Study on the Dynamic Properties of Cement–Coal Gangue Powder modified Silty Sand under Freeze–Thaw Cycles

The northeastern region of China, a typical seasonally frozen zone, contains silty sand subgrade that are subjected to long-term dynamic loads from vehicles and are highly susceptible to deformation and road distress, such as upheaval, under freeze–thaw (F–T) cycles. To enhance the dynamic properties and F–T resistance of silty sand subgrade, cement and coal gangue powder (CGP) were employed as modifiers. Through dynamic triaxial test and scanning electron microscopy (SEM) test, investigated the dynamic properties of modified silty sand with varying contents of cement and CGP, the dynamic behavior and micro-structural evolution under different F–T cycles, confining pressures, and loading cycles. As well as the correlation between micro-structural features and dynamic parameters was studied by grey relational analysis. The results revealed that the deviatoric stress amplitude of the modified silty sand increased significantly with higher cement content, while initially increased and then decreased with rising CGP content. Compared to silty sand, the modified soil maintained higher deviatoric stress amplitudes and dynamic elastic modulus across varying F–T cycles, loading cycles, and confining pressures. A significant correlation was identified between micro-structural characteristics and dynamic parameters. F–T cycles caused considerable micro-structural degradation in silty sand, whereas the modified soil exhibited less structural variation. The incorporation of cement and CGP enhanced inter-particle cementation, filled pore spaces, and improved particle arrangement and pore distribution, thereby increasing structural stability and F–T resistance. Notably, the optimal ratio for cement–CGP modified silty sand was determined to be 2% cement and 8% CGP.