Corrosion inhibition in concrete arising from its acid neutralisation capacity

Abstract It has been postulated that the most important inhibitive property of concrete affecting the level of chloride required to initiate corrosion is its ability to resist a local fall in pH that might otherwise sustain passive film breakdown at a developing pit. In this work a novel technique termed differential acid neutralisation analysis was used to characterise this property. It was noted that many solid phases in hydrated cement paste have pH dependent dissolution characteristics that may strongly influence the pore solution chemistry during corrosion initiation. While the important contribution made by calcium hydroxide has been widely recognised, other reactive hydration products contribute more than 75% of the resistance to a pH reduction to a value of 10. The resistance to a pH reduction (acid neutralisation capacity) to a value between 10 and 11 correlates reasonably well with the available chloride threshold level data. The inhibitive nature of the concrete environment, characterised by its acid neutralisation capacity, depends on the cementitious binder, decreasing in the order OPC > SRPC > PFA ≈ GGBS. Care is needed to minimise adverse effects occurring at transient pH values on the steady state data obtained in an acid neutralisation test. However the indications are that differential acid neutralisation analysis may prove to be very effective as an analytical technique.