Multi-species Ionic Diffusion in Concrete with Account to Interaction Between Ions in the Pore Solution and the Cement Hydrates

The penetration and leaching of ionic species in concrete are studied by using a model based on the Nernst-Planck equations. A finite element procedure is used to solve the coupled non-linear governing equations. A numerical example is performed in which the results are compared to measured electron probe micro analysis (EPMA) data. A close agreement of the simulated results to measured data is found for the specific studied example. The model includes the ionic species Cl-, Na+, OH-, Ca2+, K+ and SO42- and solid phases with variable composition. From the EPMA measurements the total concentration profiles of Cl-, CaO, SiO2, Na2O and SO3 are evaluated on samples exposed to a 3 wt% sodium chloride solution for one year. The main task in this investigation is to quantitatively understand the underlying mechanisms and find an accurate model that gives good correlation with the experimental results concerning the multi-species action during chloride penetration. In the model the chemical interaction between ions in solids and in pore solution is assumed governed by simple ion exchange processes only. The drawback using this approach is that the chemical part is lacking important physical relevance in terms of standard solubility thermodynamics. On the other hand the presented model is capable of accurately simulate the well documented peak behavior of the chloride profiles and the measured high content of calcium ions in pore solution under conditions when also chlorides is present. In this sense the established multi-species models for concrete based on standard solubility calculations alone is still incomplete.