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A concrete admixture is a material other than water, sand, aggregate and cement and is added to the batch immediately before or during its mixing. It is used to improve or give special properties to concrete. It should not adversely affect any properties of the concrete. The admixture is generally added to a concrete mix in a relatively minute quantity.
The degree of control must be higher to ensure that overdosages that are unlikely to occur. Excess quantity of admixture may cause harm to the properties of concrete.
2. Functions of Admixtures
Some of the important purposes for which the admixtures could be used are:
1. To accelerate the initial setting of concrete.
2. To retard the initial set.
3. To increase the strength of concrete.
4. To reduce the heat of evolution.
5. To improve the workability.
6. To inhibit the corrosion of concrete.
7. To increase the durability of concrete.
8. To increase the bond of concrete to the steel reinforcement.
9. To increase the resistance to chemical attacks.
10. To control the alkali-aggregate expansion.
11. To reduce the segregation in grout mixtures.
12. To decrease the weight of concrete per cubic meter.
13. To produce coloured concrete or mortar for coloured surfaces.
14. To produce concrete of fungicidal, germicidal and insecticidal properties.
15. To strengthen the bond between new and old concrete surfaces.
3. Types of Concrete Admixtures
There are 7 types of concrete admixtures that are commonly used:
a. Air- entraining agents:
The main objects of entraining air in concrete are:
i. To increase the workability and
ii. To improve its resistance to weathering and especially to action of frost.
There are three main ways of incorporating air or gas cells in concrete:
1. By the use of gas-forming materials such as aluminium or zinc powder, hydrogen peroxide.
2. By the use of surface-active agents which reduce surface tension. These are known as air-entraining agents and are subdivided into:
i. Natural wood resins and their soaps, e.g. vinsol resin;
ii. Animal or vegetable fats or oils, their fatty acids and their soaps;
iii. Alkali salts of sulfonated or sulphated organic compounds such as synthetic detergents, e.g. Darex.
3. By the use of cement dispersing agents.
Air-entraining should not be used unless site control is good, since the amount of air entrained may vary considerably with changes in sand grading, errors in proportioning, workability of the mix and temperatures.
The retarders, as the name suggests delay the setting time of cement. Calcium sulphate, a form of gypsum is generally added during the manufacture of cement to retard the setting.
Some other materials which are effectively used to retard the rate of hydration are Ammonium chloride, ferrous and ferric chlorides, alkali bicarbonates, salts of carboxymethyl cellulose etc.
The agents that are added to the cement to make it set and acquire strength more rapidly are called accelerators.
Calcium chloride is a commonly used accelerator.
Sodium chloride may also be used as an accelerating agent.
These may be obtained in powder, paste or liquid form and can consist of pore-filling materials or water-repelling materials. They can be chemically active or inactive.
Alkaline silicates notably silica of soda, aluminium and zinc sulphates and aluminium and calcium chlorides are the chief pore-filling materials.
These are all chemically active and hence may accelerate the setting time of the concrete thus making it more impervious at any early age.
Chalk, Fuller’s earth and talc in a very finely ground form are chemically inactive pore-filling materials.
They improve workability and improve the density of concrete. Materials in the water repellent class are soda and potash soaps.
Pozzolana is a siliceous material which while itself having no cementitious properties will either be processed or unprocessed and in finely divided form, react in the presence of water with lime at normal temperatures to form compounds of low solubility having cementitious properties.
Pozzolanas may be natural or artificial. The examples are:
Natural: Clay sand shale’s (to be calcined to become active), diatomaceous earth and opaline cherts and shale’s and volcanic tuffs and pumicites etc.
Artificial: Ground blast furnace slag and fly ash.
Following advantages are obtained through the use of pozzolanas:
1. The heat of hydration is reduced.
2. Workability with less amount of water is improved.
3. Attack from salts and sulphates are reduced.
4. Calcium hydroxide leaching is prevented.
Some of the disadvantages of pozzolanas are:
1. Strength development is slowed.
2. Drying shrinkage may be increased.
3. Durability may be impaired.
Pigments are substances added to produce coloured cement. The chief pigments used in concrete are Brown, black, red, green, blue and yellow.
To obtain a good colouring effect, the pigment should be ground with cement in a ball mill.
g. Workability agents:
Workability agents and water-reducing retarders are sometimes grouped together because many materials appear to perform all these functions. These are mineral powder ground at least as fine as the cement and usually much finer.
They are generally classified into three chemical types: Lignosulfonates, organic acid, and carbohydrates. The excessive use of these agents may increase the shrinkage of concrete.
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