Admixture which extends the time to commencement of transition of the mix from the plastic to the rigid state.
Retarding admixture are used to slow down the speed of the reaction between cement and water by slowing down the growth of the hydration products and/or reducing the rate of water penetration to the cement particles. By this, concrete stays workable for longer than it would otherwise have been.
Retarders can be used:
The use of a retarder will, in addition to increasing the setting time, delay strength development of concrete.