Admixture

ADMIXTURE: Admixture is define as the ,material, other than cement, water, and aggregate that is used as an ingredient of concrete and added on the batch immediately before or during mixing. It is classified into two types

1.      Chemical Admixture: used for specific properties enhancement.

2.      Mineral Admixture: Used for long term strength.

Some example of Admixtures are:   

a)     Fly ash:

Ø  Fly ash is a fine powder, which is bi product of burning coal in a electric generation power plant.

Ø  Fly ash is a pozzolan, a substance containing aluminious and silicious material that form cement in the presence of water.

Ø  Fly ash is an expensive replacement of Portland cement although using it improves strength, segregation, and ease of pumping concrete.

Drawbacks:

·        Slower Strength gain

·        Seasonal limitation

·        Increase in air-entraining admixtures

Advantages

·        Can be substitute of Portland cement

·        Great workability

·        Reduce heat of hydration

·        Reduce CO2 emissions

Types: ASTM Class F, ASTM Class C

      b)     Rice husk ash:

Ø  Rice husk ash is obtain by burning rice husk in a controlled manner without causing environmrnt pollution.

Ø  When properly burnt, it has high SIO2 and used as concrete admixture.

Ø  Rice husk exhibits high pozzolanic characterstics and contributes to high strength and high impermeablity of concrete.

Ø  It essientially consist of amorphous silica(90%), 5%(CO2), 2%(K2O) and has surface area between (40-100 m2/kg)

Disadvantage

·        Stength reduce to 30%

·        Transportation problem

Advantages

·        Permeablity

·        Bond strength

·        Improve compressive strength, Flexural strength.