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A phoenix from the ashes

17 Feb 15 As the supply of fresh fly ash gradually dries up, suppliers and consumers are exploring ways of exploiting historic stockpiles. David Taylor reports

There is an estimated 50 million tonnes of material stockpiled in UK ashfields
There is an estimated 50 million tonnes of material stockpiled in UK ashfields

Burning coal for power is bad for the environment – which is why for years now, government policy has been to discourage coal-fired power generation and switch to nice clean, green renewables...except that coal still accounts for almost a third of all our domestic power generation. While that continues, coal-fired power stations will continue to belch out carbon emissions. They will also continue to produce thousands of tonnes of ash every year. This ash needs to be got rid of and so most of it is buried in ashfields adjacent to the power stations, as well as in landfill sites. But a proportion of this ash is used in construction – as fill and also as a pozzolanic additive to replace a proportion of cement in concrete mixes.

Fly ash, also known as pulverised fuel ash, or PFA, is in fact an important raw material for some construction products and processes. And while it’s nice to know that it can be put to good use, some of the businesses that rely on it as a raw material are understandably nervous about future supplies as coal-fired stations give way to cleaner alternatives. That is why the UK Quality Ash Association – a trade body representing both producers and consumers of this material – is undertaking a research programme to investigate the viability of recovering and using some of the millions of tonnes of ash stockpiled all over the UK.

The research programme, undertaken in partnership with the Concrete Technology Unit (CTU) at the University of Dundee, is both identifying potential sources of stockpiled ash and testing samples for suitability as a partial cement replacement in concrete.

The CTU is also working with the UKQAA to develop processes for transforming stockpiled ash into PFA that meets the specification of EN 450, the recognised standard for the use of ash in cement and concrete.

If successful, the research could significantly boost supplies and allow the industry to make greater use of a valuable secondary material – cutting carbon dioxide emissions and reducing the need for primary raw materials in cement and concrete production.

“This is a really exciting opportunity,” says Dr Robert Carroll, technical director at the UKQAA. “Fly ash demand continues to rise to keep pace with a growing construction sector but availability is dependent on our use of coal power.

“Unlocking the potential of stockpiled ash in ashfields across the UK could increase supply, meet construction demand and exploit an otherwise underused material,” he adds.

There are certainly some powerful environmental arguments in favour of mining these neglected ashfields. Although the ash itself is the by-product of a carbon-intensive process, further carbon emissions (and further quarrying activity) are minimised when fly ash is used instead of cement.

And of course, the more ash that is consumed in the construction industry, the less there is to end up buried in the ground. The material buried in these ashfields has not always languished unused, says Carroll. “They’ve always existed and people have always used them for standard fill – for example in big embankment jobs.” Fly ash is the very fine material extracted from flue gases by precipitators located in the power station chimneys. It is taken from these precipitators and conditioned, with the addition of about 10% water, before being taken to the ashfields or delivered to the end-users.

“The power stations produce more than the construction industry can absorb, so a proportion is always stockpiled,” says Carroll. The excess material is therefore laid down, usually on land adjoining the power station, covered with soil and grassed-over. Many of these ashfields are now several decades old and some now have amenity value as part of a restored landscape. “Barlow Mound in Drax is prime example,” says Carroll. “It’s been there for more than 25 years and is now a nature reserve. We’d never be allowed to dig that up.”

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But other ashfields are available and easily accessible. As an example, Carroll cites Tilbury Power Station in Essex. This major coal-fired power station was a vital source of fly ash for a number of decades and its closure in August 2012 was bad news for the manufacturers and suppliers that depended on it for their raw material. However, there is a substantial stockpile of ash on the site and now, thanks to a recovery programme, surplus ash from the existing ashfield is being extracted, screened and tested for reuse in construction. If it’s going into cement or a concrete mix, the ash needs to conform to a rigorous standard (EN450) which means it must be dry. Members of the UKQAA therefore take their ash straight from the power station and store it in sheds or silos. Even the cleanest, driest material excavated from an ashfield won’t conform to this standard, says Carroll.

As part of the current research project, scientists at Dundee University are exploring methods of processing this reclaimed ash and drying it efficiently. “It’s not as simple as it sounds, because we need to find methods that don’t contribute too much to CO2 emissions,” says Carroll.

However, when used in manufacturing – principally in the production of aircrete blocks - the material does not have to conform to EN450. Aircrete blocks are cured in an autoclave at a temperature of 184oC. Ash from Tilbury Power Station has recently been used as fill in projects including the widening of the M25 motorway and is currently being used to make aircrete blocks by manufacturer H+H Celcon.

“Fly ash is a vital construction material and key constituent of sustainable concrete products,” says Carroll. “As a result there is a well established fly ash supply chain in the UK, with a consistent demand for raw material. With up to 50 million tonnes of fly ash stockpiled in the UK, this project has the potential to not only sustain a well established ash market but also create new ones.”

Research partners

The research project is jointly funded by the UKQAA and the Engineering & Physical Sciences Research Council and is expected to run for three and a half years. Based at the laboratories of the University of Dundee, the research programme is being run by Dr Michael McCarthy leading a team that will work in close partnership with the UKQAA and its members. “We have carried out research on fly ash in construction for more than 25 years,” says McCarthy, “and this project to investigate recovery and processing of stockpile material is in keeping with much of our other work concerned with broadening the scope of its use. We  look forward to our role in this important area.”

This article first appeared in the February 2015 issue of The Construction Index magazine. To read the full magazine online, click here.

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