pH-Wert der Porenlösung in Beton / pH-Value of the Pore Solution in Concrete

The risk for corrosion is substantially increased in carbonated concrete. Recently, methods for realcalization of carbonated structural concrete elements have been proposed and applied. A stable pH-value higher than 10 is aimed to be achieved by impregnation with aqueous solutions of alcali carbonates. Analyses of impregnated concrete elements in practice has shown, however, that the pH-value of the pore solution drops to values below 10 despite an apparently sufficient alcali content. Results described in this contribution are used to develop a hypothesis in order to explain the time-dependent decrease ofpH-value. It is concluded that the composition of the pore solution and the chemical equilibrium is changed in the presence of dissolved sulfates. Under these conditions, a comparatively high content of alcali carbonates is not sufficient any more to ascertain a pH-value higher than 10. The chemical mechanisms suggested here are used to interprete observations in practical cases. The complex interactions of dissolved ions in the pore solution of concrete has hardly been taken into account up to now. For this reason, the application of realcalization as discussed in this contribution cannot be recommended so far