Pozzolanic reaction

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The chemical reaction of calcium hydroxide and silicon dioxide to form calcium silicate hydrates is called the pozzolanic reaction . It is crucial for the post-curing of concrete over a period of years. As a result, concrete only reaches its greatest strength after several decades.

chemistry

The general reaction equation is:

x Ca (OH) 2 + y SiO 2 + z H 2 O → x CaO • y SiO 2 • (x + z) H 2 O

or in the shortened nomenclature for cement reactions :

CH + S → CSH

The reaction takes place in an aqueous environment, but does not itself bind water. This differs from the hydraulic reaction of the cement, which is responsible for the rapid hardening of the concrete.

The pH regulates the reaction. If the Ca (OH) 2 is too diluted, the basicity required to dissolve SiO 2 is not achieved . If the pH is greater than 12.6, the solubility product is exceeded and Ca (OH) 2 precipitates. Carbonate rainwater penetrating into microcracks reduces the service life of the structure via the pH value.

The reaction of calcium hydroxide (Ca (OH) 2 ) with orthosilicic acid (H 4 SiO 4 or Si (OH) 4 ) often takes place:

Ca (OH) 2 + H 4 SiO 4 → CaH 2 SiO 4 · 2 H 2 O

or in cement chemical notation:

CH + SH → CSH (the dashes indicate the varying stoichiometry )

The product is a calcium silicate hydrate (CaH 2 SiO 4 · 2 H 2 O). The ratio of calcium to silicon and the number of water molecules can vary.

Many pozzolans can contain (hydroxo-) aluminates (Al (OH) 4 - ), which react with calcium hydroxide and water to form calcium aluminate hydrates such as C 4 AH 13 , C 3 AH 6 or hydrogranates . In combination with silica , C 2 ASH 8 or strätlingite is formed. In the presence of anionic groups such as sulfates , carbonates and chlorides can ettringite phases form.

Pozzolans

Pozzolans are additives in concrete that trigger a pozzolanic reaction. Formally, they are supposed to break down the Ca (OH) 2 that is created during the hardening of cement. In addition, subsequently pressed-in pozzolans with the so-called concrete crystallization serve to seal concrete. The grain size and the surface structure of the pozzolans determine the speed of the reaction. Under normal conditions, it only sets in after several days and is completed after weeks or years.

The alkali content (Na 2 O, K 2 O) increases the pH value. The starting point is the alkalinity of pure cement, which is determined by the calcium hydroxide. It precipitates at pH values ​​above 12.6. Silicon dioxide in the form of quartz is dissolved from an even higher pH value of 13. Then SiO 2 components are dissolved more quickly and the reaction accelerated. The concrete can break up, also due to the so-called alkali-silica reaction .

In contrast to the alkali-silica reaction, the pozzolanic reaction is volume-neutral. In the case of the latter, a swellable alkali-silica gel or a swellable CSH gel forms, which breaks up the concrete from the inside by increasing its volume. It should be noted that the reactions are always in an equilibrium that depends on the pH value and that they are only reversible to a small extent due to different solubility.

See also

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