An NMR-based investigation of pore water freezing process in sandstone

Understanding the freezing pattern of pore water is the basis for revealing the mechanisms of frost damage in porous rocks. We employed low-field nuclear magnetic resonance method, aided by high-resolution temperature measurements, to investigate the pore water freezing process of in sandstone. Samples were prepared with nine degrees of initial saturation (20% to 100% at 10% intervals). The results indicate the following: 1) pore water in fully-saturated sandstone consists of bound water, capillary water and bulk water. Bound water forms the majority (76%), followed by capillary water (14%) and bulk water (10%). 2) A significant amount of pore water remains unfrozen at -20 degrees C, which is basically bound water. 3) Supercooling of pore water is distinct in samples with saturation degrees >= 70%. 4) The exotherm of rock temperature declines as the saturation degree decreases, it becomes undetectable in samples with saturation degrees (<)70%. Following further investigation of the variation of each fraction of pore water fully-saturatedduring freezing, we suggest that: 1) The entire freezing process can be broadly divided into four stages: supercooling, repid-freezing stage, stable-freezing and ceasing of freezing. 2) Small pores preferentially freeze before larger ones during the rapid-freezing stage. 3) The initial saturation degree significantly influences the pore water freezing process by determining the fraction of pore water. This study sheds new light on the pore water freezing pattern in sandstone, providing novel ideas for the analysis of frost damage mechanisms in porous rocks.