The Effect of Carbon Nanofibers on the Microstructure, Chemistry, and Pore Structure of Concrete Made with Fine Recycled Concrete Aggregates

Recycled aggregate concrete (RAC) is produced using recycled concrete aggregates (RCAs) obtained from crushed old concrete. Although RCAs offer a sustainable alternative to natural aggregates, the poor durability and mechanical performance of RAC limit its widespread application. This study investigated the enhancement of RAC's durability and performance through the incorporation of carbon nanofibers (CNFs). A novel processing method was developed to prepare high-slump CNF-modified RAC, and its chemistry, pore structure, and microstructure were analyzed using backscattered scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray powder diffraction (XRD), and mercury intrusion porosimetry (MIP). The results showed that CNFs significantly reduced the porosity and permeability, with a decrease in the porosity by 9.0 wt.% and a decrease in the water permeability by 39.3% at an optimal CNF dosage of 0.5% by weight. Furthermore, CNFs promoted the formation of calcium hydroxide and enhanced the densification of the calcium silicate hydrate (C-S-H) matrix, leading to improved resistance against environmental stressors. These findings provide a critical pathway for designing sustainable, high-durability RAC for structural applications.