An admixture is defined as a material other than water, aggregate, used as an ingredient of concrete or mortar and added to the batch immediately before or during mixing.
Admixtures vary in composition from surfactants and soluble salts and polymers to insoluble minerals. The purposes for which they are generally used in concrete include
· Improvement of workability
· Acceleration or retardation of setting time
· Control of strength development
· Enhancement of resistance to frost action and thermal cracking, Alkali-aggregate expansion, and acidic and sulfate solutions.
The admixtures can be broadly divided into two types:
· Chemical admixtures
· Air-entraining admixtures
These air entraining agents incorporate millions of no-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.
(a) Natural wood resins
(b) Animal and vegetable fats and oils, such as tallow, olive oil and their fatty acids such as stearic and oleic acids.
(c) Various wetting agents such as alkali salts or sulphated and sulphonated organic
compounds.
(d) Water soluble soaps of resin acids, and animal and vegetable fatty acids.
(e) Miscellaneous materials such as the sodium salts of petroleum sulphonic acids,
hydrogen peroxide and aluminium powder, etc.
· Water reducing admixtures
o Plasticizers
A high degree of workability is required in situations like deep beams, thin walls of water retaining structures with a high percentage of steel reinforcement, column and beam junctions, pumping of Concrete, hot weather Concreting.
Today, we have plasticizers which can help in difficult conditions for obtaining higher workability without using excess of water. The organic substances or the combinations of organic and inorganic substances, which allow a high reduction in water content for the given workability or give a higher workability at the same water content, are termed as Plasticizing Admixtures.
The basic products constituting plasticizers are:
1. Anionic surfactants such as lignosulphonates and their modifications and derivatives, salts of sulphonates hydrocarbons.
2. Nonionic surfactants such as polyglycol esters, acid of hydroxylated
carboxyl acids and their modifications and derivatives.
3. Other products, such as carbohydrates etc.
Among these, calcium, sodium and ammonium lignosulphonates are the most used.
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, depending on the dosage, initial slump of concrete, cement content and type.
o Superplasticizers (High Range Water Reducers)
Use of superplasticizer permits the reduction of water to the extent up to 30 per cent without reducing workability in contrast to the possible reduction up to 15 per cent in case of plasticizers.
The use of superplasticizer is practiced for production of flowing, self leveling, and self compacting and for the production of high strength and high performance concrete.
The mechanism of action of super plasticizers is more or less same in case of ordinary plasticizer. Only thing is that the super plasticizers are more powerful as dispersing agents and they are high range water reducers. They are called High Range Water Reducers in American literature. It is the use of superplasticizer which has made it possible to use w/c as low as 0.25 or even lower and yet to make flowing concrete to obtain strength of the order 120 Mpa or more. It is the use ofsuperplasticizer which has made it possible to use fly ash, slag and particularly silica fume to make high performance concrete.
Super plasticizers can produce:
· At the same w/c ratio much more workable concrete than the plain ones,
· For the same workability, it permits the use of lower w/c ratio,
· As a consequence of increased strength with lower w/c ratio, it also permits a reduction of cement content.
The super plasticizers also produce a homogeneous, cohesive concrete generally without any tendency for segregation and bleeding.
Classification of Superplasticizer: Following are a few polymers which are commonly used as base for super plasticizers.
o Sulphonated melamine-formaldehyde condensates (SMF)
o Sulphonated naphthalene-formaldehyde condensates (SNF)
o Modified lignosulphonates (MLS)
· Retarding admixtures
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.
Retarders are used to overcome the accelerating effect of high temperature on setting properties of concrete on hot weather concreting. The Retarders are used in casting and consolidating large number of pours without the formation of cold joints.
They are also used in grouting oil wells. Oil wells are sometimes taken up to a depth of about 6000 meter deep where the temperature may be about 2000C. The annular spacing between the steel tube and the wall of the well will have to be sealed with cement grout. Sometimes at that depth stratified or porous rock strata may also require to be grouted to prevent the entry of gas or oil into some other strata... for all these works cement grout is required to be in mobile condition for about 3 to 4 hours, even at that high temperature without getting set. Use of retarding agent is often used for such requirements.
Sometimes concrete may have to be placed in difficult conditions and delay may occur in transporting and placing. In ready mixed concrete practices, concrete is manufactured in central batching plant and transported over a long distance to the job sites which may take considerable time. In the above cases the setting of concrete will have to be retarded, so that concrete when finally placed and compacted is in perfect plastic state.
Retarding admixtures are sometimes used to obtain exposed aggregate look in concrete. The Retarder sprayed to the surface of the formwork, prevents the hardening of matrix at the interface of concrete and formwork, whereas the rest of the concrete gets hardened. On removing the formwork after one day of so, the unhardened matrix can be just washed off by a jet of water which will expose the aggregates. The above are some of the instances where a retarding agent is used.
Perhaps the most common known Retarder is calcium sulphate. It is interground to retard the setting of cement. The appropriate amount of gypsum to be used must be determined carefully for the given job. Use of gypsum for the purpose of retarding setting time is only recommended when adequate inspection and control is available, otherwise, addition of excess amount may cause undesirable expansion and indefinite delay in the setting of concrete.
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. These chemicals may have variable action on different types of cement when used in different quantities. Unless experience has been had with a retarder, its use as an admixture should not be attempted without technical advice. Any mistake made in this respect may have disastrous consequences.
Common sugar is one of the most effective retarding agents used as an admixture for delaying the setting time of concrete without detrimental effect on the ultimate strength. Addition of excessive amounts will cause indefinite delay in setting. At normal temperatures addition of sugar 0.05 to 0.10 percent have little effect on the rate of hydration, but if the quantity is increased to 0.2 percent, hydration can be retarded to such an extent that final set may not take place for 72 hours or more. Skimmed milk powder (casein) has a retarding effect mainlydue to sugar content.
Other admixtures which have been successfully used as retarding agents are Ligno sulphonic 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. This also increases 28 days compressive strength by 10 to 20 percent. Materials like mucic acid, calcium acetate and commercial products by name “Ray lig binder” are used for set retarding purposes.
· Accelerating admixtures
Accelerating admixtures are added to concrete to increase the rate of early strength development in concrete to
o permit earlier removal of formwork;
o reduce the required period of curing;
o advance the time that a structure can be placed in service;
o partially compensate for the retarding effect of low temperature during cold weather concreting;
o in the emergency repair work.
In the past one of the commonly used materials as an accelerator was calcium chloride. But, now a days it is not used. Instead, some of the soluble carbonates, silicates, fluosilicates and some of the organic compounds such as riethenolamine are used.
Accelerators such as fluosilicates and triethenolamine are comparatively expensive. The recent studies have shown that calcium chloride is harmful for reinforced concrete and prestressed concrete. It may be used or plain cement concrete in comparatively high dose.
Some of the accelerators produced these days are so powerful that it is possible to make the cement set into stone hard in a matter of five minutes are less. With the availability of such powerful accelerator, the under water concreting has become easy.
Similarly, the repair work that would be carried out to the waterfront structures in the region of tidal variations has become easy. The use of such powerful accelerators have facilitated, the basement waterproofing operations. In the field of prefabrication also it has become an invaluable material. As these materials could be used up to -100C, they find an unquestionable use in cold weather concreting.
· Water reducing and set retarding admixtures
· Water reducing and set accelerating admixtures.
· Pozzolanic or Mineral admixtures.
Ancient Greeks and Romans used certain finely divided siliceous materials which when mixed with lime produced strong cementing material having hydraulic properties and such cementing materials were employed in the construction of aqueducts, arch, bridges etc. One such material was consolidated volcanic ash or tuff found near Pozzuoli (Italy) near Vesuvius. This came to be designated as Pozzuolana, a general term covering similar materials of volcanic origin found in other deposits in Italy, France and Spain. Later, the term pozzolan was employed throughout Europe to designate any materials irrespective of its origin which possessed similar properties.
It has been amply demonstrated that the 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 water tightness;
(c) Reduce the alkali-aggregate reaction;
(d) Improve resistance to attack by sulphonate soils and sea water;
(e) Improve extensibility;
(f) Lower susceptibility to dissolution and leaching;
(g) Improve workability;
(h) Lower costs.
In addition to these advantages, contrary to the general opinion, good pozzolans will not unduly increase water requirement or drying shrinkage.
· Fly ash
Fly ash is finely divided residue resulting from the combustion of powdered coal andtransported by the flue gases and collected by electrostatic precipitator. Fly ash is the most widely used pozzolanic material all over the world.
In India, Fly ash was used in Rihand dam construction replacing cement upto about 15 per cent.
In the recent time, the importance and use of fly ash in concrete has grown so much that it has almost become a common ingredient in concrete, particularly for making high strength and high performance concrete.
The use of fly ash as concrete admixture not only extends technical advantages to the properties of concrete but also contributes to the environmental pollution control. InIndia alone, we produce about 75 million tons of fly ash per year, the disposal of which has become a serious environmental problem. The effective utilization of fly ash in concrete making is, therefore, attracting serious considerations of concrete technologies and government departments.
There are two ways that the fly ash can be used: one way is to intergrind certain percentage of fly ash with cement clinker at the factory to produce Portland pozzolana cement (PPC) and the second way is to use the fly ash as an admixture at the time of making concrete at the site of work. The latter method gives freedom and flexibility to the user regarding the percentage addition of fly ash.
· Ground granulated blast-furnace slag
Ground granulated blast-furnace slag is a non metallic product consisting essentially of silicates and aluminates of calcium and other bases. The molten slag is rapidly chilled by quenching in water ton form a glassy sand like granulated material. The granulated material when further ground to less than 45 micron will have specific surface of about 400-600m2/kg
Chemical composition:
o Calcium oxide 30-45%
o Silicon dioxide 30-38%
o Aluminium oxide 15-25%
o Ferrous oxide 0.5-2.0
o Specific gravity 2.9
In India, we produce about 7.8 million tons of blast furnace slag. All the blast furnace slags are granulated by quenching the molten slag by high power water jet, making 100% glassy slag granules of 0.4 mm size. The blast furnace slag is mainly used in Indiafor manufacturing slag cement.
There are two methods for making blast furnace slag cement. In the first method blast furnace slag is interground with cement clinker along with gypsum. In the second method blast furnace slag is separately ground and then mixed with the cement.
Clinker is hydraulically more active than slag. It follows then that slag should be ground finer than clinker, in order to fully develop its hydraulic potential. However, since slag is much harder and difficult to grind compared to clinker, it is ground relatively coarser during the process of inter grinding. This leads to waste of hydraulic potential of slag. Not only that the inter-grinding seriously restricts the flexibility to optimize slag level for different uses.
· Silica fumes
Silica fume, also referred to as micro silica or condensed silica fume, is another material that is used as an artificial pozzolanic admixture.
It is a product resulting from reduction of high purity quartz with coal in an electric arc furnace in the manufacture of silicon or ferrosilicon alloy. Silica fume rises as an oxidized vapor. It cools, condenses and is collected on cloth bags. It is further processed to remove impurities and to control particle size. Condensed silica fume is essentially silicon dioxide in noncrystalline form. Since it is an airborne material like fly ash, it has spherical shape. It is extremely fine with particle size less than 1 micron and with an average diameter of about 0.1 micron, about 100 times smaller than average cement particles. Silica fume has specific surface area of about 20000 m2/kg, as against 230 to 300 m2/kg that of cement.
· Rice husk ash
· Metakoline
The above are cementitious and pozzolanic materials.
