Abstract:The backfill formed by the solidification of the filling slurry contains a large number of pores, and the influence of pores on the mechanical properties of the backfill is not clear. In order to explore the correlation mechanism between the pore structure and the uniaxial compressive strength (UCS) of the backfill, sodium dodecyl sulfate (SDS) was used to adjust the pore content of the backfill. Low-field nuclear magnetic resonance (NMR) technology was used to test the transverse relaxation time of the internal pores, the UCS was tested by a press, and the microscopic image of the damaged surface pore structure was obtained by scanning electron microscope (SEM). The pore size distribution was analyzed, and the fractal characteristics of the pore size were studied. The results showed that the total pores in the backfill gradually decrease with the increase of curing age, and the higher the SDS content, the closer the total pores of the sample at 14 and 28 d. The proportion of multi-damaging pores decrease significantly with the increase of the curing age, and it is negatively correlated with the UCS of the sample. The proportions of non-damaging pores, less-damaging pores, and damaging pores change little during each age. When the SDS content is 0.2% of the quality of the cement used, the UCS of the sample is the highest. The number of pores with pore diameters in the range of 5~130 nm has obvious fractal characteristics, while the number of pores with pore sizes outside this range do not have fractal characteristics. The ettringite in the hydration products is clustered, and the clustered ettringite squeezes the pore space due to growth, development, and extension. This is the microscopic reason why the total number of pores and the number of multi-damaging pores decrease with the increase of curing age. The research results can provide theoretical support for a more comprehensive understanding of the strength of the backfill.