288 / 2023-02-08 12:40:26

Chloride-binding capacity of cement-GGBFS-nanosilica composites under seawater chloride-rich environments

Cementitious composite; Chloride-binding ratio; Corrosion; Nano-silica; Thermodynamic modeling

Increasing chloride-binding capacity of cementitious composites is considered as a controlling property of the reinforced concrete in the marine environment. In this paper, there were three specific objectives: firstly, to evaluate the effect of ground granulated blast furnace slag (GGBFS) on the chloride-binding capacity of cementitious composites; secondly, to examine the effect of nano-silica (NS) content on the chloride-binding capacity of GGBFS-based cementitious composites; and thirdly, to investigate the effect of different concentrations of chloride solutions (e.g. NaCl, MgCl₂, and NaCl + MgCl₂ solutions) on the chloride-binding capacity of cementitious composites.



It was found that the GGBFS replacement for cement significantly improved the chemical and physical chloride-binding capacity of cementitious composites and decreased the pH of exposure solutions (Fig. 1). Due to the pozzolanic reaction consuming Portlandite (CH) (Fig. 3), large amounts of C-S-H/C-A-S-H gels are generated, further increasing the physical chloride binding. The higher aluminum content in GGBFS increased the formation of chloroaluminate (Friedel's salts), which enhanced the chemical chloride binding (Fig. 2).



The results also illustrated that the addition of NS decreased the pH of exposure solutions, and the addition of 1.0 wt.% NS further enhanced the total chloride-binding capacity of GGBFS-based cementitious composites. However, when the content of NS is higher than 1.0 wt.%, the total chloride-binding capacity (especially, the chemical chloride-binding capacity) of GGBFS-based cementitious composites was decreased. The addition of NS increased physical chloride-binding capacity due to the formation of more C-S-H/C-A-S-H gels, while leaving less aluminium phase available for the formation of Friedel's salts (Fig. 4).



Compared to exposure to salt solutions with sodium ions, more chlorides of cementitious composites were found to be bound when exposed to salt solutions with magnesium ions. The magnesium ions increased the amount of physically bounded chloride in the gels' diffuse layer due to the decreasing pH of the MgCl₂ solutions. Except for Friedel's salts' formation, the hydrotalcite with high element proportions of Cl confirmed by EDX results is expected to be generated, further increasing the chloride binding.



The thermodynamic model (Fig. 5) could display relatively accurately in predicting most of the phase assemblage of the cement-GGBFS-NS composites exposed to different chloride-ride environments. The cementitious composite with 30 % GGBFS and 1.0% NS having the highest value of chloride-binding ratio provide a prospective way to enhance the long-term chloride-binding capacity (Fig. 6). Therefore, the utilization of GGBFS and NS in chloride binding is supposed to guide the improvement of marine constructions.