From Pore Expansion to Throat Extension: Effects of Freezing Temperature on Microstructural Evolution and Dynamic Strength Decay in Sandstone

Repeated freeze–thaw (F–T) action, together with dynamic disturbances, can progressively weaken rock masses in cold regions. However, how freezing temperature affects the relationship between microstructural evolution and dynamic strength decay remains insufficiently quantified. This study investigated yellow sandstone subjected to F–T cycles at freezing temperatures of 0, −3, −5, and −20 °C. CT-based 3D reconstruction and Split Hopkinson pressure bar (SHPB) tests were combined with grey relational analysis (GRA) to characterize pore-structure evolution, dynamic strength decay, and their relationship. The results indicated that lower freezing temperatures promoted increases in pore connectivity and structural complexity. After 60 F–T cycles at −20 °C, connected porosity increased from 11.15% to 18.67%, while the ratio of connected porosity to total porosity increased from 51.1% to 85.7%. At an impact pressure of 0.3 MPa, the dynamic strength after 60 cycles decreased by 9.51%, 20.9%, 38.1%, and 61.5% at 0, −3, −5, and −20 °C, respectively. Among the examined microstructural parameters, average throat length had the highest overall grey relational grade (0.821), suggesting that throat development is closely associated with dynamic strength decay. Lower freezing temperatures enhanced pore-ice expansion and unfrozen-water migration, promoting pore enlargement, throat extension, and crack connection. These results quantitatively link pore-network evolution to dynamic strength decay under different freezing temperatures, providing a microstructural basis for assessing the dynamic deterioration of sandstone in cold regions.