Admixture and types of admixtures

ADMIXTURE

Suitable materials known as admixtures
which may be added to concrete mix, Just
before or during the mixing to modify one
or more properties of the concrete in the
plastic or hardened states as desired.

The objective of admixture are :

• To increase the rate of strength
development at early ages.
• To retard the initial setting times.
• To increase the workability without
changing the water content.
• To increase the resistance to freezing
and thawing, vinsol resins and air
entertained admixture which is used
for this purpose which cause air to be
incorporated in the form of minute tiny
bubbles in concrete during mixing to
increase the workability and resistance
to freezing and thawing.

 DIFFERENT TYPE OF ADMIXTURE

1. Mineral Admixtures

a) Fly ash

• This increase impermeability
because it is finer than the cement
particles.
• This is by product of wooden
thermal power plant and is
produced daily in large quantities.
• These require very less water to wet
their surface.
• Same work which is obtained by
OPC is achieved by lower
water/cement ratio.

2. Chemical Admixtures

a) Accelerating Admixture:- 

A substance which increases the rate of
strength development or reduces the
setting time. CaCl2, Sodium silicate,
Sodium chloride, Calcium nitrite and
calcium nitrate are used as accelerator.
• Barium chloride acts as an
accelerator only under warm
condition
• Most frequently used accelerator is
calcium chloride (CaCl2)

b) Retarding Admixture:- 

A substance which delays the setting
time of cement paste. Gypsum,
Tartaric acid, sugar are used as
retarding admixture.
• A small quantity of sugar about 0.05
percent of the mass of cement will
act as an acceptable retarder: the
delay in setting of concrete is about
4 hours.
• A huge quantity of sugar, between 0.2 to
1 percent of the mass of cement, will
virtually prevent the setting of
cement.
• When sugar is used as a controlled
set retarder, the early strength of
concrete is severely reduced but
beyond about 7 days, there is an
increase in strength of several
percent compared with a non-
retarded mix. This is probably due
to the fact that delayed setting
produces a denser hydrated cement
gel.

Note:- 

Retarders tend to increase the plastic
shrinkage because the duration of the
plastic stage is extended, but drying
shrinkage is not affected.

c) Water Reducing Admixture: 

A substance which either increases
workability of freshly mixed mortar or
concrete without increasing water-
cement ratio or maintains workability
with reduced water-cement ratio. These
are:
• Lignosulphonic acid and its salts.
• Formaldehyde derivatives
• Hydroxylated carboxylic acids
• Calcium lignosulphonate

d) Air-entraining Admixture :- 

A substance which causes air to be
entrapped in the form of tiny bubbles in
motor or concrete, during mixing to
increasing its workability and
resistance to freezing and thawing.
Example: Vinsol resin, using of
Aluminum powder.

e) Super Plasticizing Admixture :

A substance which imports very high
workability with a large decrease in
water content (at least 20%) for a given
workability.
The resulting improvement in
workability can be exploited in two
ways. By
a) By producing concrete with a very
high workability or concrete with a
very high strength.
b) Concrete of normal workability but
with an extremely high strength
owing to a very substantial
reduction in the water/cement ratio.
• A high range water reducing admixture
is also referred to as a super plasticizer
and these are sulphonated melamine
formaldehyde condensates; sulfonated
naphthalene-formaldehyde
condensates; modified lignosulfonates;
and other such as sulfonic-acid esters
and carbohydrate esters.
• The role of super plasticizer is to
disperse, the particles remove air
bubbles, example; Sulphonated melanin
formaldehyde and to retard setting