The treated recycled aggregates effects on workability, mechanical properties and microstructure of ultra-high performance concrete Co-reinforced with nano-silica and steel fibers

To promote the application of recycled aggregates (RA) in more high-performance engineering projects, it is necessary to scientifically modify and repair the RA, which is originally damaged by mechanical crushing and contains cracks and pores. This study investigates the influence of modified RA on the workability, mechanical properties, and microstructure of ultra-high-performance concrete (UHPC). Fresh state characteristic tests (slump and flowability), hardened mechanical property tests (compressive, axial tensile, elastic modulus, and four-point bending), and analysis of microstructure (SEM images and nuclear magnetic resonance (NMR) were performed on specimens with different modifications of RA. The results showed that after three different modifications (hydrochloric acid washing, cement slurry coating, and chemical strengthening) of RA, the apparent density increased and water absorption decreased. This resulted in improved workability and fluidity of the specimens containing modified RA, with increased compressive strength and elastic modulus by 15%, as well as enhanced bending performance indicators (flexural strength, toughness, and residual strength), while the axial tensile strength was not lower than that of specimens containing natural aggregates. Additionally, SEM analysis examined the pores between the cement matrix, interfacial transition zone (ITZ), and fibers, and how they were improved. The specimens with modified RA had a complete and continuous ITZ, with more thorough hydration reactions and better interface bonding. NMR analysis revealed a distinct "three-peak" structure in the T2 spectrum and showed the evolution of four types of pores in the microstructure, including an increase in the number of small pores and a decrease in the number of large and medium pores and microcracks. These research findings provide new insights into the application of modified RA and serve as a reference for the use of a suitable amount of modified RA in crack-resistant or other high-demand engineering projects.