Study on Microstructural Evolution of Marble Under Cyclic Dynamic Impact Based on NMR

In this paper, the evolution process of microstructures of fine-grained marble and coarse-grained marble under cyclic dynamic impact is studied. The split Hopkins pressure bars (SHPB) are used to apply multiple impacts on rock samples with a constant dynamic stress. And the microstructural changes caused by each impact were detected by a nuclear magnetic resonance (NMR) system. Porosity, transverse relaxation time (T-2) spectral distribution, pore size distribution and magnetic resonance imaging under different dynamic impact cycles were obtained, and the evolution of the T-2 cutoff value (T-2c) of the rock was obtained by combining the centrifugal technique to calculate the permeability. The pores were classified into two categories according to the pore size and the fluidity of the fluid in the pores to study the effect of impact on the pore connectivity. According to the results of X-ray fluorescence (XRF) and X-ray diffractometry (XRD), it is found that the two marbles are the same in mineral composition but different in composition ratios. Due to the difference in particle size of marble and the difference in the number of pores, the magnitudes of microstructure changes of the two marbles are different, but they have a consistent trend: under the cyclic dynamic impact, the porosity of the two marbles gradually decreases, the pore size continues to decrease, the marble framework is compacted, the pore structure is gradually closed, and the permeability is weakened. The weakening of rock permeability is mainly caused by the overall decrease in pore size and loss of pore connectivity.