Pozzolana, also known as pozzolanic ash, is a fine, sandy
volcanic ash, originally discovered and dug in Italyat Pozzuoliin the region around Vesuvius, but later at a number of other sites. Vitruviusspeaks of four types of pozzolana: black, white, grey, red. It is found in all the volcanic areas of Italy, such as Naples, in various colours: black, white, grey and red.
Pozzolana is a siliceous and aluminous material which reacts with
calcium hydroxidein the presence of water to form compounds possessing cementitious properties at room temperature and that have the ability to set under water. It transformed the possibilities for making concretestructures, although it took the Romans some time to discover its full potential. Typically it was mixed two-to-one with limejust prior to mixing with water. The Roman port at Cosawas 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.
Modern pozzolanic cements are a mix of natural or industrial
pozzolans and Portland cement. In addition to underwater use, the high alkalinityof pozzolana makes it especially resistant to common forms of corrosionfrom sulfates. 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 ashfrom coal-fired power plants, silica fumefrom siliconproduction, rice husk ash from rice paddy-fields (agriculture), and metakaolinfrom oil sand 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
*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 Cosaharbor, the Lighthouse on pier 5," diagrams, and photographs. Height of Port city: 100 BC.
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