Kinetics of oxygen diffusion in hardened cement pastes

An electrochemical cell has been devised for measurement of the effective diffusivity of oxygen in water-saturated hardened cement pastes. The technique has been applied to investigate oxygen diffusion kinetics over the temperature range 15 to 35° C for Portland cement pastes of constant mineralogy but variable water-cement ratio. The values obtained for effective diffusivity and activation energy have been compared with previously published kinetic data for ionic diffusion in similar materials. Implications regarding the electrochemical behaviour of steel embedded in water-saturated cement matrices have also been examined with particular reference to prediction of limiting conditions of oxygen supply governing the transition from a state of passivity, characterized by a stable film of Fe₂O₃, to one of general dissolution, associated with a stable alkali-soluble species, Fe(OH)₃^–.