Concrete admixtures are chemicals added to concrete to improve its strength, durability, and workability. Admixtures are available as natural or manufactured compounds. They must be added at a predetermined rate to produce the desired results.
Water-reducing admixtures reduce the amount of water required in a concrete mix. This helps improve the compressive and flexural strength of the concrete. Some types of admixtures also reduce the permeability of the concrete.
Concrete accelerators speed up the strengthening process of concrete. Depending on the admixture, they can also reduce the setting time of the concrete. In addition, they can minimize the effect of high temperatures on the concrete. Their use is particularly useful in cold weather.
Retarding admixtures slow down the rate of hydration of the cement paste. These admixtures include carbohydrate derivatives, lignosulphonate acids, starch, and zinc salts.
Admixtures can also be used to prevent cracking and bleeding. However, they can also lower the initial strength of concrete. Also, they can be useful in hot climates.
Water-reducing admixtures are often used in combination with other admixtures to improve the overall moisture content of the concrete. The water-reducing admixtures usually include sequestering agents and high-range water reducers.
Bonding admixtures enhance the bond between fresh and hardened concrete. These admixtures are generally made from polymers or emulsions. Additionally, some admixtures contain fungicidal or germicidal qualities.
Superplasticizers are admixtures that produce low to normal slump concrete. These admixtures can reduce the amount of water needed by as much as 30%.