Enhancing FRP-concrete interfacial bonding for structural retrofitting via nano-Al2O3-modified alkali-activated binders: Performance, mechanisms and predictive model

Abstract

This study systematically investigates the influence of nano-Al₂O₃ (NA) incorporation on interfacial bonding characteristics of alkali-activated metakaolin-blast furnace slag composite binders. The failure mode, interfacial bond strength and microstructural evolution of specimens with varying NA dosages and curing ages are analyzed. The results show that 5% NA incorporation remarkably enhances interfacial bond strength of 28 d specimen by 29.5% compared with control specimen. Microscopic observations reveal that the incorporation of 5% NA optimizes the chemically bonded state of reaction products, promotes the generation of highly polymerized C-(A)-S-H and N-A-S-H gels, and improves the overall densification of the microstructure. Conversely, excessive NA blending induces particle agglomeration, reduces reactivity and filling efficiency, and increases surface roughness, porosity and microcracks. As curing age extends, the geopolymeric reaction proceeds more completely, and the polymerization degree and structural orderliness of gel phases are gradually improved. The predictive accuracy of existing models for interfacial bond strength of composite binders is assessed, and underlying sources of deviation are discussed. Additionally, a modified interfacial bond strength model is proposed. The predicted results align well with the measured data, with a mean prediction-to-test ratio of 0.981 and a corresponding mean squared deviation of 0.163. This study provides a theoretical basis and technical support for the rational design of nanomaterial-modified composite binders with improved interfacial performance, thereby promoting the innovation and advancement of green building materials.