Retarders are mainly used in ready-mix/ site-mix to help on workability retention and to prevent cold joints Concrete retarders are the mixture that slows down the chemical process of hydration so that the concrete remains plastic and workable for a long time, to facilitate for transportation and placing of concrete.Temperature has a significant effect on retardation. They work by

• Acting as a blocking mechanism where the admixture absorbs strongly on to cement surface slowing the formation of Silicate hydrates
• Chelation of Calcium ions in solution , preventing the precipitation of calcium hydroxide

Retarding admixtures delay the end of the dormant period and the start of setting and hardening. This is useful when used with plasticizers to give workability retention. Used on their own, retarders allow later vibration of the concrete to prevent the formation of cold joints between layers of concrete placed with a significant delay between them.