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.

Magnesium oxychloride cement (MOC) is an environment-friendly cement often used for stabilizing soft soils because of its exceptional mechanical properties. In this study, the influence of curing temperature on the strength development of MOC-solidified clay is explored, considering different MgO/MgCl2 molar ratios. Different tests were carried out to study the corresponding effects. The results show that the effect of curing temperature on the strength of MOC-solidified clay differs greatly from that of cement-solidified soil. Increasing the curing temperature leads to strength reduction, whereas decreasing the curing temperature increases the corresponding strength. Scanning electron microscope (SEM) and X-ray diffraction (XRD) analyses indicate that the variation in type and amount of hydration products of the solidified soil account for the strength development difference between MOC-solidified and cement-solidified soils. A model based on the experimental results is proposed to characterize the relationship between strength development and curing time. The strength influence factor (eta T) and the strength expedite factor (K) were introduced to demonstrate the relationship between strength development at a specific curing temperature as well as at room temperature.