Bond between alkali-activated repair mortars and OPC concrete substrate: Strength calculation, failure mode statistics and mechanisms

To assess the feasibility of using alkali-activated mortar as a repair material for existing damaged concrete, the interfacial bonding properties between alkali-activated slag/fly ash repair mortar and substrate concrete are systematically studied, focusing on the effects of slag content (40%, 60%, 80%, 100%), slant shear angle (30, 45, 60) and curing age (1–28 d) on interfacial slant shear bonding, flexural tensile properties. The results show that with the increase of slag content, the slant shear strength and flexural tensile strength of the interface are significantly improved, and the enhancement effect is the most prominent in the early age (1~3 d). The failure mode gradually changes from mixed failure of the interface and repair mortar to matrix failure. When the slag content is 80%, the main failure mode changes from interface+repair mortar failure to interface+substrate concrete failure. In the range of slant shear angle from 30° to 60°, the bond strength is increased by about 77%-116% on average, and the energy absorption capacity is enhanced. The damaged area extends from the interface area to the repair mortar side, and the damaged area is enlarged. Based on the Mohr-Coulomb criterion, the internal friction angle and cohesion of the interface during slant shear are calculated. Microscopic analysis shows that increasing slag content promotes the formation of dense C-A-S-H gel in the interface transition zone, significantly improves the microhardness and the enrichment of elements (Al, Si, Mg), thereby strengthening interfacial chemical bonding and mechanical interlocking. Compared with cement repair mortar, alkali-activated repair mortar shows better interfacial bonding performance at all ages, showing great potential as a green high-performance repair material.