Admixture is defined as a material, other than cement, water, sand and aggregates, that is used as an ingredient of concrete. It is added to the batch immediately before or during mixing. Whereas additive is a material which is added at the time of grinding cement clinker at the cement factory.

To improve the quality performance or durability of concrete, admixtures are added. These modify the properties of ordinary concrete so as to make it more suitable for any situation.

Get more knowledge about 20 types of cement used across the world.

Classification of Admixture

1. Plasticizers
2. Superplasticizers
3. Retarders
4. Regarding Plasticizers
5. Accelerators
6. Accelerating Plasticizers
7. Air entraining Admixture
8. Pozzolanic or Mineral Admixture
9. Damp proofing and Water proofing Admixture
10. Gas forming Admixture
11. Air detraining Admixture
12. Alkali aggregate expansion inhibiting Admixture
13. Workability Admixture
14. Grouting Admixture
15. Corrosion inhibiting Admixture
16. Bonding Admixture
17. Fungicidal, Germicidal, Insecticidal Admixture
18. Colouring Admixture
19. Miscellaneous Admixture

1. Plasticizers

Plasticizers can help difficult conditions for obtaining higher workability without using excess of water. One should remember that addition of excess water, will only improve the fluidity or the consistency. But it will not improve the workability of concrete. It reduces the tendency of segregation and bleeding of concrete.

Action of Plasticizers

The action of plasticizers is mainly to fluidify the mix and improve the workability of concrete, mortar or grout. The mechanisms that are involved could be explained in the following way:

Dispersion

Portland cement, being in fine state of division, will have a tendency to flocculate in wet concrete. These flocculation entraps certain amount of water used in the mix and thereby all the water is not freely available to fluidify the mix.

When plasticizers are_used, they gets_adsorb on the cement particles. The adsorption of charged polymer on the particles of cement creates particle-to-particle repulsive forces which overcome the attractive forces. This repulsive force is termed Zeta Potential, which depends on the base, solid content, quantity of plasticizer used. The overall result is_that the cement particles are_deflocculated and dispersed. When cement particles are_deflocculated, the water trapped inside the flocs gets release and now available to fluidify the mix.

When cement particles get flocculate there will_be interparticles friction between particle to particle and floc to floc. But in the dispersed condition there is water in between the cement particle and hence the interparticle friction is_reduced.

Retarding Effect

It is_mentioned earlier that plasticizer gets adsorb on the surface of cement particles and form a thin sheath. This thin sheath inhibits the surface hydration reaction between water and cement as long as sufficient plasticizer molecules are available at the particle/solution interface. The quantity of available plasticizers will progressively decrease as the polymers become entrapped in hydration products.

Many research workers explained that one or more of the following mechanisms may take place simultaneously:

• Reduction in the surface tension of water.
• Induced electrostatic repulsion between particles of cement.
• Lubricating film between cement particles.
• Dispersion of cement grains, releasing water trapped within cement flocs.
• Inhibition of the surface hydration reaction of the cement particles, leaving more water to fluidify the mix.
• Change in the morphology of the hydration products.
• Induced steric hindrance preventing particle-to-particle contact.
• It may be noted that all plasticizer are to some extent set retarders, depending upon the
• base of plasticizers, concentration and dosage used.

2. Superplasticizers (High Range Water Reducers)

Superplasticizers constitute a relatively new category and improved version of plasticizer, the use of which was_developed in Japan and Germany during 1960 and 1970 respectively. They are chemically different from normal plasticiszers. Use of superplasticizers permit the reduction of water to the extent upto 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 levelling, self compacting and for the production of high strength and high performance concrete. The mechanism of action of superplasticizers are more or less same as explained earlier in case of ordinary plasticizer. Only thing is that the superplasticizers are more powerful as dispersing agents and they are high range water reducers. They are also termed 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 of superplasticizer which has made it possible to use fly ash, slag and particularly silica fume to make high performance concrete.

The use of superplasticizer in concrete is an important milestone in the advancement of concrete technology. Since their introduction in the early 1960 in Japan and in the early 1970 in Germany, it is_widely used all over the world. India is catching up with the use of superplasticizer in the construction of high rise buildings, long span bridges and the recently become popular Ready Mixed Concrete Industry. Common builders and Government departments are yet to take up the use of this useful material.

Superplasticizers 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 superplasticizers 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 superplasticizers.

• Sulphonated malanie-formaldehyde condensates (SMF)
• Sulphonated naphthalene-formaldehyde condensates (SNF)
• Modified lignosulphonates (MLS)
• Other types

In addition to the above, in other countries the following new generation superplasticizers are also used.

• Acrylic polymer based (AP)
• Copolymer of carboxylic acrylic acid with acrylic ester (CAE)
• Cross linked acrylic polymer (CLAP)
• Polycarboxylate ester (PC)
• Multicarboxylatethers (MCE)
• Combinations of above

Effects of Superplasticizers on Fresh Concrete

It is to be noted that dramatic improvement in workability is not showing up when plasticizers or superplasticizers are added to very stiff or what is called zero slump concrete at nominal dosages. A mix with an initial slump of about 2 to 3 cm can only be fluidised by plasticizers or superplasticizers at nominal dosages. A high dosage is required to fluidify no slump concrete. An improvement in slump value can be obtained to the extent of 25 cm or more depending upon the initial slump of the mix, the dosage and cement content. It is often noticed that slump increases with increase in dosage. But there is no appreciable increase in slump beyond certain limit of dosage. As a matter of fact, the over-dosage may sometime harm the concrete.

Compatibility of Superplasticizers and Cement

It has been noticed that all superplasticizers are not showing the same extent of improvement in fluidity with all types of cements. Some superplasticizers may show higher fluidizing effect on some type of cement than other cement. There is nothing wrong with either the superplasticizer or that of cement. The fact is that they are just not compatible to show maximum fluidizing effect. Cptimum fluidizing effect at lowest dosage is an economical consideration. Giving maximum fluidizing effect for a particular superplasticizer and a cement is very complex involving many factors like composition of cement, fineness of cement etc.

Although compatibility problem looks to be very complex, it could be more or less solved by simple rough and ready field method. Incidentally this simple field test shows also the optimum dose of the superplasticizer to the cement.

Following methods could be adopted.

• Marsh cone test
• Mini slump test
• Flow table test

Factors Effecting the Workability

• Type of superplasticizers
• Dosage
• Mix composition
• Variability in cement composition and properties
• Mixing procedure
• Equipments
• Others

Type of Superplasticizers

It is_a well established fact that the average molecular weight of the plasticizer is of primary importance for its efficiency as plasticizer in concrete. The higher the molecular weight, the higher is the efficiency. However, it should be_noted that there is a maximum value of molecular weight beyond which efficiency is_expected to decrease. It may_be further noted that several intrinsic properties of the superplasticizers may influence the performance. Therefore, it is difficult to compare the efficiency of one plasticizer from the other in the absence of number of related properties of superplasticizers.

Dosage

It has_been already explained while describing the Marsh cone test that the
dosage of superplasticizer influences the viscosity of grout and hence the workability of concrete. The optimum dosage can_be ascertained from Marsh cone test if brand of cement, plasticizer and w/c ratio is_already fixed. Simple Marsh cone test can give realistic dosage than manufacturers instructions which is general in nature. In our country generally low dosage is_adopted for normal concreting operations. A dosage more than 2.5% by weight of cement is_rarely used. But in other countries much higher dosages up to 4 to 5% are_used in special situations. It has been reported in literature that upto a dosage of about 3% there are no harmful effect on the hardening properties of concrete. Higher dosage is_said to have affected the shrinkage and creep properties.

Mix Composition

The mix composition particularly the aggregate/cement ratio or richness of the mix, w/c ratio, and use of other supplementary cementing materials like fly ash or silica fume affects the workability. Wetter the mix better is the dispersion of cement grains and hence better workability. The size and shape of aggregate, sand grading will also have influence on the fluidifying effect.

Variability in Cement Composition

The variability in cement with respect to compound composition, in particular C3A content, C;S/C,S ratio, fineness of cement, alkali content and gypsum content are responsible for the lack of compatibility with a particular type of superplasticizer and their performance in concrete. Out of the above C3A content will have over-riding influence on the performance of superplasticizer.

New Generation Superplasticizers

It has been amply brought out that superplasticizers are_used to –

• increase the workability without changing the mixture composition,
• reduce the amount of mixing water, in order to reduce the w/c ratio which results in increase of strength and durability, and
• reduce both water and cement in order to cut cost and incidentally to reduce creep, shrinkage, and heat of hydration.

One of the most important drawbacks of traditional superplasticizers such as SMF or SNF or MLS, is the slump loss. Slump loss with time presents a serious limitation on the advantages of superplasticizers. More recently in Europe and Japan, new generation superplasticizers – all based on family of acrylic polymers (AP) have been investigated.

Site Problems in the use of Superplasticizers

Some of the practical site problems in the use of superplasticizers are below –

1. Slump of reference mix. (i.e., concrete without plasticizer)
2. Inefficient laboratory mixer for trial.
3. Sequence of addition of plasticizer.
4. Problem with crusher dust.
5. Problem with crushed sand.
6. Importance of shape and grading of coarse aggregate.
7. Compatibility with cement.
8. Selection of plasticizer and superplasticizer.
9. Determination of dosage.
10. Slump loss.
11. How to reduce slump loss.
12. Casting of cubes.
13. Compaction at site.
14. Segregation and bleeding.
15. Finishing.
16. Removal of form work.

3. 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. Retarders are_used to overcome the accelerating effect of high temperature on setting properties of concrete in 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.

Retarding admixtures are_sometimes used to obtain exposed aggregate look in concrete. The retarder sprayed on the surface of the formwork, prevents the hardening of matrix at the interface of concrete and formwork, whereas the rest of the concrete gets harden. On removing the formwork after one day or so, the unharden matrix can, 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 commonly known retarder is calcium sulphate. It is interground to retard the setting of cement. The appropriate amount of gypsum to be_used must 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.

4. Retarding Plasticizers

It is_mentioned earlier that all the plasticizers and superplasticizers by themselves show certain extent of retardation. Many a time this extent of retardation of setting time offered by admixtures will not be sufficient. Instead of adding retarders separately, retarders are_mixed with plasticizers or superplasticizers at the time of commercial production. Such commercial brand is_known as retarding plasticizers or retarding super plasticizers. ASTM type D is retarding plasticizers and ASTM type G is retarding superplasticizer. In the commercial formulation we have also retarding and slump retaining version.

Retarding plasticizers or superplasticizers are_important category of admixtures often used in the Ready mixed concrete industry for the purposes of retaining the slump loss, during high temperature, long transportation, to avoid construction or cold joints, slip form construction and regulation of heat of hydration.

One must_be careful in the selection of such ready made retarding admixtures. On account of heterogeneous nature and different molecular weight of retarders used with plasticizers, they tend to separate out. It happens when sugar solution is_used as cheap retarders. When retarders like gluconate is_used such separation or settlement of retarders do not happen. It is_cautioned that such retarding plasticizers should always_be shaken thoroughly or well stirred before use. There are cases that settlement of retarders from rest of the ingredients causing excessive retardation and failure of structures.

5. Accelerators

Accelerating admixtures are_added to concrete to increase the rate of early strength development in concrete to

• permit earlier removal of formwork
• reduce the required period of curing
• advance the time that a structure can be_placed in service
• partially compensate for the retarding effect of low temperature during cold weather concreting
• in the emergency repair work

6. Accelerating Plasticizers

Certain ingredients are_added to accelerate the strength development of concrete to plasticizers or superplasticizers. Such accelerating superplasticizers, when added to concrete result in faster development of strength. The accelerating materials added to plasticizers or superplasticizers are_triethenolamine chlorides, calcium nutrite, nitrates and flousilicates etc. The accelerating plasticizers or accelerating superplasticizers manufactured_by well known companies are_chloride free.

7. Air-entraining Admixture

Perhaps one of the important advancements made in concrete technology was_the discovery of air entrained concrete. Since 1930 there has been an ever increasing use of air entrained concrete all over the world especially, in the United States and Canada. Due to the recognition of the merits of air entrained concrete, about 85 per cent of concrete manufactured in America contains one or the other type of air entraining agent. So much so that air entraining agents have almost come to be considered a necessary ‘fifth ingredient in concrete making.

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 air voids present in concrete can_be brought under two groups –
(a) Entrained air
(b) Entrapped air

Air entraining agents

The following types of air entraining agents are_used for making air entrained concrete.

• Natural wood resins
• Animal and vegetable fats and oils, such as tallow, olive oil and their fatty acids such as stearic and oleic acids.
• Various wetting agents such as alkali salts or sulphated and sulphonated organic compounds.
• Water soluble soaps of resin acids, and animal and vegetable fatty acids.
• Miscellaneous materials such as the sodium salts of petroleum sulphonic acids, hydrogen peroxide and aluminium powder, etc.

Factors affecting amount of air entrainment

The manufacture of air entrained concrete is_complicated by the fact that the amount of air entrainment in a mix is_affected by many factors; the important ones are –

• The type and quantity of air entraining agent used.
• Water/cement ratio of the mix.
• Type and grading of aggregate.
• Mixing time.
• The temperature.
• Type of cement.
• Influence of compaction.
• Admixtures other than air entraining agent used.

Different air entraining agents produce different amounts of air entrainment, depending upon the elasticity of the film of the bubble produced, and the extent to which the surface tension reduced. Similarly, different quantities of air entraining agents will result in different amount of entrainments.

The Effect of Air Entrainment on the Properties of Concrete

Air entrainment will effect directly the following three properties of concreté –

(a) Increased resistance to freezing and thawing.
(b) Improvement in workability.
(c) Reduction in strength.

Incidentally air entrainment will also effect the properties of concrete in the following ways:

(a) Reduces the tendencies of segregation.
(b) Reduces the bleeding and laitance.
(c) Decreases the permeability.
(d) Increases the resistance to chemical attack.
(e) Permits reduction in sand content.
(f) Improves placeability, and early finishing.
(g) Reduces the cement content, cost, and heat of hydration.
(h) Reduces the unit weight.
(i) Permits reduction in water content.
(j) Reduces the alkali-aggregate reaction.
(k) Reduces the modulus of elasticity.

8. Pozzolanic or Mineral Admixtures

Pozzolans have been extensively used in Europe, USA and Japan, as an ingredient of Portland cement concrete particularly for marine and hydraulic structures.

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 watertightnessS;
(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.

In addition to these advantages, contrary to the general opinion, good pozzolans will not unduly increase water requirement or drying shrinkage.

Pozzolanic Materials

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.

Natural Pozzolans

• Clay and Shales
• Opalinc Cherts
• Diatomaceous Earth
• Volcanic Tuffs and Pumicites

Artificial Pozzolans

• Fly Ash
• Blast Furnace Slag
• Silica Fume
• Rice Husk Ash
• Metakaoline
• Surkhi

Other Mineral admixtures, like finely ground marble, quartz, granite powder are also used. They neither exhibit the pozzolanic property nor the cementitious properties. They just act as inert filler.

9. Damp-proofing and Waterproofing Admixture

In practice one of the most important requirements of concrete is that it must be impervious to water under two conditions, firstly, when subjected to pressure of water on one side, secondly, to the absorption of surface water by capillary action. Many investigators are of the opinion that the concrete, carefully designed, efficiently executed with sound materials will be impermeable to water. However, since the usual design, placing, curing and in general the various operations involved at the site of work leave much to be desired, it is accepted that a use of a well chosen admixture may prove to be of some advantage in reducing the permeability.

Waterproofing admixtures may be obtained in powder, paste or liquid form and may consist of pore filling or water repellent materials. The chief materials in the pore filling class are silicate of soda, aluminium and zinc sulphates and aluminium and calcium chloride. These are chemically active pore fillers. In addition they also accelerate the setting time of concrete and thus render the concrete more impervious at early age.

Some materials like soda, potash soaps, calcium soaps, resin, vegetable oils, fats, waxes and coal tar residues are added as water repelling materials in this group of admixtures. In some kind of waterproofing admixtures inorganic salts of fatty acids, usually calcium or ammonium stearate or oleate is added along with lime and calcium chloride.

10. Gas forming Agents

A gas forming agent is a chemical admixture such as aluminium powder. It reacts with the hydroxide in order to produce in the hydration of cement to produce minute bubbles of hydrogen gas throughout the matrix. Aluminium powder is used as an admixture in the production of light weight concrete. Zinc magnesium powder and hydrogen peroxide are also used as gas forming agents.

11. Air-detraining agents

There have been cases where aggregates have released gas into or caused excessive air entrainment, in plastic concrete which made it necessary to use an admixture capable of dissipating the excess of air or other gas. Also it may be required to remove a part of the entrained air from concrete mixture. Compounds such as tributyl phosphate, water-insoluble alcohols and silicones have been proposed for this purpose. However, tributyl phosphate is the most widely used air-detraining agent.

12. Alkali-aggregate expansion inhibitors

It has been seen that alkali-aggregate reaction can be reduced by thus the use of pozzolanic admixture. We have therefore already dealt about the use of pozzolanic material early in this article. There are some evidences that air entraining admixture reduces the alkali-aggregate reaction however slightly. The other admixtures that may be used to reduce the alkali aggregate reaction are thus aluminium powder and lithium salts.

13. Workability Agents

Workability is one of the most important characteristics of concrete, specially under the following circumstances –

(a) If the concrete is to be placed around closely placed reinforcement, deep beams, thin sections etc.
(b) Where special means of placement are required such as tremie, chute or pumping methods.
(c) If the concrete is harsh because of poor aggregate characteristics or grading.
(d) For making high strength concrete when w/c ratio is very low.

In the above circumstances even the cost of achieving the workability may have to be overlooked.

Some admixtures can be used to improve workability. The materials used as workabilíty agents are –

(a) finely divided material,
(b) plasticizers and superplasticizers,
(c) air-entraining agents

14. Grouting Agents

Grouting under different conditions or for different purposes would necessitate different qualities of grout-mixture. Sometimes grout mixtures will be_required to set quickly and sometimes grout mixtures will have to be in fluid form over a long period so that they may flow into cavities and
fissures. Sometimes in grout mixtures, a little water is to be_used but at the same time it should exhibit good workability to flow into the cracks and fissures. There are many admixtures which will satisfy the requirements of grout mixture.

Admixtures used for grouting are –

(a) Accelerators
(b) Retarders
(c) Gas forming agents
(d) Workability agents
(e) Plasticizers

Accelerating agenits may be used in grout to hasten the set in situation where a plugging effect is desired, In such a case calcium chloride or triethanolamine can be used.

Retarders and dispersing agents may be_used in a grout to aid pumpability and to effect the penetration of grout into fine cracks or seams. They include mucic acid, gypsum and a commercial brand known as RDA (Ray Lig Blinder) etc.

15. Corrosion Inhibiting Agents

The problem of corrosion of reinforcing steel in concrete is universal. But it is_more acute in concrete exposed to saline or brackish water or concrete exposed to industrial corrosive fumes. A patented process by Dougill was_used for the North Thames Gas Board in UK, in which sodium benzoate was_used as corrosion inhibiting admixture, therefore to protect the steel in reinforced concrete. In this process 2 percent sodium benzoate is_used in the mixing water or a 10 percent benzoate cement slurry is_used to paint the reinforcement or both. Sodium benzoate is also an accelerator of compressive strength.

It is found that calcium lignosulphonate decreased the rate of corrosion of steel embedded in the concrete, when the steel reinforcement in concrete is subjected to alternating or direct current. Sodium nitrate and calcium nitrite have been found to be efficient inhibitors of corrosion of steel also in autoclaved products. Two or three per cent sodium nitrate by weight of cement is said to serve the purpose.

16. Bonding Admixture

Bonding admixtures are_water emulsions of several organic materials that are_mixed with cement or mortar grout for application to an old concrete surface just prior to patching with mortar or concrete. Sometimes they_are mixed with the topping or patching material. Their function is to increase the bond strength between the old as well as new concrete. This procedure is_used in patching of eroded or spalled concrete or to add relatively thin layers of resurfacing. Therefore commonly used bonding admixtures are_made from natural rubber, synthetic rubber or from any organic polymers. Thus the polymers include polyvinyl chloride, polyvinyl acetate etc.

Bonding admixtures fall into two general categories, namely, re-emulsifiable types and non-re-emulsifiable types. The latter is better suited for external application since it is resistant to water.

17. Fungicidal, Germicidal and Insecticidal Admixtures

It has been suggested that certain materials may either be ground into the cement added as admixtures to impart fungicidal, germicidal or insecticidal properties to hardened cement pastes, mortars or concretes. These materials include polyhalogenated phenols dieldren emulsion or copper compounds.

18. Colouring Agents

Pigments are often added to produce colour in the finished concrete. The requirements of Suitable admixtures include

• colour fastness when exposed to sunlight
• chemical stability in the presence of alkalinity produced in the set cement
• no adverse effect on setting time or strength development. Various metallic oxides and mineral pigments are used.

Pigments should preferably be thoroughly mixed or interqround with the dry cement. They can also be mixed with dry concrete mixtures before the addition of mixing water.

RMC (India) Ltd., one of the Ready Mixed Concrete supplier markets ready mixed colour concrete, thus for decorative pavements. Sometimes they also make this colour concrete incorporating polypropelyne fibres to arrest possible cracks and craziness in the concrete floor.

19. Miscellaneous Admixtures

There are hundreds of commercial admixtures available in India. They effect more than one property of concrete. Sometimes they are ineffective and also do not fulfill the claims of the manufacturers. It is not intended to deal in detail about these commercial admixtures. However, a few of the more important admixtures are briefly described and some of them are just named.

All these commercial admixtures can roughly brought under two categories (a) Damp proofers (b) Surface hardeners, though there are other agents which will modify the properties like strength, setting tìme, workability etc.

Damp Proofers

1. Accoproof: It is a white powder to mix with concrete at the rate of 1 kg per bag of cement for the purpose of increasing impermeability of concrete structures.
2. Natson’s Cement WaterProofer: As the name indicates, it is a waterproofing admixture, thus to admix at the rate of 1.5 kg per bag of cement.
3. Trip-L-Seal: It is a white powder, in addition of which is claim to decrease permeability of concrete and mortars and produce rapid hardening effect.
4. Cico: It is a colourless liquid which when admixed with concrete, possesses the properties of controlling setting time, promoting rapid hardening, increasing strength and rendering the concrete waterproof.
5. Feb-Mix-Admix: It is a light yellow colour liquid claim to impart waterproofing quality to concrete and also increase workability and bond.
6. Cemet: It is a waterproofing admixture. The recommended dose is 3 percent by weight of cement. It is also claim that its use in concrete will prevent efflorescence and growth of fungi.

Surface Hardeners

(a) Metal Crete: Metal crete is a metallic aggregate which is tough, ductile, specially processed, size graded iron particles with or without in addition cement dispersing agent. Its claimed that it gives greater wear resistance, corrosion resistance, non-dusting and also non-slipping concrete surface.
(b) Ferrocrete No. 1: It is a surface hardener, so makes the concrete surface compact dense and homogeneous.
(c) Metal Crete Steel Patch: It is a surface hardener. When added 20 per cent by weight of cement, it is suppose to increase the compressive strength and abrasion resistance.
(d) Arconate No. 1: It is a black powder compose of iron filings.It is thus use as surface hardener in concrete.

In addition to the above, the other admixtures used as surface hardeners are –

(i) Ironite (ii) Merconite (iii) Meta Rock (iv) Purelite.

Another important admixture which has been very popular is “Lisspol N”. It is a polyetheoxy surface active agent which improves workability, strength and many other important properties of concrete when used in a very small dose of oz per bag of cement.

The commercial admixtures are not dependable. It has been common experience that many a time when these admixtures are test in a laboratory the manufacturer’s or distributors claims thus not fulfilled. So it will be wrong to have much faiths in these commercial admixtures though some of them give some encouraging results.