Retarder (concrete admixture)

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As a retarder (short: VZ ) are concrete admixtures denotes that the solidification of the cement of a fresh concrete or mortar slow down and thus increase the working time. Retarders are usually liquid substances that are used in small amounts, i.e. i.e., in quantities of a few centiliters to a few deciliters per cubic meter of fresh concrete.

commitment

On the one hand, retarders are used when the workability of fresh concrete is to be maintained over a longer period of time. This can be the case with particularly long routes between the mixer and the construction site or when large concreting sections are to be poured without construction joints (“fresh on fresh”). On the other hand, a delay means that the chemical reaction of the cement takes place more slowly and thus the heat development is reduced. This is advantageous for. B. in summer on hot days or with very bulky components.

Means used

In the production of concrete, various, both organic and inorganic chemicals are used that cause a retarding effect in the concrete. The means differ both in their effect and in the expression of "side effects", such as B. an additional liquefaction of the concrete. These side effects can definitely be viewed as positive in some areas of application.

Inorganic Agents

Organic means

  • Hydroxycarboxylic acids have both a delaying and liquefying effect and are suitable for delay times of a few hours. This substance is not permitted in the ZTV-ING area.
  • Lignosulfonates only have a weakly retarding and simultaneously liquefying effect.
  • Sucrose is particularly used to delay mortar and also enables longer delay times of over 24 hours. The dosage must be carried out particularly carefully, as the fabric tends to "turn over", i. That is, the retarding effect can turn into an accelerating effect if too much is added.

Influence on the hardened concrete

The use of retarders can change the properties and appearance of hardened concrete.

Changes that can be viewed as positive can result in increased impermeability to water as well as increased post-curing and greater final strength. Under certain circumstances, negative effects are an increased tendency to shrink as well as greater color differences on the concrete surface and more pronounced efflorescence; Changes that have to be taken into account, especially when manufacturing exposed concrete .

Individual evidence

  1. ↑ a b c cement data sheet concrete technology: Concrete additives - admixtures and additives. (PDF; 444 kB) Association of German Cement Works e. V., September 2005, accessed October 17, 2013 .
  2. ↑ a b c d Andrea Kustermann: building materials FHM - concrete additives - concrete additives. (PDF; 354 ​​kB) Institute for Building Materials - University of the Federal Armed Forces Munich, archived from the original on October 17, 2013 ; Retrieved October 17, 2013 .
  3. ↑ a b c Otto Henning: Baustoffchemie. Beuth Verlag, 2013, ISBN 978-3-410-22481-5 , p. 118 ( limited preview in Google book search).