Abstract: To investigate the influence of nano-metakaolin on the mechanical properties of recycled concrete, the experiment used the dosage of nano-metakaolin (NMK) (3%, 5%, 7%) and the replacement rate of recycled aggregate (30%, 50%, 100%) as variables to prepare nano-recycled concrete and splitting tensile and flexural strength tests were conducted. By combining optical microscopy, scanning electron microscopy (SEM), specific surface area and pore size analyzer, the microstructure and pore size distribution characteristics of the failure surface and interface transition zone (ITZ) were studied, revealing the influence of NMK on the splitting tensile and flexural strength of recycled concrete. The experimental results show that the addition of NMK has an improvement effect on the splitting tensile and flexural strength of recycled concrete with different substitution rates. The analysis of specific surface area and pore size shows that NMK filled recycled concrete has some large and mesoporous pores inside, which can reduce the total pore volume and specific surface area, refine the pore structure of recycled concrete, and improve the structural density. The microstructure analysis under the microscope shows that with the increase of NMK content, the C—S—H gel produced by hydration reaction is gradually increasing, the pores and cracks in the internal structure are reduced, and the internal microstructure of recycled concrete is more dense.

Key words: nano-metakaolin, recycled concrete, flexural strength, splitting tensile strength