Admixtures and its properties used in concrete.

1. Admixtures
CONCRETE TECHNOLOGY
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2. Admixture is defined as a material, other than cement, water and aggregates, that is used
as an ingredient of concrete and is added to the batch immediately before or during mixing in
order to control setting and early hardening, workability, or to provide additional cementing
properties. Additive is a material which is added at the time of grinding cement clinker at the
cement factory.
Type of Admixtures
Chemical admixtures - Accelerators, Retarders, Water-reducing agents, Super plasticizers,
Air entraining agents etc.
Mineral admixtures - Fly-ash, Blast-furnace slag, Silica fume and Rice husk Ash etc
CHEMICAL
ADMIXTURE
MINERAL
Accelerators Fly-ash
Retarders Silica fume
Water Reducing
Agents
Blast-furnace slag
Super plasticizers Rice husk Ash
Air entraining agents Pozzolana
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3. Plasticizers (Water Reducers)
The organic substances or combinations of organic and inorganic substances, which
allow a reduction in water content for the given workability, or give a higher workability at the
same water content, are termed as plasticizing admixtures.
The basic products constituting plasticizers are as follows:
( i ) Anionic surfactants such as lignosulphonates and their modifications and derivatives,
salts of sulphonates hydrocarbons.
( ii ) Nonionic surfactants, such as polyglycol esters, acid of hydroxylated carboxylic acids
and their modifications and derivatives.
( iii ) Other products, such as carbohydrates etc.
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4. Action of Plasticizers
The action of plasticizers is mainly to fluidify the mix and improve the workability of concrete,
mortar or grout. The mechanisms that are involved could be explained in the following way:
Dispersion: Portland cement, being in fine state of division, will have a tendency to flocculate in wet
concrete. These flocculation entraps certain amount of water used in the mix and thereby all the water
not freely available to fluidify the mix. When plasticizers are used, they get adsorbed on the cement
particles. The adsorption of charged polymer on the particles of cement creates particle-to-particle
repulsive forces which overcome the attractive forces. This repulsive force is called Zeta Potential,
depends on the base, solid content, quantity of plasticizer used. The overall result is that the cement
particles are deflocculated and dispersed. When cement particles are deflocculated, the water trapped
inside the flocs gets released and now available to fluidify the mix. Fig. 5.1 explains the mechanism.
Lubrication: When cement particles get flocculated there will be interparticle friction between particle
to particle and floc to floc. But in the dispersed condition there is water in between the cement
and hence the interparticle friction is reduced.
Retarding Effect: It is mentioned earlier that plasticizer gets adsorbed on the surface of cement
particles and form a thin sheath. This thin sheath inhibits the surface hydration reaction between water
and cement as long as sufficient plasticizer molecules are available at the particle/solution interface.
quantity of available plasticizers will progressively decrease as the polymers become entrapped in
hydration products.
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6. Super Plasticizers:
The commonly used Super Plasticizers are as follows:
1. Sulphonated melamine formaldehyde condensates (SMF) - Give 16–25% water reduction.
2. Sulphonated naphthalene formaldehyde condensates (SNF)-Typically give 16–25% water reduction.
3. Polycarboxylate ether superplasticizers (PCE) - Typically give 20–35% water reduction.
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7. Retarder
A retarder is an admixture that slows down the chemical process of hydration so that concrete remains
plastic and workable for a longer time than concrete without the retarder.
Retarders are used to overcome the accelerating effect of high temperature on setting properties of
concrete in hot weather concreting. Sometimes concrete may have to be placed in difficult conditions and
delay may occur in transporting and placing.
Perhaps the most commonly known retarder is calcium sulphate. The appropriate amount of gypsum to
be used must be determined carefully for the given job. In addition to gypsum there are number of other
materials found to be suitable for this purpose. They are: starches, cellulose products, sugars, acids or salts
of acids.
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8. Accelerators
Accelerating admixtures are added to concrete to increase the rate of early strength
development in concrete to
• permit earlier removal of formwork;
• Reduce the required period of curing;
• Advance the time that a structure can be placed in service;
• Partially compensate for the retarding effect of low temperature during cold weather
concreting;
• In the emergency repair work.
Accelerating admixtures can be divided into groups based on their performance and application:
(1) Set Accelerating Admixtures,
Reduce the time for the mix to change from the plastic to the hardened state.
(2) Hardening Accelerators,
Which increase the strength at 24 hours by at least 120% at 20ºC and at 5ºC by at least
130% at 48 hours. Hardening accelerators find use where early stripping of shuttering or very
early access to pavements is required. A hardening accelerator may be appropriate for strength
gain up to 24 hours at low temperature and up to 12 hours at ambient temperatures. Beyond
these times, a high range water reducer alone will usually be more cost-effective.
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Air-entraining Admixture
Air entrained concrete is made by mixing a small quantity of air entraining agent
or by using air entraining cement. These air entraining agents incorporate millions of
non-coalescing air bubbles of size ranging from 5 microns to 80 microns distributed
evenly in the entire mass of concrete, which will act as flexible ball bearings and will
modify the properties of plastic concrete regarding workability, segregation, bleeding
and finishing quality of concrete. It also modifies the properties of hardened concrete
regarding its resistance to frost action and permeability.
Examples
( a ) Natural wood resins ( b ) Animal and vegetable fats and oils, such as tallow, olive
oil and their fatty acids such as stearic and oleic acids. ( c ) Various wetting agents such
as alkali salts or sulphated and sulphonated organic compounds. ( d ) Water soluble
soaps of resin acids, and animal and vegetable fatty acids. ( e ) Miscellaneous materials
such as the sodium salts of petroleum sulphonic acids, hydrogen peroxide and
aluminium powder etc.

10. 10The Effect of Air Entrainment on the Properties of Concrete
Air entrainment will effect directly the following three properties of concrete:
( a ) Increased resistance to freezing and thawing.
( b ) Improvement in workability.
( c ) Reduction in strength.
Incidentally air entrainment will also effect the properties of concrete in the
following ways:
( a ) Reduces the tendencies of segregation.
( b ) Reduces the bleeding and laitance.
( c ) Decreases the permeability.
( d ) Increases the resistance to chemical attack.
( e ) Permits reduction in sand content.
( f ) Improves place ability, and early finishing.
( g ) Reduces the cement content, cost, and heat of hydration.
( h ) Reduces the unit weight.
( i ) Permits reduction in water content.
( j ) Reduces the alkali-aggregate reaction.
(k) Reduces the modulus of elasticity.

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Increased resistance to freezing and thawing
It is found that when ordinary concrete is subjected to a temperature below freezing point,
the water contained in the pore of the concrete freezes. The volume of ice is about 10 per cent
higher than the corresponding volume of water. Hence, the ice formed in the pores of hardened
concrete exerts pressure. The cumulative effect of this pressure becomes considerable, with the
result that surface scaling and disruption of concrete at the weaker section takes place. In the air
entrained concrete though the total air voids are more, the voids are in the form of minute,
discrete bubbles of comparatively uniform size and regular spherical shape. This air void system
reduces the tendency for the formation of large crystals of ice in the concrete. Secondly, the
inter-connected air voids system acts as buffer space to relieve the internal pressure.

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Effect on workability
The entrainment of air in fresh concrete by means of air entraining agent improves
workability. Better place ability of air entrained concrete results in more homogeneous concrete
with less segregation, bleeding and honeycombing. The concrete containing entrained air is
more plastic and ‘fatty’ and can be more easily handled than ordinary concrete

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Effect on strength
It can be generally stated that air entrainment in concrete reduces the compressive strength
of concrete. But when the process is applied properly, taking advantage of the benefits
accrued on account of air-entrainment, little or no loss of strength should take place.

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Reduces the segregation bleeding and laitance
Segregation, bleeding and consequent formation of laitance are reduced greatly by air
entrainment. These actions probably result from physical phenomena due to the incorporation
of a system of air bubbles. Firstly, the bubbles buoy up the aggregates and cement and hence
reduce the rate at which sedimentation occurs in the freshly placed concrete. Secondly, the
bubbles decrease the effective area through which the differential movement of water may
occur. Thirdly, the bubbles increase the mutual adhesion between cement and aggregate. Lastly,
the surface area of voids in the plastic concrete is sufficiently large to retard the rate at which
water separates from the paste by drainage.

15. 15MINERAL ADMIXTURE
Types of Mineral Admixtures
(1) Cementitious :These have cementing properties themselves.
For example: Ground granulated blast furnace slag (GGBFS)
(2) Pozzolanic : A pozzolan is a material which, when combined with calcium hydroxide
(lime), exhibits cementitious properties.
Pozzolans are commonly used as an addition (the technical term is "cement extender") to
Portland cement concrete mixtures to increase the long-term strength and other material
properties of Portland cement concrete and in some cases reduce the material cost of concrete.
Examples are : Fly ash ,Silica Fume, Rice Husk Ash, Metakaolin,

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Pozzolanic Action:
The additive act in three ways
(1) Filler; (2) Nucleating; (3) Pozzolanic
1. Filler: These additives/admixtures are finer than cement, so when added to concrete they
occupy the small pores previously left vacant.
2. Nucleating: These fine particles accelerate the rate of hydration and precipitation starts.
3. Pozzolanic: When cementing material reacts with water the following reaction take place:
C2S + H CSH + CH
C3S + H CSH + CH
CSH is responsible for strength while CH is a soluble material reacts and dissolves in water
leaving behind pores.
So when admixture is added
SiO3 or Al2O3+CH CSH
Thus it reduces the amount of CH & increase CSH
Conditions to Declare a Material Pozzolan:
Having silica + Alumina oxide+ ferrous oxide more than 70%.
Surface area on normal admixture is more than 300m²/kg.
Surface area should be more than cement used.

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Silica Fume
By-product of semiconductor industry
Silica Fume consists of very fine particles with a surface area ranging from 60,000 to 150,000
ft²/lb or 13,000 to 30,000 m²/kg, with particles approximately 100 times smaller than the
average cement particle.
Because of its extreme fineness and high silica content, Silica Fume is a highly effective
pozzolanic material. S

18. 18Rice Husk Ash:
This is a bio waste from the husk left from the grains of rice.
It is used as a pozzolanic material in cement to increase durability and strength.

19. 19Fly Ash:
The finely divided residue resulting from the combustion of ground or powdered coal. Fly
ash is generally captured from the chimneys of coal-fired power plants; it has POZZOLANIC
properties, and is sometimes blended with cement for this reason.
Fly ash includes substantial amounts of silicon dioxide (SiO2) (both amorphous and crystalline)
and calcium oxide (CaO).
Class F Fly Ash:
The burning of harder, older anthracite and bituminous coal typically produces Class F fly ash.
This fly ash is pozzolanic in nature, and contains less than 10% lime (CaO).
Class C Fly Ash:
Fly ash produced from the burning of younger lignite or subbituminous coal, in addition to
having pozzolanic properties, also has some self-cementing properties.
In the presence of water, Class C fly ash will harden and gain strength over time.

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Ground Granulated Blast Furnace Slag (GGBFS)
Ground granulated blast-furnace slag is the granular material formed when molten iron blast
furnace slag (a by-product of iron and steel making) is rapidly chilled (quenched) by immersion in
water.
It is a granular product, highly cementitious in nature and, ground to cement fineness, hydrates
like Portland cement. GGBFS is used to make durable concrete structures in combination with
ordinary Portland cement and/or other pozzolanic materials.
Concrete made with GGBFS cement sets more slowly than concrete made with ordinary Portland
cement, depending on the amount of GGBFS in the cementitious material, but also continues to
gain strength over a longer period in production conditions. This results in lower heat of
hydration and lower temperature rises, and makes avoiding cold joints easier, but may also affect
construction schedules where quick setting is required.
Use of GGBFS significantly reduces the risk of damages caused by alkali-silica reaction (ASR),
provides higher resistance to chloride ingress, reducing the risk of reinforcement corrosion, and
provides higher resistance to attacks by sulfate and other chemicals.

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