ADMIXTURES

ADMIXTURES

A material other than water, aggregates, or

cement that is used as an ingredient of concrete or mortar to control setting and early hardening, workability, or to provide additional cementing properties.

Why is admixture used?

Over decades, attempts have been made to obtain concrete with certain desired characteristics such as high compressive strength, high workability, and high performance and durability parameters to meet the requirement of complexity of modern structures.

The properties commonly modified are the heat of hydration, accelerate or retard setting time, workability, water reduction, dispersion and air- entrainment, impermeability and durability factors.

Types of Admixtures

1. Chemical admixtures - Accelerators, Retarders, Water-reducing agents, Super plasticizers, Air entraining agents etc.
2. Mineral admixtures - Fly-ash Blast-furnace slag, Silica fume and Rice husk Ash etc.

Chemical admixtures

1. Water-reducing admixture / Plasticizers:
These admixtures are used for following purposes:

• To achieve a higher strength by decreasing the water cement ratio at the same workability as an admixture free mix.
• To achieve the same workability by decreasing the cement content so as to reduce the heat of hydration in mass concrete.
• To increase the workability so as to ease placing in accessible locations.
• Water reduction more than 5% but less than 12%.
• The commonly used admixtures are Ligno-sulphonates and hydrocarbolic acid salts.
• Plasticizers are usually based on lignosulphonate, which is a natural polymer, derived from wood processing in the paper industry.

Actions involved:
Dispersion:
Surface active agents alter the physic chemical forces at the interface. They are
adsorbed on the cement particles, giving them a negative charge which leads
to repulsion between the particles. Electrostatic forces are developed causing
disintegration and the free water become available for workability.
Lubrication:
As these agents are organic by nature, thus they lubricate the mix reducing the
friction and increasing the workability.
Retardation:
A thin layer is formed over the cement particles protecting them from
hydration and increasing the setting time. Most normal plasticizers give some
retardation, 30–90 minutes
2. Super Plasticizers:
These are more recent and more effective type of water reducing admixtures
also known as high range water reducer. The main benefits of super
plasticizers can be summarized as follows:
Increased fluidity:
Flowing
Self-leveling
Self-compacting concrete
Penetration and compaction round dense reinforcement
Reduced W/C ratio:
Very high early strength, >200% at 24 hours or earlier
Very high later age strengths, >100 MPa or 15000 psi.
Reduced shrinkage, especially if combined with reduced cement content.
Improved durability by removing water to reduce permeability and diffusion.
The commonly used Super Plasticizers are as follows:
Sulphonated melamine formaldehyde condensates (SMF)
Give 16–25%+ water reduction. SMF gives little or no retardation, which makes
them very effective at low temperatures or where early strength is most
critical. However, at higher temperatures, they lose workability relatively
quickly. SMF generally give a good finish and are colorless, giving no staining in
white concrete. They are therefore often used where appearance is important.
Sulphonated naphthalene formaldehyde condensates (SNF)
Typically give 16–25%+ water reduction. They tend to increase the entrapment
of larger, unstable air bubbles. This can improve cohesion but may lead to
more surface defects. Retardation is more than with SMF but will still not
normally exceed 90 minutes. SNF is a very cost-effective.
Polycarboxylate ether superplasticizers (PCE)
Typically give 20–35%+ water reduction. They are relatively expensive per liter
but are very powerful so a lower dose (or more dilute solution) is normally
used.
In general the dosage levels are usually higher than with conventional water
reducers, and the possible undesirable side effects are reduced because they
do not markedly lower the surface tension of the water.
3. Accelerators:
An admixture which, when added to concrete, mortar, or grout, increases the
rate of hydration of hydraulic cement, shortens the time of set in concrete, or
increases the rate of hardening or strength development.
Accelerating admixtures can be divided into groups based on their
performance and application:
Set Accelerating Admixtures,
Reduce the time for the mix to change from the plastic to the hardened state.
Set accelerators have relatively limited use, mainly to produce an early set.
Hardening Accelerators,
Which increase the strength at 24 hours by at least 120% at 20ºC and at 5ºC by
at least 130% at 48 hours. Hardening accelerators find use where early
stripping of shuttering or very early access to pavements is required. They are
often used in combination with a high range water reducer, especially in cold
conditions.
Calcium chloride is the most effective accelerator and gives both set and
hardening characteristics. However, is limited due to acceleration of corrosion
of steel reinforcement and decrease resistance of cement paste in a sulfate
environment. For this reason, it should not be used in concrete where any
steel will be embedded but may be used in plain unreinforced concrete.
Chloride-free accelerators are typically based on salts of nitrate, nitrite,
formate and thiocyanate. Hardening accelerators are often based on high
range water reducers, sometimes blended with one of these salts. Accelerating
admixtures have a relatively limited effect and are usually only cost effective in
specific cases where very early strength is needed for, say, access reasons.
They find most use at low temperatures where concrete strength gain may be
very slow so that the relative benefit of the admixture becomes more
apparent.
In summary, a hardening accelerator may be appropriate for strength gain up
to 24 hours at low temperature and up to 12 hours at ambient temperatures.
Beyond these times, a high range water reducer alone will usually be more
cost-effective.
4. Set Retarders:
The function of retarder is to delay or extend the setting time of cement paste
in concrete. These are helpful for concrete that has to be transported to long
distance, and helpful in placing the concrete at high temperatures.
When water is first added to cement there is a rapid initial hydration reaction,
after which there is little formation of further hydrates for typically 2–3 hours.
The exact time depends mainly on the cement type and the temperature. This
is called the dormant period when the concrete is plastic and can be placed. At
the end of the dormant period, the hydration rate increases and a lot of
calcium silicate hydrate and calcium hydroxide is formed relatively quickly. This
corresponds to the setting time of the concrete. Retarding admixtures delay
the end of the dormant period and the start of setting and hardening. This is
useful when used with plasticizers to give workability retention. Used on their
own, retarders allow later vibration of the concrete to prevent the formation
of cold joints between layers of concrete placed with a significant delay
between them.
The mechanism of set retards is based on absorption. The large admixture
anions and molecules are absorbed on the surface of cement particles, which
hinders further reactions between cement and water i.e. retards setting. The
commonly known retards are Calcium Ligno-sulphonates and Carbohydrates
derivatives used in fraction of percent by weight of cement.
5. Air Entrained Admixtures:
An addition for hydraulic cement or an admixture for concrete or mortar which
causes air, usually in small quantity, to be incorporated in the form of minute
bubbles in the concrete or mortar during mixing, usually to increase its
workability and frost resistance. Air-entraining admixtures are surfactants that
change the surface tension of the water. Traditionally, they were based on
fatty acid salts or vinsol resin but these have largely been replaced by synthetic
surfactants or blends of surfactants to give improved stability and void
characteristics to the entrained air. Air entrainment is used to produce a
number of effects in both the plastic and the hardened concrete. These
include:
Resistance to freeze – thaw action in the hardened concrete.
Increased cohesion, reducing the tendency to bleed and segregation in the
plastic concrete.
Compaction of low workability mixes including semi - dry concrete.
Stability of extruded concrete.
Cohesion and handling properties in bedding mortars.
Mineral Admixtures in Concrete
Types of Mineral Admixtures
Cementitious
These have cementing properties themselves. For example:
Ground granulated blast furnace slag (GGBFS)
Pozzolanic
A pozzolan is a material which, when combined with calcium hydroxide (lime),
exhibits cementitious properties. Pozzolans are commonly used as an addition
(the technical term is "cement extender") to Portland cement concrete
mixtures to increase the long-term strength and other material properties of
Portland cement concrete and in some cases reduce the material cost of
concrete. Examples are:
Fly ash
Silica Fume
Rice Husk Ash
Metakaolin
Pozzolanic Action:
The additive act in three ways
Filler
Nucleating
Pozzolanic
1. Filler:
These additives/admixtures are finer than cement, so when added to concrete
they occupy the small pores previously left vacant.
2. Nucleating:
These fine particles accelerate the rate of hydration and precipitation starts.
3. Pozzolanic:
When cementing material reacts with water the following reaction take place:
C2S + H CSH + CH
C3S + H CSH + CH
CSH is responsible for strength while CH is a soluble material reacts and
dissolves in water leaving behind pores. So when admixture is added
SiO3 or Al2O3+ CH CSH
Thus, it reduces the amount of CH & increase CSH
Conditions to Declare a Material Pozzolan:
Having silica + Alumina oxide+ ferrous oxide more than 70%.
Surface area on normal admixture is more than 300m²/kg.
Surface area should be more than cement used.
3. Ground Granulated Blast Furnace Slag (GGBFS)
Ground granulated blast-furnace slag is the granular material formed when
molten iron blast furnace slag (a by-product of iron and steel making) is rapidly
chilled (quenched) by immersion in water. It is a granular product, highly
cementitious in nature and, ground to cement fineness, hydrates like Portland
cement.
(Blast-Furnace Slag: A by-product of steel manufacture which is sometimes
used as a substitute for Portland cement. In steel industry when iron ore is
molted, then in the molted state all the impurities come at its surface which
are removed called slag. It consists mainly of the silicates and aluminosilicates
of calcium, which are formed in the blast furnace in molten form
simultaneously with the metallic iron. Blast furnace slag is blended with
Portland cement clinker to form PORTLAND BLASTFURNACE SLAG CEMENT).
GGBFS is used to make durable concrete structures in combination with
ordinary Portland cement and/or other pozzolanic materials. GGBFS has been
widely used in Europe, and increasingly in the United States and in Asia
(particularly in Japan and Singapore) for its superiority in concrete durability,
extending the lifespan of buildings from fifty years to a hundred years.
Concrete made with GGBFS cement sets more slowly than concrete made with
ordinary Portland cement, depending on the amount of GGBFS in the
cementitious material, but also continues to gain strength over a longer period
in production conditions. This results in lower heat of hydration and lower
temperature rises, and makes avoiding cold joints easier, but may also affect
construction schedules where quick setting is required.
Use of GGBFS significantly reduces the risk of damages caused by alkali-silica
reaction (ASR), provides higher resistance to chloride ingress, reducing the risk
of reinforcement corrosion, and provides higher resistance to attacks by
sulfate and other chemicals.
Benefits:
Durability
GGBFS cement is routinely specified in concrete to provide protection against
both sulphate attack and chloride attack
GGBFS is also routinely used to limit the temperature rise in large concrete
pours. The more gradual hydration of GGBFS cement generates both lower
peak and less total overall heat than Portland cement.
Appearance
In contrast to the stony grey of concrete made with Portland cement, the near-
white color of GGBFS cement permits architects to achieve a lighter colour for
exposed fair-faced concrete finishes, at no extra cost.
Strength
Concrete containing GGBFS cement has a higher ultimate strength than
concrete made with Portland cement. It has a higher proportion of the
strength-enhancing calcium silicate hydrates (CSH) than concrete made with
Portland cement only, and a reduced content of free lime, which does not
contribute to concrete strength. Concrete made with GGBFS continues to gain
strength over time, and has been shown to double its 28 day strength over
periods of 10 to 12 years.
4. Fly Ash:
The finely divided residue resulting from the combustion of ground or
powdered coal. Fly ash is generally captured from the chimneys of coal-fired
power plants; it has POZZOLANIC properties, and is sometimes blended with
cement for this reason.
Fly ash includes substantial amounts of silicon dioxide (SiO2) (both amorphous
and crystalline) and calcium oxide (CaO). Toxic constituents include arsenic,
beryllium, boron, cadmium, chromium, cobalt, lead, manganese, mercury,
molybdenum, selenium, strontium, thallium, and vanadium.
Class F Fly Ash:
The burning of harder, older anthracite and bituminous coal typically produces
Class F fly ash. This fly ash is pozzolanic in nature, and contains less than 10%
lime (CaO). The glassy silica and alumina of Class F fly ash requires a cementing
agent, such as Portland cement, quicklime, or hydrated lime, with the presence
of water in order to react and produce cementitious compounds.
Class C Fly Ash:
Fly ash produced from the burning of younger lignite or subbituminous coal, in
addition to having pozzolanic properties, also has some self-cementing
properties. In the presence of water, Class C fly ash will harden and gain
strength over time. Class C fly ash generally contains more than 20% lime
(CaO). Unlike Class F, self-cementing Class C fly ash does not require an
activator. Alkali and sulfate (SO4) contents are generally higher in Class C fly
ashes.
In addition to economic and ecological benefits, the use of fly ash in concrete
improves its workability, reduces segregation, bleeding, heat evolution and
permeability, inhibits alkali-aggregate reaction, and enhances sulfate
resistance. Even though the use of fly ash in concrete has increased in the last
20 years, less than 20% of the fly ash collected was used in the cement and
concrete industries.
One of the most important fields of application for fly ash is PCC pavement,
where a large quantity of concrete is used and economy is an important factor
in concrete pavement construction.
5. Silica Fume
By-product of semiconductor industry
The terms condensed silica fume, microsilica, silica fume and volatilized silica
are often used to describe the by-products extracted from the exhaust gases of
silicon, ferrosilicon and other metal alloy furnaces. However, the terms
microsilica and silica fume are used to describe those condensed silica fumes
that are of high quality, for use in the cement and concrete industry.
Silica fume was first ‘obtained’ in Norway, in 1947, when environmental
restraints made the filtering of the exhaust gases from the furnaces
compulsory.
Silica Fume consists of very fine particles with a surface area ranging from
60,000 to 150,000 ft²/lb or 13,000 to 30,000 m²/kg, with particles
approximately 100 times smaller than the average cement particle. Because of
its extreme fineness and high silica content, Silica Fume is a highly effective
pozzolanic material. Silica Fume is used in concrete to improve its properties. It
has been found that Silica Fume improves compressive strength, bond
strength, and abrasion resistance; reduces permeability of concrete to chloride
ions; and therefore helps in protecting reinforcing steel from corrosion,
especially in chloride-rich environments such as coastal regions.
6. Rice Husk Ash:
This is a bio waste from the husk left from the grains of rice. It is used as a
pozzolanic material in cement to increase durability and strength.
The silica is absorbed from the ground and gathered in the husk where it
makes a structure and is filled with cellulose. When cellulose is burned, only
silica is left which is grinded to fine powder which is used as pozzolana.