Experimental study on freeze-thaw characteristics of lignin synergistic slag carbonization for improving loess soil

Abstract

[Objective] Loess is widely distributed in the northwestern regions of China and is characterized by its large pore structure and well-developed vertical joints. Under freeze-thaw cycles, these features make loess highly susceptible to frost heave, thaw settlement, and crack propagation, posing significant threats to the stability and durability of engineering projects in cold regions. To address these challenges, lignin and slag were used to improve the loess through a carbonation process. The freeze-thaw characteristics and microscopic mechanisms of the improved loess were investigated. [Methods] A one-dimensional freeze-thaw cycle model test was conducted to analyze the effects of freeze-thaw cycles systematically under open conditions on the temperature distribution, water migration, frost heave force, frost heave deformation, and mechanical properties of the improved loess. Additionally, scanning electron microscopy(SEM) and X-ray diffraction(XRD) were employed to examine changes in the microstructure and composition of the soil. [Results] The results demonstrated that the improved loess exhibited excellent thermal insulation properties, with temperatures at depths greater than 35 cm consistently remaining above 0℃, preventing freezing. Water migration was primarily confined to the upper 30 cm, and the water replenishment volume was lower than that of remolded loess. The frost heave force of the improved soil was concentrated within the 10 cm depth range, with a maximum frost heave deformation of 20 mm, which was 20% lower than that of remolded loess. Furthermore, the improved loess maintained a high dynamic stress level before and after freeze-thaw cycles. [Conclusion] The findings indicate that the improved loess exhibits superior frost resistance, along with high stiffness and deformation resistance, effectively mitigating structural degradation caused by cyclic loading. The synergistic improvement mechanism of lignin and slag operates on three levels: chemical reactions, physical filling, and structural enhancement. Lignin forms hydrogen bonds with soil particles through its carboxyl and hydroxyl groups, while multivalent ions released by slag complex with lignin to form a three-dimensional network structure within the soil. Additionally, carbonation of slag produces cementitious materials such as calcium carbonate, which fill soil pores, optimize the microstructure, and significantly enhance the soil's density and frost heave resistance. The freeze-thaw properties of loess soil, modified through lignin-slag synergistic carbonation, were systematically investigated. These findings establish fundamental theoretical and technical support for loess soil improvement in cold region engineering applications.

Keywords

lignin / slag / loess / freeze-thaw cycle / freeze-thaw properties / deformation / mechanical properties / cold region engineering