Adding coal ash to the sustainable construction recipe

Dr Robert Carroll, of the UK Quality Ash Association, on the role coal ash can play in achieving low carbon building.

Dr Robert Carroll

Concrete, one of the world’s most widely used construction materials thanks to its technical performance and diversity of applications, also has high embodied carbon. The extraction and manufacture of Portland cement, the primary constituent in concrete, consumes a high degree of energy and resources, and produces significant amounts of carbon dioxide.

Portland cement is the single most carbon-rich material in a concrete mix, which directly affects the overall sustainability of the end product.

But in recent years, the industry has found that one way to lower this embodied carbon, without compromising on performance, is to substitute a portion of the Portland cement with fly ash. A practical, sustainable alternative to finite virgin materials, ash can be supplied as a locally sourced secondary material, which actually improves the technical performance of concrete.

Fly ash is a by-product of the combustion process at coal-fired power stations. It is produced at a rate of around five million tonnes annually, and is easily accessible from a number of locations across the country. A fine, consistent, pozzolanic material, fly ash can be used to create a wide variety of quality, durable concrete products.

When considered in the context of the need to conserve finite raw materials, fly ash by contrast is readily available, resource efficient and environmentally-friendly to produce. Its use therefore directly addresses the need for more sustainable supply chains and products within the construction industry, without compromising on performance.

There has long been a considerable demand for coal ash products in the UK construction and engineering sectors. The UKQAA recently produced its Ash Availability Report, which showed that since the turn of the century, the construction industry has typically consumed half of what’s produced annually. This rose to 70% in 2014 as the economy showed real signs of growth and the sustainability agenda became more important to specifiers.

In 2014, more than 50% of the fly ash produced was used for construction products, varying from the manufacture of cement, Type I and II concrete additions and concrete blocks to AAC blocks and grout. In addition, around 20% was used as a construction material in engineering fill or land reclamation projects.

The report also finds that of the 5 million tonnes of fly ash produced on average annually, approximately 40 % remains surplus. This equates to around 2 million tonnes a year, adding to existing stockpiles of up to 50 million tonnes of potentially usable fly ash across the UK.

As we look ahead to an increasingly sustainability-conscious construction industry, our focus is to develop and innovate. From diversifying supply chains to investing in research programmes, the ash industry has the strength – and requisite stockpiles – to plug supply gaps and future-proof against rising demand.

The UK QAA is running initiatves like the Tilbury recovery project, which reclaimed up to 500 tonnes of usable stockpile ash a day at its peak, as well as the ongoing Innovative Processing of Stockpile Ash Project at the University of Dundee, developing a method to enhance stockpile ash into high quality material suitable for use in structural concrete.

We’re confident that more can and should be done to exploit this valuable secondary material. This includes raising awareness about the benefits of using ash among architects, specifiers and end users and leading by example.

Product specifications and design choices drive sustainable construction. By understanding the technical and environmental benefits of fly ash, construction managers can make informed decisions and use the best materials to achieve their goal.

Robert Carroll is technical director of the UKQAA, which represents the interests of UK producers and users of coal-fired power stations, co-combustion and biomass ash products. It promotes applications for ash produced from UK power stations with scientific research, and educational and legal support. Its website is here