 Fresh cement water has a pH of about 13.5 while evolved cement water pH ~ 12.5 is controlled by the dissolution of calcium hydroxide (portlandite).Carbon dioxide present in the air slowly diffuses through the concrete cover over the rebar and progressively reacts with the alkaline hydroxides (KOH, NaOH) and with calcium hydroxide leading to the carbonatation of the hydrated cement paste.As a result, the pH of the cement drops and when its value is below 10.5 – 9.5, steel surface is no longer passivated and starts to corrode.