Concrete admixtures are substances that are added to concrete in order to improve its performance. They are different from the typical components of concrete, which include aggregate, water, and cement. The purpose of adding admixtures is to modify the properties of the concrete in a desired way, such as improving its strength, durability, workability, or setting time.
There are many different types of admixtures available for use in concrete. These include air-entraining agents, water reducers, accelerators, retarders, and superplasticizers, among others. Each type of admixture has its own unique properties and benefits, and can be used in varying amounts depending on the desired outcome.
Air-entraining agents, for example, are used to increase the freeze-thaw resistance of concrete by introducing microscopic air bubbles into the mixture. Water reducers, on the other hand, are added to reduce the amount of water needed for a given amount of cement, which can improve the strength and workability of the concrete. Accelerators are used to speed up the setting and hardening of the concrete, while retarders can be used to slow down this process.
Superplasticizers are another type of admixture that can greatly improve the workability and flow of concrete without adding excess water. This can be particularly useful in situations where the concrete needs to be placed in hard-to-reach areas or where a high degree of flowability is needed.
Overall, concrete admixtures are an important tool for improving the performance and versatility of concrete. By carefully selecting and using the right admixtures, it is possible to achieve a wide range of desired outcomes and properties in concrete, making it an even more valuable and effective building material.
Types of Concrete Admixtures
Concrete admixtures are of different types and they are as follows:
- Water Reducing Admixtures
- Retarding Admixtures
- Accelerating Admixtures
- Air entraining concrete admixture
- Pozzolanic Admixtures
- Damp-proofing Admixtures
- Gas forming Admixtures
- Air detraining Admixtures
- Alkali Aggregate Expansion Inhibiting Admixtures
- Anti-washout Admixtures
- Grouting Admixtures
- Corrosion Inhibiting Admixtures
- Bonding Admixtures
- Fungicidal, Germicidal, Insecticidal Admixtures
- Coloring Admixtures
1. Water Reducing Admixtures
Water reducing admixtures are a type of substance used in concrete mixes to decrease the amount of water needed. While water is important for workability, too much water can negatively impact the strength and durability of concrete. In addition to improving workability, water reducing admixtures can also enhance concrete strength, promote good bond between concrete and steel, and prevent issues such as cracking, segregation, honeycombing, and bleeding. These admixtures are also referred to as plasticizers, and are classified into three types based on their effectiveness: normal plasticizers, mid-range plasticizers, and super plasticizers. Normal plasticizers reduce the water demand by up to 10%, mid-range plasticizers up to 15%, and super plasticizers up to 30%. Commonly used plasticizers include calcium, sodium, and ammonium lignosulphonates. However, newer types of super plasticizers have been developed using substances such as acrylic polymers, polycarboxylates, and multicarboxylate ethers.
Fig 1: Water Reducing Admixture
2. Retarding Admixtures
Retarding admixtures, also known as retarders, are substances that are added to concrete in order to slow down the rate at which cement hydrates in its initial stage. This results in an increased initial setting time of the concrete. In regions with high temperatures, concrete can set very quickly, which may cause structural discontinuities, poor surface bonding, and the formation of unnecessary voids. Retarders are therefore used to prevent these problems from occurring. Calcium sulphate or gypsum is a commonly used retarder, although other substances such as starch, cellulose products, common sugar, and salts of acids can also act as retarders.
Water reducing admixtures can also function as retarders and are referred to as retarding plasticizers. These admixtures serve the dual purpose of reducing the amount of water needed in concrete and slowing down the setting process. By reducing the amount of water needed in the mixture, they help to improve the strength and durability of the concrete, while also preventing potential problems that can arise due to quick setting.
Fig 2: Retarding Admixture (Gypsum)
3. Accelerating Admixtures
Accelerating admixtures are a type of chemical additive that is commonly used in the construction industry to speed up the initial setting time of concrete. They are also known as accelerators because they accelerate the process of hardening in the early stages of concrete. In addition to this, they can also improve the strength of concrete in its early stages by increasing the rate of hydration.
These admixtures are particularly useful in situations where early hardening of concrete is necessary. For example, they can allow for the early removal of formwork, which can save time and labor costs. They can also reduce the period of curing, which can be beneficial in emergency repair works or in constructions carried out in low temperature regions.
There are various types of accelerating admixtures, including triethenolamine, calcium formate, silica fume, calcium chloride, and finely divided silica gel. Among these, calcium chloride is the most commonly used and inexpensive accelerating admixture.
Fig 3: Accelerator (Silica Fume)
4. Air Entraining Concrete Admixture
Air entraining admixtures have been a game-changing invention in the field of concrete technology due to their ability to enhance the durability of concrete when subjected to freezing and thawing conditions. By introducing these admixtures into the concrete mix, numerous non-coalescing air bubbles are formed throughout the mix, thereby improving its properties.
One of the benefits of air entrainment in concrete is the improvement of its workability, which helps to prevent segregation and bleeding. In addition, air entrainment lowers the unit weight and modulus of elasticity of concrete, enhances its chemical resistance, and reduces the amount of cement, sand, or water needed for the mix.
The most commonly used air entrainment admixtures include vinsol resin, darex, Teepol, and Cheecol. These admixtures are typically made of natural wood resins, alkali salts, and animal and vegetable fats and oils. Their ability to improve the durability and workability of concrete has made them a crucial component in the construction industry.
Fig 4: Freezing and Thawing Effect on Concrete
5. Pozzolanic Admixtures
Pozzolanic admixtures are commonly employed in the preparation of dense concrete mixes that are best suited for water retaining structures such as dams and reservoirs. These admixtures possess the ability to reduce both the heat of hydration and thermal shrinkage, resulting in a more durable concrete structure. Optimal quantities of the best pozzolanic materials can effectively prevent or reduce several potential risks associated with concrete, such as alkali aggregate reaction, leaching, and sulfate attack.
Pozzolanic materials used as admixtures can be either natural or artificial in origin. Natural pozzolanic materials include clay, shale, volcanic tuffs, pumicite, and others. On the other hand, artificial pozzolans like fly ash, silica fume, blast furnace slag, rice husk ash, surkhi, etc., are also available for use in concrete preparation. By utilizing these materials in appropriate amounts, a concrete mixture can achieve its optimal strength, durability, and resilience.
Fig 5: Fly ash
6. Damp-proofing Admixtures
Damp proofing or water proofing admixtures are commonly utilized in construction to make concrete structures impermeable to water and prevent dampness on concrete surfaces. These admixtures not only provide water proofing properties but also act as accelerators during the early stages of concrete hardening. Damp proofing admixtures come in various forms, such as liquid, powder, and paste.
The primary constituents of these admixtures are chemically active pore fillers, such as aluminum sulfate, zinc sulfate, aluminum chloride, calcium chloride, and silicate of soda. These pore fillers work by reacting with the free lime in the concrete, forming insoluble compounds that fill the pores in the concrete. This process makes the concrete structure water resistant and prevents moisture from seeping through the surface.
Waterproofing admixtures are especially useful in areas with high humidity or frequent rainfall, where water damage and dampness are common problems. By preventing water from penetrating the concrete, these admixtures also help to increase the durability and lifespan of the structure. Additionally, the use of damp proofing admixtures can reduce the overall cost of construction by eliminating the need for external waterproofing treatments.
Fig 6: Dampness on Concrete Surface
7. Gas forming Admixtures
Gas forming chemical admixtures such as aluminum powder, activated carbon, and hydrogen peroxide are commonly used in concrete to create minute bubbles of hydrogen gas. These bubbles are formed by the reaction between the admixture and the hydroxide obtained through the hydration of cement. The size and quantity of the bubbles formed in the concrete depend on various factors such as the amount of admixture added, the chemical composition of the cement, the temperature, and the fineness of the mixture.
The purpose of adding these gas forming admixtures to concrete is to counteract the issues of settlement and bleeding. Settlement refers to the sinking of concrete, which can cause structural problems if it occurs unevenly. Bleeding, on the other hand, is the phenomenon where water from the concrete mixture rises to the surface and evaporates, leaving behind an uneven surface. Both of these problems can be mitigated by the formation of bubbles in the concrete.
Overall, the addition of gas forming chemical admixtures to concrete is an effective way to improve its durability and stability. By creating tiny bubbles of hydrogen gas, the concrete is able to resist the settling and bleeding issues that can compromise its structural integrity over time. The success of this technique depends on a variety of factors, but with the right mixture and conditions, it can lead to stronger, more reliable concrete structures.
Fig 7: Activated Carbon Powder
Gas forming admixtures are often employed to produce lightweight concrete. To improve its resistance to settlement and bleeding, a small amount of gas forming admixture is added, typically ranging from 0.5% to 2% of the weight of the cement. However, when the goal is to create lightweight concrete, a larger amount of gas forming admixture is recommended, usually at a rate of 100 grams per bag of cement.
8. Air detraining Admixtures
Air-detraining admixtures are commonly employed in the construction industry to eliminate the excess air from concrete voids. In certain cases, the aggregates used in concrete production may emit gas, leading to an air entrainment level that exceeds the necessary amount. In such situations, air-detraining admixtures are beneficial for achieving the required air content. These admixtures can include tributyl phosphate, silicones, and water-insoluble alcohols, among others. By incorporating these admixtures, construction professionals can effectively manage the air content in concrete and ensure optimal performance.
9. Alkali Aggregate Expansion Preventing Admixtures
When concrete undergoes alkali aggregate expansion, it is due to a reaction between the alkali in the cement and the silica in the aggregates. This chemical reaction results in the formation of a gel-like substance that causes the concrete to expand in volume, which can lead to cracking and disintegration over time.
To prevent the alkali-aggregate reaction from occurring, pozzolanic admixtures are often used. These admixtures can effectively reduce the risk of the reaction taking place by altering the chemical properties of the concrete. Additionally, air-entraining admixtures can also be beneficial in preventing alkali aggregate expansion in concrete.
Aluminum powder and lithium salts are commonly used admixtures to reduce the risk of alkali aggregate reaction in concrete. By introducing these admixtures into the mix, the risk of the reaction occurring is significantly reduced, which can result in a longer-lasting and more durable concrete structure. With the use of these admixtures, the potential for costly repairs or replacement due to alkali aggregate expansion is greatly diminished.
Fig 8: Effect of Alkali Aggregate Reaction on Concrete
10. Anti-washout Admixtures
Anti-washout admixtures play a crucial role in the construction of underwater concrete structures by safeguarding the concrete mix from being washed out due to the pressure of the surrounding water. These admixtures have the added benefit of improving the cohesiveness of the concrete, which is particularly important in underwater applications.
The production of anti-washout admixtures involves the use of various natural or synthetic materials, such as rubbers and cellulose-based thickeners, among others. These substances help to increase the thickness of the concrete mix, making it more resistant to the flow of water and reducing the risk of it being washed away.
Overall, the use of anti-washout admixtures in concrete is a practical and effective solution for maintaining the integrity and durability of underwater structures. By incorporating these admixtures into the concrete mix, construction professionals can ensure that their projects remain stable and reliable even when exposed to harsh underwater conditions.
Fig 9: Underwater Concreting
11. Grouting Admixtures
Grouting admixtures serve the purpose of enhancing the properties of grout materials according to the required specifications. They are utilized to produce quick or slow set grout, depending on the situation at hand. The addition of specific admixtures can significantly improve the quality and effectiveness of the grout.
Accelerators such as calcium chloride and triethanolamine are commonly employed as grout admixtures when there is a need for rapid setting. On the other hand, retarders like mucic acid and gypsum are utilized to slow down the setting time of grout, which can be useful for spreading it into deep cracks or fissures.
Gas forming admixtures, such as aluminum powder, are another type of grout admixture that is added to counteract the settling of foundations. This is achieved by creating small air bubbles within the grout material, which help to reduce its density and improve its flow properties.
In summary, the selection of the appropriate grout admixture is crucial in achieving the desired properties and performance of the grout material. Depending on the specific requirements of the project, different types of admixtures can be added to enhance the setting time, flow properties, and overall effectiveness of the grout.
Fig 10: Grouting
12. Corrosion Preventing Admixtures
Reinforced concrete structures often suffer from corrosion of their steel components, which can be a widespread and severe problem, especially in environments exposed to saline water, industrial fumes, chlorides, and other corrosive substances. To slow down or prevent this process, various types of admixtures are commonly used.
Corrosion preventing admixtures are often employed to protect reinforced concrete structures. These admixtures can significantly slow down or even prevent the corrosion of steel in concrete. Sodium benzoate, sodium nitrate, sodium nitrite, and other similar substances are some of the admixtures that are commonly used for this purpose.
The use of these admixtures is especially important in environments where the risk of corrosion is high. Such environments may include coastal regions, areas with high levels of pollution, and industrial sites with significant exposure to chemicals. By using corrosion preventing admixtures, engineers and builders can enhance the durability and longevity of reinforced concrete structures, thus ensuring their safety and reliability over the long term.
Fig 11: Corrosion of Steel in Concrete
13. Bonding Admixtures
Bonding admixtures are a type of material used to enhance the bond between old and new concrete surfaces. When fresh concrete is poured over a hardened concrete surface, there is a risk of the fresh concrete surface failing due to a weak bond with the old surface. This is where bonding admixtures come into play, as they are added to cement or mortar grout that is applied to the concrete surface just before the fresh concrete is placed.
The addition of bonding admixtures to the grout strengthens the bond between the two surfaces, making it less likely for the fresh concrete to fail. This type of admixture is commonly used for a variety of applications, including pavement overlays, screed over roof provision, and repair work.
Bonding admixtures are typically water emulsions and are made from a variety of materials, including natural and synthetic rubbers, as well as polymers such as polyvinyl chloride and polyvinyl acetate. These materials help to create a strong, durable bond between old and new concrete surfaces, ensuring that the structure remains stable and secure.
Fig 12: Concrete Pavement Overlay
14. Fungicidal, Germicidal, Insecticidal Admixtures
To inhibit the proliferation of bacteria, germs, and fungus on solidified concrete structures, it is advised to include admixtures that possess fungicidal, germicidal, and insecticidal properties in the mix. These properties can be achieved by incorporating additives such as polyhalogenated phenols, copper compounds, and dieledren emulsions, among others. By doing so, the concrete will be able to resist the growth of microorganisms and pests that could potentially cause damage to the structure and compromise its integrity. The addition of these admixtures can help ensure that the concrete remains durable and reliable, even when exposed to harsh environmental conditions.
Fig 13: Concrete affected by Fungi
15. Coloring Admixtures
Coloring admixtures are substances added to concrete to produce pigments that give the finished concrete its color. It is essential that these admixtures do not have a detrimental effect on the concrete’s strength. The process of adding coloring admixtures involves adding them to cement in a ball mill. This creates colored cement that can be used to make colored concrete.
There are various types of coloring admixtures available, and each produces a different color in the finished concrete. Some examples of coloring admixtures and their corresponding colors are presented in a table. By using these admixtures, the concrete can be made into an aesthetically pleasing material without compromising its structural integrity.
Table 1: Coloring Admixtures and their Resultant Colors
| Admixture | Color obtained |
| Iron or Red oxide | Red |
| Hydroxides of iron | Yellow |
| Barium manganite and Ultramarine | Blue |
| Chromium oxide and chromium hydroxide | Green |
| Ferrous oxide | Purple |
| Carbon black | Black |
| Manganese black , Raw umber | Brown |
Fig 14: Colored Concrete