Increasing coal fly ash use in cement and concrete through chemical activation of reactivity of fly ash

Nearly 60% of the electricity in the USA is produced by coal-fired plants, which results in the production of a large quantity of coal combustion residues, among which coal fly ash is the major component. Many applications have been developed for coal fly ash. The use of coal fly ash as a cement replacement in concrete is the most attractive one because of its high volume utilization and widespread construction. However, the replacement of cement with coal fly ash in Portland cement concrete usually increases the setting time and decreases the early strength of concrete. The addition of chemical activators such as Na2SO4 or CaCl2 accelerates pozzolanic reactions and changes pozzlanic reaction mechanisms between fly ash and lime, which results in decreased setting time, accelerated strength development, and increased strength of materials containing fly ash, especially with a high percentage of fly ash. The other promising cements include alkali-activated fly ash and alkali-activated fly ash-ground blast furnace slag cements, which can exhibit very high strength in the presence of proper alkaline activators. Thus chemical activation of reactivity of fly ash is an effective method to increase the use of fly ash in concrete.