The pozzolanic reaction is the chemical reaction that occurs in portland cement containing pozzolans. 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), and . Simply, this reaction can be schematically represented as follows:
- Ca(OH)2 + H4SiO4 → Ca2+ + H2SiO42- + 2 H2O → CaH2SiO4 · 2 H2O
or summarized in abbreviated notation of cement chemists:
- CH + SH → C-S-H
The product of general formula (CaH2SiO4 · 2 H2O ) formed 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 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 chlorine, AFm phases and AFt or ettringite phases can form.
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