Unit 2
Admixtures & fresh concrete
Module 1
Introduction
• 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.
• Additive can also be a material which is added at the time of
grinding cement clinker at the cement factory.
• These days concrete is being used for wide varieties of
purposes to make it suitable in different conditions. In these
conditions ordinary concrete may fail to exhibit the required
quality performance or durability.
• In such cases, admixture is used to modify the properties of
ordinary concrete so as to make it more suitable for any
situation.
The following admixtures are studied in this topic:
i) Plasticizers
ii) Superplasticizers
iii) Retarders and Retarding Plasticizers
iv) Accelerators and Accelerating Plasticizers
v) Air-entraining Admixtures
vi) Pozzolanic or Mineral Admixtures
vii) Damp-proofing and Waterproofing Admixtures
viii) Gas forming Admixtures
• ix) Air-detraining Admixtures
• x) Alkali-aggregate Expansion Inhibiting
Admixtures
• xi) Workability Admixtures
• xii) Grouting Admixtures
• xiii) Corrosion Inhibiting Admixtures
• xiv) Bonding Admixtures
• xv) Fungicidal, Germicidal, Insecticidal
Admixtures
• xvi) Colouring Admixtures
Plasticizers (Water Reducers):
• The use of super-plasticizer has become almost an universal practice to
reduce water/cement ratio for the given workability, which naturally
increases the strength.
• Moreover, the reduction in water/cement ratio improves the durability of
concrete.
• Calcium, sodium and ammonium lignosulphonates are the mostly used
plasticizers. 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.
• The increase in workability that can be expected, at the same w/c ratio,
may be anything from 30 mm to 150 mm slump.
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.
Calcium sulphate (gypsum) is the most commonly used
retarder. In addition to gypsum there are number of other
materials found to be suitable for this purpose.
They are: starches, cellulose products, sugars, acids or salts of
acids. Other admixtures which have been successfully used as
retarding agents are Lignosulphonic acids and their salts,
hydroxylated carboxylic acids and their salts which in addition
to the retarding effect also reduce the quantity of water
requirement for a given workability.
Accelerators:
Accelerators are added to concrete to increase the rate
of early strength development in concrete to
i) permit earlier removal of formwork;
ii) reduce the required period of curing;
iii) advance the time that a structure can be placed in
service;
iv) in the emergency repair work.
Calcium chloride, some of the soluble carbonates,
silicates fluosilicates and some of the organic
compounds such as triethenolamine are commonly
used accelerators.
Air-entraining Admixture:
Air entrained concrete is made by mixing a small quantity of
air entraining agent or by using air entraining cement.
These air entraining agents incorporate millions of non-
coalescing air bubbles, which will act as flexible ball
bearings and will modify the properties of plastic concrete
regarding workability, segregation, bleeding and finishing
quality of concrete.
It also modifies the properties of hardened concrete
regarding its resistance to frost action and permeability.
The following types of air entraining agents are used for
making air entrained concrete.
Pozzolanic or Mineral Admixtures:
Pozzolanic or Mineral Admixtures:
Best pozzolans in optimum proportions mixed with Portland cement
improves many qualities of
concrete, such as:
(a) Lower the heat of hydration and thermal shrinkage;
(b) Increase the watertightness;
(c) Reduce the alkali-aggregate reaction;
(d) Improve resistance to attack by sulphate soils and sea water;
(e) Improve extensibility;
(f ) Lower susceptibility to dissolution and leaching;
(g) Improve workability;
(h) Lower costs.
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.
Examples of Natural Pozzolans are Clay and Shales, Opalinc
Cherts, Diatomaceous Earth, Volcanic Tuffs and Pumicites.
Examples of artificial pozzolans are Fly ash, Blast Furnace
Slag, Silica Fume, Rice Husk ash,
Metakaoline, Surkhi.
