Effect of cement composition on chloride binding and corrosion of reinforcing steel in concrete

Pore solution study has been carried out on 2.43 and 14% C₃A hardened cement pastes. Data have been analyzed in conjunction with the data developed in two pore solution studies made by Page and Vennesland and Diamond using 7.37 and 9.1% C₃A mature cement pastes. The results show that C₃A and alkali contents of a cement have significant effect on its chloride-binding capacity. For similar alkali content, the levels of free chlorides in the pore solutions of 2.43 and 9.1% C₃A cement pastes are respectively 4.7 and 2.8 times more than in a 14% C₃A cement. The alkali content of a cement appears to have an inhibiting effect on its chloride-binding capacity. However, this effect is overshadowed by a conjoint strong elevation of the OH^- ion concentration in the pore solution due to cement alkalies, causing a net lowering of the Cl^-/OH^- ratio which roughly ascertains corrosion risk. Threshold chloride values have been evaluated for different C₃A cements. The threshold chloride content for a typical Type I portland cement with C₃A upto 8% and Na₂O equivalent upto 0.60%, may be taken as 0.4% chlorides by weight of cement. However, for a similar alkali cement with a high C₃A content of about 14%, the chloride threshold value is 2.5 times higher and may be taken as 1.0% by weight of cement.