The fundamental cause of frost heave and salt expansion of saline soil is the water condensation and salt crystallization during the freezing process. Therefore, controlling the water and salt content is crucial to inhibit the expansion behaviors of saline soil. Recently, electroosmosis has been demonstrated to accelerate soil dewatering by driving hydrated cations. However, its efficiency in mitigating the salt-induced freezing damages of saline soil requires further improvement. In this study, a series of comparative experiments were conducted to investigate the synergistic effects of electroosmosis and calcium chloride (CaCl2) on inhibiting the deformation of sodium sulfate saline soil. The results demonstrated that electroosmosis combined with CaCl2 dramatically increased the cumulative drainage volume by improving soil conductivity. Under the external electric field, excess Na+ and SO42- ions migrated towards the cathode and anode, respectively, with a portion being removed from the soil via electroosmotic flow. These processes collectively contributed to a significant reduction in the crystallization-induced deformation of saline soil. Additionally, abundant Ca2+ ions migrated to cathode under the electric force and reacted with OH- ions or soluble silicate to form cementing substances, significantly improving the mechanical strength and freeze-thaw resistance of the soil. Among all electrochemical treatment groups, the soil sample treated with 10 % CaCl2 exhibited optimal performance, with a 71 % increase in drainage volume, a 180 similar to 443 % enhancement in shear strength, and a 65.1 % reduction in freezing deformation. However, excessive addition of CaCl2 resulted in the degradation of soil strength, microstructure, and freeze-thaw resistance.

This study aims to solidify sodium sulfate saline loess by biomineralization of reactive magnesium oxide (MgO) binder. The impact of Na2SO4 concentration on the viability and urease activity of S. pasteurii, the mechanism and products of biomineralization of MgO, and the effectiveness of the biomineralization-MgO binder in solidifying saline loess with varying salt content (1%, 3%, and 5%) were investigated. Results showed that low and moderate concentrations of Na2SO4 favored bacterial proliferation. However, the presence of Na2SO4 inhibited bacterial urease activity, and the inhibition was more significant at higher Na2SO4 concentrations. In addition, low and moderate concentrations of Na2SO4 decreased the specific urease activity, whereas high concentrations of Na2SO4 significantly increased specific urease activity. S. pasteurii was able to use carbonate ions formed by urea hydrolysis for the mineralization of MgO and to form magnesium carbonate minerals dominated by rosettelike dypingite and hydromagnesite crystals. The primary mechanism involves microbial cells and extracellular polymeric substances leading to partial dehydration of Mg2+ ions from the Mg2+-H2O complex and allowing for further association with carbonate anions to from Mg-bearing carbonates. Unconfined compressive strength tests conducted on the saline loess samples after 7 days of curing revealed a significant influence of urea concentration on the strength of the solidified soil. The optimal urea concentration to obtain a better 7-day UCS ranged from 4 mol/L to 5 mol/L. Furthermore, solidified soil with 5% salinity yielded the highest 7-day UCS and soil with 3% salinity exhibited the lowest 7-day UCS at the same urea concentration. XRD and SEM analysis of the solidified soil samples indicated that the formation of magnesium carbonate minerals in the soil matrix by the biomineralization-MgO binder was responsible for the UCS enhancement. The remarkable 7-day UCS of saline loess solidified with biomineralization-MgO binder demonstrates the effectiveness of this material in curing saline loess.

The structural damage caused by salt weathering in loess soil is a crucial factor leading to soil erosion and geological disasters in the Loess Plateau region of China. However, the impact of salt weathering on the structural damage and strength degradation of soil under unidirectional dehumidification conditions remains unclear. To gain a comprehensive understanding of the structural damage process and strength characteristics of soil under salt weathering, this study focuses on Q(2) loess (silt loam, a loose aeolian deposit) in Fugu County, Shaanxi Province. Multiscale observations tests and direct shear tests were conducted on samples with varying sodium sulfate contents. The research findings indicate that at the macroscopic scale, The 3.0 % salt content sample experienced the most severe salt weathering, exhibiting characteristics of powdering and disintegration. Moreover, it exhibited the highest expansion displacement, reaching 22 mm. In addition, all samples go through three stages during the entire salt weathering process, namely budding, growth, and stable stages. At the mesoscopic scale, the displacements and expansions caused by salt crystallization growth on soil particles and pores were captured in real-time. Furthermore, the crystallization behavior of sodium sulfate differs on the surface and near-surface of samples as water content decreases, resulting in four distinct layering structures. At a microscopic scale, salt weathering leads to the formation of aggregates and the generation of numerous expansion pores within samples. Not only that, the shear behavior of samples transitioned from strain-softening to strain-hardening after salt weathering, with the peak strength significantly weakened compared to the residual strength. Additionally, before a salt content of 1.5 %, the cohesion of samples experiences the greatest decline, gradually slowing down thereafter, ultimately decreasing by 16 kPa. However, the decrease in the friction angle is less significant, with only a decrease of 4.8 degrees. In summary, an increase in sodium sulfate content exacerbates the occurrence of salt crystallization-induced expansion and soil drying-induced coagulation phenomena in saline soils, resulting in severe damage to soil structure and strength. This study will provide valuable insights for soil and water conservation as well as disaster prevention in the Loess Plateau region, serving as a crucial reference for future research and engineering practices in the region.