Cement chemical admixtures types and there uses

2. Chemical Admixtures
The modern construction industry uses a variety of
chemical admixtures in concrete, to reach desirable
qualities along with a view to promote sustainability in it.
ASTM C 125 defines an admixture as a material other than
water, aggregate, cement, and fiber reinforcement, used
as an ingredient of concrete mortar and added to the
batch immediately before or during mixing. Chemical
admixtures are the smallest component of a concrete mix
but impart significant beneficial effects on the concrete
properties in terms of performance, durability,
environment optimization leading to sustainability etc.

3.  The incorporation of admixtures in concrete mix has
been recognized as an important and integral means
to modify and improve performance of concrete at
both fresh as well as hardened states. Admixtures
should never be regarded as a substitute for a good mix
design, good workmanship or use of good materials.
There are several chemical admixtures that include
water reducers, accelerators, anti-freezing admixtures,
air-entraining admixtures, alkali-aggregate inhibitors,
shrinkage compensating admixture, and corrosion
inhibitors etc. Figure 1 shows some of the chemical
admixtures meant for use in cement concrete.

5.  The most common reasons for using chemical
admixtures in a concrete mix are:-To increase
workability without changing water content
 To reduce water content without changing workability
 To effect a combination of above
 To adjust setting time
 To reduce segregation and/or bleeding

6.  To accelerate the rate of early strength gain
 To improve pumpability
 To increase strength
 To improve potential durability and reduce
permeability
 To reduce the total cost of the materials used in the
concrete
 To compensate for certain poor aggregate
grading/properties
 To depress the freezing point of water in concrete at
very low temperatures (up to minus 30°C)
 To maintain adequate freezing and thawing resistance
of concrete
 To inhibit alkali-aggregate reactions (AAR)

7.  Chemical admixtures are the ingredients in concrete other
than portland cement, water, and aggregate that are added
to the mix immediately before or during mixing. Producers
use admixtures primarily to reduce the cost of concrete
construction; to modify the properties of hardened
concrete; to ensure the quality of concrete during mixing,
transporting, placing, and curing; and to overcome certain
emergencies during concrete operations.

8.  Successful use of admixtures depends on the use of
appropriate methods of batching and concreting. Most
admixtures are supplied in ready-to-use liquid form
and are added to the concrete at the plant or at the
jobsite. Certain admixtures, such as pigments,
expansive agents, and pumping aids are used only in
extremely small amounts and are usually batched by
hand from premeasured containers.

9.  The effectiveness of an admixture depends on several
factors including: type and amount of cement, water
content, mixing time, slump, and temperatures of the
concrete and air. Sometimes, effects similar to those
achieved through the addition of admixtures can be
achieved by altering the concrete mixture-reducing the
water-cement ratio, adding additional cement, using a
different type of cement, or changing the aggregate and
aggregate gradation.

10. Five Functions
 Admixtures are classed according to function. There are five
distinct classes of chemical admixtures: air-entraining, water-
reducing, retarding, accelerating, and plasticizers
(superplasticizers). All other varieties of admixtures fall into the
specialty category whose functions include corrosion inhibition,
shrinkage reduction, alkali-silica reactivity reduction,
workability enhancement, bonding, damp proofing, and
coloring. Air-entraining admixtures, which are used to
purposely place microscopic air bubbles into the concrete, are
discussed more fully in Air-Entrained Concrete.

11. Water-reducing admixtures
 usually reduce the required water content for a concrete mixture by
about 5 to 10 percent. Consequently, concrete containing a water-
reducing admixture needs less water to reach a required slump than
untreated concrete. The treated concrete can have a lower water-
cement ratio. This usually indicates that a higher strength concrete can
be produced without increasing the amount of cement. Recent
advancements in admixture technology have led to the development of
mid-range water reducers. These admixtures reduce water content by at
least 8 percent and tend to be more stable over a wider range of
temperatures. Mid-range water reducers provide more consistent
setting times than standard water reducers.

12. Retarding admixtures
 which slow the setting rate of concrete, are used to
counteract the accelerating effect of hot weather on
concrete setting. High temperatures often cause an
increased rate of hardening which makes placing and
finishing difficult. Retarders keep concrete workable
during placement and delay the initial set of concrete.
Most retarders also function as water reducers and
may entrain some air in concrete.

13. Accelerating admixtures
 increase the rate of early strength development, reduce
the time required for proper curing and protection,
and speed up the start of finishing operations.
Accelerating admixtures are especially useful for
modifying the properties of concrete in cold weather.

14. Superplasticizers
 also known as plasticizers or high-range water reducers (HRWR),
reduce water content by 12 to 30 percent and can be added to
concrete with a low-to-normal slump and water-cement ratio to
make high-slump flowing concrete. Flowing concrete is a highly
fluid but workable concrete that can be placed with little or no
vibration or compaction. The effect of superplasticizers lasts only
30 to 60 minutes, depending on the brand and dosage rate, and
is followed by a rapid loss in workability. As a result of the slump
loss, superplasticizers are usually added to concrete at the
jobsite.

15. Corrosion-inhibiting
admixtures
 fall into the specialty admixture category and are used to slow
corrosion of reinforcing steel in concrete. Corrosion inhibitors
can be used as a defensive strategy for concrete structures, such
as marine facilities, highway bridges, and parking garages, that
will be exposed to high concentrations of chloride. Other
specialty admixtures include shrinkage-reducing admixtures and
alkali-silica reactivity inhibitors. The shrinkage reducers are used
to control drying shrinkage and minimize cracking, while ASR
inhibitors control durability problems associated with alkali-
silica reactivity.

16. (Admixtures) and their
Applications
 Concrete chemicals or admixtures are materials other
than cement, aggregate and water that are added to
concrete either before or during its mixing to alter its
properties, such as workability, curing temperature
range, set time or color. Some concrete admixtures
have been in use for a very long time in concrete
construction, such as calcium chloride to provide a
cold-weather setting concrete.

17.  Based on their functions, admixtures can be classified
into the following five major categories:
 Retarding admixtures
 Accelerating admixtures
 Superplasticizers
 Water reducing admixtures
 Air-entraining admixtures

18.  Among other important admixtures that do not fit into
these categories are admixtures whose functions
include bonding, shrinkage reduction, damp proofing
and coloring. The following paragraphs provides
details on the above-mentioned categories of concrete
admixtures.

19. 1. Retarding Admixtures
 Retarding admixtures slow down the hydration of cement,
lengthening set time. Retarders are beneficially used in hot
weather conditions in order to overcome accelerating effects of
higher temperatures and large masses of concrete on concrete
setting time. Because most retarders also act as water reducers,
they are frequently called water-reducing retarders.
 As per chemical admixture classification by ASTM-ASTM C 494,
type B is simply a retarding admixture, while type D is both
retarding and water reducing, resulting in concrete with greater
compressive strength because of the lower water-cement ratio.

20.  Retarding admixtures consists of both organic and inorganic
agents. Organic retardants include unrefined calcium, sodium,
NH4, salts of lignosulfonic acids, hydroxycarboxylic acids, and
carbohydrates. Inorganic retardants include oxides of lead and
zinc, phosphates, magnesium salts, fluorates and borates. As an
example of a retardants effects on concrete properties,
lignosulfonate acids and hydroxylated carboxylic acids slow the
initial setting time by at least an hour and no more than three
hours when used at 65 to 100 degrees Fahrenheit.
 The concrete contractor, however, need not memorize these
chemical-specific results. Given the specific job requirements
and goals, the concrete supplier should offer appropriate
admixtures and concrete mixes from which to choose.

21. 2. Accelerating admixtures
 Accelerators shorten the set time of concrete, allowing
a cold-weather pour, early removal of forms, early
surface finishing, and in some cases, early load
application. Proper care must be taken while choosing
the type and proportion of accelerators, as under most
conditions, commonly used accelerators cause an
increase in the drying shrinkage of concrete.

22.  Calcium chloride is a common accelerator, used to
accelerate the time of set and the rate of strength gain. It
should meet the requirements of ASTM D 98. Excessive
amounts of calcium chloride in concrete mix may result in
rapid stiffening, increase in drying shrinkage and corrosion
of reinforcement. In colder climates, calcium chloride
should not be used as an anti-freeze. Large amount of
calcium chloride is required to lower the freezing point of
the concrete, which may ruin the concrete.

23. 3. Superplasticizers
 Superplasticizers, also known as plasticizers, include
water-reducing admixtures. Compared to what is
commonly referred to as a “water reducer” or “mid-range
water reducer”, super plasticizers are “high-range water
reducers”.
 High range water reducers are admixtures that allow large
water reduction or greater flowability (as defined by the
manufacturers, concrete suppliers and industry standards)
without substantially slowing set time or increasing air
entrainment.

24.  Each type of super plasticizer has defined ranges for the required
quantities of concrete mix ingredients, along with the
corresponding effects. They can maintain a specific consistency
and workability at a greatly reduced amount of water. Dosages
needed vary by the particular concrete mix and type of
superplasticizer used. They can also produce a high strength
concrete.
 As with most types of admixtures, superplasticizers can affect
other concrete properties as well. The specific effects, however,
should be found from the manufacturer or concrete supplier.

25. 4. Water reducing admixtures
 Water reducing admixtures require less water to make a
concrete of equal slump, or increase the slump of concrete
at the same water content. They can have the side effect of
changing initial set time. Water reducers are mostly used
for hot weather concrete placing and to aid pumping.
 A water-reducer plasticizer, however, is a hygroscopic
powder, which can entrain air into the concrete mix via its
effect on water’s surface tension, thereby also, obtaining
some of the benefits of air-entrainment (see below).

26. 5. Air-entraining admixtures
 Air-entraining agents entrain small air bubbles in the concrete.
The major benefit of this is enhanced durability in freeze-thaw
cycles, especially relevant in cold climates. While some strength
loss typically accompanies increased air in concrete, it generally
can be overcome by reducing the water-cement ratio via
improved workability (due to the air-entraining agent itself) or
through the use of other appropriate admixtures.
 As always, admixtures should only be combined in a concrete
mix by a competent professional because some of them can
interact in undesirable ways.

27. 6. Bonding admixtures
 Bonding admixtures including addition of compounds and
materials such as polyvinyl chlorides and acetates, acrylics and
butadiene-styrene co-polymers, can be used to assist in bonding
new / fresh concrete with old / set concrete.
 Coloring agents have become more commonly used, especially
for patios and walkways. Most are surface applied and often have
the additional effect of surface hardening. Such surface applied
coloring admixtures generally should not be used on air-
entrained concrete. Integrally colored concrete is also available.

28. 7. Waterproofing and damp
proofing admixtures
 Water proofing and damp proofing admixtures including
soaps, butyl stearate, mineral oil and asphalt emulsions, are
used to decrease the amount of water penetration into the
larger pores of concrete.
 Antifreeze” admixtures typically are accelerators used in
very high doses, with a corresponding high price, to achieve
a very fast set-time, though they do not have properties to
protect against freezing on their own. However, in general,
these are not used for residential work.