Realkalization Technique For Carbonated Conerete Structures

The electrochemical realkalization technology for carbonated concrete structure aiming at concrete structure aging is one research hotspots recently. This technology is in the principle of electro-osmosis, electro-migration, diffusion and capillarity to renew the alkaline condition around reinforced bar and realize the realkalization of concrete. This technique is to solve the problem according to the electrochemical corrosion mechanism of steel, and it has many advantages, for example operation is simple and convenient and has no pollution. And wide application was suggested.The electrode reactions process of reinforcement, material transfer and the changes of carbonation concrete in composition and structure during the process of electrochemical realkalization were investigated in detail combining experimental research and theoretical analysis in this paper. And obtained some research results on the realkalization process, and provide theory backup to further improvement of realkalization technology.The first part of the study shows that: the electrochemical realkalization technique can be used not only to prevent carbonated concrete reinforcement corrosion, but also to repair the concrete structures which have certain degree corrosion. After the realkalization experiment, the reinforcing steel surface will gradually formed a compact passive film which can help the reinforcing steel enter the passive state during relaxation; but the process of hydrogen generation reaction will affect the forming speed of passive film, the time of steel enter into passive state will be longer than the situation which absence of hydrogen (gas) environment ;The steel that has smooth surface has a small surface area, easy to form the passive film, and would take few time to enter the passive state. The electrolyte in the realkalization process which Ca(OH)₂ and Na₂SiO₃ have a strong alkaline can make a higher proportion of electricity in water electrolysis, which could help improving the alkaline environment around the steel, so be suitable to used in early realkalization. But in mid-and late period of the process the Na₂CO₃ solution was more suitable for iron oxide reduction.During the process of realkalization, reduction of rust on the reinforcing steel surface is concurrence, the obtained elemental iron was loose sediment layer, and it's difficult to enter the passive state; But the sediment state could ameliorate when used a combined electric field, also the stability of reinforce steel could be improved.The second part of the realkalization experiment show that the lithium-ions in electrolyte could through the concrete protective layer and reached to the reinforced surface, also some part of lithium-ions would remain in concrete. The process of ion transfer during realkalization was finished by the concentration gradient and the electric field. In the early stage of ion transfer, diffuse was the main form which caused by concentration gradient, while the latter part of the process ion migration caused by electric field turn into the main form.In the third part, different types carbonation concrete material was used to simulation accelerate realkalization experiments, the data showed that the composition and structure of the concrete carbonation would change after realkalization process. In samples of artificial carbonation, most of the partial calcium silicate (CaSiO₃) transform into positive calcium silicate (Ca₂SiO₄) after realkalization, be beneficial to the calcium silicate gel's transformation and generation; The incomplete hydration silicic acid in natural carbonation samples during the process of realkalization would continue to hydrate, the calcium carbonate soluble salts could dissolve in the role of alkaline environment and active substances, and transform into second silicone gel. These gelatinous materials which generated after re-hydration could transform to a small aperture of dry gel (layer) after drying, and attached to the reaction substrate, made the original concrete aperture small, reinforced concrete structure density and durability.