Ion transport in cement paste during electrochemical chloride removal

Abstract

To clarify open questions regarding the mechanism of electrochemical chloride removal (ECR), changes in the concentration of chloride (Cl-), hydroxide (OH-), sodium (Na+) and potassium (K+) occurring during the flow of electric current in the pore solution and of Cl-, Na+ and K+ in the solids of different zones in cement paste samples were measured. The electrochemical reaction that occurred at the electrodes resulted in a marked increase in the OH- concentration of the pore solution in the zone of the sample close to the cathode, whereas a steep decrease was observed in the zone of the sample facing the anode. These changes had different effects on the transport of chloride. In the zone towards the cathode, the rising OH- concentration caused more and more of the total current to be transported by the OH- ions as the migration progressed in time (chloride removal became much less efficient here), while in the peripheral zone towards the anode more and more current was transported by the Cl- ions (chloride removal became more efficient here). This resulted in a change of chloride concentration in the pore solution and the solids, which was in disagreement with the direction of migration of these ions. According to the direction of migration, the Cl- concentration should have decreased most slowly in the peripheral zone towards the anode, because the Cl- ions leaving the sample at this point can be replaced by others that have moved in from the zone of the sample closer to the cathode. In the case of the alkali ions sodium and potassium, the changes in concentration were in agreement with the direction of migration.