<p>Freeze–thaw damage in concrete is closely related to pore-water freezing, yet the critical pore diameter associated with characteristic freezing and the evolution of fractal characteristics in freezing-related pores remain insufficiently quantified. This study aimed to determine the critical diameter of characteristic frozen pores and clarify the evolution of pore structure under freeze–thaw cycling. Glass microspheres with different particle-size ranges were used to simulate concrete pore systems, with paraffin used as a barrier material. Freezing plateau temperatures were measured under subzero-temperature conditions, and equivalent pore diameters were calculated using a hexagonal close-packed model. Concrete specimens with a water-to-binder ratio of 0.32 were then subjected to freeze–thaw cycling, and NMR and SEM/EDS were used to characterize the pore size distribution, pore proportions, microstructure, and fractal characteristics. The results showed that the critical diameter of characteristic frozen pores was 11.49&#xa0;μm. After 100 freeze–thaw cycles, the total porosity increased by 38.12%; the proportion of harmless pores decreased by 7.3%, whereas those of freeze-damaged pores and characteristic frozen pores increased by 69 and 265.42%, respectively. The fractal dimensions of the three pore regions showed quadratic relationships with freeze–thaw cycles, with <i>R</i>² values above 0.92. This study links experimentally measured pore-freezing behavior to pore classification and segmented fractal analysis, providing a pore-scale basis for evaluating freeze–thaw deterioration in concrete.</p>