Characterisation and evolution of the full size range of pores and fractures in rocks under freeze-thaw conditions using nuclear magnetic resonance and three-dimensional X-ray microscopy

Mine slopes in high-altitude and cold regions are susceptible to damage and degradation under long-term freeze-thaw (FT) conditions, which threatens the stability of geotechnical engineering operations. The evolution of pore and fracture structures in rock masses under FT conditions crucially influences the physical and mechanical properties of rocks. However, critical information is often ignored when testing pore structures using conventional methods, owing to the limited applicable size range, which leads to an inaccurate characterisation of the pore structure. Techniques such as nuclear magnetic resonance (NMR) and three-dimensional X-ray microscopy (3D-XRM) provide improved characterisation accuracy of pores smaller than 50 μm (micropores, mesopores, macropores) and the 3D distribution of extra-large pores (>50 μm) and microfractures, respectively. This combined approach allows quantitative characterisation of the full range of fractures in rocks. On this basis, we investigated the evolution of pores and fractures in different types of rocks (granite, limestone, sandstone) under FT conditions. The results show that with increasing FT cycle, the number of mesopores increases in granite, more micropores appear and macropores are generated in limestone, and the number of micropores and mesopores generally increases in sandstone. For extra-large pores and microfractures, fractal dimensions in 2D slices, volumes, specific surface areas and volumetric porosities of pores and fractures exhibit consistent trends. Sandstone parameters are considerably larger than those of limestone and granite in the initial state, and the parameters of limestone and granite are similar. With increasing FT cycle numbers, the parameters of all three rock types increase with sandstone > limestone > granite. The mean equivalent diameters of pores and fractures gradually decrease in granite, increase linearly in limestone and increase slightly in sandstone.