Chloride penetration resistance of ultra-high performance concrete with various multi-walled carbon nanotubes

Ultra-high performance concrete (UHPC) with outstanding chloride penetration resistance (CPR) exhibits excellent prospects for extending the service life of structures in the marine environment. This study investigates the effects of various types and contents of multi-walled carbon nanotube (MWCNT) on the CPR of UHPC, including pristine MWCNT with different sizes, MWCNT with surface modification, and MWCNT with special structures. The results show that 0.25% thick-short pristine MWCNT, 0.5% short hydroxyl functionalized MWCNT, and 0.25% helical MWCNT show more significant enhancement effect, improving the CPR by 74.14%, 72.91%, and 77.17%, respectively. The correlation analysis results suggest a negative linear correlation between the CPR and pore volume of the composites. The C-S-H gels and small capillary pores with a diameter below 50 nm are the significant factors determining the CPR of the composites. This illustrates that the CPR enhancement effects of various types of MWCNT change with the refinement of pore structure of UHPC. The enhancing mechanisms can be attributed to the nano-core effects of MWCNT, reducing the CH crystal size and primary cracks, inhibiting the growth and propagation of microcracks, refining the pore structure, optimizing the compactness of the cement paste matrix and interfacial transition zone, thus reducing the channels for chloride ion diffusion, while increasing the constrictivity and tortuosity of the channels.