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Use of Chemical and Mineral
Admixtures in Concrete
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What is admixtures ?
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Admixtures are the chemical compounds in
concrete other than Portland cement, water,and aggregates that are added to the mixture
immediately before or during mixing .
Admixtures have formulated chemical
composition and special chemical action, and
are used to modify certain properties ofconcrete.
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FUNCTIONS OF
ADMIXTU!S
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Admixtures can "e classi#ed "$function as follo%s&
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The general action of accelerators is to cause more
rapid dissolution of compounds of cement , particularly
Tricalcium silicate , in water and hence facilitate morerapid hydration of these compounds.
The generally used accelerators consist of NaCl,
Na2Co, !2"#$ ,!#%.
Acceleratin' admixturesor accelerator
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An admixture used to speed up the initial set of the
concrete is called ac accelerator.
These are added in concrete either
&'( To increase the rate of hydration of cement ,and
hence to increase the rate of development of strength .
&2( To shorten the setting time
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An increase in the rate of early strength development
may help in
&'( )arlier removal of forms,
&2( *eduction of re+uired period of curing
&( )arly placement of structure in service.
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Air (entrainin' admixtures
Air entering admixtures help to incorporate a controlled
amount of air , in the form of million of minute non
coalescing bubbles distributed throughout the body ofconcrete ,during mixing, without significantly altering
the setting or the rate of hardening of concrete.
n air/entrained concrete ,the air bubbles provide
space to relieve the pressure of expanding ice as
shown fig.&'(
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Water reducin' admixturesor )lastici*ers
Water reducin' admixtures ena"le a'i+en fresh concrete mix to ha+e hi'her
,o%a"ilit$ -%or.a"ilit$/ %ithoutincreasin' the %ater content increasin'the %ater content %hich results in fasterrate of concrete 0lacement 1 eas$
0lacement in relati+el$ 0oorl$ accessi"lelocation %ithout +i"ration 1true shutter#nish for hi'hl$ reinforced mem"ers andreduction in cement content 2
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Wor.in' of su0er0lastici*ers
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T)C%NCA P)*3#*4ANC)
The performance of concrete is normally optimi0ed in
one of the following four ways 5
'. To produce flowing concrete 2. To produce concrete with varying low water cement
ratio
. To produce high performance concrete $. To produce concrete mix with reduced cement
content.