What Is Concrete Admixture?
Different types of Admixtures are used in concrete to improve the properties of concrete.
Concrete admixture is the material other than aggregate, water, and cement added to the concrete.
15 Types of concrete admixture
- Water Reducing Admixtures
- Retarding Admixtures
- Accelerating Admixtures
- Air Entraining Concrete Admixtures
- Pozzolanic Admixtures
- Damp-proofing Admixtures
- Gas forming Admixtures
- Alkali Aggregate Expansion inhibiting Admixtures
- Anti-washout Admixture
- Anti detraining Admixtures
- Grouting Admixtures
- Corrosion inhibiting Admixtures
- Bonding Admixtures
- Fungicidal, Germicidal, Insecticidal Admixtures
- Coloring Admixtures
1. Water Reducing Admixture
Water reducing admixtures, the name itself defines that they are used for minimizing the demand for water in a concrete mix. Workability is one of the most important properties of concrete which is improved with the addition of water but if the water is added more than the required quantity the strength and durability properties of concrete gets severely affected.
Water reducing admixture also improves the strength of concrete, provides a good bond between concrete and steel, prevents crack formation, segregation in concrete, honeycombing, bleeding, etc.
Water reducing admixtures are also known as plasticizers and are classified into three categories namely plasticizers, mid-range plasticizers, and superplasticizers. Normal plasticizer reduces the demand of water up to 10% in concrete, mid-range plasticizers reduce the demand of water up to 15% while superplasticizers reduce the demand of water up to 30% in concrete.
The commonly used plasticizers are Calcium, sodium and ammonium lignosulphonates. These are some of the new generation superplasticizers namely acrylic polymer-based, polycarboxylate, multi carboxylates, etc.
2. Retarding Admixture
Retarding admixtures are used to slow down the setting rate of concrete. By doing so the initial setting time of concrete is increased and the concrete mixture can stay in its workable state for longer period of time. These are also called as retarders and used especially in high-temperature zones where concrete sets quickly.
The quick setting of concrete in some situations may lead to discontinuities in structure, reduction of strength in concrete creates unnecessary voids in concrete, etc.
Calcium sulfate or gypsum is commonly used retarding admixture. Starch, cellulose products, common sugar, salts of acids are some other retarders used in concrete. Most of the water-reducing admixtures are also act as retarding admixtures and they are called retarding plasticizers.
3. Accelerating Admixture
Accelerating admixtures are generally used to reduce the initial setting time of concrete. They speed up the process of the initial stage of the hardening of concrete hence they are also called accelerators. These accelerators also improve the strength of concrete in its early stage by increasing the rate of hydration.
Earlier hardening of concrete is useful in several situations such as early removal of formwork, less period of curing, emergency repair works, for constructions in low-temperature regions, etc.
Some of the examples of accelerating admixtures are triethanolamine, calcium formate, silica fume, calcium chloride, finely divided silica gel, etc. Calcium chloride is the cheap and commonly used accelerating admixture.
4. Air Entraining Concrete Admixture
Air entraining admixtures are one of the most important inventions in concrete technology. Their primary function is to increase the durability of concrete under freezing and thawing conditions. When these admixtures are added to concrete mix they will form millions of non-coalescing air bubbles throughout the mix and improves the properties of concrete.
Air entrainment admixture in concrete will also improve the workability of concrete, prevents segregation and bleeding of the fresh concrete, lower the unit weight and modulus of elasticity of concrete, helps in improving the chemical resistance of concrete, and reduction of cement or sand or water content in concrete, etc.
Most used air entrainment admixtures are vinsol resin, darex, Teepol, Cheecol, etc. These admixtures are actually made of Natural wood resins, alkali salts, animal and vegetable fats, and oils, etc.
5. Pozzolanic Admixture
Pozzolanic admixtures are used to prepare dense concrete mix which is best suitable for water retaining structures like dams, reservoirs, etc. They also reduce the heat of hydration and thermal shrinkage.
Best pozzolanic materials in optimum quantity give the best results and prevent or reduces many risks such as alkali-aggregate reaction, leaching, sulfate attack, etc.
Pozzolanic materials used as admixtures are either natural or artificial. Some of the naturally occurring Pozzolanic materials are clay, shale, volcanic tuffs, pumicite, etc. and artificial pozzolans available are fly ash, silica fume, blast furnace slag, rice husk ash, surkhi, etc.
6. Damp-proofing Admixture
Damp-proofing or waterproofing admixtures are used to make the concrete structure impermeable against water and to prevent dampness on the concrete surfaces. In addition to the waterproof property, they also act as accelerators in the early stage of concrete hardening.
Damp proofing admixtures are generally available in liquid form, powder form, paste form, etc. The main constituents of these admixtures are aluminum sulfate, zinc sulfate aluminum chloride, calcium chloride, silicate of soda, etc. which are chemically active pore fillers.
7. Gas forming Admixture
Aluminum powder, activated carbon, hydrogen peroxide are generally used gas-forming chemical admixtures. When gas-forming admixtures are added, it reacts with hydroxide obtained by the hydration of cement and forms minute bubbles of hydrogen gas in the concrete.
The range of formation of bubbles in concrete depends upon many factors such as the amount of admixture, the chemical composition of cement, temperature, fineness, etc. The formed bubbles help the concrete to prevent the settlement and bleeding problems.
Gas-forming admixtures are also used to prepare lightweight concrete. For settlement and bleeding resistance purposes, a small quantity of gas-forming admixtures which is generally 0.5 to 2% by weight of cement is used. But for making lightweight concrete in larger quantity generally 100 grams per bag of cement is recommended.
8. Alkali Aggregate Expansion Preventing Admixture
Alkali aggregate expansion in concrete is happened by the reaction of alkali of cement with the silica present in the aggregates. It forms a gel-like substance and causes volumetric expansion of concrete which may lead to cracking and disintegration.
Use of pozzolanic admixtures will prevent the alkali-aggregate reaction and in some cases, air-entraining admixtures are also useful. Generally used admixtures to reduce the risk of alkali-aggregate reaction are aluminum powder and lithium salts.
9. Anti-washout Admixture
Anti-washout admixtures are used in concrete especially for an underwater concrete structure. It helps to protect the concrete mix from being washed out underwater pressure. It improves the cohesiveness of concrete.
This type of admixture is prepared from natural or synthetic rubbers, cellulose-based thickeners, etc.
Air-detraining Admixtures are used to remove the excess air from the concrete voids. Sometimes, the aggregates may release the gas into concrete and air-entrained is more than required then this type of admixtures are useful.
Some of the most used air-detraining admixtures are tributyl phosphate, silicones, water-insoluble alcohols, etc.
11. Grouting Admixture
Grouting admixtures are added to grout materials to improve the grout properties according to the requirement of the grout. Sometimes, there is a need for quick set grout and sometimes there is a need for slow set grout to spread into deep cracks or fissures.
Hence, different admixtures are used as grout admixtures are generally based on the situation. Accelerators like calcium chloride, triethanolamine, etc. are used as grout admixtures when the grout is to be set rapidly. Similarly, retarders like mucic acid, gypsum, etc. are used to slow down the setting time of grout.
Gas-forming admixtures like aluminum powder are added to grout material to counteract the settling of foundations.
12. Corrosion Inhibiting Admixture
Corrosion of steel in reinforced concrete structure is general and it is severe when the structure is exposed to saline water, industrial fumes, chlorides, etc. To prevent or to slow down the process of corrosion preventing admixtures are used.
Some of the corrosion preventing admixtures used in reinforced cement concrete are sodium benzoate, sodium nitrate, sodium nitrite, etc.
13. Bonding Admixture
Bonding admixtures are used for creating a bond between old and fresh concrete surfaces. In general, if fresh concrete is poured over a hardened concrete surface, there is a chance of failure of the fresh concrete surface due to weak bonds with the old surface.
For making the bond stronger, bonding admixtures are added to cement or mortar grout which is then applied on the concrete surface just before placing fresh concrete. This type of admixtures is used for pavement overlays, screed over roof provision, repair works, etc.
Bonding admixtures are water emulsions and they are made from natural rubber, synthetic rubbers, polymers like polyvinyl chloride, polyvinyl acetate, etc.
14. Fungicidal, Germicidal, Insecticidal Admixture
To prevent the growth of bacteria, germs, fungus on hardened concrete structures, it is recommended that the mix should have fungicidal, germicidal and insecticidal properties. These properties in concrete can be developed by adding admixtures like polyhalogenated phenols, copper compounds, and dieldrin emulsions, etc.
15. Coloring Admixture
Coloring admixtures are the pigments that help in the production of color in the finished concrete. Precautions should be taken that the admixtures used to produce color should not affect the concrete strength. Generally, the coloring admixtures are added to cement in a ball mill, then we will obtain colored cement which can be used for making colored concrete. Some of the coloring admixtures and their resultant colors are tabulated below.
Table 1: Coloring Admixture and their Resultant Colors
|Iron or Red oxide
|Hydroxides of iron
|Barium manganite and Ultramarine
|Chromium oxide and chromium hydroxide
|Manganese black, Raw umber