Effect of chloride-sulfate ions on reinforcement corrosion and sulfate deterioration in blended cements

There is an increasing tendency worldwide towards using cements blended with fly ash, silica fume, blast furnace slag and natural pozzolans. Incorporation of these materials in concrete makes it dense and impermeable. While the effect of chloride and sulfate ions on the durability of blended cements is well documented, very meagre data are available on the synergistic effect of high concentrations of these salts on the durability performance of these cements. Since the structural components, especially foundations in the coastal areas in some parts of the world, are subjected to high concentrations of these salts, it is imperative to investigate the performance of blended cements in such environments. In this investigation, mortar and concrete specimens made with Type I cement blended with fly ash, silica fume, and blast furnace slag were exposed to a highly concentrated chloride-sulfate (2.1% SO₄- and 15.7% CI^-solution for a period of 540 days. The performance of these cements in resisting reinforcement corrosion was evaluated by monitoring half -cell potentials and measuring corrosion rates at periodic intervals. Deterioration due to sulfate ions was evaluated by visual survey, and measuring loss in compressive strength. Results indicate that surface deterioration and loss in strength was greater in blast furnace slag and silica fume cement specimens as compared to fly ash and plain cement specimens. Severe surface scaling and considerable reduction in strength (55-75%) was observed in the former cements. Moderate surface deterioration and loss in strength of about 25% was observed in fly ash and Type I cements. Corrosion of steel in silica fume and blast furnace slag cement was, however, much lower than in fly ash blended and Type I cements.