Microscopic pore water freezing and thawing of saline silty clay under confining pressures using NMR

Unfrozen water content during freeze-thaw cycles is crucial to determining the engineering properties of frozen soils. Previous research primarily focused on the unfrozen water content without considering overburden pressure, and the variation of unfrozen water in microscopic pores of varying sizes under confining pressure was rarely investigated. This study introduces a new custom-designed sample module that allows simultaneous temperature and pressure control during Nuclear Magnetic Resonance (NMR) tests. We present unfrozen water content in pores of various sizes obtained via NMR from freezing and thawing of non-saline and saline silty clay (1 % NaCl) specimens and analyze the effects of temperature and confining pressure on the microscopic pore water freezing and thawing. Furthermore, the total unfrozen water content during a full freeze-thaw cycle and pore water freezing temperatures at selected pore sizes are presented. The results demonstrate that the water content in microscopic pores of various sizes decreases as the temperature drops, and decreasing temperature leads to reductions in the dominant pore size and the corresponding pore water content. Confining pressure significantly influences the microscopic water content distribution in microscopic pores of varying sizes. Increased pressure generally results in higher pore water content in pores of various sizes at the same temperature. However, such an effect is negligible for pores smaller than the critical pore size (i.e., about 3 nm for the study soil) due to the adsorption between soil particles and the bound water layer. Significant freeze-thaw hysteresis is observed in the unfrozen water content in microscopic pores and the total unfrozen water content. Spatially, the hysteresis is the most pronounced at the dominant pore size. In terms of temperature, it peaks near the initial freezing temperature and weakens as the temperature drops. An empirical model is proposed to predict the total unfrozen water content during freezing and thawing by accounting for the confining pressure effects. Furthermore, the pore water freezing temperatures observed from this study agree well with those predicted by a theoretical equation until the pore radius falls below 15 nm. Results from this study help understand the freezing and thawing of water in microscopic pores of saline silty clay.