In areas where sulphate attack may be present normal cement is blended to improve the performance of concrete.This is called Sulphate Resisting Cement.Reducing the risk of deterioration of the structure and structural failure,it also provides improved durability for concrete in most aggressive environments.
In particular to the action of magnesium sulphate, Ordinary Portland cement is susceptible to the attack of sulphates. Sulphates react both with the free calcium hydroxide in set-cement to form calcium sulphate and with hydrate of calcium aluminate to form calcium sulphoaluminate, the volume of which is approximately 227% of the volume of the original aluminates.It results in cracks and subsequent disruption due to expansion within the frame work of hardened cement paste. Solid sulphate do not attack the cement compound. Sulphates in solution permeate into hardened concrete and attack calcium hydroxide, hydrated calcium aluminate and even hydrated silicates.
This is sulphate attack. Sulphate attack is greatly accelerated if accompanied by alternate wetting and drying which normally takes place in marine structures in the zone of tidal variations.
To remedy the sulphate attack, the use of cement with low C3A content is found to be effective. Such cement with low C3A and comparatively low C4AF content is known as Sulphate Resisting Cement. In other words, this cement has a high silicate content. The specification generally limits the C3A content to 5 per cent.
Tetracalcium Alumino Ferrite (C3AF) varies in Normal Portland Cement between to 6 to 12%. Since it is often not feasible to reduce the Al2O3 content of the raw material, Fe2O3 may be added to the mix so that the C4AF content increases at the expense of C3A. IS code limits the total content of C4AF and C3A, as follows.
2C3A + C4AF should not exceed 25%.
In many of its physical properties, sulphate resisting cement is similar to ordinary Portland cement. The use of sulphate resisting cement is recommended under the following conditions:
(a ) In marine condition;
(b ) In foundation and basement, where soil is infested with sulphates;
(c ) Used for fabrication of pipes which are likely to be buried in marshy region or sulphate bearing soils;
(d ) In the construction of sewage treatment works.
