The ash alternative

Ashes produced at coal-fired power stations can improve the sustainability of construction and help create low carbon infrastructure. Dr Robert Carroll explains how

Concrete is one of the most widely used construction materials in the world. However it’s also one of the most resource intensive products to manufacture. Herein lies the challenge for the construction and civil engineering sectors – how can we create a truly sustainable built environment, without incurring the adverse effects of embodied carbon?

This is a question that has long been at the forefront of the industry’s mind – and as the representative of some of the industry’s largest construction product manufacturers and suppliers, UKQAA knows how important it is to specifiers, engineers and architects.

In 2008 a group of sector bodies – including UKQAA – joined forces to create the Concrete Industry Sustainable Construction Forum, producing a strategy for reducing the environmental impact of manufacturing concrete. By monitoring and reporting on trends in the industry the forum is helping to drive improvement and highlight opportunities for sustainable growth.

In the 2016 Concrete Industry Sustainability Report – the 8th in a series of annual performance reports – one of the most striking findings is the role that the alternative materials market is playing in creating a sustainable, low carbon materials supply chain. As projects like the Stirling Prize winning Everyman Theatre in Liverpool lead the way in its use of alternative materials, there’s a clear message about the value of using secondary materials like fly ash or furnace bottom ash in concrete to boost sustainable construction.

Ash opportunities
Fly ash (pulverised fuel ash or PFA) and furnace bottom ashes are produced as a result of the electricity generation process at coal-fired power stations. Fly ash is the material carried out with the flue gas following the combustion of pulverised coal, while furnace bottom ash (FBA) is slag deposited on the walls which collects at the bottom of the boiler and is flushed out periodically with water.

Due to its pozzolanic nature, fly ash complying with European Standard EN450 recovered can be used as an effective partial replacement for Portland cement in the manufacture ofconcrete, bricks and blocks – improving durability and reducing susceptibility to chemical attack. While this significantly enhances the technical performance of the end product, crucially it also reduces demand for virgin raw materials like sand and stone.

These more traditional materials are finite and resource intensive to extract and manufacture, meaning they’re less sustainable to source and produce and have higher embodied carbon as a result.

By contrast fly ash – and furnace bottom ash – are bytonnes products of energy generation, available from both operational power stations and stockpiles across the UK and require limited processing. As such they significantly improve the sustainability of construction and engineering products by reducing embodied carbon and making these products more environmentally friendly.

For example, while Portland cement has around 900kg/tonne of embodied O2, a typical fly ash cement has around 670kg/tonne. This means fly ash can reduce embodied carbon by up to 25 per cent when compared with a traditional cementitious mix. Similarly, furnace bottom ash can also be used effectively as a low carbon lightweight aggregate and in concrete, particularly within aggregate blocks.

These examples demonstrate not only the breadth of potential uses for coal ashes but also their environmental benefits – a valuable advantage in today’s environmentally conscious times.

Sustainable sourcing
In the UK we’ve a strong heritage in coal-fired generation and we believe stockpiled ash alone now amounts to some 50 million bytonnes. This surplus material may offer the industry opportunities for additional supply routes, and could potentially support demand in the construction industry for almost 20 years, even if all production ceased today.

To make the most of available stockpiles, the UKQAA is supporting a research and development project, the Innovative Processing for Stockpile Fly Ash Project, at the Concrete Technology Unit, University of Dundee. This is seeking to develop a process route by which stockpile material can be transformed into EN450 fly ash for use in quality concrete such as structural concrete. While this project is currently ongoing, we’re confident the results will prove critical in terms of our understanding of this valuable additional material and its potential uses.

As the Concrete Industry Sustainability Report highlights – fly ash is a valuable alternative material, and one which is becoming increasingly appealing to a sustainability-focused construction and civil engineering sector. It offers real opportunities for carbon savings without compromising on quality or performance. Ultimately, it’s a key ingredient to low carbon infrastructure and a more sustainable future.

Dr Robert Carroll works at the UK Quality Ash Association (UKQAA). The UKQAA represents the interests of UK producers and users of coal fired power station, co-combustion and biomass ash products. The aims of the UKQAA are to promote the scientific, technical, industrial, environmental, educational and legal nature associated with applications for ash produced from UK power stations.

For more information, please see