Interface characteristics of concrete prepared with recycled aggregates by heating

This study investigates the effects of heating temperature and water-to-binder ratio (W/B) on the interfacial transition zones (ITZ) in recycled concrete, with a focus on multiple interfaces, including the interface between old aggregate and new mortar (LG-XJ), the interface between old aggregate and old mortar (LG-LJ), and the interface between old mortar and new mortar (LJ-XJ). The results show that heating temperature significantly impacts the ITZ microstructure and mechanical properties. Lower temperatures (300°C and 500°C) promote dehydration of old mortar, leading to denser ITZ structures with increased interfacial hardness and reduced transition zone widths. Conversely, at 700°C, decomposition of hydration products increases porosity and microcracking, resulting in decreased interfacial hardness and wider transition zones. Similarly, higher W/B ratios lead to insufficient cement hydration, increased porosity, and looser ITZ microstructures, thereby reducing interfacial microhardness and widening the ITZ. SEM analysis further reveals distinct microstructural characteristics among the interfaces, with the LJ-XJ interface exhibiting better density and uniformity compared to the LG-XJ interface after heat treatment. These findings highlight the importance of optimizing heating temperature and W/B ratio to enhance the interfacial properties and overall performance of recycled concrete.