Pozzolan

Pozzolans are a broad class of siliceous and aluminous materials which, in finely divided form and in the presence of water, chemically react with calcium hydroxide (Ca(OH)₂) at ordinary temperatures to form compounds possessing cementitious properties. The reaction is known as the pozzolanic reaction.

The term derives from pozzolana, a volcanic ash first exploited by the Romans near the town of Pozzuoli (ancient Puteoli) in Italy. Modern pozzolans include both natural materials and industrial by-products.

Types of pozzolans

Natural pozzolans

Volcanic ashes and tuffs (e.g., Italian pozzolana, Santorin earth, Bavarian trass, Japanese shirasu)
Pumicite and pumice
Diatomaceous earth (diatomite)
Calcined clays and shales (metakaolin, burnt clay)
Volcanic glass

Artificial (man-made) pozzolans

Fly ash (Class F and Class C) – the most widely used artificial pozzolan worldwide
Silica fume (microsilica)
Ground granulated blast-furnace slag (sometimes considered a latent hydraulic binder rather than a true pozzolan)
Rice husk ash
Metakaolin (produced by controlled calcination of kaolin clay)
Burnt oil shale ash

Pozzolanic reaction

The general reaction can be expressed as:

Silica or silica + alumina (from pozzolan) + Ca(OH)₂ + H₂O → Calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H)

The primary product, C-S-H, is the same gel responsible for strength and durability in ordinary Portland cement hydration.<ref name="Mehta2006">Template:Cite book</ref>

Benefits in concrete

Use of pozzolans in concrete provides several advantages:

Reduced heat of hydration
Improved long-term strength and durability
Lower permeability and increased resistance to sulfate attack and alkali-silica reaction (ASR)
Reduced calcium hydroxide content (lower risk of efflorescence and chemical attack)
Lower carbon footprint when replacing part of Portland cement
Better workability with certain pozzolans (e.g., fly ash)

Standards and specifications

ASTM C618 – Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete<ref>Template:Cite web</ref>
ASTM C1240 – Standard Specification for Silica Fume Used in Cementitious Mixtures
EN 197-1 – Cement – Part 1: Composition, specifications and conformity criteria for common cements (includes pozzolanic cements)
EN 450-1 – Fly ash for concrete
IS 3812 (Part 1) – Indian Standard for Pulverised Fuel Ash as pozzolana

History

The Romans extensively used natural pozzolana mixed with lime to produce hydraulic mortar capable of setting underwater. Notable surviving structures include the Pantheon dome and many Roman harbors (e.g., Portus Cosanus).<ref name="Vitruvius">Template:Cite book</ref>

References

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