2%. The results suggest that rice straw–mortar composites may be suitable for non-load-bearing wall applications under freeze–thaw exposure conditions.