Retarders

Retarding admixtures are used in concrete practice to delay the setting times of cement paste, mortar & concrete. In hot weather concreting, delays in transport & handling between mixing & placing may result in early setting & loss of workability. in such instances incorporation of retarders becomes necessary. Retarders may be used in steam curing of concrete, to offset the long term lower strengths developed by the silicate phase in the construction of large structural units, dams, the fabrication of exposed aggregate panels. Another important application of retarders is to maintain returned concrete from ready-mixed trucks in a workable condition overnight by completely freezing the hydration. Special types of retarders are capable of controlling the slump loss in superplasticizer concrete.  
The ideal inhibitor for cementing would predictably delay the setting of cement, then suddenly allow hydration to continue at a rapid rate. A wide range of compounds shows set inhibition of the hydration of Portland cement. Some common products include tartaric acid, Sodium and Calcium Gluconate, Lignosulfonate, and organic phosphonic acids.