Study on Improving Physical–Mechanical Properties and Frost Resistance of Straw–Mortar Composite Wall Materials by Pretreatment

In recent years, China has increased the material utilization of crop straw, and the strength of straw–mortar composite wall materials is low, which limits their large-scale utilization. Pretreatment can improve the physico-mechanical and frost resistance properties of straw–mortar composite wall materials. In this study, the Box–Behnken design in the Design-Expert software was used to design and carry out a three-factor and three-level interactive experiment and freeze–thaw cycle experiment with the straw content, pretreatment time, and reagent concentration as influencing factors, and the compressive strength, water absorption rate, and dry density as response values. The results showed that the impact of each factor on the response value, from high to low, was the straw content, pre-preparation time, and reagent concentration. When the straw content was 10%, the preparation time was 5 min, and the reagent concentration was 5%, the physical and mechanical properties of the straw–mortar composite wall material were the best. At the same time, the compressive strength was 6.52 MPa, the water absorption rate was 17.7%, and the dry density was 1396.33 kg·m −3 , which was 67% higher, 31% lower, and 37% higher than that of the untreated straw–mortar composite wall materials. After the freeze–thaw cycle, the mass loss rate of the composite materials was less than 5%, which met the requirements of the frost resistance specifications; the strength loss rate of the composite materials varied between 19.7% and 27.8%, although some test blocks did not meet the requirements of less than 25% in the specification. The compressive strength was greatly improved compared with the untreated composite materials in the related research, and the water absorption rate was about 25% lower than that of the untreated straw–mortar composite wall materials. Pretreatment significantly improved the physico-mechanical and frost resistance properties of the straw–mortar composite wall materials.