Currently more than 6 million tons of coal fly ash are used annually in the United States in virtually all types of concrete products and applications. Typically, the amount of fly ash used in structural concrete applications ranges from 15% to 35% by weight of total cementitous material. Amounts up to 70% and more often are used in dams and in massive walls and girders.
Coal fly ash is a pozzolan--a material that reacts chemically with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties. Coal fly ash conforming to the requirements of ASTM C 618 will have physical and chemical characteristics that make it desirable for use with portland cement to produce concrete.
Two general methods are used to incorporate fly ash into concrete mixtures. In one method, fly ash is used prescriptively as a substitute for protland cement. In the other method, fly ash is used on the basis of information developed to optimize its effect on concrete properties and performance. Each method has a valid place in engineering practice. The optimization of the effect of fly ash on concrete characteristics and performance is approcahed on the basis of either field records or trial batches.
The use of fly ash in concrete reduces the amount of water required for a given slump. The achievement of equal or better workability with less mixing water, compared to concrete with portland cement only, means that desired desired strengths can be achieved with less drying shrinkage and cracking. The better workability of concrete containing fly ash is seen in characteristics other than the traditional measure of slump. Fly ash also improves pumpability and enhances placing and finishing characteristics.
The strength-making quality of fly ash in concrete results from its reaction with the free lime released during the hydration of portland cement. Cementitious compounds incorporating fly ash increase both the strength and durability of the concrete.