g PWUMany admixtures affect more than one property of the concrete and although they may
enhance some, they may adversely affect some others. An admixture should be employed only
after an appropriate evaluation of its effects on concrete. A degree of control must also be
exercised to ensure proper quantity of admixture.
p
The functions of admixtures are to accelerate the initial set of concrete, i.e., to speed up the rate
of development of strength at early ages, retard the initial set, increase the strength of concrete,
improve workability, reduce heat of evolution, increase durability of concrete—resistance to
freezing and thawing, control expansion caused by aggregate-alkali reaction, decrease capillary
flow of water and to make it impermeable, increase the penetration and pumpability of concrete,
reduce segregation in grouts, strengthen the bond between old and new concrete surfaces and
that between steel reinforcement and concrete, inhibit corrosion of concrete, increase resistance
to chemical attack, produce coloured and cellular concrete, produce concrete of fungicidal,
germicidal and insecticidal properties, and produce non-skid concrete surfaces.
g
Admixtures may be classified as accelerators, retarders, water proofers, workability agents,
surface active agents, puzzolanas, etc.
e
X normally reduce the setting time, accelerate the rate of hydration of cement and
consequently the rate of gain of strength. The examples of accelerators are sulphates with an
exception of calcium sulphate, alkali carbonates aluminates and silicates, aluminium chloride,
calcium chloride, sodium chloride, sodium and potassium hydroxides, calcium formate,
formaldehyde, para formaldehyde, etc.
Some substances may act as accelerators or as retarders according to the proportion added.
For example, CaCl 2 when added up to 2 per cent by weight of cement acts as accelerator, but
on increasing the proportion, it acts as retarder and leads to flash set. Similarly, triethanolamine
when added in proportion less than 0.06 per cent by weight of cement acts as an accelerator but
acts as retarder if the dosage is increased. CaCl 2 and NaCl are very useful to permit concreting
in very cold weather (–23°C). These lower the temperature at which freezing takes place, help
to keep the mixture warm by accelerating the generation of heat, and increase the ability of the
concrete to resist frost by speeding up the rate of gain of strength.
Xnormally increase the setting time and thus delay the setting of cement. Since these
reduce the rate of hydration, more water is available and better is the workability. Retarders
increase the compressive strength under freezing and thawing. Calcium sulphate, sugar, starch,
cellulose, ammonium, ferrous and ferric chlorides, sodium hexametaphosphate, lignosulphonic
acid and their salts, carbohydrates, hydrocarboxylic acids and their salts are a few examples of
retarders. The use of CaSO 4 as retarder has already been mentioned in the manufacture of
cement where it prevents the flash set. An addition of 0.2 per cent sugar by mass retards the
hydration of cement to such an extent that the final set may not take place even for 72 hours.
It has also been found that an addition of 0.1 per cent sugar by mass of cement raises the
strength of cement at 3 days and increases the 28 days strength by 30 per cent. Retarders are
very important in the situations where grouting is to be done for the voids behind the concrete
arch, tunnel lining, etc. These also ensure a better bond between successive lifts in concrete