Understanding the effect of non-carbon-based nanoinclusions on chloride penetration resistance and chloride binding capacity of ultra-high performance cementitious composite

Abstract

Ultra-high performance cementitious composite (UHPCC) with high durability has been widely employed to prevent the chloride corrosion of reinforced concrete structures exposed to the marine environment. Adding nanoinclusions is expected to fundamentally eliminate the defects (i.e., pores and microcracks) caused by the formulation principle of UHPCC, thereby reducing connected pathways for chloride diffusion. Furthermore, the diffusion resistance of chloride ions largely depends on the chloride binding capacity (CBC) of cement-based composites. Regrettably, the effects of non-carbon-based nanoinclusions on chloride penetration resistance (CPR) and CBC of UHPCC still lack comparison and analysis. This study investigated the effect of non-carbon-based nanoinclusions on the CPR and CBC as well as microstructures of UHPCC, including nano-SiO2 (NS), nano-ZrO2 (NZ), nano-TiO2 (NT), and boron nitride (BN). Correlation between pore structure parameters and CPR of composites was established based on the grey system theory. The results of rapid chloride migration (RCM) tests indicate that due to the incorporation of 1 % NS, 3 % SiO2-coated NT, 1 % NZ, and 0.5 % BN, the CPR of UHPCC is improved by 89.6 %, 66.5 %, 87.5 %, and 73.6 %, respectively. The results of the grey correlation analysis highlight the critical role of harmless and less harmful pores with a diameter below 50 nm in preventing the diffusion of chloride ions. The modifying mechanisms of nanoinclusions can be attributed to the nano-core effects, contributing to reducing chloride diffusion pathways and increasing disconnectivity and tortuosity of the pathways. Firstly, nanoinclusions can reduce pores and microcracks, improving the homogeneity of pore size distribution and inhibiting the growth of CH crystals, thus enhancing the compactness of the cement paste matrix. Secondly, nanoinclusions enriched in the interfacial transition zone (ITZ) can improve the homogeneity of ITZ by optimizing the Ca/Si ratios of C–S–H gels and improving the interfacial bond between cement paste and aggregate. Additionally, Langmuir and Freundlich isotherms can accurately describe the CBC of composites. Only NS among nanoinclusions can significantly improve the CBC of UHPCC. This can be attributed to the involvement of pozzolanic NS in cement hydration, promoting the formation of C–S–H gels for physically adsorbing chloride ions while increasing the diffusion resistance of chloride ions in the pathways.