On the fly - fly ash can replace aggregates

Clay Technology magazine
11 Feb 2013

Dr Robert Carroll from the UK Quality Ash Association explains the vital role of fly ash in a period where raw aggregates are becoming increasingly rare.  

In early October 2012, the Minerals Product Association (MPA) published its Annual Mineral Planning Survey where it highlighted a number of worrying issues facing the minerals sector as well as the wider construction industry. According to the survey, UK Government planning inertia risks harming the development of sustainable extraction industries by failing to provide sufficient land for aggregate mining. As a result, there is an inadequate supply of raw material across the country.  

As the keystone of construction, aggregate supply is a critical issue. However, with less than 50% of sand and gravel reserves and less than 67% of hard rock reserves replenished in the last 10 years, it could strike a blow to hopes of a construction recovery in the next few years. With the construction industry being the UK’s only major sector still in recession, the MPA’s call for more clarity from Government over planning for future aggregate reserves should be music to the ears of many.   

A change in planning laws to encourage the resupply of aggregate is no mean feat. It requires improved regulation from Government and greater investment from minerals companies – neither of which will happen overnight. In the short term, this leaves the sector with a lack of available raw aggregate, which could hamper recovery and leave the construction industry exposed to sudden and sustained uplift in demand. Nevertheless, alternative materials are available that could make up the shortfall and deliver the quality, consistency and durability that the sector demands.  

Secondary aggregates such as fly ash make a lot of sense – and we’re not using enough of them. Flexible, environmentally friendly and readily available across the UK, alternatives such as high quality fly ash can be used as a replacement aggregate in a range of products. Fly ash is the residue from the combustion process used at coal-fired power stations. Once pulverised coal is burned within the boiler, a series of filtration systems collect fine ash particles from the exhaust stream in hoppers, from where the material is transferred to storage silos. This fine product is fly ash, or pulverised fuel ash (PFA) as it is known in the UK. Combustion residues that contact the boiler surfaces form a glassy layer that is cooled rapidly, forming furnace bottom ash (FBA), a granular aggregate.  

Fly ash has an increasingly wide range of uses in the UK, and is used in load-bearing fill, lightweight aggregate, ground stabilisation, hard standings and grouting. It can also be used in more specialised processes, including brick manufacture. Here in particular, PFA can be used instead of raw material to create lightweight, low-carbon bricks that require less energy to manufacture. Given Government’s current commitment to boosting UK housing development, it offers huge opportunity for the sector.  

At the moment, however, it is concrete manufacture that accounts for the greatest use of fly ash. It can be added to the concrete mix instead of raw aggregates to bring about a number of significant performance improvements. For example, it is more cohesive and easier to work, and mixes can be achieved with reduced water, giving increased strength and high durability. Contractors can also achieve a better surface finish, with crisper corners and cleaner edges. Reduction in the overall weight of the final product may also be achieved by using FBA as a lightweight aggregate. As a result, major highrise projects, such as the Burj Khalifa in Dubai and the Shard in London, have both used fly ash to enhance strength and stability in demanding conditions.  

Green ash you like  
This is good news, but what contractors have also realised are the significant environmental benefits available through the use of fly ash-based concretes. As one of the world’s most energy and resource intensive industries, the construction industry is under increased pressure to improve its environmental credentials. In 2008, the UK Quality Ash Association (UKQAA), along with a number of partnership organisations, helped develop the Concrete Sustainability Strategy. This seeks to reduce the sector’s environmental impact without harming product quality or consistency, and targets a 30% reduction in CO2 and a 90% reduction in waste, and ensures that 95% of all concrete is accredited to BRE standard BES 6001.  

These are tough but necessary targets and many manufacturers and contractors are finding fly ash helps deliver against the Strategy. As well as reducing the amount of water required in the mixture, we limit the need for natural resources. In fact, studies conducted by the UKQAA show that fly ash can be included in the concrete mix at rates of up to 80% of the cement content, further reducing the need for raw material and water. This also has a direct impact on the carbon footprint. While cement creates around one tonne of carbon in the manufacturing process and contains 913kg per tonne of embodied carbon, fly ash contains a mere 4kg per tonne.  

In major concrete-intensive developments, these figures can have a huge impact on the overall sustainability of the finished project. The Shard, for example, uses its high proportion of fly ash content to target a BREEAM Excellent rating – a dramatic achievement for such a significant part of the London skyline. However, these achievements are nothing unless backed by a comprehensive set of quality guidelines. In the UK, work carried out by the UKQAA and the Waste and Resources Action Programme means that fly ash is extremely well controlled, due to the Waste Framework Directive and the Quality Protocol for PFA and FBA. To ensure consistency of fly ash and FBA, these standards exist to monitor the control, transportation and use of all ash products, ensuring consistency of the material and ultimately giving end users complete confidence in its application. These quality assurances are vital and provide a framework for the increased use of fly ash, proving it is a viable alternative (or addition) to natural, raw aggregates on a global scale and in globally recognisable projects.  

While the MPA’s survey highlights some serious concerns for the future of the construction sector, we mustn’t lose sight of other options that are available. With an increasing need to balance economic growth and an ever-developing population with sustainability, materials such as fly ash can help the construction industry to reduce its impact on the environment while building just as effectively.  

For further information, contact Dr Robert Carroll, enquiries@ukqaa.org.uk