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Rising From the Ashes

Published by
World Cement,


Introduction

Quality ash could not be more important to the construction industry. As well as providing vital technical and structural benefits in applications that range from soil stabilisation to the partial replacement of Portland cement in concrete, the fact it is a byproduct has benefitted an industry that has worked hard to improve its sustainability.

However, the shift away from high-carbon to low-carbon sources of energy generation means the main way to produce ash – coal power – is being phased out, and phased out quickly. In November 2015, Amber Rudd, then the UK Energy Secretary, announced plans that would see coal-fired power plants closed by 2025, with restrictions put in place by 2023.

If that feels like it is some time away, then you would be only half right; the shift towards low-carbon sources is already having an impact on the coal industry. As recently as 7 December last year, offshore wind accounted for 20% of UK energy demand and, in the summer, half of all UK electricity generated came from low carbon sources of power. The impact is that, even with seven coal plants still operating, they are not necessarily running at full capacity.

The decarbonisation of the energy industry is an important way to meet the UK’s environmental targets, but doing so may actually end up making it harder for the construction industry to help combat climate change. This appears paradoxical, but the figures are worth reflecting on. A 2015 Concrete Centre report noted that, while Portland cement (CEM I) had an embodied CO2 of 913 kg/t, flyash carried around 4 kg/t. As a result, even using relatively low levels of flyash in the concrete mix can significantly reduce the embodied carbon of a finished building and contribute towards environment targets set by clients, as well as divert a material from the waste stream.

However, alongside the environment and technical benefits of flyash, it has also been a cost-effective material. While the UKQAA is not involved with the pricing of flyash, it fully expects the market to remain competitive.

Reasons to remain positive

There are some excellent reasons why flyash is likely to remain a valuable constituent of construction products. While it remains true to say that the immediate production of ash in the UK will drop, as is already being seen, demand for it remains very high, and that encourages innovation. The first major change to the market is likely to be an increase in ash imports, something the industry has not traditionally needed to do.

Despite the fact that coal-fired power is closing here in the UK, that is not necessarily true elsewhere in the world. Even within the EU, coal power is an important part of the energy mix few countries will, or can, abandon quite yet. In Germany, for example, the decision to move away from nuclear energy in 2011 means ash is still widely produced and available for export to new markets. Elsewhere in Europe it is a similar story, but there are also markets further afield, such as China, India, and the US, and globally the amount of ash produced currently exceeds demand.

However, there are technical issues to consider. Within Europe, EN450 is a recognised standard for high quality flyash that can be used as a pozzolana in the concrete mix, but further afield the same specifications may not apply. Furthermore, in the UK, the production, storage, and processing of flyash is carefully regulated to ensure that both producers and users protect the environment, while still being able to exploit the benefits the material can bring to the construction industry. It is important to ensure that high-performance specifications and environmental standards are maintained and adhered to. Ultimately, this remains the aim of the UKQAA: to provide technical advice and guidance on the use of flyash.

However, if imports and the potential technical and logistical challenges sound like too big a change for the industry, then there is a danger that the benefits of flyash are being underestimated. While the economics and environmental benefits are part of the story, it does not end there. For almost 2000 years, the technical benefits of using pozzolanas, such as ash, in concrete have been well-known. The most dramatic example of this is the Pantheon in Rome, but ash-based concretes are widely known for their ability to remain durable, even when used in aggressive environments.

For example, Portland cement (CEM I) in concrete produces lime, which can react with sulfate ions and form products that cause deterioration. By contrast, flyash in the mix combines with the excess lime and makes less permeable concrete, reducing the movement of sulfate ions. The overall result is a reduction in expansive pressures, stronger concrete, and increased sulfate resistance. In applications such as road, rail, and offshore developments, such as wind turbine bases, these sulfate resisting properties are critical and are not easy to replace.

It is for these reasons that imports are likely to become more significant in the UK supply chain, but there is a growing move towards ash extraction from stockpiles at coal-fired plants and various sites across the UK. While demand for ash has long been high, over the past 50 years more ash has been produced than the construction industry could use and, until recently, around 2 million t of ash have been added to national ash stockpiles per year. Today, it is estimated that there is around 50 million t of stockpiled ash material, which represents huge potential for the ash and construction industries, from the point of production and manufacture right through to design and specification.

Because much of this ash is historical and storage techniques vary, quality is likely to be an issue, so the UKQAA is supporting a project at the University of Dundee’s Concrete Technology Unit that is looking closely at how stockpiled flyash could be recovered and used in cementitious applications. In particular, the aim is to find ways to recover as much of this material as possible to meet EN450 specifications. This involves taking samples from ash fields across the UK and assessing key properties as pozzolanas when used as a partial cement replacement in concrete, as well as investigating methods of improving performance where quality is an issue. This work will be vital in exploring the potential for stockpiled ash in the UK, but the good news is there is already precedent for using ash from storage. In the US, ash extraction and recovery has had a successful recent history and, even in the UK, the ash piles held in storage at the former Tilbury power plant in Essex have been recovered for use in concrete blocks.

Generation at Tilbury, a former RWE plant on the bank of the Thames, ceased in 2013, after nearly 50 years of operation. Rather than decommissioning and landscaping the ash fields, RWE Generation took the opportunity to make use of the surplus flyash stock lying dormant in the ash fields by developing a process by which stockpiled flyash could be used to support well-established supply chains. By carefully excavating, screening, and testing the ash recovered at Tilbury, the team was able to process ash to the point it met EN 13055-1, the industry standard required for lightweight aggregates in concrete. The recovery programme was then able to supply around 500 t of quality ash a year directly to H+H, enough for around 20 million aerated concrete blocks a year.

Conclusion

This year will be a challenging year for a lot of reasons, but the UKQAA is confident that, as this period of transition is entered, the benefits of flyash will ensure demand remains strong. Work, such as that undertaken at Tilbury and at the University of Dundee, shows that innovation and ingenuity have a role to play in maintaining flyash as a valuable asset. Not only can this future-proof supply and sustain confidence in the market, it also shows there is life beyond the decommissioning of a coal-fired power plants, thereby breathing new life into old infrastructure to develop and support supply chains. How this affects the industry will have to be seen, but it is unlikely that this internationally important material will vanish anytime soon.

Read the article online at: https://www.worldcement.com/special-reports/27032017/rising-from-the-ashes/


 

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