United Kingdom Quality Ash Association

What is Pulverised Fuel Ash and Furnace Bottom Ash?

When pulverised coal is burned in a coal fired power station the ash is carried out with the flue gases. These pass through electrostatic precipitators that remove the fine ash or PFA from the flue gases. The PFA is collected in silos where it is either sold dry for use in concrete or moistened (called conditioning) in applications such as for fill, grouts, etc. Some stations may mix the ash with large quantities of water and pump the resulting slurry to lagoons. Eventually lagoon ash is allowed to drain and it is also sold for fill and grouting applications.

FBA is molten ash that clings to the boiler tubes and falls into the bottom of the furnace. This extracted wet in UK power stations. It is graded and sold to make lightweight concrete blocks. For more information read "... power behind PFA"

See - Where ash comes from... web pages for an interactive description

What is fly ash?

Fly ash is a term used for the fine ash carried over in the gases from a furnace. It can mean the ash from any furnace, however; usually for furnaces other than coal fired power stations it is normal to prefix the words with the type of burning, e.g. Incinerator Ash, Paper Sludge Fly Ash, etc. PFA is the preferred term in the UK, but fly ash is widely used throughout the rest of the world for coal fired power station ash and in all European Standards.

Care must be taken to not confuse fly ash from other other furnaces with coal fired power station fly ash, which may have VERY different properties. The information on this web site refers only to PFA/fly ash from coal fired power stations.

What does the word Pulverised mean in PFA?

The word 'Pulverised' refers to the coal, which pulverised before being burned in the furnace to a fine talcum like powder. This is to ensure it burns efficiently and quickly, usually only taking 2 to 4 seconds. It burns at a temperature of between 1250C and 1600C in the types of furnaces used in the UK.

What's happened to BS3892 PFA and what's this BS EN450-1:2005?

BS3892 Part 1 (PFA for concrete) will be withdrawn as from 1 January 2007. After many years finally a 'harmonised standard' for PFA, or fly ash as it is commonly known in the rest of Europe has been agreed. This standard in BS EN450-1:2005. This standard permits a wide range of fly ash to be used in concrete. However, the finer material as required by the old BS3892 Part 1 will not disappear - but it will have a new name. It will be called EN450-1:2005 Category S fly ash. Think of the 'S' as meaning special ash. In addition there will be a Category N fly ash to EN450-1:2005 which will increasingly become available in the UK.

Further details about the changes can be found in our Technical Datasheet 1.1

How much does PFA or FBA cost?

It is impossible to give a price for PFA and FBA. The costs depend on the location of the contract, the proximity of the power stations, the availability of the material, the type of material required, e.g. is it processed like BS3892 Part 1 PFA, the quantities required, the timescale of the contract, etc.

The UKQAA is not a commercial organisation and only promotes the technical aspects of the material. In order to obtain prices please contact the members of the UKQAA directly, who will be only too pleased to help in giving competitive prices for their materials.

There are few coal fired power stations left. Is there enough PFA available?

There are 18 coal fired power stations in the UK. While there has been a move towards other forms of electricity generation, coal has proven to be one of the most cost effective and reliable fuels. The production of electricity from coal has remained stable over recent years and there is still a considerable amount of PFA available for use in the various applications. There is a considerable amount of PFA on long term stockpile that could be used for fill, grouting and similar applications. In addition, with new regulations on NOx emissions being enforced during 2007 more PFA will be processed to reduce the LOI and increase its suitability for use in concrete.

There are few coal fired power stations left. Is there enough FBA available?

Furnace Bottom Ash is produced at all the power stations and has been used in the manufacture of lightweight concrete blocks used for building for many years. It has proven to be a effective aggregate producing a strong and durable product and yet using a recycled material. All the FBA produced in the UK is fully utilised in block manufacture. Unfortunately there is unlikely to be any surplus material for other applications in the foreseeable future.

How much PFA can I put into a concrete mix?

The simple answer is 'as much as you like!' There are recognised limits to how much PFA can be counted as being part of the cement content. These limits are covered within BS EN197-1 for factory made cements and BS 8500 Parts 1 and 2 for mixer blended concretes with 55% of the total cementitious being the maximum. However, more PFA may be added as a Type I addition, that is its assumed to be an inert material, effectively acting as a fine aggregates replacement. There is no technical reasons why large proportions of PFA cannot be used, in fact High Volume Fly Ash concrete (HVFA) is being used in the USA with up to 70% PFA of the cementitious material for environmental impact reasons.

What is the 'drainage blanket' for beneath PFA when used as a fill material?

The UKQAA recommends a drainage blanket beneath PFA when used as a fill material because PFA, like any fine grained material, will tend to draw water from the ground beneath by capillary suction. This water will rise to ~600mm and may cause stability problems in thin layers of fill. However, where thick layers of PFA, say >2m, are to be used it has been shown that this layers can be removed and a stable fill be be achieved. The Specification for Highway Works requires a 6D granular fill as a starter layer beneath PFA fill. For more details see Technical Datasheet No.2.

What is the steepest slope that can be created using PFA as a fill material?

When fly ash is excavated from lagoons and stockpiles, vertical faces can be created without failures occurring. However, this is an extreme and couldn't be considered safe for normal construction applications. The maximum slope used in recent construction projects, to our knowledge, has been 45°, which was used at the Black Country Route in the West Midlands. The moisture content, the source, the degree of compaction and the shear strength of the PFA all effect the possible slope. As PFA gains strength with time, due to the pozzolanic reaction, the shear strength increase very significantly with some ashes. Typical values for shear strength are given in Technical Datasheet No. 2.

Vertical faces are possible by the use of geogrid for reinforced earth applications. Such grids have to be resistance to the high alkalinity of PFA. For further details of suitable geogrid materials contact Tensar who produce such products.

What is the environmental impact of using PFA as a fill material

PFA, when used as a fill material, usually replaces virgin aggregates. As the winning of such aggregates has impacts associated with their extraction and processing, (~21kg/tonne of aggregates of CO2), using PFA can reduce significantly the overall environmental impacts. In addition it help preserve aggregates reserves for future generations.

As coal is a naturally occurring material, PFA contains similar trace elements as would be found in soils, except they are concentrated to some extent. Most of the elements are held in the glassy matrix of the PFA particle and not available for leaching. In addition the low permeability of compacted PFA means water only percolates through the material at a very low rate. The results is there is little detectable leaching from PFA in fill applications. More details are given in our Generic Environmental Risk Assessment and Nottingham University Leachates from Embankments project report.

For a general assessment of Environmental & Sustainability issues click here

What is the environmental impact of using PFA in concrete?

PFA is pozzolanic and can be used as a partial cementitious material. As such it reduces the quantity of Portland Cement required. Due to the chemistry of Portland cement and the temperatures required to produce cement, for every tonne of PC made produces ~900kg of CO2. By replacing 30% of the total cement content with PFA the overall environmental impact has been shown to be reduced by at least 17% for the same 28 day strength. With some types of concretes for certain exposure classes (See BS EN206-1 and BS 8500) reductions in emissions of >50% are possible. With High Volume Fly Ash (HVFA) concrete, which contains between 55 and 70% PFA, or when the specified strength is at 56 days or more, even greater reductions in the environmental impact are achievable.

When PFA is incorporated in structural concrete there is a reduction in leaching potential. The pozzolanic reaction reduces the permeability effectively preventing any significant leaching from either the PFA, the Portland cement or the aggregates.

For a general assessment of Environmental & Sustainability issues click here

What is the environmental impact of using PFA grouts?

The range of grouts recommended by the UKQAA are such there should be little risk of a significant level of leachates from PFA grouts. As with concrete the leachates will be trapped by the hydration of the Portland cement and the pozzolanic interaction. However, during the construction process, any bleed water from the grout may have environmental consequences, especially if close to high quality aquifers. Reference should be made the the Generic Environmental Risk Assessment for the leachates associated with the PFA and some assessment of the leaching from the cement should also be made. A full environmental risk assessment may be required to obtain approval from the Environment Agency. We would suggest the BRE Environmental Code of Practice for Grouts could form the basis of such an assessment.

For a general assessment of Environmental & Sustainability issues click here

Is there much sulfate in PFA?

Typically fresh PFA contains between 0.35 and 2.5% sulfate as SO3. This puts the material in Design Class 2 or 3 sulfates when considered as a soil to BRE Special Digest 1. The sulfate in PFA exists as gypsum, which has limited solubility in water. Therefore, when used as a fill material it has the potential to cause sulfate attack, but in practice there is no known case of such attack in concrete adjacent to PFA. Also, in concrete the low solubility of the gypsum does not cause any specific problems such as excessive retardation or expansion. However, when used as a fill material it can attack some metals, e.g. mild steel and this is why the Specification for Highways states that PFA should not be placed within 500mm of metallic items, e.g. over culverts, etc.

Lagoon PFA, which effectively has been washed with copious amounts of water, has a very low sulfate content <0.1g/l, which would equate to DC 1 sulfates.

It is normal to either protect concrete from sulfate attack, e.g. by using a drainage layer and bitumen sealing coat, for example when building embankments adjacent to retaining walls, etc. Where foundation trenches are dug into PFA, the use of concrete designed to resist DC3 class of sulfates is recommended unless it can be verified the sulfates content of the ash is lower, e.g. lagoon material or similar..

Can PFA attack some metals?

Yes, PFA is an alkaline material with a pH of between 7 (for lagoon ash) to 11 for fresh material, depending on the source of the ash. If a metal is known to be attacked by alkalis such as calcium, sodium or potassium hydroxides, such as aluminium or lead, PFA will also react with it - in the presence of water. In addition PFA can contain sulfates as gypsum, see the question above, and it will attack carbon steel, though not stainless steel.

I want to import PFA (Fly ash) into my country, who should I contact?

You should contact the members of the UKQAA directly. There is a list of contacts on our Full Members and Associate Members pages of this web site. However, if you find you are having problems please send details of the fly ash you are looking for to Enquiries@UKQAA.org.uk.

I / my company would be interested in becoming a members of the UKQAA?

The membership of the UKQAA is only for corporate members, e.g. businesses and we do not have any individual members. You should contact us by letter or e-mail explaining your interest in PFA/fly ash. There are 3 categories of membership of the UKQAA - Full Members, who own and operate coal fired power stations or are 'Total Ash Managers' on behalf of a coal fired power station. Additionally there is Associate Membership that for users of PFA/fly ash or smaller operators of coal fired power stations. The third category of membership is affiliate members - this was introduced in December 2005. This is again for users, hauliers, suppliers, etc to the ash industry who do not wish to be involved and receive the information the associate members do. For full details follow this link <Affiliate Membership>.

Applicants for Full and Associate membership will be required to give a presentation to the Executive committee meeting explaining their interest in the UKQAA and their connections with the PFA/fly ash industry. A decision will be made based on the details given and the presentation. Details of the rules of membership and the benefits of membership will be provided upon request.

How much PFA and FBA are produced in the UK?

The UKQAA does publish limited data on the utilisation of PFA, FBA and gypsum. Data is available of this web site for 1997, 1999, 2000, 2001, 2002, 2003 & 2004. However, in recent years these statistics are restricted to the proportions used in the various applications. The UKQAA has a policy of not revealing the total production figures because of various authors misquoting and creating misunderstandings of the data. <Back to Top>

Does the chromium VI directive for cement effect PFA in anyway?

The EU Directive, 2003/53/EC, restricts the marketing and use of cement and cement-based preparations, where they contain when hydrated, more than 0.0002 % (2 ppm) chromium VI of the dry weight of cement. It is the responsibility of the cement producer to ensure this limit is adhered to by testing their products and adding an appropriate reducing agent. The cement industry have full details about Chromium VI, see the British Cement Association web site, including the timetable of implementation and advice on reducing agents.

The chromium VI content of PFA is generally very low, though occasional samples are found with values close to the 0.0002% limit. We have published a Position Paper on this issue which outlines some of the problems found with the EU Directive.

REACH - How does this affect the industry?

Under REACH, the burden of proof for demonstrating the safe use of chemicals will be transferred from Member States to industry to ensure that risks to human health and environment are avoided or adequately controlled. This regulation will replace over 40 existing legal acts. Enterprises that manufacture or import more than one tonne of a chemical substance per year will be required to register the chemical in a central database. This database will be operated by the EU Chemicals Agency (ECHA) which will be based in Helsinki, Finland. A dossier will have to be submitted containing information on properties, uses and safe ways of handling the chemical being registered. This affects about 30,000 differing producers involving some 100,000 chemical substances.

The UKQAA is working with ECOBA, the European Ash Association, to prepare the dossier that will be required from all producers of substances. Each individual producer of a substance will be required to pre-register that substance between 1 June 2008 and Dec 2008. This process will take until 2018 for all the dossiers to be prepared for the substances involved. These dossiers will have to take into account all possible uses of the materials.

Downstream users will receive Safety Data Sheets (SDS) that will explain how to use PFA and FBA safely from UKQAA members in due course. In the interim we would suggest referring to Technical Datasheet No.9 - the COSHH datasheet. This explains safety and chemical requirements relating to PFA and FBA. While the members of the UKQAA will take into account all known and obvious uses of PFA, cenospheres and FBA. However, if you are a downstream user with a use that may be unique/unknown to the ash suppliers, then advise them of the use so that this can be included at pre-registration.

If you are an importer from outside the EU of coal fired power station products (or of any chemical/substance), then you will be required to pre-register and produce your own dossier for ECHA.

Further information on REACH can be found at: http://ec.europa.eu/environment/chemicals/reach/reach_intro.htm and at http://echa.europa.eu