A set retarding admixture is an additive that delays the initial setting of concrete by extending the time for which the mixture remains in a plastic state. This admixture does not adversely affect the development of concrete’s latter strength and is usually made from lignosulfonate, carbohydrate or inorganic salts. It is used when high temperature or long transport distance cause early slump loss or for mass concrete pours to prevent the formation of cold joints. If too much retarder is added the concrete will start to exotherm quickly and form a cold joint, therefore it should be used with care.

The retarding action of the admixture is caused by its ability to form a film around the cement compounds and inhibit or slow their reaction with water. The amount by which hydration is retarded depends on the thickness of the retarding film. Once the retarding film breaks down normal hydration begins. This enables concrete to be placed, compacted and finished within the desired workability range.

Retarders are particularly useful in hot weather concreting, as they increase the slump of the concrete and delay its set. However, it is important to remember that once the effects of the retarder and any superplasticizer wear off concrete will set rapidly.

Fritz Pak offers a variety of different set retarders to suit your project needs. The most popular is Standard Delay Set, which can be added to the mix on site or poured directly into the concrete. This admixture increases slump by up to 7 inches and retards the set by an hour or so. The use of this product can also reduce segregation and bleeding and improve pumpability.