Ion transport in cement paste during electrochemical chloride removal

concentration of chloride (Cl-), hydroxide (OH-), sodium (Na+) and potassiu m (K+) occuring during the flow of electric cur I ent in the pore solution and of Cl-, Na+ and K+ in the solids of different zones in cement paste sam ples were measured The electrochemical reaction that occurred at the electr odes resulted in a marked increase in the OH- concentration of the pore sol ution in the zone of the sample close to the cathode, whereas a step decrea se was observed in the zone of the sample facing the anode. These changes h ad different effects on the transport of chloride. In the zone towards the cathode, the rising OH- concentration caused more and more of the total Cur rent to be transported by: the OH- ions as the migration progressed in time (chloride removal became much less efficient here), while in the periphera l zone towards the anode more and more current was transported the Cl- ions (chloride removal became more efficient here). This resulted in a change o f chloride concentration in the pore solution and the solids, which was in disagreement with the direction of migration of these ions. According to th e direction of migration, the Cl- concentration should have decreased most slowly in the peripheral zone towards the anode, because the Cl- ions leavi ng the sample at this point can be replaced by others that half moved in fr om 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,l t with the direction of migration.