What is a pozzolan?
A pozzolan is a fine siliceous (often also aluminous) material that has little or no binding power on its own, but hardens into strong, water-resistant compounds when combined with lime (calcium hydroxide) and water. This is called the pozzolanic reaction.
The name comes from Pozzuoli near Naples. The Romans mixed volcanic ash from the area with lime to make mortar and concrete that would even set under water.
Types of pozzolan
| Type | Examples |
|---|---|
| Natural | Volcanic ash, trass (ground volcanic tuff, for example from the Eifel region) |
| Industrial by-product | Pulverised fuel ash (fly ash) from coal-fired power stations, silica fume from silicon production |
| Calcined | Calcined clay and metakaolin, crushed brick |
Note the difference from ground granulated blast-furnace slag (GGBS): slag is latent hydraulic, meaning it can set by itself given an activator. A pozzolan genuinely needs lime to react.
How it works in cement and concrete
As Portland cement hydrates it produces calcium hydroxide. A pozzolan turns that into additional binder. Cements containing pozzolans have their own designations in EN 197-1, such as CEM II/A-V (Portland fly ash cement) and CEM IV (pozzolanic cement).
Advantages
– Denser concrete with lower permeability
– Better resistance to chlorides and alkali-silica reaction
– Less clinker, so a lower carbon footprint
– Lower heat of hydration in massive pours
Disadvantages
– Slower strength gain in the first days
– More sensitive to drying out and to cold weather
Practical tips
- Curing matters even more. Keep concrete containing pozzolans moist and covered for longer, especially in wind and sun.
- Allow for striking times. Slower early strength can mean formwork has to stay in place longer.
- Trass in lime mortar gives a more water-resistant mortar, useful for plinths and masonry in damp conditions.
- Don’t add random ash. Wood ash, fireplace ash or untested residues are not reliable pozzolans and may contain harmful substances. Use certified products.
Related terms
- Fly ash
- Portland cement
- Lime mortar
- Blast furnace cement
- Curing concrete
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