Extraction of chlorides at free and bound ones under electrochemical treatment in concrete

Abstract

only a chloride concentration in the bulk concrete has been measured rather than a profiling at different depths.

The aim of the present study is to quantify the concentration of chlorides which could be extracted by the electrochemical treatment in concrete contaminated by chlorides, of which the level exceeds the critical concentration for the onset of corrosion. As the effect of chloride extraction depends on the type of chlorides in the cement matrix, the binding of chloride ions in concrete was preliminarily determined. Then, transport of chlorides under the electrochemical treatment was identified in a closed circuit of the treatment with no electrolyte exposure to restrict the loss of chlorides in a given concrete. Substantially, the extraction of chloride ions in concrete was identified at free and bound ones under electrochemical treatment.

As a result, it was found that the chloride binding capacity was affected by the cement types in terms of C3A content in clinker. An increase in the C3A resulted in an increase in the chloride binding capacity. Simultaneously, increased chloride binding capacity produced a higher build-up of the surface chloride, leading to an increase in the gradient on the chloride concentration. Thus, the diffusivity of chlorides was increased by C3A content in cement, despite the denser pore structure. Also, chloride extraction was maximised at the end of the electrodes in terms of cathode and anode. In particular, a removal of chloride was significant at the depth of the steel. Moreover, there was an impact of binder type. Concrete containing pozzolanic materials, for example, ground blast furnace slag enhanced the chloride extraction. After the treatment, corrosion of steel was more or less repassivated, ensured by the corrosion rate and mass loss. However, it did not seem that the transient electrochemical treatment has effect to 100% guaranteed repassivation of steel embedment. Additionally, the extraction of chloride ions was measured at free and bound. It was evident that free chlorides were mostly removed by the electrochemical treatment, whilst bound chlorides were partially released into free and then removed away from the concrete body. A release of bound chloride seems restricted chlorides physically adsorbed on the hydration surface, which has lower bond strength with chlorides. Thus, with no acidification of the pore solution, chlorides would turn mobile under electrochemical treatment.; The present study concerns the effect of chloride extraction under electrochemical treatment in concrete contaminated by chloride ions to impose the risk of chloride-induced corrosion. To repair concrete structures under steel corrosion, electrochemical treatment is often used to remove rust and simultaneously the causes of corrosion such as chloride ions, carbonation in terms of acidified pore solution and formation of the protective layer of hydration products. By providing a source of electrons, negative ions such as chloride, hydroxyl and carbonate are forced to move toward the concrete surface, while positive ions such as calcium, sodium and potassium move towards the cathode. From a literature review, the electrochemical treatment has successfully been used to repair the concrete structures under chloride-induced corrosion by extract chloride ions, confirmed by an electrochemical measurement such as the half-cell potential of the steel. However, chloride transport in the electrochemical treatment was not fully identified at free and bound chlorides in the cement matrix