Buettner Nathanial, Iyacu Gass, Dal Poggetto Giovanni, Akono Ange-Therese
Civil and Environmental Engineering, Northwestern University, Evanston, 60208, USA.
Civil, Construction, and Environmental Engineering, North Carolina State University, Raleigh, 27606, USA.
Sci Rep. 2025 Jul 29;15(1):27544. doi: 10.1038/s41598-025-11530-9.
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.
本研究探讨了碳纳米纤维（CNFs）对细粒再生骨料混凝土（RAC）致密化和增韧机制的影响。RAC是使用由破碎旧混凝土制成的再生混凝土骨料（RCA）生产的混凝土。尽管RCA提供了一种通过原材料回收来减少混凝土碳足迹的方法，但RCA中的孔隙率和残余水泥往往导致RAC的耐久性和性能较差。先前的研究试图使用纳米材料改善RAC的性能，但缺乏对分子机制的基本理解。利用新颖的合成方法和先进的纳米级力学表征方法，如划痕测试和网格纳米压痕，我们研究了CNFs对RAC增韧机制以及硅酸钙水合物（C-S-H）分布的影响。具体而言，我们表明CNFs通过增加高密度和超高密度C-S-H促进RAC的致密化。随着分别添加0.1 wt%、0.2 wt%和0.5 wt%的CNFs，RAC中高密度和超高密度C-S-H的总相对含量分别增加了45.3%、23.8%和62.5%。CNFs降低了RAC的C-S-H凝胶孔隙率。0.1 wt%和0.5 wt%的CNFs分别导致孔隙率降低6.3%和7.3%，而0.2 wt%使孔隙率增加1.9%。CNFs通过在断裂过程中桥接水化产物和细化孔隙结构来提高RAC的断裂韧性。与未改性的FRCA相比，CNFs使断裂韧性提高了4.0%（0.1 wt%）、6.7%（0.2 wt%）和1.3%（0.5 wt%）。这些结果对于设计具有更高耐久性、力学性能、更低维护成本和更低成本的纳米改性RAC非常重要。因此，在RAC中使用CNFs可改善力学性能，同时有可能将二氧化碳排放量降低31%，并将生命周期成本降低2%，使其成为传统混凝土更耐用、更可持续的替代品。
