On the fly - improving the green credentials of concrete

Materials World magazine
1 Aug 2013

Concrete is one of the construction sector’s most innovative materials – big, bold and ideal for creative designers and architects. However, its manufacture carries with it some substantial environmental impacts. In a world with increasingly tough sustainability targets, is it still relevant? Dr Robert Carroll from the UK Quality Ash Association (UKQAA) says yes, and there’s never been a better time to use it…

Concrete is one of the world’s oldest building materials. Originally made using raw stone, sand or volcanic ash mixed with water, the Romans found concrete was a cheap and adaptable material that allowed them to create larger structures without the limitations of brick and wood.

Today, the global use of concrete is more than double that of steel, wood, glass and aluminum combined despite the growth of the steel-and-glass skyscraper. With economies in the west beginning to see improvement after a protracted period of recession, we can expect concrete use to increase as new building projects – roads, railways, and power stations – receive funding.

This can only be seen as positive news, because a healthy concrete sector is the backbone of a healthy construction industry. However, cement manufacture can be incredibly energy intensive and accounts for nearly 5% of global CO2 emissions – more than air travel. It’s also a vast consumer of natural resources, including quarried aggregate and water, which require sourcing, processing and transportation.

In response, in 2008 the Concrete Centre launched the Concrete Industry Sustainable Construction Strategy to counter perceptions of the sector and highlight opportunities to reduce the impact of industry on the environment. The UK Quality Ash Association – UKQAA – is one of the partners behind the Strategy and we’ve worked to encourage the use of fly ash in the cement and concrete production processes as one way to improve the green credentials of the finished concrete product.

As a secondary resource, fly ash is inherently sustainable. Produced as a result of the combustion process used at coal-fired power stations, fly ash – sometimes known as pulverised fuel ash (PFA) – is a very fine and consistent material. Rather than using processed raw aggregates, fly ash can be used in cement and concrete products at rates of up to 55%. This immediately reduces the need for raw materials and CO2 emissions to drop significantly, both in terms of quarrying and transporting materials, and also in the processing of aggregate.

Fly ash-based concrete also requires much less water because the fine,spherical fly ash particles pack between aggregate grains, providing a cohesive mix. This means the final cured concrete has low porosity, is less prone to cracking, develops high strength and is highly durable. Durability and strength are fundamental requirements in high quality concrete, and fly ash-based concrete in particular can be effective when used in more challenging environments, such as sea defences, tunnels, bridges and power stations.

For example, one potential application the UKQAA has been exploring is the use of fly ash in gravity bases used in offshore wind turbine developments. Used to secure the huge turbines to the sea floor, these bases need to be structurally sound and able to withstand the pressures of a working turbine and the unpredictable nature of the seabed. Given these demands, the potential for fly ash is significant here as it is effective in resisting chloride, alkali silica and sulphate attacks and reaches higher levels of strength far earlier than Portland cement products. What’s more is that we already know fly ash can be used in these circumstances – North Sea oil platforms have used fly ash in the past to ensure strength below sea level, so we expect to see this market grow in future years.

Similarly, at Pembroke Power Station in South Wales, contractor Lafarge used almost 7,000 tonnes of fly ash (sourced from nearby coal-fired power station Aberthaw) to construct the power station’s five turbine halls. Rather than using traditional Portland cement, the contractor wanted to make use of a more environmentally sustainable material that was quick drying, easy to work and resistant to cracking and shrinkage – crucial to the demands of a power station.

However, outside of major infrastructure projects, fly ash-based cement products have a key role to play. For example, they’re often more workable and flexible than products based only on Portland cement due to their lower water content. This means contractors can create the sharper lines, edges and surfaces often required in modern construction projects – the 2012 Olympic aquatic centre is an example of this style of construction – and still hit the sustainability targets a client may demand. In fact, while Portland cement carries around 900kg/tonne of embodied CO2, fly ash cement carries around 600–700kg/tonne, which is a considerable drop.

These are significant benefits for the modern contractor and make concrete extremely competitive with other core building products – such as steel and glass – without harming the strength and durability of the material. Undoubtedly such benefits ensure concrete will continue to be a relevant construction material for some time to come.

For more information, call 01902 373 365 or visit www.ukqaa.org.uk