Published: Jan 1966
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Admixtures are generally used to provide an economical means of improving one or more properties of fresh or hardened concrete. Among the effects sought are: reduction in bleeding, increased workability, acceleration or retardation of hydration or setting, increased strength, reduction in heat of hydration, added resistance to freezing and thawing, increased impermeability, improved resistance to aggressive waters and soils, and reduction of expansion caused by reactive aggregate and alkalies in cement. It should be noted that many of these improvements may be obtained without the use of admixtures if proper steps are taken in the design of concrete for the desired uses. However, admixtures designed for specific uses may effect the desired improvements in properties at a lower cost. Economy is, therefore, one of the matters of prime consideration. Admixtures are defined under ASTM Definitions of Terms Relating to Concrete and Concrete Aggregates (C 125) as materials other than water, aggregates, and portland cement (including air-entraining portland cement and portland blast-furnace slag cement) that are used as ingredients for concrete and are added to the batch immediately before or during mixing. A wide variety of materials are so classified. Among these are organic compounds, such as triethanolamine, sulfonated lignins, oils, fats, resins, and carbohydrates; inorganic compounds, such as calcium chloride, borax, sodium carbonate, and sodium silicate; and finely divided minerals. Only mineral admixtures are discussed in this paper. Mineral admixtures include any essentially insoluble material other than cement and aggregate, which is used as an ingredient for concrete, and is added to the batch immediately before or during mixing. Mineral admixtures include natural materials, processed natural materials, and artificial materials. They are finely divided and so form pastes to supplement portland cement paste, in contrast to soluble substances which act as chemical accelerants or retardants during the hydration of portland cement or otherwise modify the properties of the mixture.
Higginson, E. C.
Chief, Bureau of Reclamation, Denver, Colo.