Sulphates present in soil or water can cause serious damages to concrete. The methods given below can be adopted to protect concrete from sulphate attack.
Use of Sulphate Resisting Cement
The most efficient method of resisting the sulphate attack is to use cement with the low C3A content. It has been found that a C3A content of 7% gives a rough division between cement of good and poor performance in sulphate waters.
A well designed, placed and compacted concrete which is dense and impermeable exhibits a higher resistance to sulphate attack. Similarly, a concrete with low water/cement ratio also demonstrates a higher resistance to sulphate attack.
Use of Air-entrained Admixture
Use of air-entrainment to the extent of about 6% has beneficial effect on the sulphate resisting qualities of concrete. The beneficial effect is possibly due to reduction of segregation, improvement in workability, reduction in bleeding and in general better impermeability of concrete.
Use of Pozzolana
Incorporation of or replacing a part of cement by a pozzolanic material reduces the sulphate attack. Admixing of Pozzolana converts the leachable calcium hydroxide into non-leachable cementitious product. This pozzolanic action is responsible for impermeability of concrete. Secondly the removal of calcium hydroxide reduces the susceptibility of concrete to attack by magnesium sulphate.
High Pressure Steam Curing
High pressure steam curing improves the resistance of concrete to sulphate attack. This improvement is due to the change of C3AH6 into a less reactive phase and also to the removal or reduction of calcium hydroxide by the reaction of silica which is invariably mixed when high pressure steam curing method is adopted.
Use of High Alumina Cement
The cause of great resistance shown by high alumina cement to the action of sulphate is still not fully understood. However, it is attributed in part to the absence of any free calcium hydroxide in the set cement, in contrast to Portland cement. High alumina cement contains approximately 40% alumina, a compound very susceptible to sulphate attack, when in normal Portland cement. But this percentage of alumina present in high alumina cement behaves in a different way. The primary cause of resistance is attributed to formation of protective films which inhibit the penetration or diffusion of sulphate ions into the interior. It should be remembered that high alumina cement may not show higher resistance to sulphate attack at higher temperature.