Reinforcing biomimetic cementitious composites by aligned hierarchical carbon fibers

Cementitious composites reinforced by carbon-based fibers of varied length scales still face limitations of a single dimension, anisotropic alignment, smooth surface, or poor dispersion. In this study, we developed a bioinspired, hierarchical reinforcing structure that enables multiscale coupling of mechanical properties and directional fiber alignment, significantly improving the load-bearing capacity of cementitious composite. The aligned hierarchical reinforcing structure was synthesized by grafting CNTs onto microscale CFs-COOH via an esterification reaction and a nozzle-injected technique. Parameters for synthesizing the CNTs grafted aligned CF (ACF-CNTs) were optimized considering oxidation solutions, grafting techniques, CNTs geometry, and CNTs concentrations. The chemical bond between CF and CNTs was examined by Laser Raman Spectrometer (LRS), Fourier Transform Infrared Spectrometer (FTIR), and X-ray photoelectron spectroscopy (XPS), respectively. Results indicate that CNTs are uniformly and densely grafted onto CF via ester linkage, forming a hierarchical reinforcing structure. Mechanical measurements show that 1 wt% ACF-CNTs results in a maximum flexural strength of 34.76 MPa and a maximum improvement of 297.12 % (compared to plain cement paste) in the biomimetic cementitious composites. Analysis based on fiber alignment, interface transition zone, and failure mode of CF-CNTs/CNTs suggests that multiscale coupling of mechanical properties and directional fiber alignment are the main reinforcing mechanisms.