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Strong salt expansion and frost heave are induced to make the infrastructure in the salted region damaged with water or salt phase change at low temperature. Laboratory test based on differential scanning calorimetry is used to investigate salt and water phase transformation and their crystallization mechanism in sodium sulfate soils and solutions. During the experimental process, crystallization heat release, crystallization period and supercooling are measured. According to the conservation principle of mass and heat, salt and ice crystallization are separated, and unfrozen water content is calculated at different temperatures. Moreover, variations of unfrozen water content, as well as the supercooling degree of sodium sulfate soils with different size are compared and analyzed based on heterogeneous nucleation theory. The results demonstarte occurance of the ice crystallization before salt crystallization in minor soil samples; however, salt crystallization appears first in pure solution at high concentration. The interval at which the ice and salt crystallization begin decreases as the salt content increases, and the supercooling decreases as the salt content or sample size increases. Additionally, water freezing in small soil samples is more difficult than that in larger soil samples, and the unfrozen water content increases as the salt content or sample size decreases.

Saline soils is porous medium whose thermal conductivity is affected by components and particle shape. However, the particle shapes in geomaterials are various, and the component and its content ar...

Thermal conductivity of soils is significant on the thermal simulations in cold region engineering. Based on the generalized thermal conductivity for geotechnical materials, a thermal conductivity model for saline soil was proposed in this paper. At the micro level, the microstructure composition of sodium sulfate soils in the proposed model was redefined. Macroscopically, the variation of salt crystals and ice crystals with temperature and the relationship between the arrangements of components (i.e., liquid water, ice, soil particles) of saline soil were studied. Shape parameters α, β, τ were adopted to define the volume proportion of needle shape soil particles, spherical shape soil particles and disk shape soil particles, respectively. Besides, the proposed thermal conductivity model considered the heat loss due to the heat radiation in the measurement process. Furthermore, the accuracy of the proposed model was verified by the experimental data. The results showed that the soil particles can be defined as SWCA (i.e., soil particles, water, crystals mixture of salt and ice, air) four-layer concentric structure in the calculation of the thermal conductivity model of saline soil. Shape parameters have significant influence on simulation results. By comparing test data with calculated values, it can be found that at positive temperature, silty clay soil particles are mainly spherical (β = 0.5) and disc-shaped (τ = 0.5), while the sandy soil and soil-rock particles are more similar to the needle shape (α = 1.0). With the decrease of temperature, the particle parameters of silty clay soil change, β decreases to 0, τ increases to 0.75, and α increases to 0.25. However, at negative temperature, the soil particles of sandy soil and loess are still mainly needle-shaped (α = 1.0).

Salt migration and accumulation are the main sources of salt expansion. To study the role of salt transport in soil, laboratory tests were conducted to simulate salt transfer under real conditions. Temperature, water content, salt content and soil displacement of a sodium sulfate soil were measured during the freezing process/freeze-thaw cycles. Meanwhile, Salt concentration was regressed bases on test data to investigate the movement of salt diffusion, in the process, the Pitzer ion model was employed to calculate the freezing point of saline soils to determine the frozen depth. Moreover, the amount of crystallization was estimated by the saturation curve of a sodium sulfate solution and the detected salt content. The results show that salt transfer in soil occurs as a result of numerous physicochemical processes and that the maximum salt transport occurred in the frozen fringe zone in the soil. Salt crystallization increases the effect of salt transfer in the soil unidirectional freezing process. In addition, salt expansion had an accumulative effect, and it increases as the number of freeze-thaw cycles increases. The quantity of salt that is transported increases as the salt content increases.

The unfrozen water content is a result of soil freezing, and its variation is closely related to temperature. To study the change in unfrozen water content, a relationship between water pr...