Just what is a retarder, as referred to in "Waste Materials in Concrete"? (September 1971, page 372.) What is it chemically and what does it do to the strength characteristics of the concrete?
Retarders for use on forms or concrete surfaces are likely to be made up of such substances as sugars, dextrins, hydroxylated carboxylic acids or their salts, or lignosulfonates, the same classes of materials used in retarding admixtures. When formulated for use on forms and surfaces, commercial products are sufficiently concentrated to delay the set of the cement at the surface for perhaps 8 to 24 hours. They are effective to depths of 1/8 or 1/4 inch for ordinary operations. Deeper etches can be obtained with certain materials when used according to the manufacturer's directions. Below the effective depth the main body of the concrete does not come into contact with the retarder. It sets and gains strength at the normal rate. As for the chemical action of retarders used as admixtures, when used in the proper proportions they slow down the chemical reactions that cause setting, and those that contribute to early strength gain as well. Early strengths are likely to be slightly low. With proper curing, ultimate strengths should be equivalent to or even higher than those of the same concrete without a retarder.