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Concrete Admixtures A menu of chemical additives can custom-tailor concrete for the job at hand. by Bruce McIntosh Anyone who works with concrete should know something about admixtures -their advantages and limitations, and how they affect the properties of the concrete. After all, the chances are good that you already use admixtures whenever you order concrete from your local ready-mix supplier. According to the National Ready-Mixed Concrete Association, 39 percent of all ready-mixed-concrete producers routinely use fly ash, and 65 to 70 percent of all ready-mixed concrete contains a water-reducing admixture. An admixture is, simply, any ingredient in concrete-other than cement, water, and aggregates-that is added during mixing. Admixtures are used to achieve special properties, such as greater workability or longer setting time. Admixtures are