Admixture is defined as a material besides concrete ingredients like aggregates, water, cement, and sand, mixed with the concrete to improve some of its properties in a hardened or fresh state. The concrete properties that are commonly modified are the setting times, workability, air entrainment, dispersion, and rate of hydration.
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Types of Concrete Admixtures
Concrete admixtures are divided into two categories; mineral admixtures and chemical admixtures.
These are insoluble materials used in concrete to attain specific engineering characteristics. They include fly ash, rice husk ash, furnace slag, and silica fume.
Chemical admixtures are added to concrete mortar before or during mixing to alter the concrete’s properties and performance. They are usually used to reduce the cost of concrete casting and improve the quality of concrete. However, the effectiveness of chemical admixtures largely depends on the water content, amount of cement, mixing time, air temperature, slump, and batching techniques used.
Chemical admixtures are grouped according to their functions;
- Water-reducing Admixtures – Water-reducing admixtures can reduce the demand for water in a concrete mix by about 5% to 10% percent. Workability is the vital element of concrete, which is enhanced by adding water. However, adding too much water can affect the strength, connectivity, and durability of the concrete.
- Plasticizers or Superplasticizers – These admixtures are also known as high-range water reducers – HRWR and can reduce the demand for water by up to 30%. The effect of Superplasticizers can last only for a few minutes, so it’s used at the job site to overcome slump and workability loss.
- Accelerating Admixtures – Accelerator admixtures enhance the strength of concrete at the early stages, enhance concrete operations, and minimize the time required for adequate curing. These admixtures are particularly important for altering the properties of concrete mixes in cold seasons.
- Air-entraining Admixtures – These admixtures are used to increase the durability and strength of the concrete and enhance resistance to freezing and thawing. Once added to concrete, air-entraining mixtures form several air bubbles that help improve workability and chemical resistance and prevent concrete bleeding.
- Retarding Admixtures – Adding retarding admixtures to concrete reduces the rate and heat of hydration and delays the initial setting rate of the concrete. These mixtures are often used in high-temperature zones with an increased rate of hardening.
- Specialty Category – This group includes all other types of admixtures whose work includes shrinkage reduction, bonding, colouring, and reduction of alkali-silica reactivity. They can also be used in workability enhancement, damp proofing, corrosion inhibition, and more.
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