Chemical realkalization in concretes subjected to natural carbonation over 20 years: Analysis of effectiveness and changes produced

Chemical realkalization (CRA) is used to combat the carbonation front from reaching the steel bars of reinforced concrete. This treatment may also promote restructuring of the passive film of reinforcing steel. An alkaline solution is applied on the surface of the concrete, and the pH of the pore solution is reconstituted through the transport mechanisms of capillary absorption and diffusion. The present study sought to identify the main mechanisms of the CRA front and estimate the duration of the treatment through the velocity of treatment advancement in the concrete pore system. Beam prototypes subjected to natural carbonation for more than 20 years were used, with different water/binder ratios (0.55 and 0.70) and supplementary cementitious materials (SCM): silica fume, metakaolin, rice husk ash, and blast furnace slag. Statistical analysis of the intervening factors indicated the predominant mechanism. The microstructural tests (TG, XRD, WDXRF, ICP-OES, electrical conductivity, and pH) provided an overview of the changes in the cementitious matrix and pore solution. After realkalization, the recarbonation process was monitored, and the estimated recarbonation coefficients resulted much higher than the initial carbonation. High water/binder ratio and low electrical resistivity significantly increased the realkalization and recarbonation velocity. Among the SCMs, only the blast furnace slag significantly changed the realkalization and recarbonation coefficient. Finally, the changes in the pore solution and cementitious matrix indicated the precipitation of new products, such as portlandite, vaterite, and aragonite, as well as alkali adsorption in the decalcified C-S-H. One of the main findings of the work was that restoring the pH of carbonated concrete is quite feasible but not recovering the initial alkalinity of the concrete (prior to the carbonation process). • CRA restores the pH in the pore solution of different carbonated concretes in 15 days. • The recovery of concrete pH is a relevant action for the repassivation of reinforcement. • CRA quickly restores the concrete pH but does not restore the system to full alkalinity. • CRA promotes changes in the cement matrix, with new phases impacting the microstructure. • Recarbonation in realkalized layers presents higher speeds due to the lower resistivity. • The alkaline reserve is lower in realkalized layers, favoring faster recarbonation.