Fly ash is a finely divided residue made from the combustion of pulverized coal that can be used to increase concrete durability and workability, while reducing permeability.
As fly ash contains aluminous and siliceous components, it may form cement when mixed with water if the calcium content of the fly ash is high enough. Similarly, mixing fly ash with lime and water can create a substance similar to Portland cement.
Fly ash can be separated into two main types, known as:
Class F fly ash contains particles covered in a type of melted glass allowing it to reduce the risk of concrete expansion and increase resistance to sulphates and alkali-aggregate reactions.
Class C fly ash contains a higher percentage of calcium oxide, making it more effective in strengthening structural concrete.
