Resistance of concrete to chemical attack:
ü The cement composition used in the concrete.
ü Conditions under which the cement paste hardened
ü All determine properties of concrete
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.
1.1 Chemical mechanism
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.
The reaction of some forms of silica and carbonates inaggregates with the alkalis in cement produces a gel, which causes expansion and cracks.
2.1Mechanism of Alkali-aggregate reaction
This is called alkali carbonate reaction. Certain carbonate rock aggregates have been reative in concrete. The results of these reactions have been characterized as ranging from beneficial to destructive.
The destructive category is apparently limited to reactions with impure dolomitic aggregates and are silt of either dedolomitization reactions. Visual examination of those reactions that are serious enough to disrupt the concrete in a structure will generally show map or pattern cracking and a general appearance, which indicates that the concrete swelling. A distinguishing feature which differentiates alkali-carbonate rock reaction from alkali-silica reaction is the lack of silica gel exudations at cracks. Typical alkali aggregate reaction damage is as shown fig.
ü Size of the aggregate particles
ü Alkali content in cement
ü Fineness of cement particles
ü Porosity of the aggregate particles