Wednesday, 10 April 2019

ADMIXTURES

ADMIXTURES

Admixture is defined as a material other than cement, water, and aggregates, that is used as an ingredient of concrete and is added to batch immediately before or during mixing.

Types of Admixtures

Mineral Admixtures
·       Flu ash
·       Silica Fume
·       Ground granulated blast furnace slag ( GGBS)
·       Stone powder etc.

Chemical Admixtures

1. Accelerators :
·       Increase the rate of strength development or reduces the setting time.
·       Calcium chloride ( cacl2) is widely used accelerator.
Other examples include tri-ethenolamine ,fluosilicates etc.

2. Retarders:
·       Delays the setting time of cement paste.
Examples: Gypsum ( calcium sulphates) , mucic acid , calcium acetate , sugar etc.

3. Plasticizers( water reducers) :
·       Increases the workability of concrete without altering the water /cement ratio.
Examples: Lingo sulphuric acids in form of calcium or sodium salts, oleate etc.

4. Super plasticizers ( high range water reducers:
·       Imparts very high workability with large decreases in water content ( at least 20%) for a given workability.
Examples: Hydroxylated carboxylic acids, for maldehyde derivates,such as melamine formaldehyde , napthalene sulphonate formaldehyde etc.

Uses of Admixtures
Admixtures are used to:
1.      Accelerate or retard setting time.
2.      Decreases heat of evolution , rate of bleeding segregation.
3.      Increase the rate of hydration and strength development.
4.      Increase water tightness and reduce capillary flow etc. 



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