Sulphate Resisting Cement(SRC)
Ordinary Portland cement susceptible to the attack of sulphates, in particular to the action of magnesium sulphate(MgSo4,7H20). Sulphate react with free calcium hydroxide in set-cement to form calcium sulphate(reaction 1) and with hydrate of calcium aluminate to form calcium salphoaluminate(3Cao Al2O3, 3CaSo4,31H20)(reaction 2), the volume of which is 227% of the volume of the original aluminates .There expansion in the framework of hardened cement paste results in cracks and subsequent disruption.Solid sulphate does not attack the cement compound. Sulphate in solution penetrate into the hardened concrete and attack calcium hydroxide,hydrated calcium aluminate and hydrated silicates(reaction 3) illustrated as by the following reaction leading to sulphate attack:
3CaO Al2O3,9H2O+3[CaSO4,2H20}+16H2O—–>3CaO Al2O3,3CaSO4,31H2O
Most predominate sulphate attack is due to calcium aluminate hydrate.Sulphate attack is greatly accelerated if accompanied by alternate wetting and drying which normally occurs in marine structures in the zone of tidal variation.
To remedy the sulphate,the use of cement with low C3A content is formed to be effective.Such cement with low C3A and comparatively low C4AF content is known as Sulphate Resisting Cement.
- Use of Sulphate Resisting Cement is mandatory in marine structure.
- In foundations and basement when soil is infested with sulphates.
- In concete used for casting of pipes which is likely to be burried in marshy region of sulphate bearing soil.
- In concreting work for construction of sewage treatment plant.