<p>Sand-bentonite mixtures are commonly used as vertical barriers to prevent contamination. However, their impermeability can be affected by dry–wet cycles. To analyze the impact of dry–wet cycles on the impermeability of sand-bentonite mixtures, this study adopts a microscopic perspective. Five experimental groups were established to investigate the effects of moisture content variations and the influence of heavy metal ions (Zn²⁺) on the impermeability of sand-bentonite mixtures. Permeability tests, SEM tests, and CT tests were conducted after 2, 4, 6, and 8 dry–wet cycles for each specimen. SEM images reveals that dry–wet cycles cause bending and loosening of bentonite particles, which weakens their bonding with sand particles and alters the skeletal structure, ultimately resulting in reduced impermeability. Moreover, CT analysis indicates that volumetric porosity, total pore area, total pore volume, average pore radius, and average throat radius are the main factors affecting the impermeability of sand-bentonite mixtures.</p>