Multifractal Characteristics of Sandstone Microstructure after Freeze-Thaw Damage

In order to study the damage and deterioration mechanism of freezing and thawing of rocks in cold regions, the deterioration law of internal pore structure, the change of surface microstructure and the multifractal characteristics of sandstone after different freeze-thaw (FT) cycles (FT0, FT20, FT40, FT60, FT80, FT100) are discussed based on nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM). Study results show that: The number of small pores in the freeze-thaw sandstone gradually increases and accounts for 99% of the newly added pores. When the freeze-thaw exceeds 80 times, the initiation rate of small pores exceeds the expansion rate, and the distribution of small pores changes from gradually uniform to non-uniform. The macropores in the freeze-thaw sandstone have fractal characteristics, and the overall NMR fractal dimension decreases with the increase of freeze-thaw cycles. After the action of freeze-thaw, the crystal surface inside the sandstone becomes rough, the intragranular cracks increase, the grains gradually fall off. The average lacunarity of the local structure of sandstone continues to increase after freeze-thaw 20 times, and the growth rate reaches the maximum at freeze-thaw 80-100 cycles, which is 0.1%. The SEM images of freeze-thaw sandstone show good multifractal characteristics. The weaker the rock, the more pronounced the multifractal features.