Sulphate Resistant Cement (SRC)

Sulphate Resistant Cement (SRC)

The normal OPC is susceptible to sulphate attack particularly magnesium sulphate. The sulphate present in the soil or surrounding environment reacts with free Ca(OH)2 available in the concrete and CaSO4 is formed. There is no dearth of free Ca(OH)2 as it is available in abundance in the set cement. The CaSO4 thus produced reacts with hydrate of calcium aluminate and form an expansive compound called calcium sulpho-aluminate which causes expansion and cracks in the set cement. Sulphate attack is further accelerated if it is accompanied by alternate wetting and drying also, which normally takes place in marine structures of the tidal zone.

The cement can be made sulphate resistant if quantities of C3A and C4AF are controlled as given below :

 C3A < 5% (Normally it is 8-12% in OPC) and 3 C3A + C4AF < 25%

 While controlling the quantities of C3A and C4AF, a precaution is to be taken when chloride attack is also perceived in addition to sulphate attack. In this situation, the quantity of C3A should be limited to 5-8%. Similarly when only chloride attack is there and no sulphate attack, then sulphate resistant cement (SRC) should never be used.

 The quantity of C3A in OPC can be controlled simply by blending with slag cement.

 SRC is recommended in

(1)Marine condition where both sulphate and chloride attacks are there

(2)Construction of sewage treatment plants

(3)Foundations and basement in soil having sulphate attack

(4)Chemical factories