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ADMIXTURES IN CONCRETE
Admixtures are materials or chemicals which are added to concrete at the mixing stage to
modify some of the properties of the mix
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Admixtures should never be regarded as a substitute for good mix design, good
workmanship, or use of good materials
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To reduce water content without changing workability
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To adjust setting time
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To improve pumpability
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To increase strength
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To reduce the total cost of the materials used in the concrete
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CLASSIFICATION OF ADMIXTURES:
Admixtures are generally classified into Chemical Admixtures and Mineral Admixtures
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Types of Mineral Admixtures
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Pozzolanic (Low-calcium FA)
Cementitious (Granulated iron blast-furnace slag)
Both pozzolanic and cementitious (Hihgcalcium FA)
Natural materials (Volcanic Ash)
By-product Material

POZZOLANS
Pozzolanas 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 Ca(OH)2 at ordinary temperatures to form compounds possessing
cementitious properties
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The
pozzolanic reaction is slow, so the rate of the strength development and the heat of hydration
associated with this reaction are low
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Ecological Advantage: Power plants using coal as fuel and metallurgical furnaces producing cast
iron, silicon metal, and ferrosilicon alloys are the major sources of byproducts that are being
produced at the rate of millions of tonnes every year in many countries
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Hence their usage as an admixture avoids environmental pollution
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With proper
quality control, large amounts of many industrial by-products can be incorporated into concrete,
either in the form of blended portland cement or as mineral admixtures
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Natural Pozzolans: Except diatomaceous earth, all natural pozzolans are derived from volcanic
rocks and minerals
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Classification
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•
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•

Volcanic glasses: Santorini Earth of Greece, Bacoli Pozzolan of Italy, and Shirasu
Pozzolan of Japan are examples of pozzolanic materials which derive their lime-reactivity
characteristic mainly from the unaltered aluminosilicate glass
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Temperatures on the order of 600 to 900 C, in kilns is required for this purpose
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Diatomaceous earth: This group of pozzolans is characterized by materials of organic
origin
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The largest known deposit is
in California
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Significance: Countries like China, India, the United States, Russia, Germany, South Africa, and
the United Kingdom, are among the biggest producers of fly ash which, at the current (Year 2000)
rate of production, some 500 million tonnes a year constitutes the largest industrial waste product
in the world
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The dust-collection system removes the fly ash, as a fine particulate residue, from the
combustion gases before they are discharged into the atmosphere
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Some of
the minerals agglomerate forming the bottom ash, but most of the fine particles fly out with the
flue gas stream and are called fly ash
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Effect on Heat of Hydration
Replacement of cement by fly ash results in a reduction in the temperature rise in fresh Concrete
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Effect of fly ash on the structural properties of hardened concrete: At early ages fly ash
exhibits very little cementing value, at later ages cementing activity becomes apparent and a
considerable Contribution to strength may results
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Classification
Fly ashes can be divided into two categories which differ from each other mainly in calcium
content
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The ash typically containing 15 to 35 percent analytical CaO, is
generally a product of combustion of lignite and subbituminous coals
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The partial devitrification of glass in low-lime fly ashes
accounts for the presence of crystalline aluminosilicates
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High-calcium fly ash: It is more reactive because it contains most of the calcium in the
form of reactive crystalline compounds
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Microstructure: Most of the particles in fly ash occur as solid spheres of glass
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As
alkalies and sulfate tend to occur in a relatively larger proportion in the high-calcium fly ashes, the
deposition of alkali sulfates on the surface of spherical particles accounts for their dirty appearance
of the spheres
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The particle size
distribution, morphology, and surface characteristics of the fly ash selected for use as a mineral
admixture exercise a considerable influence on the water requirement and the workability of fresh
concrete, and the rate of strength development in hardened concrete
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01μm)
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Silica fume is a highly pozzolanic materials
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Comparison of particle size distributions of Portland cement, fly ashes, and condensed
silica fume

Particle size distribution: Compared to normal portland cement and typical fly ashes, silica fume
samples show particle size distributions that are two orders of magnitude finer
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When quenched rapidly with
water or air to a glassy state and finely ground, it develops the property of latent hydraulicity
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Due to hydraulicity,
therefore, its use contributes not only to improvement in concrete performance, but also to resource
and energy savings
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Heat of hydration of concrete containing slag powder decreases with
the increase in the slag power content
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RICE HUSK ASH
Rice husks, also called rice hulls, are the shells produced during the dehusking operation
of paddy rice
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As they are bulky, the
husks present an enormous disposal problem for centralized rice mills
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The white rice husk ash has better quality compared to the black rice husk ash due
to the existence of carbon affecting adversely on the reactivity
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A highly pozzolanic ash can be
produced by controlled combustion when silica is retained in non-crystalline form and a cellular
microstructure
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The product
is a highly reactive pozzolan of white color that is especially suitable for use in architectural
concrete
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Grain Refinement: without pozzolans large CH crystals develop
Pore Refinement: reduction of porosity

Workability improvement
With fresh concrete mixtures that show a tendency to bleed or segregate, the incorporation of finely
divided particles generally improves the workability by reducing the size and volume of voids
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The small size and the glassy texture of
fly ash and slag makes it possible to reduce the amount of water required for a given consistency
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For a given consistency of
concrete, the use of very high surface area materials, such as pumicite, rich husk ash, and silica
fume increases the water requirement
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These admixtures do not react to a significant degree
for several days
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Durability to chemical attack
The permeability of concrete plays a fundamental role in determining the rate of deterioration due
to destructive chemical actions such as the alkali-aggregate expansion and attack by acidic or
sulfate solution
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Durability
Mineral admixtures improves the resistance of the concrete to acidic water, sulfate water, and
seawater
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MULTICOMPONENT BLENDED CEMENT
It is developed by mixing two or more kinds of mineral admixtures with the aim of further
improving the performance of blended cement
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IMPROVED CONCRETE QUALITY WITH COMBINATION OF FLY ASH AND
SILICA FUME
Adding relatively small amount of silica fume to fly ash concretes with w/c of 0
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45,
results in concretes with satisfactory strength and very low permeability at 28 days
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The three important areas of concrete performance are strength, durability and fluidity and
the materials affecting the physical properties of cement matrix are cement, mineral admixture and
chemical admixture
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CHEMICAL ADMIXTURES
Types of Chemical admixtures are:
•
•
•
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Super-plasticizers
Air-entraining agents
Accelerators
Retarders

AIR ENTRAINING ADMIXTURES
An air-entraining agent introduces air in the form of minute bubbles distributed uniformly
throughout the cement paste
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Air entraining admixtures have a large use in Europe, North America, Japan and such countries in
the higher latitudes
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Air entraining admixtures are made from:
(i)
(ii)
(iii)

Natural wood resins
Synthetic detergents
Salts of petroleum acids

Air Entrainment
The of a large number of small bubbles of air production within the concrete mix is referred to as
air entrainment
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The bubbles must be stable
and remain in the concrete as it is transported and placed so that they are still present in the
hardened mass of concrete
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Mechanism of Entrainment
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•
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Anionic surfactants have a hydrophobic and a hydrophilic end
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This reduces surface tension
and allows the formation of a stable bubble
The hydrophilic end is polar and is attracted by charges on the surface of cement and
aggregate particles
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Air is not added to the mix
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Combinations of materials are used to provide stability under different conditions and to
produce a desired range of bubble sizes
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Mix cohesion problems
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•
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Cohesion problems occur when the internal structure in a concrete mix is insufficient to
hold the mix together
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If there is a lack of materials of a particular size in the grading of the aggregate then the
physical support may not be complete
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If there is insufficient attraction between the particles then there can be a separation

Role of air entrainment in attaining cohesion
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Typically 1 or 2 % additional air over that normally present in the mix will be sufficient to
give improvements in cohesion
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Air tends to form at the right size to replace any gaps in the sand grading
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Electrostatic action between air bubbles and aggregate acts to hold mix components
together, Air bubbles act as elastic buffers in the concrete mix, allowing mix components
to move past each other more easily
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Effects of improved Cohesion
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Segregation reduces, minimizing problems of settlement of aggregates
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Practical considerations
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Air entrainment may reduce the strength of concrete and overdosing can cause major loss
of strength
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It is therefore

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•
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important that mixes be specially designed for air entrainment and that the percentage of
air entrained during construction be monitored
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A change in cementitious content, in the grading or proportions of the fine fractions of sand
will normally alter the volume of air entrained
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Forced-action mixers entrain larger volumes of air than other types
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The use of ground granulated blast furnace slag (GGBS) and fly ash (FA) tends to reduce
the amount of air entrained
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Reduction in strength
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1% air will usually reduce compressive strength by 5 to 6% requiring an allowance to be
made when designing the concrete mix
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Low cohesion or harshness in a mix may increase water demand as more water is added in
an attempt to improve the compaction of the mix
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Among the main types of accelerators
are chloride based, non-chloride bases and shotcrete accelerators
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Non-chloride based: 500 ml
to 2000 ml per 100 kg of cementitious material
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Refer to the
supplier’s directions regarding dosage
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g
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Where rapid turnover of moulds or formwork is required
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Practical considerations
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All chloride-based accelerators promote corrosion of reinforcing steel and should not be
used in:
o reinforced concrete

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•
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o water-retaining structures
o pre-stressed concrete
o steam-cured concrete
Overdosing with these materials can cause instant setting of the concrete resulting in
equipment damage
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RETARDERS
These admixtures slow the chemical reaction of the cement and water leading to longer
setting times and slower initial strength gain
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Dosage: Typical dosages for retarders are between 150 ml and 500 ml per 100 kg cementitious
material
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To prevent cold joints due to duration of placing
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Practical considerations
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If a mix is overdosed beyond the limit recommended by the supplier, retardation can last
for days
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Delayed addition of retarders can result in extended retardation
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Reasons for acceleration
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To overcome the slower setting of concrete at low temperatures
To meet the needs of particular construction methods, such as floor finishing
To produce rapid early strength gain
To allow rapid turnover of moulds

Chemicals used:
 Retarders - mostly organic chemicals
– Carbohydrates
– Hydroxycarboxylic acids and salts
– Phosphates

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 Accelerators - mostly inorganic chemicals
– Calcium chloride, Nitrates
– Thiocyanates
– Silicates
– Aluminates
Calcium chloride is the most effective accelerator
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It affects:
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Cement hydration
Setting time
Working life
Strength gain

Although retardation and acceleration are primarily affecting the chemistry of the hydration
reaction, it is usually the physical results of this that are required
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It is important that
the exact needs are determined before and attempt at a solution made
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Temperature
Cement chemistry
Starting workability

PLASTICIZERS
When added to a concrete mix, plasticizers (water-reducing agents) are absorbed on the
surface of the binder particles, causing them to repel each other and deflocculate
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The main types of plasticizers are lignosulphonic acids and their salts, hydroxylated
carboxylic acids and their salts, and modifications of both
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Uses
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Plasticizers usually increase the slump of concrete with a given water content
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The addition of a plasticizer makes it possible to achieve a given strength with a lower
cement content
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Practical considerations
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A number of plasticizers contain a retarder and can cause problems if overdosed
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Where plasticizers are used to increase workability, the shrinkage and creep will invariably
be increased
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When they are used to produce flowing concrete a rapid
loss of workability can be expected and therefore they should be added just prior to placing
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Dosage: The normal dosage of a superplasticizer is between 750 ml and 2500 ml per 100 kg of
cementitious material
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Where a self-levelling consistence facilitates placing
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Practical considerations
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Special mixes must be designed for superplasticizers and their use must be carefully
controlled
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They have a relatively high unit cost
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High range water reducers/ super plasticizers
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Whatever is true of plasticizers more of that with superplasticizers
Higher dosages; greater benefits in properties of fresh concrete

Air entraining agents
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Workability increases
Mixes are more cohesive, Finishability better
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EFFECTS OF CHEMICAL ADMIXTURES ON HARDENED CONCRETE
Water reducer / Plasticizer
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when used as plasticizer no significant effect on 28 day strength, modulus of elasticity or
permeability
When used as water reducer- reduced creep and shrinkage
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When W/C is reduced enhanced strength, higher modulus of elasticity and all round
improvement in all properties including permeability
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Air entraining agents
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Lead to lower unit weight of concrete, inversely proportional to air entrained
At the same W/C ratio, compressive strength, flexural and tensile strengths are reduced
Approximately, for every 1% air entrained (by volume), 5% loss of strength
Loss of strength overcome by a small reduction in W/C ratio made possible by reducing
water content to a small extent
Durability is better since capillary absorption of water is blocked by air bubbles
Freeze-Thaw resistance, Resistance to aggressive solutions

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OTHER ADMIXTURES
Other admixtures with different chemical compositions and effects are available (e
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pumping aids, pigments, expansion aids and grouting admixtures)
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DESIGN OF MIXES
A laboratory tests should be carried out to optimize dosages of admixtures with the
cements/blended cements and aggregates which will be used on the site
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The control mix (i
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one without admixture),
should be prepared before any other to prevent trace contamination with the admixture under test
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g
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If more than one effect is desired,
then more than one admixture may be required
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STORAGE AND HANDLING
Most admixtures are aqueous solutions of active chemicals that are biodegradable
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Admixtures should be protected from frost, as most
have freezing points at or below 0°C
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It is also necessary to protect
admixtures from heat because their degradation is accelerated at elevated temperatures
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Admixtures generally contain preservatives to reduce biodegradation and extend shelf life
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Therefore they should be used on a first-in-first-out basis
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Bulk liquid storage tanks are often supplied, serviced and
maintained by the admixture supplier
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