Fly ash

Ash is produced as a result of burning coal for the production of electricity. Coal is used to supply around 37% of the UK's electricity, and therefore fly ash is an abundant material. It is a product of the soil and minerals laid down with the coal many millennia ago.

In a pulverised fuel coal fired power station there are two types of ash produced, Pulverised Fuel Ash (PFA), also known as fly ash in many countries, and Furnace Bottom Ash (FBA). They are used in differing ways as follows.

Furnace bottom ash

All the UK coal fired power stations have what's known as 'wet bottom' furnaces, where the ash is flushed from the furnace using water. This means the FBA is washed in copious quantities of water making it suitable for use as an aggregate. Just over 1,000,000 tonnes of FBA is produced annually and virtually all the UK production of FBA for many years has been used in the manufacture of lightweight concrete blocks. Due to its method of production all leachable materials are removed and as a result there are no known risks to the environment or the user from FBA.

Pulverised fuel ash
PFA represents the largest proportion of the ash produced from a power station, with about six times the FBA being produced. This is a fine powder (like talcum), grey to dark grey in colour. It is used in a wide variety of applications. For more information on the uses please consult the UKQAA Technical Datasheets.

For a variety of reasons not all PFA is used, with about half of the annual production being landfilled. Large proportions of ash are used in cementitious applications, e.g. cement manufacture, concrete addition, block making, precast concrete and grouting.
Environmental sustainability and fly ash

It is likely that coal fired power stations will continue to generate a proportion of UK electricity for the foreseeable future. With the development of 'clean coal' power stations using carbon capture and storage technologies emissions of carbon dioxide into the atmosphere from these power stations will be reduced. Therefore, one can expect UK produced fly ash to be available to specifiers in the construction industry for the foreseeable future.

Furthermore, using fly ash as an aggregate or as a cementitious material has many other sustainability benefits. For one thing its use in concrete is a benefit to the environment. If it is not meaningfully used then it has to be landfilled. In fact, there is more fly ash produced annually than is used and thus there is still potential for greater use.

When fly ash is used to extend CEM I cement the saving in carbon dioxide emissions per tonne of concrete are in the region of 20-30% for mixes designed with equal 28 day strength.

Further information

More information on Fly Ash can be found at the UK Quality Ash Association website.