The leakage problem of underground concrete structures has long been a chronic issue in the engineering field. Especially, acid root ions in groundwater can severely erode the repair interface, making it difficult to maintain the leakage repair effect for a long time. This is mainly due to the insufficient density of the concrete structure, which provides channels for harmful ions. This problem is particularly prominent at the repair interface after repair. Because the repair interface is relatively weak. In this regard, this paper adopts expansive geosynthetic concrete for leakage repair. Meanwhile, the interface slotting technology is utilized to constrain the expansion, causing the self-stress of expansion to exert a directional squeezing effect on the repair interface, achieving a densification effect. This effect can effectively reduce the porosity of the repair interface, increase its density, and thereby enhance its impermeability and erosion resistance. The influence laws and mechanisms of different types and dosages of expansive agents as well as grooving on the impermeability and erosion resistance of the geopolymer concrete repair interface were studied through experiments. The test results show that the impermeability performance of the repaired interface reaches the best when the slotting treatment size is 10 mm×10 mm, and the corresponding maximum particle size of the aggregate is 20 mm. Therefore, in actual engineering, the slotting size of the interface treatment process should be half of the maximum particle size of the repair material. When 10%HCSA or 5%MgO expansive agent is added to the repair material, the resistance to chloride ion penetration at the repair interface increases by 31.3% and 30% respectively. The effects of the two are significant and similar. However, in terms of resistance to sulfate erosion, the effect of 10%HCSA (corrosion resistance coefficient 85%) is significantly better than that of 5%MgO (corrosion resistance coefficient 78%). Therefore, in actual engineering, the cost-effectiveness and performance requirements should be comprehensively considered to rationally select the type and dosage of the expansive agent.