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From muck to materials

16 Jun She’s at it again…Heriot-Watt University’s Professor Madero is leading a new quest to find profitable, sustainable uses for construction waste. David Taylor reports

The geoenvironmental engineer behind the revolutionary K-Briq, Professor Gabriela Madero of Heriot-Watt University, is now heading up a project to explore ways of making new construction materials from the muck left over from recycled waste.

The university has teamed up with Livingston-based Brewster Bros, one of Scotland’s leading waste recycling businesses, to reprocess the clay materials arising from demolition and general construction waste.

This article was first published in the May 2021 issue of The Construction Index magazine. Sign up online.

Brewster collects waste from construction sites across the Livingston-Edinburgh region and processes it in accordance with the relevant waste regulations, recycling as much as possible. 

Clay can account for up to 25% of the output produced when excavation waste is recycled via a washing process; and this by-product usually ends up in landfill.   

“They can separate everything from the very fine particles to gravel and rubble and recycle them. But the very last product in the process is clay, and there’s currently no use for that,” explains Madero.

Now a £250,000 Knowledge Transfer Partnership funded by Innovate UK aims to develop the circular economy approach further by carrying out research and development into ways of creating new sustainable products from waste streams. 

The project will also include the creation of a hazardous soil treatment centre, believed to be the first of its kind in Scotland.  

Clay has traditionally been used to create impermeable barriers to cap contaminated sites or manage water and drainage, but the material Madero is working with requires a lot of treatment to ensure it is clean and free of harmful contamination – making it far too expensive to use for these conventional applications. “We need to look more broadly and find out how we can use this clay for other purposes,” she says.

Madero says that recycled clay is a promising but relatively unexplored material. She believes that finding new uses for it could significantly reduce the amount of waste sent to landfill each year.

“Through this project, we will rigorously test the properties and behaviour of the recycled clay to prove it meets Building Standards for product specification by performing a life-size case study on its performance attributes,” says Madero.

  She continues: “Additionally, we will investigate the use of spent oil shale and incinerator bottom ash as secondary aggregates, producing research evidence in the processing and application techniques which we hope will allay any concerns about their future use.” 

Spent oil shale has been used as a general fill material in road construction for decades. However, like recycled clay, this abundant material could be utilised for higher value applications, believes Madero.

  The use of incinerator bottom ash is currently constrained by regulation but Madero says that further research into this material and improvement processes could provide evidence to allow current restrictions to be relaxed. 

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“Incinerator bottom ash will become a more prevalent waste product in the coming years as an increasing number of energy-from-waste incinerators are used in response to landfill bans,” she says.

 The research will initially focus on methods of cleaning and reprocessing the materials to explore their physical and chemical properties. Madero is careful not to speculate about exactly what sort of construction products might made with them.

“Yes, potentially you could make bricks – but we have plenty of other ideas to add value to the material,” she says. “We are being very creative.”

There are already well-established methods for processing clay arisings from excavations, such as Trenchmod, a patented process developed by London demolition contractor Keanes. This processes cohesive clay subsoils to create a granular fill that is equivalent to Type 1.

However, Madero says that, although she has looked into various methods of making aggregate from clay, they all involve the addition of cementitious materials which adds cost and involves higher carbon emissions. “Also you’ve got to make sure that any cementitious materials won’t leach out over time,” she says.

Scott Brewster, managing director of recycling firm Brewster Bros, says: “The more ambitious and innovative we become when creating new products from waste, the more technical knowledge we will require from experts in their field. 

“This Knowledge Transfer Partnership will not only enhance our company’s capabilities and offering but also provide a vast body of knowledge that will benefit the wider industry as we collectively focus on achieving net zero targets,” he says.   

Making new products out of waste material helps create a ‘circular’ business model which eases pressure on landfill capacity and finite mineral resources while helping customers avoid paying landfill tax and the aggregates levy, says Brewster. 

“By collaborating with Heriot-Watt University, we aim to overcome restrictions on use like waste legislation and obtain end-of-waste approval. However, understanding the true potential of these waste materials is also a key driver,” he says. “We hope to discover if they can be used for higher-value applications or as raw materials in added-value products.” 

Brewster Bros’ West Lothian site will provide the location of the new treatment centre, which will use various remediation techniques to transform hazardous soils into a non-hazardous state so that they can then be recycled and reused.

 According to the Scottish Environmental Protection Agency, 1.17 million tonnes of soils were disposed of in Scotland in 2019, comprising 39% of all waste sent to landfill. 

This article was first published in the May 2021 issue of The Construction Index magazine. Sign up online.

The Knowledge Transfer Partnership (KTP) between Heriot-Watt University and Brewster Bros will run for two years. The team will include Wini Obande, KTP associate, who will conduct the research and Melis Sutman, academic support and assistant professor at Heriot-Watt University.

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