A comprehensive review of the new-to-old interface formed between sustainable alkali-activated materials (AAMs) and existing concrete substrate: Bonding strength and mechanism, enhancement strategies, and long-term durability

The aging of global concrete infrastructure has precipitated an urgent demand for efficient repair and rehabilitation strategies. Alkali-activated materials (AAMs) have emerged as a promising alternative to Ordinary Portland Cement (OPC) binders. However, the effectiveness of repaired systems is predominantly governed by the integrity of the new-to-old interface between new repair materials and old existing concrete, which remains the most vulnerable region in repaired composites. This review systematically examines recent progress and advances in the use of AAMs as sustainable repair binders. Interfacial bonding behavior is systematically examined at multiple scales, from macroscopic bonding strength and failure modes to microscale products, microstructures and micro-mechanical properties. Reasons for vulnerability mechanisms of the new-to-old interface are discussed and the Overlay Transition Zone (OTZ) is quantitatively defined based on products evolution and quantitative phase distribution. Strategies to enhance interfacial performance are summarized from the aspects of AAMs mix optimization and substrate surface treatments. Furthermore, the durability of composites repaired using AAMs, including to high temperatures and chloride attack, is reviewed to examine their long-term service life. Finally, comparative analyses of costs and embodied carbon are used to highlight the potential of AAMs as eco-friendly and cost-effective solutions for sustainable infrastructure rehabilitation. Future research still requires application-oriented and intelligent design of AAMs, long-term and large-scale performance evaluation, and use of advanced interfacial characterization techniques to allow assessment and optimization of the bonding performance of AAMs.