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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: black, white, grey and red, all of which can be found in the volcanic areas of Italy, such as Naples.

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 and that have the ability to 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.

Modern pozzolanic cements are a mix of natural or industrial pozzolans and Portland cement. In addition to underwater use, the high alkalinity of pozzolana makes it especially resistant to common forms of corrosion from 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 ash from coal-fired power plants, silica fume from silicon production, rice husk ash from rice paddy-fields (agriculture), and metakaolin from 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.

Pozzolanic reaction

At the basis of the Pozzolanic reaction stands a simple acid-base reaction between calcium hydroxide, also known as Portlandite, or (Ca(OH)2), and silicic acid (H4SiO4, or Si(OH)4). For simplifying, this reaction can be schematically represented as following:

Ca(OH)2 + H4SiO4 —> Ca2+ + H2SiO42- + 2 H2O —> CaH2SiO4 · 2 H2O

or summarized in abbreviated notation of cement chemists:

CH + SH —> CSH

The product of general formula (CaH2SiO4 · 2 H2O ) formed is a calcium silicate hydrate, also abbreviated as CSH in cement chemist notation. The ratio Ca/Si, or C/S, and the number of water molecules can vary and the hereabove mentioned stoichiometry may differ.

As the density of CSH is lower than that of portlandite and pure silica, a consequence of this reaction is a swelling of the reaction products. This reaction may also occur with time in concrete between alkaline cement porewater and poorly-crystalline silica aggregates. This delayed process is also known as alkali silica reaction, or alkali-aggregate reaction, and may seriously damage concrete structures because the resulting volumetric expansion is also responsible for spalling and decrease of the concrete strength.

See also

References

  • Cook D.J. (1986) Natural pozzolanas. In: Swamy R.N., Editor (1986) Cement Replacement Materials, Surrey University Press, p. 200.
  • 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.
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