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Retarding admixtures

Retarding admixtures have little or no effect other than to delay the setting of the cement. They do not plasticise significantly and have no effect on the water demand or other properties of the concrete. Retarding plasticising admixtures not only delay the setting of the cement but are also efficient plasticising water-reducers. Most commercially available retarders are of this type. Retarding plasticisers are used to give workability retention to the concrete, delay the setting time and increase initial workability.

As with plasticisers, ultimate strength gain increases as water is reduced. Set retardation allows the slower formation of a more ordered, smaller, denser cementitious matrix. This increases ultimate strength relative to an unretarded mix with the same w/c ratio.

Provided that the concrete is correctly cured, retarded concrete should be stronger and just as durable as equivalent plain concrete. However, because of the extended plastic stage, more attention needs to be paid to protecting it before it sets. Retarded plasticised concrete will also contain less water than the equivalent plain concrete and will be correspondingly more durable.

Acknowledgement: The Concrete Society

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TR18 A guide to the selection of admixtures for concrete