Arrest of alkali leaching from concrete structures in an aquatic environment

Abstract

The increasing attention on the alkali leaching into external water has been achieved, due to its impact on concrete properties and ecological contamination. In particular, cations such as Ca2+ ions can be dissolved in the pore solution, which is subsequently diffused into external water. In this process, ionic leaching takes place. The Ca2+ ion leaching in the C-S-H gel may decompose the hydration products, while OH- ions in the pore solution can be leached out into external water with no dissolution from hydration products. Instead, a precipitation of Ca(OH)2 is less likely to form in the cement matrix. Likewise, the loss of alkali ions can lower the solidification of hydration products through leaching and thus a reduction of concrete properties is usually accompanied. In this study, OH- ion leaching was mainly dealt with to determine the leaching behaviour in terms of the leaching capacity and leaching region. As a result, it was found that the OH- ion leaching took place at an early exposure at about 10 hours to reach 12.0 in the pH or beyond. Also, the influential depth for alkali leaching accounted for 5.0 mm. To arrest OH- ion leaching, the electrochemical treatment was applied at 1000 mA/m2 for 1, 2 and 4 weeks. Then, it was found that an increase in the duration of the treatment resulted in a decrease in the alkali leaching into external water; the cumulative OH- ion concentration was lower by 4 order than control. Also, the influential depth was lower, presumably due to a barrier in the cement matrix. After the completion of the electrochemical treatment, the specimen had a marginal leaching, reaching only 8.0 in the pH in external water, whilst control indicated 13.0 in the pH or beyond. In the microscopic analysis on the surface of cement paste, the distribution of pores indicated that the electrochemical treatment formed less the pores in the vicinity of surface, implying that the treatment could modify the pore structure. Simultaneously, the C-S-H gel was less decomposed by Ca2+ dissolution in the pore solution. Also, Ca(OH)2 was more precipitated. Substantially, the porosity turned denser, while hydration products kept more solid, thereby leading to a reduction of the alkali leaching.