Chemical disruption
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
1Sulphate 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.
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.
2Alkali reaction
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.
Factors:
ü Size of
the aggregate particles
ü Alkali
content in cement
ü Fineness
of cement particles
ü Porosity of the aggregate particles
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