Pozzolanic reaction

The Pozzolanic reaction is the chemical reaction that occurs in hydraulic cement, a mixture of slaked lime (calcium hydroxide) with amorphous siliceous materials (namely, pozzolan or pozzolana, a finely divided volcanic ash, rich in obsidian, a mineral glass commonly found in lava), forming non-water-soluble calcium silicate hydrates. It is the main reaction involved in the Roman concrete invented in Ancient Rome and used to build, for example, the Pantheon.

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). Simply, this reaction can be schematically represented as follows:

Ca(OH)₂ + H₄SiO₄ → Ca²⁺ + H₂SiO₄^2- + 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.
• 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.
• Mertens, G.; R. Snellings, K. Van Balen, B. Bicer-Simsir, P. Verlooy, J. Elsen (2009). "Pozzolanic reactions of common natural zeolites with lime and parameters affecting their reactivity". Cement and Concrete Research 39 (3): 233–240. doi:10.1016/j.cemconres.2008.11.008. ISSN 0008-8846. http://www.sciencedirect.com/science/article/B6TWG-4VC14XR-4/2/3383178a708024fc4d80b7e98f28c61f. Retrieved 2009-03-23.