Pozzolana

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.

 

Source: Wikipedia