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Admixtures and purposes of their using in concrete

Other than cement, water and aggregate we add other substances to concrete mixes for the purpose of altering properties of fresh or hardened concrete that is admixture. For specific conditions or circumstances, admixtures may sometimes be necessary to alter specific properties of the concrete.

Most commonly used admixtures in residential concrete construction are-
          Chemical admixtures,
          Air-entraining agents and
          Coloring pigments
Chemical admixtures: most commonly used chemical admixtures are
                     a) Set accelerators, b) Set retarders, and c) Water reducers 

Set accelerators speed up the setting time and early strength development of concrete. The most widely used chemical accelerator is calcium chloride.

Set retarders slow down the hydration process so that the concrete stays plastic and workable for a longer time after mixing.

Water reducer slower the amount of mixing water required without decreasing workability.

Air-entraining agents: Air-entraining agents improve workability of concrete. They are particularly effective in lean mixes and in concrete mixes containing poorly graded or sharp and angular aggregates. Air-entrainment reduces segregation slows the rate of bleeding and shortens finishing time. Either a separate air-entraining agents or air-entrained cement may be used for this purpose.

Coloring pigments: To produce colorful concrete, Coloring pigments are added to concrete mix. For this purpose, synthetic oxide pigments or natural oxide pigments are used. But, the pigments must be insoluble in water, free from soluble salts and acids, chemically stable in the alkaline cement paste, and have no adverse effect on the setting time, strength development or durability of the concrete.

Mixing admixtures with concrete must be carefully controlled to avoid adversely affecting the concrete. so, it is best to use admixtures in concrete with experienced person.


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