To improve the performance of concrete, a variety of admixtures are utilized. Concrete admixture is anything that is added to concrete in addition to the aggregate, water, and cement.
1. Water Reducing Admixtures
2. Retarding Admixtures
3. Accelerating Admixtures
4. Air entraining concrete admixture
5. Pozzolanic Admixtures
6. Damp-proofing Admixtures
7. Gas forming Admixtures
8. Air detraining Admixtures
9. Alkali Aggregate Expansion Inhibiting Admixtures
10 .Anti-washout Admixtures
11. Grouting Admixtures
12. Corrosion Inhibiting Admixtures
13. Bonding Admixtures
14. Fungicidal, Germicidal, Insecticidal Admixtures
15. Colouring Admixtures
Types of Admixture Used in Concrete
1. Water Reducing Admixtures
The term “water-reducing additive” suggests that water reducers are used to make concrete, mortar, or grout more workable or flowable without adding more water.
Plasticizers, also known as water lowering admixtures, are divided into superplasticizers, mid-range plasticizers, and plasticizers. Common plasticizers cut the water requirement by up to 10%, mid-range plasticizers cut it by up to 15%, and superplasticizers cut it by up to 30%. Plasticizers including calcium, sodium, and ammonium lignosulphonates are frequently employed. A few of the next-generation superplasticizers are multicarboxylatethers, polycarboxylates, and acrylic polymer-based materials.
2. Retarding Admixtures
Admixtures that retard cement’s initial rate of hydration and prolong cement paste’s setting time are known as retarders. Certain kinds of concrete admixtures can be employed with frequent temperature variations, and concrete must be carried over large distances.
Gypsum and calcium sulphate are two often used retarding admixtures. Other substances utilized as retarders include starch, cellulose derivatives, ordinary sugar, acid salts, etc.
3. Accelerating Admixtures
The application of accelerating admixtures speeds up concrete’s initial setting time. They are also known as accelerators because they hasten the early stages of concrete hardening. By speeding up the hydration process, these accelerators also increase the early-stage strength of concrete.
These concrete additives use silica fume, calcium chloride, finely divided silica gel, etc., as accelerators. Cost-effective and widely used as an accelerating additive is calcium chloride.
4. Air entraining concrete admixture
Increased concrete durability during freezing, dewatering, and thawing conditions is the main goal of air-entraining admixture. When used in the concrete mix, these admixtures create millions of air bubbles that enhance the concrete’s characteristics.
Triethanolamine, calcium formate, silica fume, calcium chloride, finely divided silica gel, and others are some examples of accelerating admixtures. A cheap and widely used accelerating additive is calcium chloride.
5. Pozzolanic Admixtures
Pozzolanic admixtures are utilized to create a dense concrete mix ideal for water-retaining structures like dams, reservoirs, etc. They also lessen thermal shrinkage and hydration heat.
The naturally occurring pozzolanic materials used in admixtures include clay, shale, volcanic tuffs, pumice, etc. Fly ash, silica fume, blast furnace slag, rice husk ash, surkhi, etc., are artificial pozzolanic materials that are readily available.
6. Damp-proofing Admixtures
In order to make the concrete structure impermeable to water and to stop dampness on the concrete surface, damp proofing or waterproofing admixtures are utilised.
In general, damp proofing admixtures can be found in liquid, powder, paste, etc. form. Aluminium sulphate, zinc sulphate, aluminium chloride, calcium chloride, soda sulphate, and other chemically active pore fillers are the principal components of these admixtures.
7. Gas forming Admixtures
Activated carbon, hydrogen peroxide, and aluminium powder are often used chemical admixtures for gas formation. When gas-forming admixtures are introduced, they react with the cement-derived hydroxide to produce tiny hydrogen gas bubbles in the concrete.
Chemical admixtures that create gases frequently include hydrogen peroxide, aluminium powder, and activated carbon.
Lightweight concrete is also made with the use of admixtures that produce gas.
8. Air detraining Admixtures
To eliminate the extra air from the concrete voids, air-detraining admixtures are utilised. When air-entrained is more than necessary in concrete due to the escape of gas from aggregates, this additive is helpful.
Tributyl phosphate, silicones, water-insoluble alcohols, and others are some of the most popular air-entrained admixtures.
9. Alkali Aggregate Expansion Inhibiting Admixtures
Alkali aggregate expansion in concrete is caused by a reaction between the silica in the aggregates and the alkali in the cement. It causes concrete to expand in volume and turn into a gel-like substance, which can cause cracking and disintegration.
Alkali-aggregate interaction can be avoided by using pozzolanic admixtures, and in some circumstances, air-entraining admixtures are also beneficial. Aluminium powder and lithium salts are frequently employed as admixtures to reduce the possibility of an alkali-aggregate reaction.
10. Anti-washout Admixtures
Concrete is mixed with anti-washout additives, particularly for submerged concrete structures. It guards against water pressure washing away the concrete mix. Concrete becomes more cohesive as a result. These admixtures are made from thickeners based on cellulose, natural or synthetic rubbers, and other materials.
11. Grouting Admixtures
To increase the qualities of grout in accordance with requirements, grouting admixtures are added to grout ingredients. When filling deep fissures or gaps, slow-setting grout is occasionally required instead of quick-setting grout.
To shorten the time grout takes to set, retarders like gypsum and muriatic acid are utilised. To counteract foundation settle, grout material is used with gas-forming additives like aluminium powder.
12. Corrosion Inhibiting Admixtures
In reinforced concrete constructions, corrosion of the steel is a common problem that gets worse when the building is exposed to chlorides, saline water, industrial odours, etc. Admixtures that prevent corrosion are used to stop or slow down the process.
Sodium benzoate, sodium nitrate, sodium nitrite, and other corrosion-preventing admixtures are some of the ones utilised in reinforced cement concrete.
13. Bonding Admixtures
In order to create a solid bond between the surfaces of old and new concrete, bonding admixtures are required. In general, if new concrete is poured over an existing concrete surface that has already hardened, the new concrete surface has a probability of failing because of the weak links with the existing surface.
For paving overlays, screed over roof provision, repair work, etc., this type of admixture is employed. Water emulsions that are created from natural rubber, synthetic rubber, and polymers like polyvinyl chloride and polyvinyl acetate are called bonding admixtures.
14. Fungicidal, Germicidal, Insecticidal Admixtures
It is advised that the mix contain fungicidal, germicidal, and insecticidal qualities to stop the growth of bacteria, germs, and fungus on hardened concrete buildings. By using admixtures such as polyhalogenated phenols, copper compounds, dieldrin emulsions, etc., these characteristics in concrete can be enhanced.
15. Colouring Admixtures
The pigments used in colouring admixtures give the final concrete its colour. The strength of the concrete shouldn’t be impacted by the colouring agents utilised. In a ball mill, colouring admixtures are typically added to cement to produce coloured cement that can be used to create coloured concrete. The following table lists some of the colouring admixtures and the colours they produce.
In addition to cement, sand, and aggregate, concrete can also contain other building materials called mixes. They enhance concrete’s ability to withstand freezing, sulphate attack, controlled setting, hardening, workability, strength, etc.
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