The Microscopic Pore Structure Change and Its Correction with the Macroscopic Physicomechanical Properties of Sandstones after Freeze–Thaw Cycles

Freeze–thaw damage of rocks causes many engineering problems in cold regions. During the cyclic freeze–thaw process, the frost-heaving pressure, which is produced by 9% volumetric expansion of pore water, will drive the expansion and growth of microscopic pores. However, the change law of microscopic pore structure and how it affects the macroscopic properties of rocks under freeze–thaw are still not clear. In this study, the microscopic pore structure of five sandstones after different freeze–thaw cycles were continuously measured by mercury intrusion porosimetry (MIP). It shows that the microscopic pore structure of these sandstones has obvious fractal characteristics under freeze–thaw conditions. The fractal dimension has a gradual reduction with increasing freeze–thaw cycles. It illustrates that the distribution of the pore size is more and more uniform and that the complexity of the pore structure decreases. In addition, a bimodal exponential model can be well used to characterize the size distribution of nano- and micropores for these sandstones. The characteristic radii of nano- and micropores calculated by the bimodal exponential model have an obvious increasing trend with increasing freeze–thaw cycles. In addition, although the pore radii in these sandstones increase due to the freeze–thaw action, the volumetric fractions of nanopores and micropores are not changed remarkably. By using the least squares regression method, the porosity, P-wave velocity, and uniaxial compressive strength (UCS) are highly correlated with the fractal dimensions and characteristic pore radii. The P-wave velocity and UCS decrease with the increase of characteristic pore radius or reduction of fractal dimension. This study has filled the gap between microscopic pore structure change and deterioration of macroscopic physicomechanical properties under freeze–thaw, and it also provides a better understanding of the macro–micro frost damage mechanism for rocks.