Mechanical Properties and Freeze–Thaw Cycling Degradation of Loess Improved with an Ionic Curing Agent and Cement Composite

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 (<i>C</i>_c), F1 dosage (<i>C</i>_F), number of freeze-thaw cycles (<i>N</i>_F-T), and confining pressure (<i>σ</i>₃) 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 <i>C</i>_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 (<i>ε</i>_f), thereby achieving simultaneous improvements in strength and ductility. An appropriate mix proportion was identified as <i>C</i>_F = 0.2 L/m³ and <i>C</i>_c = 6%. After 7 d curing, FCIL exhibited a UCS of 1.35 MPa, a cohesion (<i>c</i>) of 205 kPa, an internal friction angle (<i>φ</i>) of 36.2°, and <i>ε</i>_f 1.8 times that of loess improved with <i>C</i>_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 <i>C</i>_c and <i>σ</i>₃ increase, triaxial peak strength (<i>q</i>_max) and secant modulus (<i>E</i>₅₀) increase significantly. Compared with natural loess (NL), FCIL shows a markedly lower porosity (<i>n</i>), 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.