This paper presents a study of the effectiveness of electro-chemical realkalisation (ECR) treatment on the microstructure of cement paste/ steel cathode and near-surface zones of carbonated hardened cement pastes. ECR was set-up for carbonated hardened OPC cement paste prisms of 0.70 w/c ratio using 0, 1 and 5 A/m for 14 days, 1 A/m for 0, 7 and 14 days and 1 mole sodium carbonate as an electrolyte. The effect of using other electrolytes such as sodium phosphate, sodium hydroxide and lithium hydroxide was also investigated. Fragmental samples taken around the steel cathode and at near-surface was then analysed by mercury intrusion prosometry (MIP) and differential thermal analysis (DTA), thermo-gravimetric (TG). The results showed that ECR treatment resulted in a marked densification of the pore structure of near-surface and steel/cement paste transition zones. This effect was more pronounced with current density, period of treatment and particularly with the use of sodium phosphate as an electrolyte.