Pozzolanic ash is an alumino-siliceous material which reacts with calcium hydroxide in the presence of water to form compounds possessing cementitious properties at room temperature, producing C-S-H. This allowed it to be used in the Roman Empire to make cement by combining with lime and water. Pozzolanic ash occurs naturally in volcanic deposits from Pozzuoli and Naples. See also pozzolana.

Today, pozzolanic ash can also be obtained from industrial waste products and used as a replacement for Portland cement in concrete. Some industrial sources of pozzolanic ash are: Class F fly ash from coal fired power plants, silica fume from silicon production, rice husk ash from rice paddies (agriculture), and metakaolin from oil sands operations. Metakaolin, a powerful pozzolan, can also be manufactured (from kaolinite clays), and is valued for improving white concrete.

Pozzolanic substances react with C-H in the cement paste, converting it to C-S-H and resulting in increased concrete strength. Most pozzolans, with the exception of fly ash, result in high early strength in concrete, but reduced workability and high heat of hydration. This necessitates the use of superplasticizer and chilled water when mixing such admixtures into concrete. Class F fly ash, however, results in low early strength but reduced heat of hydration and improved workability. One optimal solution for "green concrete" is to use both fly ash as well as silica fume at the same time, though this requires careful mix proportioning.

Other industrial waste products used for partial cement replacement include Class C fly ash and blast furnace slag. However, these substances are not pozzolanic, and should not be mistaken as such.

See also

• Pozzolana
• Cenospheres

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