Pozzolans are added to cement to increase set strength and reduce cement use.
Pozzolanic materials must be used in concrete in fine powder form. Lumps of hard material only work as aggregate.
Upto 30% of portland cement may be replaced by pozzolan. Use of a waste material pozzolan such as flyash can reduce the cost of concrete.
Pozzolanic cement mixes take longer to achieve full strength.
Fly ash is available as class C and class F. Only class F should be used for cement mortar mixes. This improves the strength and durability of the mortar, though both plain and pozzolanic mortars have life advantages and disadvantages under different conditions, so it does not always lead to improved life.
Concrete made with pozzolan has lower pH, and can corrode metalwork buried in it. Hence it is necessary to sleeve any metal pipework buried in the slab.
Pozzolans require very thorough mixing with the cement to be effective. Hand mixing is inadequate, extended dry mixing time in a powered mixer is required.
List of Pozzolans
- Fly ash
- Coal ash
- Volcanic ash
- Silica fume
- Brick dust
- Tile & crockery dust
- Some clays
- Cannabis fibre