Critical role of carbon nanofibers in boosting C-S-H density and fracture toughness in fine recycled aggregate concrete.

This work investigates the influence of carbon nanofibers (CNFs) on densification and toughening mechanisms in fine recycled aggregate concrete (RAC). RAC is concrete produced using recycled concrete aggregates (RCA) produced from crushed old concrete. Although RCA offers a way to reduce the carbon footprint on concrete via raw resource recycling, the porosity and residual cement within RCA often lead to poor durability and performance of RAC. Prior studies have attempted to improve the properties of RAC using nanomaterials, yet a fundamental understanding of molecular mechanisms is lacking. Using novel synthesis methods and advanced nanoscale mechanical characterization methods such as scratch testing and grid nanoindentation, we investigate the influence of CNFs on toughening mechanisms and the distribution of calcium silicate hydrates (C-S-H) within RAC. Specifically, we show that CNFs promote densification in RAC via an increase in high-density and ultra-high-density C-S-H. The combined relative amount of high-density and ultra-high-density C-S-H in RAC increased by 45.3%, 23.8%, and 62.5% with the additions of 0.1 wt%, 0.2 wt%, and 0.5 wt% CNFs, respectively. CNFs decrease the C-S-H gel porosity of RAC. CNFs at 0.1 wt% and 0.5 wt% led to reductions of 6.3% and 7.3%, respectively, whereas 0.2 wt% increased porosity by 1.9%. CNFs enhance the fracture toughness of RAC by bridging hydration products during fracture and refining the pore structure. CNFs enhanced fracture toughness by 4.0% (0.1 wt%), 6.7% (0.2 wt%), and 1.3% (0.5 wt%), compared to unmodified FRCA. These results are important for designing nanomodified RAC with enhanced durability, mechanical properties, lower maintenance, and lower costs. Therefore, the use of CNFs in RAC improves mechanical properties while potentially reducing CO emissions by 31% and lowering lifecycle costs by 2%, making it a more durable and sustainable alternative to traditional concrete.