Experimental study on mechanical properties and microstructure of lime-lignin-amended loess under freeze-thaw cycles

The widely distributed loess in Yili region is prone to structural damage under freeze-thaw cycles, which seriously threatens the stability of engineering in cold regions. The combination of lime and lignin can improve the frost resistance of soils, but the optimal dosage of the two and the synergistic mechanism under freeze-thaw conditions are not clear in the existing studies. In order to address the problem of insufficient frost durability of traditional single-doped hardener-improved loess, it is necessary to find out the doping threshold of lime-lignin composite doping system and its performance evolution in freeze-thaw cycles. The macroscopic and microscopic properties of loess improved with different lime-lignin dosages (0%:0%, 2%:1%, 4%:2%, 6%:3%, 8%:4%) were systematically analyzed by conducting 0, 1, 5, 10, 15 freeze-thaw cycle tests, combined with triaxial shear tests (perimeter pressures of 100, 200, and 300 kPa) and scanning electron microscope observations. The study focuses on the influence of lime-lignin dosing and the number of freeze-thaw times on the mechanical indexes such as shear strength parameter and modulus of elasticity of the improved soils, and simultaneously reveals the evolution characteristics of hydration products and pore structure. The research results show that: under the same circumferential pressure, the shear strength of improved loess shows a tendency of increasing and then decreasing with the increase of lime-lignin doping, and the highest destructive strength of the specimen is found when the doping is 2%:1%. With the increase of the number of freeze-thaw cycles, the destructive strength of each dosage of improved loess showed a first decrease and then tend to stabilize, while the shear strength index showed a decreasing trend. Under the action of freeze-thaw cycles, the modulus of elasticity of 2%:1% lime-lignin-amended loess was higher than that of plain soil, which had a better improvement effect on loess. In terms of microstructure, lime and lignin generated a more stable structure by reacting in the soil, which reduced the porosity between the soil and lignin could also fill the internal pores of the soil, resulting in a more stable overall structure of the soil. The results of this study can provide a reference basis for the application of lime and lignin to improve loess in actual projects.