Investigation of the bond strength and microstructure of the interfacial transition zone between cement paste and aggregate modified by Bayer red mud

• Propose a procedure of RM modifying aggregate to improve mechanical properties significantly. • Compressive and flexural strength of concrete prepared with AMBRM increased by over 10.00 %. • Wettability theory used to reveal formation mechanism of ITZ microscopic structure. • Harmful micropores with >100 nm in the concrete prepared with AMBRM were significantly fewer. In this study, a method is proposed for modifying aggregate with Bayer red mud (RM), and the bond strength and microstructure of the interfacial transition zone (ITZ) in the concrete prepared using the modified aggregate is determined. Compared to concrete prepared using natural basalt aggregate, concrete prepared with RM-modified basalt aggregate aged for 7 and 28 days had a 25.08 % and 21.75 %, respectively, higher compressive strength and a 39.53 % and 15.30 %, respectively, flexural strength. Compared to concrete prepared using natural limestone aggregate, the compressive and flexural strengths of concrete prepared with RM-modified limestone aggregate increased by over 10.00 % and 20 % respectively, after aging of both 7 and 28 days. The RM had a higher wettability to cement paste than basalt and limestone, implying that cement paste on the surface of RM-modified aggregate had a correspondingly stronger microflow and filling capacity. In addition, concrete prepared with the RM-modified aggregate had a low voidage, a compact ITZ structure and strong interfacial adhesion, resulting in considerably enhanced mechanical properties. This study provides novel applications for RM that can be widely used in building materials and waste reduction and a new method for improving the mechanical properties of concrete.