Set Retarding

Definition EN 934 Part 2:

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:

  • in hot weather (above 20 °C) to prevent early stiffening
  • to increase workability time, when used in conjunction with plasticizers
  • when a large pour of concrete need several hours to place
  • to place concrete in several layers without cold joints
  • for mass concrete, to reduce maximum temperature
  • to extend the possible time between mixing and placing (e.g. for long transport time)
  • prevent setting of the concrete in the truck in case of delay or damage.

The use of a retarder will, in addition to increasing the setting time, delay strength development of concrete.