Pozzolanic reaction

A pozzolanic reaction, refers to the chemical reaction of calcium hydroxide and silicon dioxide to calcium silicate hydrates. It is essential for the post-curing of concrete in a time period of years. The greatest strength is achieved by concrete only after several decades.

Chemistry

The general reaction equation is:

Or in the abbreviated nomenclature for cement reactions:

The reaction proceeds in an aqueous environment, binds itself but no water. This is different from the hydraulic reaction of the cement, which is responsible for the rapid curing of the concrete.

The pH value controls the reaction. If the Ca ( OH) 2 diluted too much, the necessary alkalinity is not reached to solve SiO2. The pH value is greater than 12.6, the solubility is exceeded, and Ca (OH) 2 is precipitated. In micro-cracks penetrating carbonate rainwater lowers over the pH value the life of the structure.

Pozzolans

Pozzolans, aggregates, concrete, trigger a pozzolanic reaction. Formally, they should reduce the Ca (OH) 2, which is formed during the curing of cement. Moreover subsequently serve pressed pozzolans with the so-called crystallization concrete for sealing concrete. The particle size and the surface structure of the pozzolans determine the reaction rate. Under normal conditions, it does not begin until after several days and is completed after weeks or years.

The alkali content ( Na2O, K2O) increases the pH. The starting point is the alkalinity of pure cement, which is determined by the calcium hydroxide. It falls at pH values ​​of about 12.6. Silicon dioxide in the form of crystal is dissolved from a still higher pH of 13. Then SiO2 fractions are dissolved more quickly and accelerates the reaction. This can lead to break up of the concrete due to the well known alkali -silica reaction.

The pozzolanic reaction volume is directly in contrast to alkali-silica reaction. In the latter, a swellable alkaline silica gel, or a swellable CSH gel breaks through the concrete volume increase from the inside is formed. Note that the reactions are in an always dependent on the pH balance and due to different solubility are reversible only to a small extent.