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 H2Oor summarized in abbreviated notation of cement chemists:
CH + SH → C-S-HThe 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.