Fly ash (or pulverized-fuel ash, pfa) is used as a constituent of blended cement or in combination with Portland cement, PC. As a blend or a combination the normal proportion of fly ash is 20 to 35% of the total cement. Compared with Portland cement concrete at equal w/c ratio, use of fly ash in concrete will give:

•lower heat of hydration
•enhanced resistance to sulfate attack
•enhanced resistance to chloride ingress
•more effective mitigation of damaging alkali-silica reaction
•lower compressive strength
•less resistance to carbonation.

The mix design of concrete containing fly ash needs to be modified to take account of changes in the concrete performance that results. Fly ash has a long history of successful use in concrete, and requirements for its use in concrete are given in BS EN 450 [1].

Fly ash is a by-product of coal-fired power stations. Approximately 5.5 million tonnes are produced per annum, of which only 50% currently finds use. This compares with the 15 million tonnes of Portland cement produced each year. Increased use of fly ash as a cement component in concrete will contribute towards policies of sustainable development by:

•reducing overall greenhouse gas emissions
•reducing natural aggregate usage
•reducing Portland cement consumption
•improving some aspects of concrete durability and prolonging service
life of structures.