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Hanson proves secondary aggregate's potential in concrete used on major London site

Hanson explains how innovation in the development of advanced sustainable concretes is boosting the use of recycled and secondary materials in concrete.

We have done a lot of work in the past few years to develop concrete products with lower embodied carbon.

We have also been using our technical expertise to design concretes that incorporate a greater proportion of recycled or waste materials, but can still perform to high industry standards.

The benefits of secondary aggregates

These approaches have been combined in the supply of advanced sustainable concrete mixes using very high proportions of secondary aggregate to Bloomberg’s new UK headquarters in Walbrook Square, London.

The secondary aggregate is china clay waste – a by-product from the extraction of kaolinite, which is sourced from Cornwall.

“This is the first permanent works concrete in the capital to contain 100 per cent secondary material as aggregate”

In some of the concrete mixes, all the sand and stone aggregate used on the project was china clay waste, making it the first permanent works concrete in the capital to contain 100 per cent secondary material as aggregate.

There have been concerns in the past that concretes with high secondary aggregate content may not meet exacting quality and technical standards, particularly in terms of durability and performance in aggressive ground conditions.

But the range of bespoke concretes we developed in conjunction with Cementation Skanska at the Bloomberg site shows that it can be done – and done successfully.

Sustainability credentials

We have already supplied concrete containing 42,250 tonnes of secondary aggregate and it has comfortably achieved strength grades of C32/40, with an average embodied carbon content 20 per cent lower than conventional mixes.

Concrete mixes using recycled aggregates normally target the stone fraction only and limit themselves to specific sizes of stone.

But you also need sand, so by procuring a specialist secondary aggregate that provides stone and sand, the concrete’s recycled content can be significantly increased. The material does not require many specialist modifications; the key factor is to achieve an agreed uniform grading.

What we can learn from the Bloomberg project

The Bloomberg project has shown that high-quality, sustainable concretes can be sourced and provided in sizeable quantities in a timely and cost-effective manner, and we expect further take-up of low-carbon and high recycled content concretes for other projects.

“The Bloomberg project has shown that high quality, sustainable concretes can be sourced and provided in sizeable quantities”

Looking ahead, we believe there is also great potential for the increased use of secondary aggregates in foundation concretes, which still predominately rely on virgin aggregates. 

We accept that there are cost implications in this – secondary materials are not always cheaper. But as a society we are progressively assigning greater value to sustainable products and concrete should be no exception.

Jasen Gauld is technical manager at Hanson Concrete

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