A retarding superplasticizer extends the period of time it takes concrete to set and harden. They are a very useful tool for mass concrete pours and help reduce problems such as cold joints that can occur when unretarded concrete sets too quickly. This is particularly important in applications that require a high workability such as precast concrete or in cases where long placing periods are needed.
These admixtures are based on synthetic water-soluble polymers such as sulfonated naphthalene formaldehyde (SNF), sulfonated lignosulfonate formaldehyde (SLF) or modified sugar-free lignosulfonate (MLP). They also have dispersing capabilities that improve the performance of cement and aggregate. Most retarding admixtures are multifunction products and have both plasticizing and retarding properties. Some such as gluconate and sugar are purely retarders with no plasticizing action while others, such as phosphotes and some SNFs, have some retarding ability but only minor plasticizing effects.
In order to provide the liquefaction benefits of these chemicals they are adsorbed on particle surfaces in the cementitious system. The comb shaped polymers shield attractive forces between the particles of the concrete and aid in a well-dispersed and homogeneous mixture. This is the basis for modern high performance concretes with low water:cement ratios.
The adsorption also causes a change in the behavior of water in the concrete. The water molecules are hindered from interacting with the clinker phases of the cement and this inhibits hydration. This has a detrimental effect on the strength gain rate and can result in lower early strength than would be expected from other high quality concretes.