The freezing temperature of soil is an important index to judge whether it is in a frozen state or not, and plays a key role in distinguishing salt expansion from frost heave. Freezing experiments on sulfate saline soil and its ion solution indicate that the initial freezing temperature of the soil is mainly determined by the properties of the ion solution. The effects of saline soil types and saline solution concentrations on the freezing temperature are also investigated. An equation to calculate the freezing temperature of the sulfate salt solution is deduced from the Pitzer ion model and its validity is verified by the experimental results. Under the assumption that the soil freezing temperature is determined by the freezing temperature of the corresponding ion solution and the energy of the ice-liquid interface, a precise relation between the soil freezing temperature and the radius of the ice-liquid interface is obtained and a general equation to determine the freezing temperature of the silty clay of the Qinghai-Tibet Plateau is proposed.

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This paper presents a computational model of the freezing point of soil and the nucleation rate of ice based on the soil volume for saline soils. Physicochemical methods and crystallization theory are used to explore the macroscopic mechanism of water freezing induced by ice crystallization, and the influence of water undercooling is discussed with regard to different soil volumes and salt contents. Growth sites, shapes, and sizes of ice crystals are determined by an assumption of heterogeneous nucleation, and the contact angle is introduced to represent a wide range of soils. The relationship between the initial contact angle and the salt content is analyzed quantitatively through an indoor cooling test and thermal difference analysis. Moreover, the van Genuchten model is applied to provide an equation for the unfrozen water content in salty soils, and the probability of ice formation is introduced to investigate the variation of contact angle during the soil freezing process. This study shows that the initial radius of an ice crystal decreases as the salt concentration decreases. The temperature of ice nucleation is determined mainly by the contact angle of ice crystals, which decreases as the soil volume increases and results in decreased supercooling.