1 Sulphate Attack
Mechanism-sulphates are found in most of the soils as calcium, potassium, sodium and magnesium sulphates. Sulphate attack occurs when pore system in concrete is penetrated by solution of sulphates.
The effect of sulphate on concrete can be mainly, chemical and physical and they are closely related. The sulphate attack or reaction is indicated by the characteristic whitish appearance on the surface. As a result of the chemical reactions between sulphate and hydration products, changed in the microstructure and pore size distribution of the cement paste takes place. Sulphate converts calcium hydroxide into large of calcium sulphate.
Na2So4.10H2O +Ca(OH)2 -- -- > CaSO4.2H2O +2NaOH +8H2O
The second hydration hydration produc, tricalcium aluminates hydrate reacts with sulphate solution to form sulpho aluminates hydrate, which has a greater volume than that of the original compound.
2(3CaO.Al2O3.12H2O) + 3(Na2So4 .10H2O) -- - > 3CaO.Al2O3.3CaSo4.31H2O + 2Al(OH)3 +6NaOH+17H2O
When concrete cracks, its permeability increases and the aggressive water penetrates more easily in to the interior, thus accelerating the process of deterioration.
2 Salt attack/Weathering
Solid salts do not attack concrete, but when present in solution they can react with hardened concrete. It is a more general problem in masonary structures. Efflorescense is awhitish crystalline deposit on the surface. Efflorscence is the formation of calcilum carbonate precipitate on the concrete surface owing to carbonation
ü Using sound materials free from salts
ü Proper concrete proportioning
ü Consolidation and Curing
ü Preventing the access of moisture to the structure