Electrokinetic desalination of porous building materials

The deterioration of porous building materials and structures by the crystallization of water soluble salts is a well known phenomenon. The threats posed by salts to building materials can be minimized either by controlling the environment or by removing the salts from the deteriorated zone. In building materials electro kinetic desalination aims to remove salts from the affected zone, mainly by electro migration, by application of an external electric field. However, in addition to enhancing the transport of dissolved salt ions, the applied electric field might also introduce new ionic species, due to reactions at the electrodes. In particular, H and OH ions can be introduced at the positively and negatively biased electrodes, respectively. This results in acidic and alkaline fronts entering the material, which can affect the desalination process. In addition, the acidic environment can induce corrosion in reinforced concrete and damage the mortar in masonry structures. In this thesis the effect of an applied DC electric field on the migration of salt (Na) ions in fired-clay brick has been investigated by performing non-destructive measurements of the time evolution of the moisture and Na profiles using a nuclear magnetic resonance (NMR) technique. Furthermore, the effect of an applied electric field on the evolution of the acidic and alkaline fronts and the electric potential gradients within the brick sample has been studied. In order to interpret the experimental observations a numerical model, based on the Poisson-Nernst-Planck equations, has been developed. The results of this model were in good agreement with the experimental data. The results of the study indicate that the evolution of the acidic and alkaline severely affects or even halts the transport of salt ions, which drastically limits the applicability of the electro kinetic method to the desalination of porous building materials.