Evolution in sandstone pore structures with freeze-thaw cycling and interpretation of damage mechanisms in saturated porous rocks

Measuring the evolution of pore structures is the most straightforward approach to assessing frost damage in porous rocks. Systematic and fine measurement of pore structures is critical to the interpretation of frost damage mechanisms. We conducted experiments where sandstone samples were subjected to 50 freeze-thaw cycles and their T2 spectra measured after each cycle via the nuclear magnetic resonance (NMR) method. This allowed the evolution in pore size distribution, porosity (pore volume), permeability (pore connectivity), tortuosity (pore curvature), and pore size uniformity coefficient (pore non-uniformity) to be comprehensively estimated. The results show that: (1) variation in pore structures is mainly caused by the propagation and expansion of nanopores and micropores and generation of new nanopores. (2) The porosity of sandstone has a three-stage trend of increasing-stabilizing- increasing, while tortuosity exhibits an opposite trend of decreasing-stabilizing-decreasing with freeze-thaw cycling. (3) The permeability and surface roughness of sandstone increase with freeze-thaw cycling, while the pore size uniformity coefficient decreases linearly. Based on the above results and scanning electron microscopic observations of the pore structure of sandstone, we found that: (1) the pore structure of layered sandstone can be simplified as a main stem type. (2) In this pore structure, there are two main damage processes: (a) where water in secondary cracks migrates to stem cracks and freezes, causing propagation and expansion of stem cracks; and (b) where unfrozen water in secondary cracks freezes in-situ, leading to the propagation, expansion and generation of secondary cracks. The above two damage processes coexist and alternately play pivotal roles during repeated frost activity, resulting in multistage changes to the porosity, permeability, and tortuosity of sandstone.