Solid salts do not attack concrete but when present in solution they can react with hardened concrete. Some clay contain alkali, magnesium and calcium sulphate and the ground water in such a clay produces a sulphate solution. Attack on concrete can thus take place the sulphate reacting with Ca(OH)2 and with calcium aluminate hydrate. The products of the reactions gypsum and calcium sulpho-aluminate have a considerably greater volume than the compounds they replace so that the reactions with the sulphates lead to expansion and disruption of the concrete.
The rate of sulphate attack on concrete increases with an increase in the strength of the solution. Concrete attacked by sulphate has a characteristic whitish appearance. The damage usually starts at edges and corner and is followed by progressive cracking and spalling which reduce the concrete to a friable state or even soft state.
They vulnerability of concrete to sulphate attack can be reduced by:
- Use of cement of low C3A.
- Addition of pozzolana material with cement.
- Use of dense and impermeable concrete.
- Heep of high pressure steam curing and
- Addition of calcium chloride to the concrete mix.
Sulphate Attack on Concrete
Sea water contains sulphate and attack on concrete similar to the chemical action, crystallization of the salts in the pores of concrete may result in its disruption owing to the pressure exerted by the salt crystals. Because crystallization takes place at the point of evaporation of water this form of attack occurs in the concrete above the water level. Since the salt solution rises in the concrete by capillary action, the attack takes place only when water can penetrate into the concrete. So, impermeability of concrete is once again its most important attribute. In tropical climates sulphate attack on concrete is more rapid.
In some cases the action of sea water thus sulphate attack on concrete is accompanied by the destructive agents of frost, wave impact and abrasion and all these tend to aggravate the damage of the concrete.
In case of reinforced concrete the absorption of salts established anodic and cathodic areas: the resulting electrolytic action leads to an accumulation of the corrosion products on the steel with a consequent rupture of the surrounding concrete. So, the effects of sea water are more severe on reinforced concrete than on plane concrete. It is therefore, re-essential to provide a sufficient cover to reinforcement (3 inch preferably) and to use dense and impermeable concrete. Sulphate resisting Portland blast furnace and Portland pozzolana cements give good results to resist sulphate attack on concrete.