## 1 Introduction
Additionally, when dealt with appropriately, it can be transformed into a harmless substance used for recycled aggregate reinforcement, reducing the mining of natural aggregates and protecting the environment. [PERSON] et al. [19] tested silica fume and fly-ash-doped concrete and found that the capillary water absorption of concrete increased more after freeze-thaw cycles, and [PERSON] et al. [20] found that freeze-thaw cycles accelerated the destruction of concrete. In the northern region, many soils contain a large number of Cl-, SO4-, and other salt ions due to the huge temperature difference between day and night, so the concrete is not only subjected to freezing and thawing damage but is also subjected to the erosion of salt ions, which greatly shortens the service life of the building. [PERSON] et al. [21] found that the penetration rate of chloride ions was accelerated after freeze-thaw cycles, which exacerbated the damage inside the concrete. [PERSON] et al. [22] found that there was a decrease in the freeze-thaw durability of demolition construction concrete in de-icing salts. [PERSON] et al. [23] performed freeze-thaw cycle tests after immersing the specimens in salt water and found that the specimens survived for a shorter period of time. [PERSON] et al. [24] performed freeze-thaw experiments by placing three different types of concrete in water and 5% Na2 SO4 and found that sulfate accelerated the freeze-thaw damage of concrete. [PERSON] et al. [25] found that sulfate decreases the frost resistance of cement mortar, and [PERSON] et al. [26] found that the compressive strength of concrete decreased by 22% when it was immersed in sulfate solution for 365 days. [PERSON] et al. [27] made concrete that was cured in different solutions, and it was found that the strength of the concrete decreased more in 5% sodium sulfate and 5% sodium chloride solutions, and the internal reinforcement was damaged by corrosion. In summary, recycled aggregates have more defects, the natural environment in cold regions is more severe, and the application of recycled concrete in cold regions needs to be considered. In recent years, more research has been conducted on aggregate modification, water freezing, and salt freezing, and the effect is more obvious. However, there is less research on single modification and composite modification combined with water freezing and salt freezing. The ability to absorb capillary water is one of the important indexes for evaluating the durability of concrete [28; 29]. It is closely related to the internal pores and cracks of concrete, and the freeze-thaw cycle has a large influence on the capillary water absorption ability of concrete. Therefore, the objective of this study is to investigate the durability of modified recycled concrete under different freeze-thaw environments. In this paper, 5% water glass solution, 8% silane solution, and 5% water glass solution + 8% silane solution were used to modify the recycled aggregate. The test specimens were placed in water, 3.5% sodium chloride solution, and 5% sodium sulfate solution for the freeze-thaw cycle test, and then a capillary water absorption test was conducted to measure the water absorption of the specimens within 7 days, and electron microscope experiments were carried out on the recycled concrete after freezing and thawing to analyze the durability of the concrete both superficially and microscopically.