Admixture is a term used in construction to describe materials added to concrete, mortar, shotcrete and cement to enhance their properties. Admixtures can improve building materials’ workability, durability, strength, and aesthetics. Admixtures can be classified into different types based on their chemical and physical properties. The most common types of admixtures include chemical admixtures, mineral admixtures, and fibre admixtures. We will discuss the types of admixtures, their benefits, applications, selection, and dosage.
Admixture: Types
Chemical admixtures
Chemical admixtures are the most common type used in construction. They are added to concrete, mortar, shotcrete, and cement to enhance their properties. Chemical admixtures can be further classified into different types based on their chemical composition and function. The most common chemical admixtures include water reducers, retarders, accelerators, air-entraining agents, and superplasticisers.
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Water reducers
Water reducers, also known as plasticisers, are chemical admixtures that are added to concrete, mortar, and shotcrete to reduce the amount of water required to achieve the desired workability. Water reducers increase the strength and durability of concrete and reduce the risk of cracking.
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Retarders
Retarders are chemical admixtures added to concrete, mortar, and shotcrete to slow down the setting time. Retarders are useful when the concrete sets too quickly in hot weather conditions. Retarders are also useful when more time is needed to finish the surface of the concrete.
Accelerators
Accelerators are chemical admixtures added to concrete, mortar, and shotcrete to speed up the setting time. Accelerators are useful in cold weather conditions, where the concrete sets too slowly. Accelerators can also be used in emergency repairs, where the concrete needs to be set quickly.
Air entraining agents
Air entraining agents are chemical admixtures added to concrete, mortar, and shotcrete to introduce small air bubbles into the mixture. The air bubbles increase the workability of the mixture and reduce the risk of cracking. Air entraining agents are useful when the concrete is exposed to freezing and thawing cycles.
Superplasticisers
Superplasticisers are chemical admixtures added to concrete, mortar, and shotcrete to improve workability. Superplasticisers increase the flowability of the mixture without increasing the amount of water. Superplasticizers are useful when the concrete needs to be pumped or placed in difficult-to-reach areas.
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Mineral admixtures
Mineral admixtures are materials added to concrete, mortar, and cement to enhance their properties. Mineral admixtures can be further classified into different types based on their chemical composition and function. The most common mineral admixtures include fly ash, silica fume, slag cement, and metakaolin.
Fly ash
Fly ash is a byproduct of coal combustion that is collected from power plants. Fly ash is added to concrete to increase strength, reduce permeability, and improve workability. Fly ash is also useful in reducing the amount of cement required in concrete, which reduces the carbon footprint of the building.
Silica fume
Silica fume is a byproduct of silicon and ferrosilicon production. Silica fume is added to concrete to increase its strength, reduce permeability, and improve durability. Silica fume is also useful in reducing the amount of cement required in concrete, which reduces the carbon footprint of the building.
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Slag cement
Slag cement is a byproduct of the steel-making process. Slag cement is added to concrete to increase its strength, durability, and sulphate resistance. Slag cement is also useful in reducing the amount of cement required in concrete, which reduces the carbon footprint of the building.
Metakaolin
Metakaolin is a calcined clay added to concrete to increase its strength, reduce permeability, and improve durability. Metakaolin is also useful in reducing the amount of cement required in concrete, which reduces the carbon footprint of the building.
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Fibre admixtures
Fibre admixtures are materials added to concrete, mortar, and shotcrete to enhance their properties. Fibre admixtures can be further classified into different types based on their chemical composition and function. The most common fibre admixture types include synthetic, natural, and steel.
Synthetic fibres
Synthetic fibres are made from polypropylene, polyester, and nylon. Synthetic fibres are added to concrete, mortar, and shotcrete to improve their durability, impact, and crack resistance.
Natural fibres
Natural fibres are made from wood, coconut husks, and sisal. Natural fibres are mixed with concrete, mortar, and shotcrete to improve their durability, impact, and crack resistance.
Steel fibre
Steel fibres are made from stainless steel and carbon steel. Steel fibres are added to concrete to increase strength, durability, and impact resistance. Steel fibres are also useful in reducing the risk of shrinkage cracking.
Admixture: Benefits
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Admixtures offer several benefits to building materials, including:
Improved workability: Admixtures enhance the workability of building materials, making them easier to place and finish. It improves the quality and appearance of the finished product.
Increased strength and durability: Admixtures increase the strength and durability of building materials, making them more resistant to wear and tear. It increases the lifespan of the building and reduces maintenance costs.
Reduced permeability: Admixtures reduce the permeability of building materials, making them more resistant to water and chemicals. It reduces the risk of damage from water infiltration and chemical attack.
Improved resistance to freeze-thaw cycles: Admixtures enhance the resistance of building materials to freeze-thaw cycles, reducing the risk of damage from expansion and contraction due to temperature changes.
Reduced shrinkage cracking: Admixtures reduce the risk of shrinkage cracking in building materials, which can be caused by moisture loss during the curing process.
Enhanced aesthetics: Admixtures can improve the appearance of building materials, making them more attractive and enhancing the overall design of the building.
Admixture: Applications
With the growing demand for sustainable and eco-friendly construction, admixtures are becoming an increasingly important component in reducing the carbon footprint of buildings. By reducing the amount of cement required and increasing the lifespan of the building, admixtures offer a cost-effective and sustainable solution for construction projects.
Admixtures are used in a wide range of applications in construction, including:
- Construction of bridges, tunnels, high-rise buildings, and pavement.
- Building masonry walls, plastering, and tile setting.
- Construction of slope stabilisation and tunnel lining.
- Soil stabilisation, grouting, and precast concrete.
FAQs
What is an admixture?
An admixture is an additive used in the construction industry to modify the properties of concrete, mortar, or other building materials. Admixtures can improve these materials' strength, durability, workability, and other characteristics.
What are the different types of admixtures?
There are many different types of admixtures, each with its specific purpose. Some common types of admixtures include water-reducing admixtures, which can reduce the amount of water needed in the mix without compromising workability; air-entraining admixtures, which can improve the freeze-thaw resistance of concrete; and accelerating admixtures, which can speed up the rate of hardening and strength gain.
How are admixtures added to concrete or other building materials?
Admixtures can be added to concrete or other building materials at the ready-mix plant or on-site. They are typically added during the mixing process, either as a dry powder or a liquid. The exact addition method will depend on the specific admixture and the application's requirements.
What are the benefits of using admixtures?
Admixtures can provide several benefits for concrete and other building materials. They can improve the strength and durability of the material, reduce the amount of water needed in the mix, improve workability and completion, and provide other performance benefits.
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