An experimental study of the influence of loess salinity on combined water film thickness based on NMR and nitrogen adsorption technique

Bound water film is a kind of binding water, which has an important influence on the permeability and mechanical properties of deep layer loess. Its thickness is an extremely critical factor, and the saturation of the binding water and specific surface area are particularly important parameters in the calculation of water film thickness. In this paper, a new testing approaches for calculating parameters of water film thickness is attempted. Based on the analysis of freezing point of the bound and free water by the NMR technology, a reliable time limit of free and bound water relaxation is obtained, and the bound water saturation in pore fluid samples with different NaCl contents is calculated. The test results show that (1) the combined water and free water have different thermodynamic properties. The low-temperature freezing-thawing test shows that the freezing point of loess internal bound water is about -3.3 ℃, while the freezing point of free water is higher. Meanwhile, the relaxation time of the two in the T2 relaxation curve is also different, and the relaxation time limit (T2 cut-off value) is 1.65 ms. (2) The thickness of water film in loess is negatively correlated with the salt content in pore solution of soil. With the increasing salt content, the content of bound water decreases and the thickness of bound water film also decreases, leading to the decrease of the shear strength of the bound water and the increase of the permeability of the deep layer of loess. In this paper, the Nuclear Magnetic Resonance technology combined with nitrogen adsorption method is used for the first time to study the relationship between salt content and the film thickness of bound water. This method has no damage to the samples, and the test is faster and more convenient with higher reliability, providing a new idea for the microscopic study of water occurrence in saline soil.