1. What is coal ash?
Coal ash is produced during the combustion process used at coal-fired power stations. Three types of coal ash are available in the UK:
• Pulverised Fuel Ash (PFA)/ Fly Ash
• Furnace Bottom Ash (FBA)
These by-products can be incorporated into a range of building products with little need for further refinement.
2. What is Pulverised Fuel Ash?
Pulverised Fuel Ash (PFA) is the fine ash produced during the combustion process used at coal-fired power stations.
When coal is burned, the fine ash is carried out with the flue gas. This passes through electrostatic precipitators that remove these fine particles as Pulverised Fuel Ash (PFA).
PFA is then collected in silos where it is either sold dry for use in concrete or moistened (called conditioning) for applications such as 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.
3. What does the word ‘pulverised’ mean in PFA?
The word ‘pulverised’ refers to the coal used at coal-fired power stations which is pulverised to a fine talcum powder type consistency before being burned in the furnace. This is to ensure it burns efficiently and quickly, usually only taking 2 to 4 seconds. Temperatures of between 1250C and 1600C are used in the types of coal-fired furnaces common in the UK.
4. What is Fly Ash?
Fly ash is another term for Pulverised Fuel Ash (PFA), used to describe the fine ash carried out with the flue gases from a furnace during the combustion process.
In the UK, PFA is the preferred term to describe this ash but fly ash is widely used throughout the rest of the world to refer to coal-fired power station ash and in all European Standards.
Fly ash can also mean ash from furnaces other than coal-fired power station furnaces. However it is usual to refer to the type of burning involved to create it, e.g. incinerator ash, paper sludge fly ash. This distinction is important as fly ash from other furnaces have very different properties from the ash produced at coal-fired power stations.
Please note, the information on this website refers only to PFA/fly ash from coal-fired power stations.
5. What is Furnace Bottom Ash?
Furnace Bottom Ash (FBA) forms from the molten material that clings to the boiler tubes and falls into the bottom of the furnace during the combustion process at coal-fired power stations. This granular product is extracted wet from UK power stations, graded and sold as lightweight aggregate for use in concrete blocks etc.
6. What are cenospheres?
Cenospheres are a by-product from coal ash, recovered from ash ponds or lagoons typically located at the same site as a coal-fired power station. They are fine, rigid hollow spheres and – often filled with air or inert gas. They are often used as low density fillers and in the creation of lightweight concrete.
7. What is biomass ash?
Biomass ash is a by-product from the combustion of biomass feedstock at a combustion plant.
8. With a large number of coal-fired power stations shutting down, is coal ash still readily available?
Yes, coal ash is still available in large quantities and will be available into the future. There are 17 coal-fired power stations represented by the UKQAA and while there has been a move towards other forms of energy generation, coal-fired power still accounts for over a third of the energy generated in the UK.
However, as we move towards a greener economy, coal-fired power stations will either need to close or invest in carbon saving technologies, such as carbon capture and storage (CCS). While CCS is a new technology, proposed schemes such as the White Rose Project at Drax look promising. Coal may therefore remain a core part of our energy mix, but will be significantly cleaner, more efficient and provide high quality PFA and FBA as by-products.
9. How is PFA and FBA used in the UK?
Coal ash is used in a variety of building materials, from filler, cement and grouts to highway construction and block making. The UKQAA publishes data on the different applications of PFA, FBA in the UK, which is available here.
We no longer share figures on total production due to the difficulties of interpreting the data.
10. How much does PFA or FBA cost?
It’s not possible to give a standard price for PFA or FBA as it depends on a number of factors including the location of the contract, the proximity of the power station, the availability of the material, the type of material required, the quantities required and the time-scale of the contract.
The UKQAA is not a commercial organisation and only promotes the technical aspects of the material. In order to obtain current prices, please contact the members of the UKQAA directly.
11. What’s happened to BS3892 PFA and what is BS EN450-1 & EN450-1?
As with all cement products, coal-ash based cement products are defined by agreed industry standards. These documents seek to standardise the composition, specification and conformity criteria for various coal-ash based cement products and help to clarify what levels of coal ash are contained in particular products, what its properties are and therefore what applications it can be safely used for.
BS3892 Part 1 (PFA for concrete) was withdrawn on 1 January 2007. This followed a lengthy process to agree a harmonised European standard for PFA/fly ash, resulting in BS EN 450-1 and BS EN 450-2.
BS EN450-1 & BS EN450-2
BS EN450-1 gives the specifications and categories for PFA/fly ash for use in concrete.
The finer material, previously identified as BS3892 Part 1, is now called EN 450-1 Category S fly ash.
In addition there is a Category N fly ash to BS EN 450-1 which is also available in the UK, however, this does not have the guarantee of water reducing properties as with other categories of coal ash.
Further details about the changes can be found in our Technical Datasheet No.1.1
BS EN450-2 describes the conformity evaluation, which defines the quality control and statistical compliance.
12. How much PFA can I put into a concrete mix?
As a type II addition, with cementitious properties, the proportions of PFA/fly used in concrete are generally determined by EN 197-1, for factory made cements, or BS 8500 Parts 1 and 2 for mixer blended concretes. Up to 55% of Portland cement may be replaced by an EN 450-1 fly ash.
PFA/fly ash may be used in concrete as a type I addition, as an inert filler aggregate. Many coal-fired power stations can supply PFA/fly ash as lightweight filler aggregate to EN 13055-1.
High Volume Fly Ash concrete (HVFA) is being used in the USA with up to 70% replacement of Portland cement by PFA/fly ash. This technology reduces the environmental impact of manufacturing concrete. HVFA has also been used in the UK for secant piling applications and has proven very successful.
If you would like more information or examples of the use of PFA in various concrete products, please get in touch.
13. What is the ‘drainage blanket’ for beneath PFA when used as a fill material?
The drainage blanket beneath PFA-based fill material prevents the filler from leaching ground water.
The UKQAA recommends the use of drainage blankets or capillary breaks when using thin layers of PFA-based filler because PFA, like any fine grained material, will tend to draw water from the ground beneath by capillary suction. This water could rise to approximately 600mm and could cause stability issues.
However, where thick layers of PFA-based filler (greater than 2m) are to be used, it has been shown that this precaution can be removed and a stable fill achieved.
The use of PFA as a fill material is covered by an Environment Agency regulatory Position Statement in England, Wales and Northern Ireland.
Please note, the Specification for Highway Works requires a 6D granular fill as a starter layer beneath PFA fill.
For more information, please see Technical Datasheet No.2.
14. What is the steepest slope that can be created using PFA as a fill material?
The maximum slope used in recent construction projects to our knowledge has been 45 degrees, used at the Black Country Route in the West Midlands.
The suitability of ash-based products for such an application depends on a variety of factors. The moisture content, source, degree of compaction and shear strength of the PFA all determine the degree of slope that can be created using PFA-based fill material. It is important to note that as PFA gains strength with time due to the pozzolanic reaction, the shear strength of a PFA-based product can vary significantly between ashes. For typical shear strength values, please see our Technical Datasheet No. 2.
When PFA/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. Vertical faces can be achieved safely by the use of geogrid for reinforced earth applications. Such grids have to be resistant to the high alkalinity of PFA. For further information on suitable geogrid materials, please contact the manufacturer.
Environment & Health
15. What is the environmental impact of using PFA as a fill material?
There are a number of environmental benefits to using PFA as a fill material based both on the process by which it is recovered compared with other aggregates and its natural properties which make it a safe and practical building material.
When used as a fill material, PFA usually replaces virgin aggregates. As the extraction and processing of such aggregates is a resource intensive process, producing approximately 21kg of CO2 per tonne of aggregate recovered, using PFA can significantly reduce the overall environmental impact.
As coal is a naturally occurring material, PFA contains similar trace elements as found in soil. The glassy matrix of the PFA particle means that there is limited scope for leaching. PFA is a stable, durable and non-harmful aggregate for fill materials in a variety of environments. Its low permeability is further reduced in compacted PFA products.
More details are given in the Nottingham University Leachates from Embankments project report. Also see the Quality Protocol page.
16. What is the environmental impact of using PFA in concrete?
There are a number of environmental benefits to using PFA in concrete based on the reduced need for other process-intensive aggregates and its intrinsic properties which make it a safe, and practical building material.
PFA is pozzolanic and has significant cementitious properties. As such it reduces the quantity of Portland cement required to create concrete. For every tonne of Portland cement made, approximately 900kg of CO2 is produced. By replacing 30 per cent of the total cement content with PFA, the overall environmental impact has been shown to be reduced by at least 17 per cent for the same 28 day strength.
With some types of concretes (See BS EN206-1 and BS 8500) reductions in emissions of greater than 50 per cent are possible. With High Volume Fly Ash (HVFA) concrete, which contains between 55 and 70 per cent PFA, or when the specified strength is at 56 days or more, even greater reductions in the environmental impact are achievable.
As with PFA-based filler, PFA in structural concrete ensures that there is a considerable reduction in leaching potential of the concrete and the pozzolanic reaction reduces permeability, preventing any significant leaching from either the PFA, Portland cement or the other aggregates.
17. What is the environmental impact of using PFA grouts?
The key environmental benefit of PFA-based grout is – like PFA-based fill and concrete – the limited risk of leaching. The leachates are trapped by the hydration of the Portland cement and the pozzolanic interaction.
However, it is important to bear in mind that during the construction process, any bleed water from the grout may have environmental consequences, especially if close to high quality aquifers.
If you require more detail on the leachates associated with the PFA, then please contact us.
Under the Quality Protocol for Bound and Grouts, best practice is required for PFA grouts, which refers to BR 509, the BRE Environmental Code of Practice for Grouts. This contains a risk assessment procedure, which defines when a grout can be shown to be compliant, ceases to be a waste.
18. Is there much sulfate in PFA? What class of sulfates for the concrete in foundations is required?
Yes, typically fresh PFA contains between 0.35 and 2.5 per cent sulfate as SO3. This puts the material in Design Sulfate Class 2 or 3 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. However, in practice there is no known case of such attack in concrete adjacent to PFA. In addition, in concrete, the low solubility of the PFA 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, for example mild steel, and this is why the Specification for Highways states that PFA should not be placed within 500mm of metallic items, such as over culverts.
Lagoon PFA, which effectively has been washed with copious amounts of water, has a very low sulfate content, less than 0.1g/l, which would equate to DC 1 sulfates.
It is normal to either protect concrete from sulfate attack by using a drainage layer and bitumen sealing coat, for example, when building embankments adjacent to retaining walls. 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 sulfate content of the ash is lower, e.g. lagoon material or similar.
19. 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 and aluminium or lead, PFA will also react with it in the presence of water. In addition PFA can contain sulfates such as gypsum (see Question 18) and it will attack carbon steel, though not stainless steel.
20. Does the chromium VI directive on cement effect PFA?
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 per cent (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 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.
For more information about chromium VI, see the Mineral Products Association website, including the timetable of implementation and advice on reducing agents.
21. REACH – how does this affect the industry?
As a chemical substance, coal ash is subject to REACH – a European Union regulation concerning the Registration, Evaluation, Authorisation & Restriction of Chemicals.
For more information about REACH, please visit the REACH page.
22. I want to import PFA (Fly ash) into my country, who should I contact?
If you need assistance on locating producers or suppliers, please contact us.
23. I / my company would be interested in becoming a member of the UKQAA?
If you are interested in becoming a member, please visit our Members page to find out more about the different levels of membership we offer and to view our full lists of members. If you would like to apply for a membership, please visit the Join page.