To address the engineering problems of high cement content, high brittleness, and weak frost resistance of cement-improved loess in the seasonal frozen soil area of Northwest China, F1 ion curing agent (F1) and cement composite improved loess (FCIL) were used in this paper. Through unconfined compressive (UC) strength tests, consolidated undrained (CU) triaxial shear tests, and microscopic pore characteristics analysis, the mechanical properties, freeze–thaw cycle deterioration law, and microscopic pore structure of FCIL were studied. The effects of cement content (C_c), F1 dosage (C_F), number of freeze–thaw cycles (N_F-T), and confining pressure (σ₃) on the strength, deformation behavior, and pore characteristics of FCIL were analyzed. The synergistic improvement mechanism of FCIL, as well as the freeze–thaw damage mechanism, was elucidated. The results show that C_c is the primary factor controlling the strength of improved loess. The incorporation of F1 can further increase UCS and markedly enhance the failure strain (ε_f), thereby achieving simultaneous improvements in strength and ductility. An appropriate mix proportion was identified as C_F = 0.2 L/m³ and C_c = 6%. After 7 d curing, FCIL exhibited a UCS of 1.35 MPa, a cohesion (c) of 205 kPa, an internal friction angle (φ) of 36.2°, and ε_f 1.8 times that of loess improved with C_c = 6% cement alone. CU triaxial shear tests indicate that, under all tested conditions, the stress–strain responses of FCIL exhibit σ3-sensitive strain-softening behavior. As C_c and σ₃ increase, triaxial peak strength (q_max) and secant modulus (E₅₀) increase significantly. Compared with natural loess (NL), FCIL shows a markedly lower porosity (n), a substantial increase in the proportion of micropores, and reductions in medium and small pores. After multiple freeze–thaw cycles, the evolution of the pore structure is effectively restrained. This indicates that the combined use of F1 and cement promotes the formation of a dense layered stacking structure, significantly improves the microscopic pore-size distribution, and enhances the mechanical performance of loess under freeze–thaw environments.

Under the influence of water and external loads, the microstructure of loess changes, leading to a loss of strength and ultimately frequent engineering accidents. The fractal characteristics and influencing factors of the microscopic pore structure of unsaturated loess were investigated using nuclear magnetic resonance (NMR) test and fractal theories. The results show that the microscopic pore structure of the sample at different dry densities is dominated by small and medium pores, and increasing the dry density can significantly reduce the medium pores and large pores of the soil. However, the effect on the micro and small pores was small, resulting in the fractal dimension of the medium pores (D₃), and the fractal dimension of the large pores (D₄) increased with increasing dry density, whereas the fractal dimension of the small pores (D₂) did not change significantly. The moisture content significantly affected the medium pores and large pores of the soil, decreasing the fractal dimension of small pores (D₂) and large pores (D₄) with increasing moisture content and the fractal dimension of medium pores (D₃) with increasing moisture content. The medium pore structure of the soil was more sensitive to the change in moisture content when the dry density was smaller, and increasing the dry density effectively reduced the effect of moisture content on the large pore structure of the soil.

Loess has bad engineering characteristics due to its special forming environment, particle size distribution and mineral composition, so it needs to be reinforced to meet the engineering construction demand. As part of the study of the physical and mechanical properties of Formula1 ionic stabilizer (F1) reinforced loess, the variation of basic physical parameters, strength characteristics, and microstructure characteristics was examined by using test methods such as unconfined compressive strength tests, unconsolidated undrained triaxial tests, scanning electron microscope tests, nuclear magnetic resonance tests, and X-ray diffraction tests. F1 reduces water film thickness, reduces optimal water content, increases maximum dry density, and improves soil hydraulic properties and compaction properties. The UCS of both F1NL and F1BL increased rapidly with increasing F1 dosage up to an optimum value of 0.3 L/m ³ , beyond which it gradually decreased. The optimal dosage of F1 reinforced loess was 0.3 L/m ³ , and the maintenance duration has a relatively small impact on UCS. When the dosage of F1 was 0.3 L/m ³ , compared with the loess, average shape coefficients of F1NL and F1BL pores increase, the friction angle increases by 18.9% and 28.6%, the cohesion increases by 41.6% and 147.4%, and the pore area decreases by 17.2% and 81.32%. The results show that F1 can reduce intergranular pores, increase soil compactness, and enhance cohesion and bonding between soil particles. As a result of the action of F1, a series of chemical reactions take place between Albite and Dolomite in the loess, which results in the increase of calcite, chlorite and white mica content in the reinforced loess and enhances the cementation between soil particles.