Zum Einfluss von Langzeitverzögerern auf der Basis von Phosphonsäure auf die Hydratation einzelner Klinkerphasen Portlandzementklinker und Portlandzemente In addition to the large group of plasticizing admixtures such kind of admixtures are required which enable a very long time of workability of concrete. The workability of concrete over a period of more than 90 hours can be achieved by long term retarders. This new type of admixture is based on phosphonic acid. By systematic investigations profound knowledge about the operative mechanisms of long term retarders could be drawn. The retarding action is due to a formation of sparingly soluble calcium phosphonate which seal upœ the particle surface. Whether the addition of long term retarder leads to an immediate formation of calcium phosphonate or to a short termed acceleration of the hydration of reactive clinker phases depends on the supply of solved calcium. Because of the adjusted calcium sulphate content in cement the concentration of calcium is high enough for an immediate formation of sparingly soluble calcium! phosphonate. In combination with the formation of primary ettringite the calcium sulphate agent is the basis for the desired mode of action of the long term retarder.