Pozzolana blended Portland cement produce a lower porosity concrete, which makes it more resistant to water absorption and spalling. It is also resistant to forms of corrosion induced by sulfates.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). For simplifying, this reaction can be schematically represented as following:
- Ca(OH)2 + H4SiO4 —> Ca2+ + H2SiO42- + 2 H2O —> CaH2SiO4· 2 H2O
or summarized in abbreviated notation of cement chemists:
The product of a general formula (CaH2SiO4 · 2 H2O ) formed is a calcium silicate hydrate, also abbreviated as CSH in cement chemist words. The ratio Ca/Si, or C/S, and the number of water molecules can vary also the above mentioned stoichiometry may differ slightly.