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Pozzolana from Bacoli in the Bay of Naples

Pozzolana, also known as pozzolanic ash, is a fine, sandy volcanic ash, originally discovered and dug in Italy at Pozzuoli in the region around Vesuvius, but later at a number of other sites. Vitruvius speaks of four types of pozzolana. It is found in all the volcanic areas of Italy in various colours: black, white, grey and red.

Finely ground and mixed with lime it creates a hydraulic cement and can be used to make a strong mortar that will also set under water. It transformed the possibilities for making concrete structures, although it took the Romans some time to discover its full potential. Typically it was mixed two-to-one with lime just prior to mixing with water. The Roman port at Cosa was built of Pozzolana that was poured underwater, apparently using a long tube to carefully lay it up without allowing sea water to mix with it. The three piers are still visible today, with the underwater portions in generally excellent condition after 2100 years.

Pozzolana is a siliceous and aluminous material which reacts with calcium hydroxide in the presence of water to form compounds possessing cementitious properties at room temperature. 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.

Modern pozzolanic cements are a mix of natural or industrial pozzolans and Portland cement. In addition to underwater use, the pozzolana's high alkalinity makes it especially resistant to common forms of corrosion from sulphates. Once fully hardened, the Portland cement-Pozzolana blend may be stronger than Portland cement due to its lower porosity, which also makes it more resistant to water absorption and spalling.

Some industrial sources of materials with pozzolanic properties are: Class F (silicious) fly ash from coal fired power plants, silica fume from silicon production, rice husk ash from rice paddy-fields (agriculture), and metakaolin from oil sands operations. Metakaolin, a powerful pozzolan, can also be manufactured, and is valued for making white concrete.

Other industrial waste products used in Portland composite cements include Class C (calcareous) fly ash and ground granulated blast furnace slag.

• Aerated autoclaved concrete
• Cenospheres

• McCann, A.M. (1994). "The Roman Port of Cosa",(273 BC), Scientific American, Ancient Cities, pp. 92–99, by Anna Marguerite McCann. Covers, hydraulic concrete, of "Pozzolana mortar" and the 5 piers, of the Cosa harbor, the Lighthouse on pier 5, diagrams, and photographs. Height of Port city: 100 BC.