Evolution characteristics of unfrozen water content and damage for saturated sandstone during freezing–thawing cycle process based on the in-situ nuclear magnetic resonance

The accurate determination of unfrozen water content is significant to evaluating the freezing process of pore water and to revealing the freezing damage mechanism. In this study, an in-situ low filed nuclear magnetic resonance (LF-NMR) testing system equipped with a low temperature thermal cycle system were designed to conduct freezing–thawing cycle tests. The pore water freezing process of three sandstone samples (Coarse-grained sandstone, Medium-grained sandstone, Fine-grained sandstone) was studied by using the LF-NMR technology. The results indicate that the free water content of the three sandstone samples decreases sharply as the temperature decrease, the free water signal intensity and peak region decrease significantly from 0℃ to -5 ℃, whereas bound water that exists in small pores needs a much lower temperatures to freezing. The larger the equivalent average pore size, the faster the water–ice conversion rate. The more movable water the sample contains, the greater the frost heaving force generated inside the sample. At the temperature range of 0 ~ -10 ℃, the freezing-heaving strain increasing sharply, and gradually become gentle as the temperature declining. Coarse-grained sandstone with the highest porosity has the largest freezing-heaving strain, while the Fine-grained sandstone with the lowest porosity has the smallest freezing-heaving strain. The freezing damage of Coarse-grained sandstone is more serious than the other two sandstone. P-wave velocities and scanning electron microscope (SEM) test also indicate that Coarse-grained sandstone has more serious damage than Fine-grained sandstone subjected to freezing–thawing cycle.