Fractal Characteristics of Pore Structure of Coal Gangue Aggregate Concrete under Freeze–Thaw Environment

In the accumulation of coal gangue, a substantial amount of heavy metal elements is released, contaminating the environment. However, crushing and screening coal gangue for concrete preparation presents a green and more sustainable alternative. The practical application of coal gangue concrete heavily relies on its frost resistance, which is intricately linked to its pore structure. To investigate the impact of freeze–thaw cycles on frost resistance and pore structure, silica fume-modified coal gangue aggregate concrete was employed. The evolution of the pore structure during freeze–thaw cycles was examined using nuclear magnetic resonance testing. Fractal theory was employed to evaluate the complexity of the pore structure, revealing the freeze–thaw damage mechanism of silica fume-modified coal gangue aggregate concrete. The findings indicate that silica fume effectively enhances the number of freeze–thaw cycles that coal gangue aggregate concrete can endure, increasing the endurance from 75 to 150 cycles. Additionally, it was observed that silica fume increases the integral area of the T2 spectrum of concrete, elevates the proportion of gel pores, small pores, and medium pores while reducing the proportion of large pores. Notably, the gel pores of coal gangue concrete do not exhibit fractal characteristics. However, the fractal dimensions of small pores, mesopores, and macropores initially increase and then decrease with an increasing silica fume content, peaking at 10%. This trend closely aligns with the relative dynamic elastic modulus. During the freeze–thaw process, the proportion of large pores in coal gangue concrete gradually increases, causing a decrease in the fractal dimension of each pore type and a smoothing of the pore structure. Ultimately, this leads to freeze–thaw damage in the concrete.