What Is Admixture?
Admixture is artificial or natural materials added to concrete, in addition to cement, water, and aggregates, to improve certain properties of concrete during casting, laying, or the service stage.
To improve the properties of the concrete needed. It can be divided into two groups which are
Types of Admixtures used in Concrete
Concrete Admixtures are of different types and are as follows:
1. Water Reducing Admixture
Water-reducing Admixture, the name itself defines that they are used to minimize the demand for water in a concrete mix.
Workability is the important property of concrete, which is improved by adding water, but if the water is added more than necessary, the strength and durability properties of concrete will be affected.
In addition, to increase workability, it also improves the strength of concrete, the good connection between concrete and steel, prevents cracking, segregation, honeycombs, bleeding, etc.
Water reducing Admixture are also called plasticizers and are classified into three types: plasticizers, mid-range plasticizers, and superplasticizers.
The normal plasticizer reduces water demand by 10%, mid-range plasticizers reduce water demand by up to 15%, while superplasticizers reduce water demand by up to 30%.
Calcium, sodium, and ammonium lignosulfonates are commonly used plasticizers. Some of the new generation superplasticizers are based on acrylic polymers, polycarboxylates, multicarbovyl ethers, etc.
2. Retarding Admixture
Retarding Admixture decrease the rate of cement hydration in its initial stage and increase the initial hardening time of the concrete.
They are also called retarders and are used especially in high-temperature areas, where concrete hardens quickly.
The quick configuration in some situations can lead to discontinuities in the structure, the lack of connection between the surfaces creates unnecessary voids in the concrete, etc. Retarders are useful to eliminate this type of problem.
Retardant additive commonly used is calcium sulfate or plaster. Starch, cellulose products, common sugar, acid salts are some other retarders.
Most water-reducing mixtures also act as a retardant additive and are called retardant plasticizers.
3. Accelerating Admixture
Accelerating Admixtures are used to reduce the initial hardening time of the concrete.
They speed up the process of the initial phase of concrete hardening, so they are also called accelerators.
These accelerators also improve the strength of the concrete in the initial stage, increasing the rate of hydration.
The previous hardening of the concrete is useful in several situations, such as early formwork removal, shorter curing period, emergency repairs, for buildings in low-temperature regions, etc.
Some of the accelerator mixtures are triethanolamine, calcium formate, active silica, calcium chloride, finely divided silica gel, etc. Calcium chloride is an inexpensive and commonly used accelerator mixture.
4. Air Entraining Admixtures
Air entrainment Admixture is one of the most important inventions in concrete technology. Its main function is to increase the durability of the concrete under freezing and thawing conditions.
When added to the concrete mix, these mixtures will form millions of non-coalescent air bubbles throughout the mix and improve the properties of the concrete.
The entry of air into the concrete will also improve the workability of the concrete, prevent segregation and bleeding, reduce the weight of the unit and the elastic modulus of the concrete, improve the chemical resistance of the concrete and reduce the content of cement or sand or water in the concrete, etc.
The most commonly used air entrainment Admixture is vinsol resin, darex, Teepol, Cheecol, etc.
These Admixtures are actually made from natural wood resins, alkaline salts, animal and vegetable fats, and oils, etc.
5. Pozzolanic Admixture
Pozzolanic mixtures are used to prepare a dense concrete mix that is most suitable for water retention structures, such as dams, reservoirs, etc. They also reduce the heat of hydration and thermal shrinkage.
The best pozzolanic materials in an ideal quantity provide better results and avoid or reduce many risks, such as the reaction of alkaline aggregates, leaching, attack of sulfates, etc.
The pozzolanic materials used as Admixture are natural or artificial.
Naturally occurring pozzolanic materials are the clay, shale, volcanic tufts, pumice, etc. and artificial pozzolans available are fly ash, silica fume, blast furnace slag, rice husk ash, etc.
6. Moisture–proof Admixture
Moisture-proof or waterproofing Admixture is used to make the concrete structure impervious to water and to prevent moisture on concrete surfaces.
In addition to the waterproof property, they also act as accelerators in an early stage of concrete hardening.
Moisture-proof Admixture is available in liquid, powder, paste, etc.
The main constituents of this Admixture are aluminum sulfate, aluminum chloride, zinc sulfate, calcium chloride, sodium silicate, etc., which are chemically active pore fillers.
7. Gas Formation Admixture
Aluminum powder, activated carbon, hydrogen peroxide are commonly used chemical gas-forming Admixtures.
When gas-forming Admixture is added, it reacts with the hydroxide obtained by hydrating the cement and forms small bubbles of hydrogen gas in the concrete.
The range of bubble formation in the concrete depends on many factors, such as the amount of mixture, the chemical composition of the cement, the temperature, the fineness, etc.
The bubbles formed help the concrete to fight the problems of settlement and bleeding.
Gas forming Admixture is also used to prepare lightweight concrete. For purposes of resistance to settlement and bleeding, a small amount of gas-forming Admixture is used, which is generally 0.5 to 2% by weight of cement.
However, in order to manufacture light concrete in greater quantity generally, 100 grams per bag of cement is recommended.
8. Air Detraining Admixtures
Air detraining mixtures are used to remove excess air from concrete voids.
Sometimes aggregates can release gas in the concrete, and the entrained air is more than necessary, so this type of mixture is useful.
Some of the most widely used detraining mixtures are tributyl phosphate, silicones, water-insoluble alcohols, etc.
9. Alkaline Aggregate Expansion that Prevents Admixture
The expansion of the alkaline aggregate in the concrete occurs by the reaction of the alkaline cement with the silica present in the aggregates.
It forms a gel-like substance and causes volumetric expansion of the concrete, which can lead to cracks and disintegration.
The use of pozzolanic Admixture will prevent the reaction of alkaline aggregates, and, in some cases, air entrainment Admixture is also useful.
Admixture commonly used to reduce the risk of the reaction of alkaline aggregates are aluminum powder and lithium salts.
10. Anti–Wash Admixture
Anti-wash Admixture is used in concrete, especially for an underwater concrete structure.
It protects the concrete mixture from being washed under pressure underwater. It improves concrete cohesion.
This type of mixture is prepared from natural or synthetic rubbers, thickeners based on cellulose, etc.
11. Grouting Admixture
Grout additions are added to the grout materials to improve the grout properties according to the grout requirement.
Sometimes there is a need for quick grouting, and sometimes t
here is a need for slow grouting to spread into deep cracks or fissures.
Therefore, different Admixture is used as grout Admixture based on the situation.
Accelerators such as calcium chloride, triethanolamine, etc. are used as grout Admixture when the grout must be adjusted quickly.
Likewise, retarders such as mucic acid, plaster, etc. are used to shorten the grout hardening time.
Gas-forming Admixture, such as aluminum powder, are added to the grouting material to neutralize the laying of foundations.
12. Corrosion Prevention Admixture
Corrosion of steel in the reinforced concrete structure is general and severe when the structure is exposed to saline water, industrial fumes, chlorides, etc. To prevent or delay the corrosion process, Admixture is used.
Some of the corrosion prevention additions used in reinforced concrete are sodium benzoate, sodium nitrate, sodium nitrite, etc.
13. Bonding Admixture
Bonding Admixture is used to create a bond between old and fresh concrete surfaces.
In general, if fresh concrete is poured onto a hardened concrete surface, there is a chance of failure of the fresh concrete surface due to weak connections with the old surface.
To make the bond stronger, bonding the Admixture is added to the cement or mortar grout that is applied to the concrete surface before laying the fresh concrete.
This type of additive is used for floor overlays, screed over roofing, repair work, etc.
The binding Admixture is water emulsions and is made of natural rubber, synthetic rubbers, polymers such as polyvinyl chloride, polyvinyl acetate, etc.
14. Fungicidal, Germicidal, and Insecticidal Admixture
To prevent the growth of bacteria, germs, and fungi in hardened concrete structures, it is recommended that the mixture has fungicidal, germicidal, and insecticidal properties.
This property can be developed by adding mixtures such as polyhalogenated phenols, copper compounds, and dieldrin emulsions, etc.
15. Coloring Admixture
Coloring Admixture is the pigments that produce color in the finished concrete. The mixtures used to produce colors should not affect the strength of the concrete.
Generally, the color mixes are added to the cement in a ball mill, and then colored cement can be obtained that can be used to make colored concrete.