Admixtures and types of admixtures

 

Admixtures

In this article  we disscused the what is admixtures and types of admixture with examples in detail.

Anything added in concrete other than binding Material, fine aggregate, coarse aggregate and water in order to modify its properties are termed as admixture

Admixture are further classified as

1 Chemical admixture

2 Mineral admixture / Pozzolona's

 

Chemical Admixture

 

 

1. Plasticizers( water reducers)

2. Super Plasticizers

3. Retarders

4. Retarding plasticizers

5. Accelerators

6. Accelerating plasticizers

7. Air entraining admixture

 

 1. Plasticizers( water reducers)

 

It is a mixture of organic and inorganic substances which permits the reduction in water cement ratio at same workability on offers higher workability at same water cement ratio in either of the case it provides high strength concrete on highly workable concrete

These plasticizers act as a deflocculating reagent hence gets adsorbed over the cement particle carries out repulsion between them thereby makes the entrapped water free from the voids which participate in the hydration process and modify the property of concrete Dose of plasticizer varies in the range of on 0.41% by wt. of cement at which they can permit reduction in water content by 5-15% or increases  the slump value by 30-150 mm

Commonly used plasticizer includes

·        Lignosulphate

·        Polyglycolester

·        Carbohydrates

·        Hydroxylated carboxylic acid

NOTE

Action of plasticizers includes

·        Dispersion

·        Retarding affect

Many research explains the working of plasticizer as follows

·        Reduction in surface tension of water

·        Induced electrostatic repulsion between particles

·        Lubricating film between the particles

·        Dispersion of cement grains releasing water and air trapped between the particles

·        Inhibition of surface hydration reaction

·        Change in the morphology of hydration particles

·        Induced steric hindrance preventing particle to particle content

 

2. Super Plasticizers

Super plasticizers are same as that of plasticizers in terms  of their action but are chemically  different from plasticizers

The effect of super plasticizers is comparatively more than that of plasticizer such that for same dose they can permit the reduction of water content upto by 30%

Their application is same as that of plasticizers

 Commonly used super plasticizers includes

·        Modified lignosulphate

·        Sulphonated Malanic formaldehyde( SMF)

·        Sulphonated Napthalene formaldehyde( SNF)

3. Retarders

These are the admixtures which slows down the chemical process of hydration so that concrete can remain workable for more duration

They can also be used to overcome the effect t of acceleration temperature over the hydration process Their normal dose in range of 0.05-0.02% by wt. of cement at which they can delay the hydration process upto 72 hrs

They find their application where transportation period required is more and in the casting of oil well (where temp may go upto 200 ^OC )

Commonly used retarder includes

·        Calcium sulphate

·        Tartaric acid

·        Starch

·        Cellulose

·        Sugar

4. Retarding plasticizers

 Plasticizers also act as retarders as they form the layer over cement particles and delay the hydration process

Thereby acting as retarder One must be very careful in the selection of such retarding admixture as their improper dose may impart undesirable properties in concrete

Retarding plasticizers or super plasticizers category of admixture is often used in ready mix  concrete RMC

5. Accelerators

These are the type of admixture which increases the rate of hydration  

They find their application in

·        Pavement construction

·        Pre fabricated structures

·        Cold weather concreting

·        Where form work is to be re used for speedy construction

Their normal dose varies in the range of 0.1-O.2% of wt. of cement

Commonly used accelerators includes

·        Calcium Chloride

·        Silicates

·        Fluro silicates

·        Tri-ethanol amine

6. Accelerating plasticizers

These admixtures are added to accelerate the strength development of concrete via higher workability Such accelerating plasticizers when added to concrete increases rate of development of higher strength

The accelerating materials added to the plasticizer and superplasticizer are

·        triethanol amine,

·        calcium nitrate ,

·        nitrates and fluro silicates, etc.

7. Air entraining admixture

These are the type of admixture which entrain millions of air bubbles in voids of concrete which act as flexible ball bearing and slip pass over each other thereby modify almost every property of concrete

These are now days used as fifth ingredients in the preparation of concrete

Commonly used air entraining admixture includes

·        Aluminum powder

·        Hydrogen peroxide

·        Plants and animal fatty acid

·        Natural wood resin

·        Oelic acid and ferric  acid

 

These admixture effect following properties

Ø  Increase resistance of freezing and thawing

Ø  Increases workability

Ø  Reduces strength

Ø  Reduces tendency of segregation and bleeding

Ø  Decreases permeability

Ø  Increases resistance to chemical attack

Ø  Reduces unit weight and alkali aggregate reaction