Sulphates and Chlorides in concrete - IS 456

Sulphates occur in aggregates ,soil and water. They lead to whitish appearance on concrete surface. Magnesium sulphate is most dangerous to concrete. It leads for formation of sulphuric acid in concrete. It increases the vol of concrete and leads to surface cracks and patches of small size. Cooling towers (NDCT ) contains more sulphates in its water, Thus causes irritation in eyes when stand nearby.

SO3 should not exceed 4 % in concrete mix ( including aggregates, water, admix etc) It can be controlled by low w/c ratio, sulphate resisting cement, use of PPC or by steam curing

Chlorides leads to rusting in steel. They are present in aggregates, cement , water and admixture. In warm moist conditions there is risk of chloride attack on concrete. A protective layer of oxide is present on reinforcement due to high alkality of concrete. This layer can be destroyed by chloride attack and leads to rusting. Higher pH value ( more than 6 - 10 ) protects concrete from rusting as pH value falls below than 6 chances become more for rusting.

For reinforced concrete limits of chlorides is  0.6 kg/cum. It can be controlled by lowering w/c ratio. In pre stressed concrete chlorides should be 0.06% of mass of cement.

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