Admixtures are not new. But general acceptance of admixtures had been slow, partly because many are waste products- fly ash, sulfite liquors from wood pulp production, by-products from resin production or from petroleum. Some were introduced without proper purification or control. As recently as 1963, the portland cement industry opposed the use of any admixture as a means of reducing cement content. Good pozzolans used in optimum amounts improve many qualities of concrete. They contribute to strength, improve workability, reduce heat of hydration and thermal shrinkage, increase watertightness and improve resistance to sulfate soils and sulfate waters. Entrained air improves the resistance of concrete to spalling caused by freezing and thawing and by the action of deicing slats. Commercially produced air entraining agents are made from a number of waste byproducts. Two of the most common are a by-product in the preparation of wood resin from pine stumps and a by-product in the refinement of petroleum to which is added triethanolamine. Most chemicals used as water reducing admixtures tend to retard the set. Such admixtures would naturally be classified as water reducing and retarding admixtures. To overcome the retardation and produce a water reducing admixture, manufacturers commonly add just enough accelerator to accomplish that purpose. By using still more accelerator in the formulation the manufacturer can produce a water-reducing accelerating admixture. Retarding admixtures may be made for various inorganic salts, but since there is no limitation of water reduction for type B admixtures, type D admixtures also qualify as type B. Over 500 trade name admixtures have been marketed in this country during the last 50 years. But the total cost of concrete, including maintenance, has been an overriding consideration in their use. Our present knowledge permits the selection of an admixture that will produce the desired effects on the concrete mixture. Admixtures are now recognized as a necessary adjunct to the production of good economical concrete.