Conditioned fly ash can be used in concrete as a filler (Type I [inert] addition), fine aggregate and as a Type II (pozzolanic) addition. Conditioned fly ash only differs from dry fly ash in that it has been moistened and cannot be stored and transported dry, but otherwise has similar properties. Compared with Portland cement (CEM I) concrete at equal w/c ratio, use of conditioned fly ash in concrete will give:

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

The mix design of concrete containing conditioned fly ash needs to be modified to take account of changes in the concrete performance that results.

Conditioned fly ash is a by-product of coal-fired power stations that has been stored in lagoons and stockpiles. This practice has existed for over 50 years and it is estimated that over 120 million tonnes could be recovered for use in concrete. This compares favourably with the 15 million tonnes of Portland cement produced in the UK each year. Increased use of conditioned fly ash in concrete will contribute towards policies of sustainable development by:

• reclaiming a material that has previously been landfilled
• 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.