Fly ash to construction assets

Materials World magazine
2 Aug 2016

How can ashes produced by coal-fired power generation boost sustainable construction? Dr Robert Carroll explains.

Coal ashes are by-products of the combustion process at coal-fired power stations. Following decades of research and development into possible uses, these are now regularly used as alternatives to virgin raw materials in the manufacture of construction products, including concrete, bricks and blocks, grout, structural fill and land reclamation. Ultimately, this safeguards finite virgin raw materials and makes use of an available secondary resource. As such, the use of coal ashes can provide a range of technical, practical and important environmental benefits for the construction and engineering sectors.

As shown in the 2016 Concrete Industry Sustainability Report, the alternative materials market is becoming increasingly important in creating a sustainable, low-carbon materials supply chain for the construction sector – and coal ashes are a key ingredient.

The production process

In the UK, the energy generation process at coal-fired power stations involves burning pulverised coal in a furnace to generate steam. At temperatures of around 1,400°C, two materials are obtained from the molten mineral residue of the coal particles – fly ash or pulverised fuel ash (PFA) and furnace bottom ash (FBA). 

Fly ash is the fine material carried out of the furnace with the flue gases following the combustion of pulverised coal. It largely comprises microscopic spherical particles of alumina-silicate glass, which contain minor amounts of iron. Its composition depends on a variety of factors including the combustion temperature, type and fineness of coal, and the length of time the minerals were retained in the furnace. 

Around 80–85% of the ash produced is PFA, which is subsequently extracted by mechanical and electrostatic precipitators and stored in silos as a dry powder. Alternatively, it is conditioned by adding water for transport to stockpiles or made into a slurry and pumped to lagoons. The remaining 15–20% of the molten material forms a slag layer on the internal surfaces of the furnace, which falls to the bottom as FBA and is flushed out periodically with water. It is then passed through a crusher and delivered to ash pits to drain. FBA is a granular, glassy material and its composition is comparable with fly ash.   

Manufacture of quality materials

Several grades of high quality coal ashes are supplied to the construction industry. PFA produced to European Standard EN 450, in particular, is a much sought after constituent for concrete. It can be used as a partial replacement for Portland cement, reducing the amount of water involved in its manufacture and improving the cohesiveness of the wet mix. The setting concrete generates less heat, which prevents thermal stress and reduces the incidence of cracking – particularly important in large volume pours. A high-strength, low-porosity and durable concrete is produced with low migration rates of chloride ions and other deleterious species through the matrix. 

EN 450 PFA is obtained by testing, selecting and segregating dry ash of the required specification in storage silos. Ash may also be processed, or beneficiated, to achieve the necessary characteristics. For example, electrostatic separation can reduce the amount of unburnt carbon and size classification will increase fineness.  

Both FBA and PFA can be produced as lightweight aggregates to European Standard EN 13055-1. FBA is used extensively as granular lightweight aggregate for concrete, particularly concrete blocks, while PFA is used as a fine lightweight aggregate. However, a large amount of EN 13055-1 compliant PFA is supplied as the main siliceous constituent of autoclaved aerated concrete. PFA is also used to manufacture sintered fly ash aggregates, such as Lytag, which is used as a lightweight aggregate. Hydraulically bound mixtures (HBM) are often used in road construction or for soil stabilisation. Dry or conditioned PFA to European Standard EN 14227-4 can be used for these applications.

Coal ashes can enhance the technical quality of construction products and help to create high-performance building materials suitable for a range of uses – from road construction and bridges to tunnels and flood defences.

Improving environmental efficiency

While technical performance and a variety of applications are important, in terms of safety, reliability and usability, the practical and environmental benefits of using coal ash-based products and materials are key factors driving uptake. 

The construction and engineering sectors have made wide use of PFA and FBA for a number of years, but in these environmentally-conscious times, the sectors are increasingly governed by regulation targeted at improving the sustainability of infrastructure and materials used to build, striving to reduce embodied carbon and promote sustainable sourcing.

A significant environmental benefit of using PFA is a reduction in the amount of Portland cement used in concrete. All the constituents of concrete consume large amounts of energy during the extraction and manufacture of raw materials. In particular, the calcination of limestone or chalk during the manufacture of cement clinker emits substantial quantities of CO2 directly into the atmosphere.  

By contrast, PFA is relatively easy to access from silos, stockpiles and lagoons and is more resource efficient to manufacture. As a result, concrete made with a substantial replacement of Portland cement by PFA has up to 25% less embodied carbon compared with conventional concrete. Similarly, FBA can also be used effectively as a low carbon alternative material in the manufacture of lightweight aggregate and concrete aggregate blocks, with comparative environmental savings.

Aside from the processes involved in extracting and manufacturing materials used to make construction products, sustainable sourcing is vital in today’s materials marketplace. Demand for materials is high and its consumption needs careful management and planning. All supply streams have come under increased pressure to meet the construction industry’s growing appetite, and as new quarries continue to be one of the most unpopular forms of development in the UK (according to the recent Development Intelligence report), the marketplace needs to retain a diverse, sustainable supply mix.

Coal ash is produced by a number of existing coal-fired power stations across the UK and, thanks to our strong coal-fired heritage, we’ve also amassed surplus coal ash over the past half century. This has led to stockpiles of up to 50 million tonnes of potential additional material – possibly enough to meet demand in the construction and engineering sectors for up to 20 years, should all production cease immediately.

The use of this stockpile ash is a relatively recent development in the UK but is already proving the capacity that power stations have for supplying additional sources of building material. For example, Tilbury power station in south-east England ceased operation in 2013 but, thanks to an ash recovery programme that extracted and screened ash stocks lying dormant in the ash fields, the station produced around 500 tonnes of usable recovered ash a day. Much of this material was used to produce aerated concrete blocks. 

The ash market offers increasingly diverse supply routes and a strong appetite for innovation. This is helping to make the most of available material and extend our understanding of how material diverted from the waste stream can be used to beneficial effect. Whether it’s the Channel Tunnel, the Shard, the Burj Khalifa or London’s Crossrail – projects across the world have gained from the technical, practical and environmental benefits of coal ashes. These materials have been used to create stunning design, robust architecture and sustainable buildings. Ultimately, they contribute significantly to the sustainable future both the industry and government strive for – and we the ash industry are eager to take on the challenge.

Dr. Robert Carroll joined the UKQAA in October 2012 as Technical Director. He has more than 30 years’ experience in the concrete industry, including 27 years with Hanson Building Products as Head of Concrete Services.

Robert has responsibility for representing the UKQAA and its membership to industry, actively promoting the use of coal ash and biomass ash from UK power stations.