Damage characteristics of sandstone pore structure under freeze-thaw cycles

To study the damage characteristics of sandstone pore structure under freeze-thaw cycles, five rock specimens were selected to conduct 100 freeze-thaw cycles, and the pore structure of sandstone was measured by nuclear magnetic resonance technology (NMR). The mesostructure characteristics such as T2 spectrum distribution and sandstone porosity were obtained under the freeze-thaw effect. According to the distribution of pores, the pore size was divided into three categories: mini-pores, meso-pores, and macro-pores. Meanwhile, the diffusion electric double layer theory was used to analyze the distribution of pore water with different pore sizes. The results show that as the number of freeze-thaw cycles increases, the T2 distribution of NMR shifts to the right and the porosity of the sandstone increases. At the same time, some minerals are dissolved in the pore water under the water-rock effect, which causes the ion concentration to increase in the pore water and results in a large number of secondary pores within the rock. With the increase of pore size, the bound water content in the pores gradually decreases, and the bound water content of small pores is much larger than that of macro-pores. At a low temperature, the free water freezes before the bound water, and the increase of ion concentration of bound water in small pore is lower than that of macro-pores, which leads to a difference in ion concentration. As a result, the water molecules in the small pores migrate to the macro-pores and the damage rate of small pores is much smaller than that of macro-pores. Therefore, the mini-pores continuously deteriorate under the effect of water-rock interaction and the frost heaving pressure; the macro-pores are rapidly developed and expanded under the frost heaving until the macroscopic damage of the rock samples.