Pozzolanic reaction

The pozzolanic reaction is the chemical reaction that occurs in portland cement upon the addition of pozzolans. It is the main reaction involved in the Roman concrete invented in Ancient Rome and used to build, for example, the Pantheon. The pozzolanic reaction converts a silica-rich precursor with no cementing properties, to a calcium silicate, with good cementing properties.

In chemical terms, the pozzolanic reaction occurs between calcium hydroxide, also known as portlandite (Ca(OH)₂), and silicic acid (written as H₄SiO₄ or as Si(OH)₄):

Ca(OH)₂ + H₄SiO₄ → CaH₂SiO₄·2 H₂O

or summarized in abbreviated notation of cement chemists:

CH + SH → C-S-H

The product CaH₂SiO₄·2 H₂O is a calcium silicate hydrate, also abbreviated as C-S-H in cement chemist notation, the hyphenation denotes the variable stoichiometry. The ratio Ca/Si, or C/S, and the number of water molecules can vary and the above-mentioned stoichiometry may differ.

Many pozzolans may also contain aluminate, or Al(OH)₄⁻, that will react with calcium hydroxide and water to form calcium aluminate hydrates such as C₄AH₁₃, C₃AH₆ or hydrogarnet, or in combination with silica C₂ASH₈ or strätlingite (cement chemist notation). In the presence of anionic groups such as sulphate, carbonate or chloride, AFm phases and AFt or ettringite phases can form.

See also

References

• Cook D.J. (1986) Natural pozzolanas. In: Swamy R.N., Editor (1986) Cement Replacement Materials, Surrey University Press, p. 200.
• Lechtman H. and Hobbs L. (1986) "Roman Concrete and the Roman Architectural Revolution", Ceramics and Civilization Volume 3: High Technology Ceramics: Past, Present, Future, edited by W.D. Kingery and published by the American Ceramics Society, 1986; and Vitruvius, Book II:v,1; Book V:xii2.
• 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.