Type E: Expansion through the formation of the mineral ettringite
Ettringite is a Calcium-aluminium-sulphate (CSA) produced by calcium, aluminium and sulphated compounds. When all three components are dissolved in water in the ideal composition, long needles of ettringite are formed.
The reaction with water leads to the formation of the mineral ettringite. This process takes place during the curing in the first 3-4 days and is mostly completed after 28 days. Therefore, no late expansion due to unreacted relicts need to be expected.
Caution: In cement mixtures with a retardation agent, the formation of ettringite may already happen without effect in the liquid stage or influence time and amount of ettringite formation.
The availability of water is crucial for the formation of ettringite. In order to fully react, ettringite needs 32 molecules of water to hydrate, meaning a w/c = 0,85 (water to concrete ration). With the low w/c factors used today, there is often not enough water to build ettringite. The after-treatment must therefore ensure sufficient supply of water.
A disadvantage of Ettringite is its sensitivity to heat. The mineral starts to decompose at 50° C – a temperature that can easily be obtained in massive concrete.