Admixtures in concrete/Types of admixtures

Advanced Concrete Technology
Admixtures in Concrete
By
Dr. S.K

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.
OR
Admixtures are chemicals they, could be minerals as called as mineral
admixtures, they are added to mortar or grout at the time of mixing to
modify properties either in the wet state or after the mix is hardened.
Additive is a material which is added at the time of grinding cement
clinker at the cement factory.
• Used to enhance quality and durability to suit the application
• Used since 1930, popularized with Plasticizers around 1970
• Popular in India from 1985 in higher strength concrete for high rise
buildings and bridges.
Admixtures

Fig 1. Admixtures (Source: Internet)

Fig 2. Adding of admixtures (Source: Internet)

Functions of Admixtures in Concrete
Followings are the main purposes for which admixtures should be added
in the concrete mix:
 To increase or decrease the setting time of the fresh concrete mix.
 To make better or enhance the workability or flowability of concrete
mix which is the main property of the concrete.
 To maximize the strength and durability of the concrete.
 To reduce the heat of hydration
 To lowers the segregation and bleeding which may occur during the
placing of concrete.
 To reduce the permeability of concrete
 To achieve other desirable properties

□As per the report of the ACI(Association of Construction Inspectors),
admixtures have been classified into 15 groups according to type of
materials constituting the admixtures, or characteristic affect of
the use
Admixtures
• Plasticizers
• Superplasticizers
• Retarders and Retarding Plasticizers
• Accelerators and Accelerating Plasticizers
• Air-entraining Admixtures
• Pozzolanic or Mineral Admixtures
• Damp-proofing and Waterproofing Admixtures
• Gas forming Admixtures

• Air-detraining Admixtures
• Alkali-aggregate Expansion Inhibiting Admixtures
• Workability Admixtures
• Grouting Admixtures
• Corrosion Inhibiting Admixtures
• Bonding Admixtures
• Fungicidal, Germicidal, Insecticidal Admixtures
• Colouring Admixtures

There are two main categories in which different types of
admixtures used in concrete classified
1.Chemical admixtures
A chemical admixture is any chemical additive to the
concrete mixture that enhances the properties of concrete in
the fresh or hardened state. It does not typically include
paints and protective coatings
2.Minerals admixtures
According to the definition of ASTM, Pozzolans are siliceous
or alumina materials, they behave no or less cementitious
properties. They are present in fines which react with
calcium hydroxide in the presence of water at ordinary
temperature results in the formation of compounds
possessing cementitious properties.

Plasticizers (Water Reducers)
• Requirement of right workability is the essence of good concrete.
• Concrete in different situations require different degree of workability.
• A high degree of workability is required in situations like deep beams,
thin walls of water retaining structures with high percentage of steel
reinforcement, column and beam junctions, tremie concreting, pumping of
concrete etc.,
• The conventional methods followed for obtaining high workability is by
improving the gradation, or by the use of relatively higher percentage of
fine aggregate or by increasing the cement content.
• There are difficulties and limitations to obtain high workability in the field
for a given set of conditions.
• The easy method generally followed at the site in most of the conditions is
to use extra water unmindful of the harm it can do to the strength and
durability of concrete.
• One must remember that addition of excess water, will only improve the
fluidity or the consistency but not the workability of concrete.

The plasticized concrete will improve the
desirable qualities demanded of plastic concrete.
• The practice all over the world now is to use plasticizer or
superplasticizer for almost all the reinforced concrete and even for
mass concrete to reduce the water requirement for making concrete
of higher workability or flowing concrete.
• Moreover, the reduction in water/cement ratio improves the
durability of concrete.
• Sometimes the use of plasticizers is employed to reduce the cement
content and heat of hydration in mass concrete.
• 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.

Action of plasticizers
The action of plasticizers is mainly to fluidify the mix
and improve the workability of concrete, mortar or
grout.
• Dispersion
• Retarding Effect
12

Dispersion
 Portland cement-have a tendency of flocculate.
 flocculation entraps certain amount of water.
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
13

Retarding Effect
 The plasticizer will get 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.
14

Fig.3 Mechanism of plasticizers (Source: Internet)

Addition of plasticizers may involve one or more of
the following mechanisms to take place simultaneously:
• Reduction in the surface tension of water.
• Induced electrostatic repulsion between particles of cement.
• Lubricating film between cement particles.
• Dispersion of cement grains, releasing water trapped
within cement flocs.
• Inhibition of the surface hydration reaction of the cement
particles, leaving more water to fluidify the mix.
• Change in the morphology of the hydration products.
• Induced steric hindrance preventing particle-to-particle
contact.

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.
• Among these, Calcium, Sodium and Ammonium
Lignosulphonates are the most used.
• Plasticizers are used in the amount of 0.1% to 0.4% by
weight of cement. At these doses, at constant workability
the reduction in mixing water is expected to be of the order
of 5% to 15%. This naturally increases the strength.

Superplasticizers (High Range Water Reducers)
• Use of Superplasticizers permit the reduction of
water to the extent upto 30 per cent without
reducing workability in contrast to the possible
reduction up to 15 per cent in case of plasticizers
• It is the use of superplasticizer which has made it
possible to use w/c as low as 0.25 or even lower
and yet to make flowing concrete to obtain strength
of the order 120 Mpa or more

Superplasticizers can produce:
• At the same w/c ratio much more workable concrete than
the plain ones,
• For the same workability, it permits the use of lower w/c
ratio,
• As a consequence of increased strength with lower w/c
ratio, it also permits a reduction of cement content
Factors Affecting the Workability
• Type of Superplasticizers
• Dosage
• Mix composition
• Variability in cement composition and properties
• Mixing procedure
• Equipments

Dosage
• Dosage of superplasticizer influences the viscosity
of grout and hence the workability of concrete
• The optimum dosage can be ascertained from
Marsh cone test if brand of cement, plasticizer
and w/c ratio is already fixed
• Optimum dosage of 2.5% - 3% by weight of
cement, dosages up to 4 to 5% used in special
situations

1. Field tests to determine optimum dosage
of the superplasticizer
1. Marsh cone test
2. Mini slump test
3. Flow table test.
21

Marsh cone test
• Cement slurry is made and its flow ability is found out.
• In concrete, cement paste that influences flow ability.
• The presence of aggregate will make the test more
complex and often erratic.
• using grout alone will make the test simple, consistent and
indicative of the fluidifying effect of superplasticizer with
cement
22

• Marsh cone is a conical brass vessel, which has a
smooth aperture at the bottom of diameter 5 mm.
23
Fig.4 Marsh Cone (Source: Internet)

Procedure
• Take 2 kg cement, proposed to be used at the project.
• Take one liter of water (w/c =0.5) and say 0.1% of
plasticizer.
• Mix them thoroughly in a mechanical mixer
(Hobart mixer is preferable) for two minutes.
• If hand mixing is done, the slurry should be
sieved through 1.18 sieve to exclude lumps.
24

 Take one liter slurry and pour it into Marsh cone duly
closing the aperture with a finger.
 Start a stop watch and simultaneously remove the finger.
 Find out the time taken in seconds, for complete flow out
of the slurry.
 The time in seconds is called the “Marsh cone time”
 Repeat the test with of plasticizer.
25

 The dose at which the Marsh cone time is lowest is
called the saturation point.
 The dose is the optimum dose for that brand of
cement and plasticizer or super plasticizer for that
w/c ratio.
26

Fig.5 Marsh Cone (Source: Internet)

Fig.6 Comparison of plasticizers ,Super plasticizers ,New generation
plasticizers(Source: Internet)

Mechanism of Super Plasticizers
• More or less same in case of ordinary plasticizer.
• Super plasticizers are more powerful as
dispersing agents and they are high range water
reducers.
• With super plasticizers It possible to use w/c as low
as 0.25 or even lower.
• Use of superplasticizer - fly ash, slag and
particularly silica fume to make high performance
concrete.
29

Classification of superplasticizer
• Sulphonated melamine-formaldehyde condensates
(SMF)
• Sulphonated naphthalene-formaldehyde condensates
(SNF)
• Modified lignosulphonates (MLS)
30

Effects of superplasticizers on fresh concrete
• No dramatic improvement in workability-zero slump.
• Initial slump of about 20 to 30 mm - fluidized by plasticizers or
super plasticizers at nominal dosages.
• High dosage is required to fluidify no slump concrete.
• An improvement in slump value can be obtained to the extent of
250mm or more depending upon the initial slump of the mix, the
dosage and cement content.
31

Compatibility of superplasticizers and
cement
• All super plasticizers are not showing the same extent
of improvement in fluidity with all types of cements.
• They are just not compatible to show maximum
fluidizing effect.
• Optimum fluidizing effect at lowest dosage is an
economical consideration.
32

Effect of super plasticizers on the
properties of hardened concrete
• Once the effect of adsorbed layer is lost, the hydration continues
normally.
• No bad effect upto 3% by weight of cement.
• Only if the bad quality lignosulphonate based plasticizers.
• Since plasticizers and super plasticizers improve the workability,
compatibility and facilitate reduction on w/c ratio, and thereby
increase the strength of concrete.
• Hence the use of superplasticizers is a pragmatic step to improve
all-round properties of hardened concrete. 33

RETARDERS
• 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.
• The retarders are used in casting and consolidating large number of
pours without the formation of cold joints.
• They are also used in grouting oil wells. Oil wells are sometimes
taken upto a depth of about 6000 meter deep where the temperature
may be about 200°C.
• The annular spacing between the steel tube and the wall of the well
will have to be sealed with cement grout.

• Sometimes at that depth stratified or porous rock strata may also require to
be grouted to prevent the entry of gas or oil into some other strata.
• For all these works cement grout is required to be in mobile condition for
about 3 to 4 hours, even at that high temperature without getting set.
• Use of retarding agent is often used for such requirements.
The most commonly known retarders are,
• Calcium sulphate.
• Gypsum
• Starches,
• Cellulose products,
• Sugars, (0.05 to 0.10 per cent)
• Acids or salts of acids.
• These chemicals may have variable action on different types of cement
when used in different quantities.
• use of admixture should not be attempted without technical advice.

Different Retarding Agents:
1. Ligno sulphonic acids and their salts
2. hydroxylated carboxylic acids and their salts.
In addition to the retarding effect also reduce the quantity of
water requirement for a given workability.These days’
admixtures are manufactured to combine set retarding and
water reducing properties
36

Retarding Plasticizers
 plasticizers and super plasticizers by themselves show
certain extent of retardation.
 Many a time this extent of retardation of setting time
offered by admixtures will not be sufficient.
 Retarders are mixed with plasticizers or super
plasticizers at the time of commercial production.
plasticizers or retarding super plasticizers.
 Such commercial brand is known as retarding

Extreme weather concreting:
There are two major extreme weather
conditions,
1. Hot weather concreting
2. Cold weather concreting

EXTERME WEATHER CONCRETING:-
• In countries which experience extreme weather condition
special problems are encountered in preparation, placement and
curing of concrete.
• India has regions of extreme hot weather (hot –humid and hot-
arid) as well as cold weather .
• The Indian standards dealing with extreme weather concreting
are:-
IS: 7861 (Part 1-1975)- Hot weather concreting IS:
7861 (Part 2-1981)- cold weather concreting

HOT WEATHER CONCRETEING:-
• Hot weather is any combination of the following
conditions that tends to impair the quality of freshly mixed
or hardened concrete by accelerating the rate of moisture
loss and rate of cement hydration, or otherwise causing
detrimental results:
• High concrete temperature;
• Low relative humidity;
• Wind speed
• Solar radiation.
• High ambient temperature.

Difficulties in Hot Weather:-
(a)Rapid rate of hydration of cement, quick setting and
early stiffening.
(b)Rapid evaporation of mixing water.
(c ) Greater plastic shrinkage.
(d) Less time for finishing.
(e) Reduced relative humidity.
(f)Absorption of water from the concrete by the
subgrade and formwork.
(g) Difficulty in continuous and uninterrupted curing.
(h) Difficulty in incorporation of air entrainment.

Precautions:-
• Use materials and mix proportions that have a good
record in hot weather conditions.
• Cool the concrete or one or more of its ingredients.
• Use a concrete consistency that allows rapid placement.
• Reduce the time of transporting, placing, and finishing as
possible.
• Schedule concrete placements to avoid extreme weather,
such as at night or during favorable weather conditions.
• Consider the methods to limit moisture loss during placing
and finishing such as sunshades, wind screens, fogging,
and spraying.

Effect of High Concrete Temperature:-
• As concrete temperature increases there is a loss in slump that is often
unadvisedly compensated for by adding water to the concrete at the
jobsite. At higher temperatures a greater amount of water is required to
hold slump constant than is needed at lower temperatures.
• increase the rate of setting and shorten the length of time within
which the concrete can be transported, placed, and finished.
• Setting time can be reduced by 2 or more hours with a 10°C increase in
concrete temperature
Fig.7 High Concrete Temperature(Source: Internet)

• There is an increased tendency for cracks to form both before
and after hardening.
• Rapid evaporation of water from freshly placed concrete can
cause plastic-shrinkage cracks before the surface has
hardened.
• Cracks may also develop in the hardened concrete because of
increased drying shrinkage due to higher water contents or
thermal volume changes as the concrete cools.
Where
T=Temperature of the feshly mixed concrete ,Celsius
Ta,Tc,Tw and Twa=Temperature of aggregates, cementing
materials, added mixing water and free water on aggregates,
respectively

Fig.8 Adding ice for substituting
water in the
concrete mix (Source: Internet)
Fig.9 Adding liquid nitrogen
(Source: Internet)

Preparation Before Placing:-
• Mixers, chutes, conveyor belts, hoppers, pump lines, and
other equipments for handling concrete should be shaded,
painted white, or covered with wet burlap to reduce solar
heat.
• Forms, reinforcing steel, and subgrade should be fogged or
sprinkled with cool water just before concrete is placed.
• Restrict placement of concrete to early morning, evening,
or night time hours, especially in arid climates. This will
help in minimizing thermal shrinkage and cracking of
thick slabs and pavements.

Transporting, Placing, and Finishing:-
• Should be done as quickly as practical
weather.
during hot
• Delays contribute to the loss of slump and increase in
concrete temperature.
• Prolonged mixing should be avoided.
• If delays occur, stopping mixer and then agitating can mi
• Setting of concrete is more rapid in hot weather.
• Extra care must be taken with placement techniques to
avoid cold joints.
• Temporary sunshades and windbreaks help to minimize
cold joints. nimize the heat generated by mixing.

Curing in Hot Weather :-
• The need for moist curing of concrete slabs is greatest during the first
few hours after finishing.
• To prevent the drying of exposed concrete surfaces, moist curing should
commence as soon as the surfaces are finished.
• When the air temperature is at or above 27°C, curing during the basic
curing period should be accomplished by water spray or by using
saturated absorptive fabric
• For mass concrete, curing should be by water for the basic curing period
when the air temperature is at or above 20°C, in order to minimize the
temperature rise of the concrete.
• If approved, the application of the curing compound should be preceded by
24 hours of moist curing.
• Crazing cracks are very fine and barely visible except when the
concrete is drying after the surface has been wet. They do not penetrate
much below the surface.

Admixtures:-
• A retarding admixtures can be very helpful in
delaying the setting time, despite increased rate of
slump loss resulting from their use.
• A hydration control admixture can be used to stop
cement hydration and setting.
• As a general rule a 5°C to 9°C temperature rise per 45 kg
of Portland cement can be expected from the heat of
hydration.

Cold weather concreting:-
• Concrete can be placed safely without damage from freezing
throughout the winter months in cold climates if certain precautions
are taken.
• Cold weather is defined by ACI Committee 306 as a period when
for more than 3 successive days the average daily air temperature
drops below 5°C (40°F) and stays below 10°C (50°F) for more than
one-half of any 24 hour period.
• Under these circumstances, all materials and equipment needed for
adequate protection and curing must be on hand and ready for use
before concrete placement is started.
• Normal concreting practices can be resumed once the ambient
temperature is above 10°C (50°F) for more than half a day.
• During cold weather, the concrete mixture and its temperature
should be adapted to the construction procedure and ambient
weather conditions.
• Preparations should be made to protect the concrete; enclosures,
windbreaks, portable heaters, insulated forms, and blankets should
be ready to maintain the concrete temperature.

• Forms, reinforcing steel, and embedded fixtures must be
clear of snow and ice at the time concrete is placed.
• Thermometers and proper storage facilities for test
cylinders should be available to verify that precautions
are adequate.
Fig.10 Cold Weathering Concrete
(Source: Internet)

EFFECT OF FREEZING FRESH CONCRETE:-
• Concrete gains very little strength at low temperatures.
Freshly mixed concrete must be protected against the
disruptive effects of freezing until the degree of saturation of
the concrete has been sufficiently reduced by the process of
hydration. The time at which this reduction is accomplished
corresponds roughly to the time required for the concrete to
attain a compressive strength
• Concrete that has been frozen just once at an early age can
be restored to nearly normal strength by providing
favourable subsequent curing conditions.
• The critical period after which concrete is not seriously
damaged by one or two freezing cycles is dependent upon
the concrete ingredients and conditions of mixing, placing,
curing, and subsequent drying.
• For example, air-entrained concrete is less susceptible to
damage by early freezing than non air- entrained concrete.

• Up to 50% reduction of ultimate strength can occur if
frozen - - Within a few hours
- Before reaching a strength of 3.5 MPa (500 psi)
• Frozen only once at an early age -
-With curing nearly all strength can be restored
- Less resistance to weathering
- More permeable
Thumb Rule
• “For every 10°C (18°F) reduction in concrete temperature,
the times
increasing
of setting of the concrete double, thus
the amount of time that the concrete is
vulnerable to damage due to freezing.”

• “Accelerating admixtures are added to concrete shortening
setting time and/or increasing early strength development”
• The rates of chemical reactions between clinker materials in
cements and water, often referred to as cement hydration reactions,
may be altered by adding small amounts of chemical substances to
the cement-water mix
• Substances affecting these rates to give an overall increase in the
hydration rate, i.e. an accelerating effect, are termed accelerating
admixtures or simply accelerators
• Hence, an accelerator is added to concrete for the purpose of
shortening setting time and/or increasing early strength
development
Accelerators

• Set accelerating admixture : Admixture which decreases the time to
commencement of transition of the mix from the plastic to the rigid
state
• Hardening accelerating admixture : Admixture which increases the
rate of development of early strength in the concrete, with or without
affecting the setting time
Benefits provided by accelerators
The benefits of a reduced setting time may include :
• Earlier finishing of surfaces
• Reduction of hydraulic pressure on forms
• More effective plugging of leaks against hydraulic pressure

The benefits of an increase in the early strength may include :
•Earlier removal of formworks
•Reduction of the required period of curing and protection
•Earlier placement in service of a structure or a repair
• Partial or complete compensation for the effects of low
temperatures on strength development

Usage:
• Accelerating admixtures are added to concrete to increase
the rate of early strength development in concrete
• 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 emergency repair work

Constituent
• In the past one of the commonly used materials as an accelerator was
calcium chloride but found harmful for reinforced concrete and pre-
stressed concrete
• Some of the soluble carbonates, silicates, fluosilicates and some of
the organic compounds such as triethenolamine are used
• Accelerators such as fluosilicates and triethenolamine are
comparatively expensive
Applications
• Powerful accelerator makes cement set into stone hard in a matter
of five minutes or less
• Under water concreting has become easy
• Repair work that would be carried out to the waterfront structures in
the region of tidal variations has become easy

• Powerful accelerators have facilitated, the basement
waterproofing operations
• In the field of prefabrication also it has become an invaluable
material
• Could be used up to 10°C, they find an unquestionable use in
cold weather concreting
• Sprayed concrete and shotcreting operations
Fig.11 Accelerator admixture
(Source: Internet)

Accelerating Plasticizers
• Ingredients are added to accelerate the strength
development of concrete to plasticizers or super
plasticizers.
• The accelerating materials added to plasticizers or super
plasticizers are triethenolamine chlorides, calcium nitrite,
nitrates and fluosilicates etc.
60

Air-entraining admixture
 Air entrained concrete is made by mixing a small quantity of
air entraining agent or by using air entraining cement.
 air entraining agents incorporate millions of no- coalescing
air bubbles.
 Modify the properties of plastic concrete regarding
workability, segregation, bleeding and finishing quality of
concrete.
 Its resistance to frost Daepcarttmieontn of Caivnil d
permeability.
61

Fig.12 Air entrained concrete
(Source: Internet)
Fig.13 Entrained & Entrapped Air (Source:
Internet)
Fig.14 Entrained & Entrapped Air (Source: Internet)

Air voids in concrete
 Entrained air - size ranging from 5 microns to 80 microns
distributed evenly in the entire mass of concrete.
 Entrapped air- size may range from 10 to 1000 microns or
more and they are not uniformly distributed throughout the
concrete mass
63

Air entraining agents
• Natural wood resins.
• Animal and vegetable fats and oils, such as tallow, olive oil and their
fatty acids such as stearic and oleic acids.
• Various wetting agents such as alkali salts or sulphated and sulphonated
organic compounds.
• Water soluble soaps of resin acids, and animal and vegetable fatty acids
• Miscellaneous materials such as the sodium salts of petroleum sulphonic
acids, hydrogen peroxide and aluminium powder, etc.
64

Commercial air entraining agents
• MC-Mischoel LP
• MC-Michoel AEA
• Complast AE 215
• Roff AEA 330
65
The effect of air entrainment on the
properties of concrete
• Increased resistance to freezing and thawing-
• Improvement in workability.
• Reduction in strength.
• Effect on segregation & bleeding and laitance

• Effect on permeability.
• Effect on chemical resistance.
• Effect on sand, water and cement content.
• Alkali-aggregate reaction
• Modulus of elasticity
• Abrasion resistance
66

NEW GENERATION SUPERPLASTICIZERS
A new generation of superplasticizers, based on polycarboxylate
ether polymers (CE) which allow for a reduction of water content of
up to 40 % and at the same time give an extended slump retention
will make the production of such high performance concretes a
realistic task.

Fig.15 Correlation between water cement ratio,workability and
compressive strength (Source: Internet)

MECHANISM OF NEW GENERATION SUPERPLASTICIZERS
• The dispersion mechanism of polycarboxylate based
superplasticizers is mainly due to two different types of
repulsion forces between the cement particles : electrostatic
repulsions due to the presence of the negative charge given by
the carboxylic groups and steric repulsion effect due to the
main and long chains of the polymers.
• The electrostatic repulsion force for CE is half of the value
measured for SNF superplasticizers and the dispersion is
mainly due to the very strong steric repulsion effect

The chemical structure of polymers present in the new CE
superplasticizers consists of a main flexible backbone,
containing negatively charged carboxylic groups, and a large
number of side chains.
Fig.16 Mechanism of super plasticizer (Source:
Internet)

The opportunities offered by CE based superplasticizers.
Their influence on some of the key factors (strength, w/c
ratio and slump retention) has a great impact on the
performance characteristics of concrete. Other parameters
which are indirectly positively influenced by the use of
CE based superplasticizers - which are also called
"advanced superplasticizers" - are those of shrinkage,
creep and of elasticity modulus

Pozzolanic or Mineral admixtures
• According to the definition of ASTM, Pozzolans are
siliceous or alumina materials, they behave no or less
cementitious properties.
• They are present in fines which react with calcium
hydroxide in the presence of water at ordinary temperature
results in the formation of compounds possessing
cementitious properties.
• It was recognized long time ago, that the suitable
pozzolans used in appropriate amount.
73

• Ancient Greeks and Romans used certain finely divided
siliceous materials which when mixed with lime produced
strong cementing material.
• After the development of Portland cement in the early
19th century, the practice of using pozzolans declined.
• But in more recent times, Pozzolans have been extensively
used in Europe, USA and Japan, as an ingredient of
Portland cement concrete particularly for marine and hydraulic
structures.
• Pozzolan(MineralAdmixtures)+CalciumHydroxide+Water=C-
S-H (Gel)
• This reaction is called pozzolanic reaction. The consumption of
calcium hydroxide improves the durability of concrete by
making more denser and impervious paste.
74

Advantages of Pozzolans
 Lower the heat of hydration and thermal shrinkage.
 Increase the water tightness.
 Reduce the alkali-aggregate reaction.
 Improve resistance to attack by sulphonate soils and sea
water.
 Improve workability.
 Lower costs.
75

Types of Pozzolans
1.Natural Pozzolans- The natural pozzolans are naturally available
on earth.
• Clay and shales
• Volcanic tuffs
• Opaline Cherts
2.Artificial Pozzolans-
• Fly ash
• Rice husk ash
• Silica Fume
• Blast furnace slag

Fly ash
• “Fly ash is finely divided residue resulting from the
combustion of powdered coal and transported by the
flue gases and collected by electrostatic
precipitator.”
• The fly ash is an industrial waste, its use in concrete
significantly improves strength in long-term
• The fly ash used in cement additives have high
fineness, low carbon content and highly reactive
forms 77

• Fly ash is the most widely used pozzolanic material all over
the world.
• Hungry Horse dam in America in the approximate amount of 30
per cent by weight of cement.
• Rihand dam construction replacing cement upto about 15 per cent
• Bandra worli Bridge upto 50%
• use of fly ash has become a common ingredient in concrete,
particularly for making high strength and high performance
concrete.
• The use of fly ash as admixture not only extends technical
advantages but also contributes to the environmental pollution
control.
78

Utilization of fly ash
Two ways that the fly ash can be used:
– One way is to intergrind certain percentage of fly ash with
cement clinker- Portland pozzolana cement (PPC).
– Use the fly ash as an admixture at the time of making
concrete at the site of work.
79
ASTM classification
• Class F- Burning antharacite or bituminous coal usually has
less than 5% CaO.
• Class C - Burning lignite or sub-bituminous
coal, some fly ash may have CaO content in excess of 10%.
• Fly ash, has to be tested in accordance with IS: 1727- 1967.

Effect of fly ash on fresh concrete
• Good fly ash with high fineness, low carbon content, highly
reactive forms only a small fraction of total fly ash collected.
• Use of right quality fly ash:
• reduction of water demand
• reduction of bleeding and drying shrinkage
• heat of hydration is reduced
80

Effects of fly ash on hardened concrete
• Contributes to the strength of concrete due to its
pozzolanic reactivity.
• Pozzolanic reaction proceeds slowly, the initial strength of fly
ash concrete tends to be lower than that of concrete without fly
ash.
• Pozzolanic reaction can only proceed in the presence of water
enough moisture should be available for long time.
• fly ash concrete used in under water structures
81

Durability of concrete
• Sufficiently cured concrete containing good quality fly ash shows
dense structure which offers high resistivity to the infiltration.
• Reduces the calcium hydroxide content, which results in
reduction of passivity to the steel reinforcement.
• secondary cementitious material formed makes the paste
structure dense, and thereby gives more resistance to the
corrosion of reinforcement.
• Although fly ash is an industrial waste, its use in concrete
significantly improve the long term strength and durability and
reduce heat of hydration.
• In other words, good fly ash will be an indispensable mineral
admixture for high performance concrete
67

Silica fume
 Silica fume, also referred to as micro silica or
silica fume, is an artificial pozzolanic
condensed
admixture.
 It is a product resulting from reduction of high purity quartz with
coal in an electric arc furnace in the manufacture of silicon or
ferrosilicon alloy.
 It is extremely fine with particle size less than 1 micron and with
an average diameter of about 0.1.
83

Properties of silica fume
• Micro silica is initially produced as an ultra fine
undensified powder.
• At least 85% SiO2 content.
• Mean particle size between 0.1 and 0.2 micron
• Minimum specific surface area is 15000 m2/kg
• Spherical particle shape.
84

Pozzolanic action
• Micro silica is much more reactive than fly ash or any other
natural Pozzolana.
• The reactivity of a Pozzolana can be quantified by measuring
the amount of calcium hydroxide in the cement paste at
different times.
85

Influence on fresh concrete
• Water demand increases in proportion to the amount of
micro silica added.
• About 1% for every 1% of cement substituted
86
Influenceon hardened concrete
• Concrete containing micro silica showed outstanding
characteristics in the development of strength.

Ground granulated blast furnace slag
(GGBS)
• Non metallic product consisting essentially of silicates
and aluminates of calcium and other bases.
• The molten slag is rapidly chilled by quenching in water ton
form a glassy sand like granulated material.
• Grounded to less than 45 micron will have specific
surface of about 400-600m2/kg
87

Chemical composition
• Calcium oxide 30-45%
• Silicon dioxide 30-38%
• Aluminium oxide 15-25%
• Ferrous oxide 0.5-2.0
• Specific gravity 2.4
88

Utilization of GGBS
Two methods for making blast furnace slag cement:
• Blast furnace slag is interground with cement clinker
along with gypsum.
• Blast furnace slag is separately ground and then mixed
with the cement.
• Recently for marine outfall work at Bandra, Mumbai, GGBS
has been used as an admixture to replace cement to the tune of
70%.
• Growing popularity of RMC, the scope for using GGBS should
also become popular. 89

Fresh concrete
• GGBS will reduce the unit water content necessary to obtain the
same slump.
• water used for mixing is not immediately lost, as the surface
hydration slag is slightly slower than that of cement.
• Reduction of bleeding if the slag has a fineness of 6000 sqcm/g
and above.
Hardened concrete
• Reduced heat of hydration.
• Refinement of pore structures.
• Reduced permeability to the external agencies.
• increased resistance to chemical attack. 90

Other Mineral Admixtures
Rice Husk Ash
• Rice husk ash is a product which is obtained by
burning rice husk in a controlled manner without
causing environmental pollution.
• Rice husk ash has high SiO2 content when it is
properly burnt.
• Rice husk ash shows high pozzolanic characteristics
and contributes to high strength and high
impermeability of concrete.

Metakaolin
• Metakaolin is the un purified thermally activated
ordinary clay and kaolin clay.
• Metakaolin is not highly reactive pozzolanic
material, but it shows high pozzolanic reactivity
after removing unreactive impurities by water
process.
• The colour of metakaolin is white or cream. With
metakaolin, cement paste undergoes distinct
densification and it helps to increases the strength
and decreases the permeability.

Surkhi (Calcined Clay Pozzolana)
• In India, Surkhi, was and is the commonest pozzolanic material. It
has been used along with lime in many of old Indian structures
before modern Portland cement was used. Nowadays the
terminology “calcined clay Pozzolana” is used instead of the word
surkhi. Surkhi is an artificial pozzolanic material which is made
by powdering bricks or burnt clay balls.
• For large scale production of surkhi, clay balls are specially burnt
and then powdered. Surkhi is a very complex material and differs
widely in its qualities and performances by its nature.
• Its characteristics are greatly influenced by the constituent
mineral composition of soil because it is derived from the soil and
is also influenced by the degree of burning and fineness of
grinding.
• Surkhi was one of the main ingredients in waterproofing
treatments in combination with lime and sometimes even with
cement for extending valuable pozzolanic action to make the

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