Impact of Cation Type of Chloride Salts on Diffusion Behavior and Binding Capacity of Chloride Ions in Concrete Containing Sodium Nitrite and Disodium Hydrogen Phosphate

Abstract

Chloride diffusion in concrete induces depassivation of steel reinforcement, which can jeopardize the service life of concrete structures subjected to aggressive environments. This paper investigates the effect of a cation type of chloride salts, sodium nitrite (SN) (NaNO2) and disodium hydrogen phosphate (DHP), on the diffusion behavior and binding capacity of chloride ions in concrete prepared from ordinary portland cement (OPC) and portland pozzolana cement (PPC) and exposed to different chloride salt solutions for an exposure period of 756 days. The results showed that the cation type of chloride salts and admixed NaNO2 and DHP have a significant effect on the chloride diffusion coefficient and chloride binding capacity of concrete. The apparent chloride diffusion coefficient (Dapp) of concrete decreased in the order Dapp: NaCl>CaCl2>MgCl2, whereas chloride binding capacity (R) decreased in the order R: CaCl2>MgCl2>NaCl. The microstructure of concrete examined through X-ray diffraction, Fourier transform infrared spectroscopy, and field emission scanning electron microscope analyses indicated the formation of nitrite- and phosphate-based compounds in concrete containing NaNO2 and DHP, respectively. The addition of NaNO2 or DHP decreased Dapp and increased chloride binding capacity of concrete. NaNO2 showed a lower chloride diffusion coefficient than Na2HPO4 (DHP). However, DHP exhibited a higher chloride binding capacity than NaNO2. This was due to the dominant effect of the formation of chlorapatite and a higher amount of Friedel’s salt in concrete containing DHP over the effect of a lower extent of physical binding of chloride ions due to the partial replacement of adsorbed nitrite ions from C─ S─ H by chloride ions. It was also due to the formation of a lower amount of Friedel’s salt due to the partial replacement of nitrite ions in nitrite–aluminate-ferrite-monosubstituent (AFm) by chloride ions in concrete containing NaNO2.