Admixtures are used to modify the properties of fresh and hardened concrete. They are classified as chemical and mineral admixtures. Chemicals admixtures are used in construction industry for building strong, durable and waterproof structures. Depending on their use, chemical admixtures are used for the following four main purposes.
1) Some chemicals are mixed with concrete ingredients and spread throughout the body of concrete to favorably modify the moulding and setting properties of the concrete mix. Such chemicals are generally known as chemical admixtures. Admixtures are added to concrete to give it certain desirable properties in either the fresh or the hardened state. Most admixtures result in modifying more than one intended property.
2) Some chemicals are applied on the surface of concrete to protect it during or after its setting.
3) Some chemicals are applied on the surfaces of moulds used to form concrete to effect easy mould-releasing operation.
4) Some chemicals are applied to bond or repair broken or chipped concrete.
Mineral admixtures are silicious materials which have fine particle size. They are added to concrete either as a filler or to improve favourably certain desired properties such as durability. Mineral admixtures are classified as either pozzolanic or cementitious. They are either natural materials or by-products such as fly ash of industries.