The erosion effect of acid ions in groundwater on the repaired interface leads to recurrent leakage and difficulty in the repair of underground concrete structures. To address this persistent engineering issue, this paper proposes the use of expansive geopolymer concrete for structural leakage repair. Interface grooving technology was employed to apply directional constraints on the material’s expansion effect, prompting the expansive self-stress to exert precise compression on the repaired interface. This effectively enhances the interface compactness and bond performance, thereby significantly improving the impermeability and erosion resistance durability of the repaired interface, and ameliorating the bonding condition between the new and old materials. Experimental research were conducted to investigate the effects and mechanisms of different types and dosages of expansive agents, as well as grooving, on the impermeability and erosion resistance of geopolymer concrete repaired interfaces. The research results show that the optimal impermeability performance of the repaired interface is achieved with a groove size of 10 mm × 10 mm, under which the maximum aggregate size is 20 mm. Therefore, it is recommended that in practical engineering, the groove size for interface treatment be set to half the maximum particle size of the repair material. When 10% HCSA or 5% MgO expansion agent is added to the repair material, the chloride ion penetration resistance of the repaired interface is respectively improved by 31.3% and 30.0%, with both agents exhibiting significant and comparable effects. In terms of sulfate erosion resistance, the incorporation of 10% HCSA results in a corrosion resistance coefficient of 85%, which is significantly superior to that of 5% MgO (78%). Therefore, in practical engineering applications, the type and dosage of expansive agent should be rationally selected based on a comprehensive consideration of cost-effectiveness and performance requirements.