Chloride Ion Penetration Resistance of Matrix and Interfacial Transition Zone of Multi-Walled Carbon Nanotube-Reinforced Concrete

In this paper, the methods of rapid chloride migration (RCM) coefficient and immersion were used to investigate the chloride penetration resistance of multi-walled carbon nanotubes (MWCNTs)-reinforced concrete. The chloride diffusion depth on the longitudinal section of concrete specimens was measured to calculate the chloride diffusion coefficient. When the content of MWCNTs reaches 0.15% (by mass of concrete), the diffusion depth and diffusion coefficient of 28-day concrete's chloride ion reduces by 25.7% and 19.1%, respectively. In the four different ages of natural immersion, the chloride ion concentrations inside MWCNTs-reinforced concrete are always lower than the control group. The results indicate that the concentration of free chloride ions and the chloride ion diffusion coefficient at concrete each depth decrease with the MWCNTs content, and incorporating MWCNTs enhances the concrete's chloride ion penetration resistance. Moreover, the chloride ion diffusion capacity of the concrete interface transition zone (ITZ) is higher than that of the matrix, while the interface effect range is getting smaller with the MWCNTs content. The microhardness test results indicate that the microhardness of the interface is much lower than that of the matrix, but increases after MWCNTs penetration. When the MWCNTs content reaches 0.15%, the microhardness of the interface increases by 13.2%. Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) tests were used to investigate the microscopic enhancement mechanism of concrete chloride ion penetration resistance by MWCNTs. The analysis shows that MWCNTs have certain bridging and filling effects and contribute to the improvement of the microstructure and chloride ion penetration resistance by inhibiting the crack diffusion and compacting the pore structure of concrete.