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Essentially water reducing admixtures perform of plasticize concrete more by physical
interaction than by chemical interaction. The chemical interaction is generally responsible for
slump retention or retardation of concrete.
When admixtures are added, the initial hydration of aluminate and silicates is delayed since
some chemicals strongly affect the formation and growth of crystals of the hydrated phases.
The chemical admixture first gets absorbed on the hydrated phases containing alumina and
depending on the quality of the admixture and C 3 A content of cement it is further absorbed on
other hydrated phases of silicates. This inhibition of the surface hydration reaction of the cement
particles leaves more water to fluidise the mix and thereby increase the workability of concrete
and also retain the workability for a longer period. Numerous organic compounds such as
lignosulphonates, hydroxy carboxylic and carboxylic acids and their salts etc. cause retardation.
3X are hydrodynamic lubricants which impart high workability by reducing
friction between the grains or by reducing the amount of water to be added. They are improved
version of plasticizers interact both physically and chemically with cement particles. The
mechanism of action of super plasticizers is same as that of plasticizer. Superplasticisers are
anionic in nature and impart a negative charge to the cement particles, causing them to repel
each other. Once the superplasticiser is added, the concrete should not be agitated as it will lead
to an early loss of workability. The workability of the superplasticised concrete decreases more
with time than that of the ordinary concrete. The strength, water-cement ratio and creep of
concrete with the additive are same as that of concrete before the addtion of plasticizer, while
shrinkage and surface absorption are reduced slightly and the resistance to thawing and
freezing is improved. By addition of superplasticisers it is possible to obtain same strength
with a reduced cement content. However, the durability is reduced. The water-cement ratio of
the superplasticised concrete may be reduced up to 30 per cent to obtain the initial equality of
workability.
In addition to enhancing workability, there are many other advantages of using
superplasticisers. Since these are found not to be affected by accelerated curing, they help to
release prestressing wires and early demoulding. They can be of assistance in placing concrete
under water as the spreading of concrete under water is avoided. They are most advantageous
when the concrete is to be pumped. They are not recommended for slabs, the upper surface of
which slopes more than 3° or for extruded concrete.
The quantity of the superplasticiser is generally specified by the supplier. In the absence of
such data. A dose of 1 to 6 litres per cubic metre of concrete may be used. If they are used in
excess of the specified quantity, the rate of evolution of heat is increased, but the total heat of
hydration is not much affected. Superplasticisers are added immediately before use.
Some of the examples of superplasticisers are sulphonated melamine formaldehyde
condensates, naphthalene sulphonate formaldehyde condensates, modified lignosulphonate,
and mixture of saccharates and acid amines.
33 are siliceous materials which are themselves inactive but react, in the presence of
water, with lime to form compounds having cementitious properties. The examples of puzzolana
are lime, fly ash, burnt clay and blast furnace slag. Puzzolanas react with free lime in cement
and improve the durability of concrete, and reduce the rate of hardening of concrete, which is
the principal objection to its use. These are dealt in details in Chapter 9.
i" 2 2e "
are used to counteract the drying shrinkage of concrete.
Granulated iron and chemicals are most effective.