Traditional inorganic curing agents have long been utilized to improve the mechanical properties of loess for engineering applications in regions abundant with loess. However, the unique climatic conditions in northwest China, marked by low temperatures and substantial temperature variations, make improved loess prone to structural degradation, which can result in brittle failure and subsequent engineering challenges. This study combines the principles of reinforced soil mechanics with conventional soil improvement techniques to conduct unconfined compressive (UC) tests, freeze-thaw (F-T) cycle tests, triaxial shear tests, and microstructure analyses under various initial conditions using cement and polypropylene fiber composite improved loess (CFIL) as the test material. The research aimed to examine the mechanical properties and the internal damage mechanisms of CFIL subjected to F-T environments. Results indicated that an increase in the length and content of polypropylene fibers significantly improved the unconfined compressive strength (UCS) of CFIL. This enhancement initially showed a rapid rate of improvement before experiencing a subsequent decrease. The addition of fibers significantly mitigates the degree of strength attenuation in the specimens subjected to F-T cycles compared to cementimproved loess. This effect is attributed to the overlapping and interweaving of polypropylene fibers in CFIL, which, along with loess particles embedded in cement hydrates enveloped by the fibers form a robust skeleton that enhances both strength and deformation resistance. Based on the variations in strength across different fiber lengths (Lf), fiber contents (Cf), and cement contents (Cc) before and after F-T cycles, the optimal Lf is identified as 12 mm, while the optimal Cf and Cc are 0.3 % and 2 %, respectively. The stress-strain curve of CFIL displays strain-softening behavior, although this behavior is notably less pronounced than in cement-improved loess. Furthermore, the initial tangent modulus and triaxial strength of CFIL decrease nonlinearly as the number of F-T cycles (NF-T) increases, with the rate of decrease gradually slowing over time. A decrease in freezing temperature (T) exacerbates the deterioration of the mechanical properties of improved soil. Microstructural test results indicate that as the NF-T increases, the porosity (n) of CFIL rises, accompanied by an increase in the proportion of macropores and mesopores, while the proportion of micropores and small pores diminishes. Utilizing the binary medium theory, the F-T damage mechanism of CFIL was investigated, and a damage equation that captures the dual impacts of F-T cycles and loading was formulated. Building on the Lemaitre equivalent strain principle and the nonuniform medium homogenization theory, a binary medium model for CFIL considering F-T cycles was developed. The proposed constitutive model effectively characterizes the stress-strain relationship of CFIL under F-T conditions, as demonstrated by the comparison between experimental results and calculated data.

In order to study the improvement effect of the CG-2 curing agent and cement on loess, a series of physical and mechanical property tests and microstructure tests were carried out on loess improved with different dosages of curing agent and cement to study the physical and mechanical properties, durability and microscopic pore characteristics of the CG-2 curing agent and cement-improved loess. The results show that the unconfined compressive strength of improved loess increases gradually with the increase in curing agent and cement dosage, and the higher the compaction degree and the longer the curing age, the higher the unconfined compressive strength. In the case of the same cement content, the higher the dosage of curing agent, the more the unconfined compressive strength of improved loess increases. Under the condition of reaching the same unconfined compressive strength, the addition of curing agent can significantly reduce the amount of cement. The more the content of cement and curing agent, the less the unconfined compressive strength decreases after a certain number of freeze-thaw cycles, and the higher the dry-wet cycles index after a certain number of dry-wet cycles, indicating that the addition of curing agent can significantly improve the ability of the sample to resist freeze-thaw cycles and dry-wet cycles. According to the microscopic test results, it is found that the addition of curing agent can reduce the porosity of soil particles, change the contact and arrangement mode between soil particles, and enhance the agglomeration and cementation characteristics between soil particles, and obviously improve the physical and mechanical properties of soil. The research results can provide new ideas and methods for the improvement technology of loess.