It has been postulated that the most important inhibitive property of concr
ete affecting the level of chloride required to initiate corrosion is its a
bility to resist a Local fall in pH that might otherwise sustain passive fi
lm breakdown at a developing pit. In this work a novel technique termed dif
ferential acid neutralisation analysis was used to characterise this proper
ty. It was noted that many solid phases in hydrated cement paste have pH de
pendent dissolution characteristics that may strongly influence the pore so
lution chemistry during corrosion initiation. While the important contribut
ion made by calcium hydroxide has been widely recognised, other reactive hy
dration products contribute more than 75% of the resistance to a pH reducti
on 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 concr
ete environment, characterised by its acid neutralisation capacity, depends
on the cementitious binder, decreasing in the order OPC > SRPC > PFA > GGB
S. Care is needed to minimise adverse effects occurring at transient pH val
ues on the steady state data obtained in an acid neutralisation test. Howev
er the indications are that differential acid neutralisation analysis may p
rove to be very effective as an analytical technique. (C) 2000 Elsevier Sci
ence Ltd. All rights reserved.