Chloride induced corrosion behavior of carbon steel rebar in seawater mixed Portland cement-SF-MK ternary pastes with low w/b ratio

Abstract

Utilizing seawater in the formulation of ultra-high-performance concrete (UHPC) presents promising opportunities in marine civil engineering. This study investigates the chloride-induced corrosion behavior of carbon steel within seawater-mixed Portland cement-SF-MK ternary pastes with a low water-to-binder ratio, through electrochemical measurements. The findings reveal that the incorporation of supplementary cementitious materials (SCMs) such as SF (Silica Fume) and MK (Metakaolin), at appropriate replacement levels, significantly enhances the chloride binding capacity. This improvement stems from the pozzolanic reaction and the formation of Friedel's salt. The corrosion of steel is effectively mitigated by partially substituting cement with SF or MK, attributed to the reduction in oxygen diffusion and the concentration of free chlorides in the pore solution. This enhancement in corrosion resistance is further bolstered by the refinement of the microstructure and the increased chloride binding capacity. However, the effectiveness of corrosion mitigation is found to be dependent on the composition of the binder. Regardless of whether it's a binary or ternary system, steel corrosion exacerbates as the total replacement level of SF and MK reaches 30 %, primarily due to the consumption of portlandite. Therefore, the judicious selection of SF and MK replacement levels is crucial. Optimally, SF and MK replacement levels of around 10 % are recommended to improve the resistivity (R₁) of the paste, increase the impedance (R₂) of the passivation film or corrosion products, and enhance the charge transfer impedance (R₃) of the steel significantly. This inhibits the redox reaction of steel in seawater-mixed Portland cement-SF-MK ternary pastes with low water-to-binder ratio.