The various admixtures used in concrete can be categorized into two major groups as follows;
- Chemical Admixtures
- Mineral Admixtures
Modern development in chemistry and high understanding of the mechanism of hydration and strength of cement enforce in the use of different chemicals in concrete for altering the properties of concrete to fulfill specific requirements. These chemicals are called chemical admixtures. Some admixtures with the combined property (such as set retarding and air-entraining and water reducers and air entraining etc) are also available in the market.
Chemical admixtures can be classified as;
a) Set accelerators; two types
- Accelerates setting time e.g. sodium carbonate for flash set (Na2Co3)
- Accelerates to early gain in strength e.g. calcium chloride (CaCl2)
b) Set retarder
- Use of hot weather concreting to prevent the rapid setting.
- Used to keep equipments like mixtures free from malfunctioning. E.g. most commonly available is sugar but not in use nowadays.
c) Water reducers and super-plasticizers
Water reducers and super-plasticizers are used to reduce water cement (w/c) ratio for the same workability. About (10-25%) water is reduced with these water-reducing admixtures.
d) Air entraining agents: to entrain in concrete.
- These are added in concrete mix for partial replacement to the ordinary cement.
- These admixtures have cementitious or pozzolanic effect.
Some mineral admixtures are;
- Fly ash; generally used to increase workability.
- Ground blast furnace slag
- Condensed silica fume (vapor)
- Coal ash
- Rice husk ash
3) These are used in many countries to produce high strength concrete.
4) Reduces heat of hydration.