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Crunching carbon | The dash for ash

PFA being tipped at Celtic Manor Ryder Cup practice area Cropped

Ash for concrete is in increasingly short supply, which means new challenges for suppliers seeking to cut carbon.

Used in cement, autoclaved aerated concrete (AAC) blocks, and a variety of other applications, fly ash, or Pulverised Fuel Ash (PFA), is an important material for the construction industry. But the closure of coal fired power stations has seen a reduction in the amount of material available, and presents a serious challenge for suppliers.

In November of last year, then UK energy secretary Amber Rudd announced a consultation to “set out proposals to close coal by 2025 – and restrict its use from 2023”. During 2016, three large coal plants were switched off. And for the remaining seven, time is running out.

Coal is by far the most carbon intensive fuel, with gas power producing half the emissions while generating the same amount of electricity. The UK is bound by law to reduce its carbon dioxide emissions to 80% below 1990 levels by 2050, and, as Rudd argued: “One of the greatest and most cost-effective contributions we can make to emission reductions in electricity is by replacing coal fired power stations with gas.”

Ironically, as the government strives to meet its sustainability targets, many in the construction industry may, as a direct consequence, struggle to meet theirs.

Coal fired power stations produce PFA as a by-product of generating electricity. The material undergoes a pozzolanic reaction when mixed with cement and water. In simple terms, this reaction enables PFA to act as a secondary constituent for cement.

The UK Quality Ash Association represents many of the producers and users of coal-fired power station ash, and its technical director Robert Carroll believes that the sustainability of the product is one of its major assets: “There are significant environmental benefits, because you are using a secondary material, diverting a material that would have gone into the waste stream. It has a very low embodied CO2 value, because it is a by-product.”

Ratcliffe power stn

Ratcliffe power station

Ratcliffe is one of the last seven remaining coal fired power stations in the UK, pictured with ash stockpiles.

A report published last year by The Concrete Centre, part of the industry body Mineral Products Association, calculated that Portland cement had an embodied CO2 value of 913kg/t, whereas the same quantity of fly ash produced only 4kg. Effectively, using a cement which is a third fly ash, reduces the embodied carbon by almost a third.

Not only does PFA offer huge environmental benefits, it is also significantly cheaper than cement. But the decrease in the number of coal fired power stations in the UK has led to a shortage of a material that has been in abundance since the Victorian era. The inevitable result has been an increase in the price of the material.

Power Minerals is the UK’s leading independent ash marketer, and its sales and technical manager Ivan Skidmore acknowledges that “there has been more economic advantage in the past [for conusmers], when there was more material available.”

“It is important to make a clear distinction between EN450 quality material and other lower grades of ash,” continues Skidmore, “i.e. the cementitious quality ash for use in concrete and the other more general quality material for use in geotechnical activities or as fillers. Obviously there is a limit to how it can compete with key materials. However, there has been room for the EN450 prices to increase and still be competitive.”

graph Cropped  1

UKQAA graph detailing the various uses of ash

How different UK sectors used fly ash in 2014, according to the UK Quality Ash Association.

While the production of fly ash is being curtailed by the switch to cleaner energy, Power Minerals is investing in infrastructure that will allow PFA to be available more readily. Power Minerals managing director Nigel Waldron is keen to stress that the company’s key concern at the moment is availability rather than price: “In the past we have never really been in the position to offer a consistent all year round supply to the customers, but now we have invested in storage solutions and have got ash imported at a sensible price. We can sit down with customers and say, the price has risen but there is good reason for that.”

The importation of ash is figuring increasingly as a solution to the dwindling supply of the material in the UK.

Skidmore lists Germany, Holland, Italy, Spain and Portugal as the largest sources of imported ash.

“We import from western Europe mainly, but we do see the possibility of ash coming from further afield using much larger ships,” says Skidmore. ”[At the moment] we would bring ash in on ship sizes from 4,000t to 8,000t, but to go further afield we would need to bring in much larger shipments. That is one for the future.”

An ash drying facility at Lunen

An ash drying facility at Lunën

Source: Power Minerals

Th ash drying facility in Lunën Germany can process up to 200,000t of moistened ash a year.

Germany, to give one example, used coal to generate 45% of their power in 2014, whereas the UK’s coal fired plants produced only 29% of the country’s energy that year. This is partly due to the German public’s aversion to nuclear power, heightened by the Fukushima nuclear disaster in 2011, which has led to the government committing to close all nuclear plants by 2022.

How far in the future is a moot point. The problem with importing from other countries is one of scale and, as Carroll points out: “The demand, in the developed world, for fly ash is somewhat distant from where the maximum production is.”

Though PFA’s economic and environmental advantages are part of what creates demand for the material, they are not the sole benefits of fly ash. Carroll highlights its “many technical benefits. It increases the performance and makes a very durable concrete. For example, if you have an aggressive soil, particularly one that contains a lot of sulphate, using fly ash gives a resistant concrete.”

Perhaps the best testament to the durability of concrete that involves a pozzolanic reaction can be found a short walk from the Trevi fountain, where the Pantheon has survived 2,000 years, the Great Fire of Rome, and the Visigoths.

Ash stockpiles being recovered for use as lightweight aggregate Cropped

Ash stockpiles

Source: Power Minerals

Here, stockpiles of the material are being recovered for use as a light aggregate.

The sulphate resisting properties are just as, if not more, important, though they offer somewhat less spectacular examples, such as the M1-M6 junction. Concrete that uses CEM I produces an excess of lime, making it porous and chemically vulnerable to soluble sulphates. PFA combines with the lime making a stronger and less permeable concrete, therefore less susceptible to chemical attack.

The Romans used volcanic dust rather than PFA in their concrete, found near Naples at the base of Mount Vesuvius. Given the shortage of fresh fly ash, we in the UK may have to start looking to the ground also.

Coal has been burnt to generate power in the UK for well over a century, and the sheer amount of PFA produced means that large amounts of the material were stockpiled near power stations. If that stockpiled material could be excavated and used, it would go a long way to solving the shortage of ash in the UK.

“Our prime objective is to support the UK supply chain, and utilise every tonne of fresh fly ash that is produced” says Carroll. “But also, there is 50M tonnes of stockpiled material, which has got great potential for recovery. We are supporting a project at the University of Dundee looking at the technical challenges of recovering that material, and ensuring it still has the pozzalanic properties to be used in concrete.”

“It has been done in the United States, so it is not total blue sky thinking,” says Skidmore of recovering stockpiled ash “it’s whether it can be done economically, and we think that it can be done.”

The technical benefits of cement that contains fly ash means that demand is unlikely to disappear completely, but a material that has been taken for granted previously, is likely to be prized more highly in the future. Carroll even suggests “it is possible that fly ash might become an international tradable commodity.”

Readers' comments (2)

  • Fly ash has evolved from being merely a cheap filler to replace more expensive Portland cement to being a strategic material for the production of durable concrete. Fly ash, like other supplementary cementitious materials, enhances Portland cement performance yielding benefits not available when using only Portland cement. Because of this strategic importance to the production of high performance concrete recovery and processing of fly ash from landfills and surface impoundments makes sense where supply from operating power plants cannot meet market demand. International trade is on the horizon as well.

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  • I note much of the comment in this article refers to the benefits to the production of concrete, especially showing the saving in CO2. One of the main uses of PFA is shown to be AAC blocks used in structural drainage. Maybe this is a time to consider alternatives to concrete as a whole. Studies have shown that locally manufactured drainage geocomposites with high void ratio can be used instead of concrete blocks with overall cost savings and vastly improved whole life CO2 reduction, not least in transportation CO2 costs of heavy bulky concrete blocks.

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