Research on Capillary Water Absorption Characteristics of Modified Recycled Concrete under Different Freeze–Thaw Environments

Recycled coarse aggregate is processed through the second crushing, which causes some internal damage, resulting in its physical indicators being far worse than natural coarse aggregate; its durability is relatively poor, and in the northern region, the soil contains a large number of acidic salt ions from the erosion of concrete, resulting in a decline in its durability. In this test, concrete was made from the single and composite immersion of recycled coarse aggregate using 5% water glass and 8% silane solution and subjected to a rapid freeze–thaw test in water, 3.5% NaCl solution, and 5% Na2SO4 solution, followed by a capillary water absorption test. The study was conducted to test the durability of recycled concrete, establish the initial capillary water absorption prediction model under freeze–thaw in different media, and analyze the internal structure of the RAC group after freeze–thaw using SEM. The test results showed that the composite-modified water absorption decreased the most, which can effectively improve the durability of recycled concrete, and the chloride salt caused the greatest erosion of recycled concrete and had the least clear water. The predictive model has high accuracy and can be used as a reference for capillary water absorption experiments on recycled concrete.