admixtures 31-7-21

7/31/2021 Chemical admixture 2
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and/or from the other sources accessible to the author
have been freely taken for the purpose of teaching and
dissemination of knowledge to the students.
These are gratefully acknowledged.

Concrete Technology
Admixtures
• Admixtures are chemicals, added to concrete, mortar or grout at the
time of mixing, to modify properties either in the plastic state or after
mix has hardened
• Can be a single chemicals or blend

Concrete Technology
Admixtures and additives
• Admixtures is defined as a material, other than cement, water and
aggregates, that is used as an ingredients of concrete and is added to
the batch immediately before or during mixing
• Additives are the material which is added at the time of grinding
cement clinker at the cement factory

Concrete Technology
Types of Admixtures
• Chemical Admixture
• Mineral Admixtures

Concrete Technology
Chemical Admixtures
• Plasticizer (Water Reducing Agent)
• Super-plasticizer (High Range Water Reducing Agent)
• Retarders and Retarding Plasticizers
• Accelerators and Accelerating Plasticizers
• Air Entraining Admixtures
• Other Admixture (Viscosity modifier etc.)

Concrete Technology
Mineral Admixtures
• Fly Ash
• Blast Furnace Slag
• Silica Fume
• Metakaoline
• Rice Husk Ash

Concrete Technology
History of Admixtures
• Plasticizer, 1932
• Air Entraining Admixture, 1942
• Super-Plasticizers, 1963

Concrete Technology
Reason for using Admixture
• To achieve certain properties in concrete more effectively than other
means
• To maintain the quality of concrete during stages of mixing,
transporting, placing, compacting and curing in adverse weather
conditions
• To overcome certain emergencies during concreting operation
• To reduce the cost of concrete construction

Concrete Technology
ASTM classifications of Chemical Admixtures
Type Specification
Type A Water Reducer
Type B Retarders
Type C Accelerators
Type D Water reducer and retarder
Type E Water reducer and accelerator
Type F High range water reducer or Super
plasticizer
Type G Super plasticizer and retarder

Concrete Technology
Classification based on action
• Chemical interaction in Hydration (Accelerators and Retarders)
• Adsorption on cement surface causing better particle dispersion
(Plasticizer and Super-plasticizer)
• Affecting surface tension of water (increased air entrainment)
• Affecting rheology of water (Viscosity modifier)
• Special chemicals acting on hardened concrete (Corrosion inhibitors
and water repellent)

Concrete Technology
Plasticizers (Water Reducing Agent)
• Concrete in different situations require different degree of workability
• Conventional method for obtaining high workability
• Improving gradations, increasing proportion of fine aggregate, increasing
cement content
• There are difficulties and limitations to obtain high workability in
the field for given set of conditions
• The easy methods generally followed at the site in most of the
conditions is to use extra water unmindful of the harm it can do on
strength and durability of concrete
• Un-engineering way to use extra water

Concrete Technology
• Addition of excess water will only improve the fluidity or the
consistency but not the workability of concrete
• Excess water will not improve inherent good quality such as
homogeneity and cohesiveness which reduces the tendency of
segregation and bleeding

Concrete Technology
• To reduce the quantity of mixing water content usually by 5 or 10 %
and sometimes up to 15 %.
• Allow reduction in w/c ratio while retaining the desired workability.
• At a given w/c ratio , reduce cement content making concrete
economical.
• Improve workability at a given w/c ratio.
• Improve the workability of concrete made with fairly poorly graded
aggregate.
• Used in pump concreting

Concrete Technology
Mechanism of Plasticizers
• Dispersion of cement grains
Cement
+Water
Tendency
to
flocculates
Flocculation
entraps
certain
amount of
water
So, All water is
not available to
fluidify the mix

Concrete Technology
Mechanism of Plasticizers
Plasticizer

Concrete Technology
Action of Plasticizers
More water available to fluidize the mix
Release of Entrapped water
Causes deflocculation of floc
Leads to repulsion of cement particles
Get adsorbed on cement particles
Plasticizer added in Concrete

Concrete Technology
Flocculated Dispersed

Concrete Technology
• Plasticizers are surface acting agents negatively charged (-ve)
adsorbed on cement grains, gives the particles negative charge
leading to repulsion
• Repulsive force is called Zeta Potential
• Zeta Potential is potential difference between dispersion medium and
stationary layer of fluid attached to dispersed particles

Concrete Technology
Retarding effect of Plasticizers
Hence retardation in setting of concrete
Prevent the Hydration process as long as Plasticizer molecules are
present on particle
Forms a thin sheath
Plasticizer get adsorbed on surface of cement particle

Concrete Technology
Higher strength of Plasticizers at early age
So, Hydration progresses at higher rate on early stage, consequently
higher early strength gain
Plasticizer disperses them so, larger surface area is available for
hydration
In flocculated cement, the touching surfaces of cement particles are
not available for early hydrtion

Concrete Technology
Doses and Side effect of plasticizers
• General dose is 0.1 % to 0.4 % by weight of cementitious material
• Plasticizer permits the reduction of water up to 15 % at particular
workability
• Some plasticizers entrains air but good plasticizer doesn’t cause air-
entrainment more than 1 or 2 %
• Higher dose causes excessive retardation and entrains more air,
causing segregation and lower strength

Concrete Technology
Uses of Plasticizers in different requirement
S (28 days)= A
Workability= B
S (28 days)> A
Workability= B
S (28 days)= A
Workability >= B
S (28 days)>= A
Workability= B
S (28 days)= A
Workability= B
S (28 days)= A
Workability >= B
+ C
+ W+C
+ WRA, -W
W/C reduced
+ WRA, -W,-C
W/C same
+WRA

Concrete Technology
Superplasticizer (High Range Water Reducer)
• Improved version of plasticizer
• Developed in Japan ( in 1960) and Germany ( in 1970)
• Permit the reduction of water up to 30 % without educing workability
in contrast to the possible reduction up to 15 % in case of plasticizer
• Mechanism of action is same as Plasticizer, only thing is that
superplasticizer are more powerful dispersing agent
• India is catching up superplasticizer in the construction of high-rise
building

Concrete Technology
Superplasticizer (High Range Water Reducer)
• Better dispersion of particles causes better hydration
• Produce homogenous, cohesive concrete without any tendency to
segregation and bleeding
• Produces more workability than plain ones at same W/C ratio
• Permits use of lower W/C ratio for same workability

Concrete Technology
Effect of superplasticizers on workability and
strength
Water content- Kg/m3
Flow
table
spread-mm
Source: Shetty, M. S. (2005). Concrete Technology Theory & Practice, Published by S. CHAND & Company, Ram Nagar, New Delhi

Concrete Technology
Classification of Superplasticizer
• Sulphonated malanine-formaldehyde condensates ( SMF)
• Sulphonated napthalene-formaldehyde condensates ( SNF)
• Modified lignosulphonates ( MLS)
• Sulfonic esters and carbohydrate esters etc.

Concrete Technology
Mechanism of Superplasticizer
• Similar to plasticizer
• Superplasticizer are water soluble organic polymers having long
molecules of high molecular mass
• These long molecules wrapped around cement grain and give them a
highly negative charge so that grains repel each other

Concrete Technology
Effects of Superplasticizers on Fresh Concrete
• Very stiff mix or zero slump mix cannot fluidized by nominal dosage
and a high dose is required
• A mix with an initial slump of about 20 to 30 mm can only fluidized by
using nominal dosages
• Slump increases with increase in dosage but there is no appreciable
increase in slump beyond certain limit of dosage
• An improvement in slump can be obtained to the extent of 250 mm
or more depending upon the initial slump of the mix, dosage and
cement content

Concrete Technology
Effects of Superplasticizers on Fresh Concrete
10
12
14
16
18
20
22
0 0.2 0.4 0.6 0.8 1 1.2
Slump
in
cm
% of Superplasticizer by weight of cement
Effect of additions of SP on the workability of concrete

Concrete Technology
Strength Gain at early stage for given workability
SP Content by % weight of cement
Compressive
Strength
MPa
W/C ratio

Concrete Technology
Compatibility of Superplasticizers and Cement
• All SP are not showing the same extent of improvement in fluidity
with all types of cements
• Some SP may show higher fluidizing effect on particular type of
cement than other cement type
• There is nothing wrong with neither cement or that of SP, but they
are incompatible
• Optimum fluidizing effect at lowest dosage is an economical
consideration

Concrete Technology
Marsh cone Apparatus

Concrete Technology
Marsh cone time and Dosage
40
50
60
70
80
90
100
110
120
0 0.5 1 1.5 2
Marsh
cone
time
in
sec
% of Superplasticizer by weight of cement
Marsh cone time and Dosage
Saturation Point Point
Sa w/c =0.35
T= 22 0C

Concrete Technology
Slump loss
• Slump at mixing point is not very important
• Slump at placing point is of primary importance
• Often there is delay between mixing and placing of concrete
• Slump loss occurs with time at a given temperature
• But slump loss is rapid in super-plasticized concrete

Concrete Technology
Slump loss
• The effectiveness of SP in preventing re-agglomeration of cement
grains is there as long as sufficient amount of SP molecules are
available
• SP molecules are entrapped in products of hydration with time and
availability of SP molecules become inadequate so there is rapid loss
in slump
• So measures should be taken to reduce loss of slump

Concrete Technology
Measures for reducing slump loss
• By keeping initial high slump
• By repetitive dosing
• By keeping temperature low
• Using retarding plasticizer or superplasticizers
• By using new generation superplasticizers
• By using compatible SP with cement

Concrete Technology
By keeping initial slump high
0
50
100
150
200
250
300
0 10 20 30 40 50 60 70
Slump
in
mm
Elasped time in sec
1% SP
2% SP
3% SP
This is simple
but
uneconomical
and not
generally used

Concrete Technology
By Repetitive Dosing
• This is the most common practice in industry
• is advantageous to add the SP in two or even three operations i.e. add
in repetitive dose
• disadvantages of re-dosing are possibility of bleeding and segregation,
or air entrainment in the mixture

Concrete Technology
By Repetitive Dosing
Time- Min
Slump,
mm

Concrete Technology
Using New generation Superplasticizers
• Acrylic polymer based (AP)
• Carboxylic acrylic ether based ( CAE)
• Multicarboxylethers based ( MCE)
It increase the workability more than superplasticizers and lower slump
loss with time

Concrete Technology
Effect of Super plasticizer on the Properties of
Hardened Concrete
• Do not participate in any chemical reactions with cement
• Their action is only to fluidize the mix even at particular w/b ratio
• Fluidifying action last as long as Mix is in plastic stage
• But due to improvement in workability, better distribution of cement
particles and reduction in w/c ratio lead to higher strength
• But, If SP are used in larger amount say more than 4 % the strength of
hardened concrete may reduce due to air entrainment caused by SP
• Early age strength will be affected when larger SP dose is used which
adversely retard the setting time

Concrete Technology
Effect of Super plasticizer on the Properties of
Hardened Concrete
Admixture, % by weigth of cement
Compressive
Strength
(MPa)
Source: Shetty, M. S.
(2005). Concrete
Technology Theory &
Practice, Published by S.
CHAND & Company, Ram
Nagar, New Delhi

Concrete Technology
Accelerator
Accelerator
Strength Accelerator Set Accelerator Both

Concrete Technology
Accelerator
shorten the setting time
greater heat evolution
Leads to higher early strength
Accelerator added to increase the rate of hydration

Concrete Technology
Accelerator-Mechanism
Higher early strength
greater heat evolution
Increases the rate of hydration of C3S and C3A
Accelerating Admixture

Concrete Technology
Accelerator uses
• Cold weather concreting
• Early removal of formwork
• At normal temperatures , accelerators can be used in conjunction
with super-plasticisers where very early age strength is required
• urgent concrete repairs and in sea defence work, to ensure early
stiffening of concrete in the tidal zone

Concrete Technology
Commonly Used Accelerator
• Calcium chloride was used as accelerator in past. It accelerates the
setting and hardening both. Due to chloride it is not recommended
for RCC and pre stressing works
• Calcium nitrate and calcium nitrite
• Thiocyanate salts
• Calcium formate

Concrete Technology
Advantages of Accelerator
• Permit earlier removal of formwork
• Allow earlier start, of finishing works and curing
• In cold weather concreting for partially compensating the retarding
effect of low temperature
• In the emergency repair work
• Allow the structure to be placed in service at early age

Concrete Technology
Disadvantages of Accelerator
• Possibility of cracking due to higher heat evolution
• Possibility of corrosion of embedded reinforcement when chloride
free admixtures are not used

Concrete Technology
Retarder
Concrete remains plastic and workable for longer time
Slow down the chemical process of hydration

Concrete Technology
Use of Retarder
• Used in hot weather to reduce any premature stiffening of the
concrete and consequent loss of workability
• Avoid formation of cold joints
• Delaying the initial set of concrete or grout when difficult or unusual
conditions of placement occur, such as placing concrete in large piers
and foundations, cementing oil wells, or pumping grout or concrete
over considerable distances

Concrete Technology
Commonly used Retarder
• Sugars, Starches and Cellulose products
• Ligno sulphonic acids and their salts
• Hydroxylated carboxylic acids and their salts
• Tartaric acid and salts
• Borax

Concrete Technology
Commonly Sugar as Retarder
• Sugar (about 0.05 percent of mass of cement) delay the setting time
up to 4 hours
• But 0.2 % to 1 % delay the setting time for more than 72 hours
• So it is used as an inexpensive “kill” for preventing concrete to set
when concrete can not discharged due to mixer or agitator has been
broken down

Concrete Technology
Disadvantages of Retarder
• Early age strength will be low
• Increase plastic shrinkage due to extension of duration of plastic stage
• If properly not monitored and dosages are not adjusted carefully by
conducting trial mixes , it affect the concrete adversely

Concrete Technology
Air Entraining Admixture
• Air-entraining admixtures are used to purposely (intentionally)
introduce and stabilize microscopic air bubbles in concrete
• Dramatically improve the durability of concrete exposed to cycles of
freezing and thawing
• Air entraining agents incorporate millions of non-coalescing air
bubbles
• Act as flexible ball bearings and will modify the properties of plastic
concrete regarding workability, segregation, bleeding and finishing
quality of concrete

Concrete Technology
Air Entraining Admixture
• It also modifies the properties of hardened concrete regarding its
resistance to frost action and permeability
Air void in
concrete
Intentionally
incorporated
Entrained air
bubble
Un-intentionally
incorporated
Entrapped air
bubble
5 to 80 microns
evenly distribute
10 to 1000 microns,
due to insufficient
compaction

Concrete Technology
Air Entraining Admixture type
• Natural wood resins
• Animal and vegetable fats and oils, such as tallow, olive oil and their
fatty acids
• Various wetting agents such as alkali salts or sulphated and
sulphonated organic compounds
• sodium salts of petroleum sulphonic acids
Air entrained concrete has been used in the construction of Hirakund
dam, Koyna dam, Rihand dam etc.

Concrete Technology
Factors affecting amount of air entrainment
Amount of air entrainment in a mix is affected by many factors:-
• The type and quantity of air entraining agent used
• Water/cement ratio of the mix
• Type and grading of aggregate
• Mixing time
• Temperature
• Type of cement
• Influence of compaction
• Admixtures other than air entraining agent used

Concrete Technology
The Effect of Air Entrainment on the Properties
of Concrete
Air entrainment will effect directly the following three properties of
concrete:
• Increased resistance to freezing and thawing
• Improvement in workability
• Reduction in strength

Concrete Technology
The Effect of Air Entrainment on the Properties
of Concrete
Incidentally air entrainment will also effect the properties of concrete
in the following ways:
• Reduction in tendency of segregation
• Reduces the bleeding and laitance
• Decreases the permeability
• Increases the resistance to chemical attack
• Permits reduction in sand content
• Improves place-ability, and early finishing

Concrete Technology
Resistance to Freezing and Thawing
• High resistance of hardened concrete to scaling due to freezing and
thawing
• The ice formed in the pores of hardened concrete exerts pressure
• Cumulative effect of this pressure becomes considerable,
consequently, surface scaling and disruption of concrete at the
weaker section

Concrete Technology
Resistance to Freezing and Thawing

Concrete Technology
Effect on workability
• The place ability of air entrained concrete having 7.5 cm slump is
superior to that of non-air entrained concrete having 12.5 cm slump
• For adequate workability of concrete, aggregate particles must be
spaced so that they can move past one another with comparative
ease during mixing and placing

Concrete Technology
Effect on workability
Fig- Effect of entrained air on compaction factor of concrete
Source: Shetty, M. S. (2005). Concrete
Technology Theory & Practice, Published
by S. CHAND & Company, Ram Nagar,
New Delhi

Concrete Technology
Effect on strength
Entrained air content
%
age
Reduction
in
strength
Source: Shetty, M. S. (2005). Concrete
Technology Theory & Practice, Published
by S. CHAND & Company, Ram Nagar,
New Delhi

Concrete Technology
Effect on Segregation, Bleeding and Laitance
• Segregation and bleeding of concrete are different manifestations of
loss of homogeneity
• Reduces the compressive strength of concrete
• Segregation usually implies separation of coarser aggregate from
mortar or separation of cement paste from aggregates
• Bleeding is the autogenous flow of mixing water within, or its
emergence to the surface from freshly placed concrete, usually, as a
result of sedimentation of the solids due to compaction and self
weight of the solids

Concrete Technology
Effect on Segregation, Bleeding and Laitance
• Bleeding results in the formation of a series of water channels some
of which will extend to the surface
• A layer of water will emerge at the surface of the concrete, often
bringing some quantity of cement with it
• The formation of this layer of neat cement particles is called laitance
Segregation, bleeding and consequent formation of laitance are
reduced greatly by air entrainment

Concrete Technology
Effect on Permeability
• Greater uniformity of concrete with entrained air due to its increased
workability, modified pore-structure of the air entrained concrete,
reduction of water channel due to reduction in bleeding
• So, permeability characteristics improved hence reduction in
permeability takes place

Concrete Technology
Effect on Chemical Resistance
• In view of lower permeability and absorption, the air entrained
concrete will have greater resistance for chemical attack than that of
normal concrete

Concrete Technology
Effect on Sand, Water and Cement content
• The minute spheroidal air bubbles act as fine aggregates and enable
the reduction of fine aggregates. The reduction of fine aggregate
further enables the reduction of water requirement without
impairing the workability and slump
• Sand content by weight of total aggregate may be reduced by one per
cent for each per cent increase in air entrainment up to about 8 per
cent

Concrete Technology
Unit Weight and Alkali-Aggregate Reaction
• Comparing two mixes, one ordinary concrete and the other air
entrained, which have the same workability and strength, the air
entrained concrete will contain 5 per cent less of solid material, and
hence will be lower in unit weight
• Air entrainment reduces the alkali-aggregate reaction
Desirable air content is ranging from 3 to 6 percent.

Concrete Technology
Disadvantages of air entraining admixture
• Air entrainment in concrete reduce the compressive strength of
concrete
• But by using optimum amount of air entrainment and proper care
affect the strength negligible.
• 3 to 4 % air content is good for improving workability

Supplementary cementing materials

Concrete Technology
Pozzolanic or Mineral Admixtures
Pozzolanic Materials:
Pozzolanic materials are siliceous or siliceous and aluminous materials,
which in themselves possess little or no cementitious value, but will, in
finely divided form and in the presence of moisture, chemically react
with calcium hydroxide liberated on hydration, at ordinary
temperature, to form compounds, possessing cementitious properties.

Concrete Technology
Advantages of Pozzolans
• Improved Workability
• Economy
• Reduced Alkali-aggregate Reaction
• Increased Sulphate Resistance

Concrete Technology
Pozzolanic Reactions
Hydraulic reaction
C2S/C3S + water CSH gel + Ca(OH)2
Pozzolanic reaction
Pozzolana + Ca(OH)2 + water CSH gel
The reduction in Ca(OH)2 increase the durability

Concrete Technology
Pozzolanic Reactions
• Additional CSH gel modify the microstructure and densify the matrix
•
• Resulted in reduction in permeability and hence better durability
• The characteristic feature of pozzolanic reaction is firstly slow, with
the result that heat of hydration and strength development will be
accordingly slow

Concrete Technology
Classification of Pozzolanic materials
Pozzolanic materials
Natural Pozzolans Artificial Pozzolans

Concrete Technology
Natural Pozzolana
• Clay and Shales
• Opalinc Cherts
• Diatomaceous Earth
• Volcanic Tuffs and Pumicites

Concrete Technology
Artificial Pozzolans
• Fly ash
• Blast Furnace Slag
• Silica Fume
• Rice Husk ash
• Metakaoline
• Surkhi

Concrete Technology
Artificial Pozzolans- 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
• Fly ash is the most widely used pozzolanic
• Fly ash was first used in large scale in the construction of Hungry
Horse dam in America in the approximate amount of 30 percent by
weight of cement
• Fly ash was used in Rihand dam construction replacing cement up to
about 15 percent

Concrete Technology
Artificial Pozzolans- FLY Ash
• Nowadays Fly Ash is widely used as supplementary cementitious
material for making of high strength and high performance concrete
• The use of fly ash as concrete admixture not only extends technical
advantages to the properties of concrete but also contributes to the
environmental pollution control
• 7% of the world’s carbon dioxide emission is attributable to Portland
cement industry [Concrete Technology-MS Shetty,1996]
• There is a need to economise the use of cement
• Solutions to economise cement is to replace cement with
supplementary cementitious materials like fly ash and slag

Concrete Technology
Fly Ash
• One way is to intergrind certain percentage of fly ash with cement
clinker at the factory to produce Portland pozzolana cement (PPC)
• second way is to use the fly ash as an admixture at the time of
making concrete at the site of work
• The second method gives freedom and flexibility to the user regarding
the percentage addition of fly ash
• As per ASTM classification… ‘Fly ash C’ and ‘Fly ash F’ two type is
available

Concrete Technology
Effect of Fly Ash on fresh concrete
• Use of right quality fly ash, results in reduction of water demand for
desired slump
• Heat of hydration can be reduced through replacement of part of the
cement with fly ash

Concrete Technology
Effect of Fly Ash on fresh concrete
Age (Days)
Temprature
rise
(
0
C)

Concrete Technology
Effects of Fly Ash on Hardened Concrete
• Fly Ash contributes to the strength of concrete due to its pozzolanic
reactivity
• Since the pozzolanic reaction proceeds slowly, the initial strength of
fly ash concrete tends to be lower than that of concrete without fly
ash
• Due to continued pozzolanic reactivity, concrete develops greater
strength at later age, which may exceed that of the concrete without
fly ash

Concrete Technology
Effects of Fly Ash on Hardened Concrete
• The pozzolanic reaction also contributes to making the texture of
concrete dense, resulting in decrease of water permeability and gas
permeability
• It should be noted that since pozzolanic reaction can only proceed in
the presence of water enough moisture should be available for long
time
• fly ash concrete should be cured for longer period
• fly ash concrete used in under water structures such as dams will
derive full benefits of attaining improved long term strength and
water-tightness

Concrete Technology
Effects of Fly Ash on Durability of Concrete
• Dense structure, high resistivity to the infiltration of deleterious
substances
• More resistance to the corrosion of reinforcement
• Prevention of alkali-aggregate reaction leads less expansion

Concrete Technology
Conclusion-Fly Ash
• Although fly ash is an industrial waste, it’s use in concrete significantly
improve the long term strength and durability and reduce heat of
hydration
• Good fly ash will be an indispensable mineral admixture for high
performance concrete

Concrete Technology
Silica Fume
• 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
• Silica fume rises as an oxidised vapour
• It cools, condenses and is collected in cloth bags
• It is further processed to remove impurities and to control particle
size
• Silica fume has specific surface area of about 20,000 m2/kg

Concrete Technology
Silica Fume
• It does contribute to the strength
property by being very fine pozzolanic
material and also creating dense
packing and pore filler of cement paste
Silica fume powder

Concrete Technology
Pozzolanic action of Silica Fume
• Microsilica 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 Ca(OH)2 in the cement paste at different times
• Most research workers agree that the C – S – H formed by the
reaction between microsilica and Ca(OH)2 appears dense

Concrete Technology
Silica fume- Influence on fresh concrete
• Water demand increases in proportion to the amount of microsilica
added
• The addition of microsilica will lead to lower slump but more
cohesive mix
• The microsilica make the fresh concrete sticky in nature and hard to
handle
• It was also found that there was large reduction in bleeding and
concrete with microsilica could be handled and transported without
segregation

Concrete Technology
Silica Fume-Curing
• Curing is probably the most important aspect of microsilica concrete
as the material undergoes virtually zero bleeding
• If the rate of evaporation from the surface is faster than the rate of
migration of water from interior to the surface, plastic shrinkage
takes place
• In the absence of bleeding and slow movement of water from
interior to the surface, early curing by way of membrane curing is
essential

Concrete Technology
Ground Granulated Blast Furnace Slag (GGBS)
• Ground granulated blast-furnace slag is a nonmetallic product
consisting essentially of silicates and aluminates of calcium
• The molten slag is rapidly chilled by quenching in water to form a
glassy sand like granulated material
• The granulated material when further ground to less than 45 micron
will have specific surface of about 400 to 600 m2/kg

Concrete Technology
GGBS effects on hardened concrete
• Reduced heat of hydration
• Reduced permeabilities to the external agencies
• Increased resistance to chemical attack

Concrete Technology
Rice Husk ash
• Obtained by burning rice husk in a controlled manner without causing
environmental pollution
• properly burnt rice husk has high SiO2 content and can be used as a
concrete admixture
• Rice husk ash exhibits high pozzolanic characteristics and contributes
to high strength and high impermeability of concrete
• Rice husk ash (RHA) essentially consist of amorphous silica (90%
SiO2), 5% carbon, and 2% K2O.

Concrete Technology
Rice Husk Ash
Controlled
Burning
Rice Husk Rice Husk Ash

Concrete Technology
Rice Husk ash
• India produces about 122 million ton of paddy every year. Each ton of
paddy producers about 40 kg of RHA
• In U.S.A., highly pozzolanic rice husk ash is patented under trade
name Agrosilica and is marketed. Agrosilica exhibit superpozzolanic
property when used in small quantity i.e., 10% by weight of cement
and it greatly enhances the workability and impermeability of
concrete

Concrete Technology
Surkhi
• Surkhi is an artificial pozzolana made by powdering bricks or burnt
clay balls
• In some major works, for large scale production of surkhi, clay balls
are specially burnt for this purpose and then powdered
• Now the terminology “calcined clay Pozzolana” is used instead of the
word surkhi
• Surkhi was one of the main constituents in waterproofing treatments
in conjunction with lime and sometimes even with cement for
extending valuable pozzolanic action to make the treatment
impervious

Concrete Technology
Surkhi in powdered form

Concrete Technology
In Bhakra Nangal Dam scientifically made surkhi (burnt clay Pozzolana) was used about 100 tons
per day at the rate of 20% Cement replacement

admixtures 31-7-21

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