Calculation of ionic diffusion coefficients on the basis of migration test results

Migration tests are now commonly used to estimate the diffusion coefficients of cement-based materials. Over the past decade, various approaches have been proposed to analyze migration test results. In many cases, the interpretation of test data is based on a series of simplifying assumptions. However, a thorough analysis of the various transport mechanisms that take place during a migration experiment suggests that some of them are probably not valid. Consequently, a more rigorous approach to analyze migration test results is presented. The test procedure is relatively simple and consists in measuring the evolution of the electrical current passing through the sample. Experimental results are then analyzed using the extended Nernst-Planck-Poisson set of equations. A simple algorithm is used to determine for each experiment the tortuosity factor that allows to best reproduce the current curve measured experimentally. The main advantage of this approach resides in the fact that the diffusion coefficients of all ionic species present in the system can be calculated using a single series of data. Typical examples of the application of this method are given. Results indicate that the diffusion coefficients calculated using this approach are independent of the applied voltage and depends only slightly on the concentration level. and the chemical make-up of the upstream cell solution.