Pozzolanic Chemistry in Cement

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⁺ + H₂SiO₄^2- + 2 H₂O —> CaH₂SiO₄· 2 H₂O

or summarized in abbreviated notation of cement chemists:

 

CH + SH —> CSH

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.

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.