The £330m project to dual an 8.7-mile section of the A30 between Chiverton and Carland Cross in Cornwall has been in the news recently for its carbon-saving credentials. By processing a material called ‘stent’, a by product of the China clay industry in the area, the scheme will be using just 20% virgin aggregate.
Stent, which is heaped in white ‘mountains’ near clay pits in Cornwall, has been processed and used in road and building projects for decades. But with awareness of embodied carbon growing, and carbon reduction target dates looming, it is more in demand than ever. It is impressive to see the A30 project, being constructed by Costain Jacobs with material supplied by Aggregate Industries, using such a high proportion of stent.
The National Highways press release calls the material ‘recycled aggregate’, but a better description might be ‘secondary aggregate’. According to the Mineral Products Association (MPA), recycled aggregates are the product of processing inert construction and demolition waste, whereas secondary aggregates are derived from other industrial processes.
China clay – or kaolin – is found in granite (it’s a weathered part of the rock) and is removed from the hard rock using high pressure water jetting. Stent is the rock material by-product from the process, lumps of granite that vary in size from less than 100mm to over 2m in diameter. Other waste materials from the process include sand and mica (sub-sand sized particles).
Stent has been used in Cornish road building – and beyond – for decades. For instance, crushed stent was used as Type 1 sub-base for the A30/A30 Indian Queens and Fraddon Bypass, a 7km stretch of dual carriageway which was constructed in 1993 to 1994. In its latest use on the A30, it will be deployed in higher layers of the road too.
Brookland Sand & Aggregates, which is based at Littlejohns Pit in St Austell, where the A30’s material is coming from, was formed in 2010 to process the waste from china clay production into secondary aggregate. And its parent company Norman Cleave Plant Hire was processing secondary aggregate even before then, in the mid-1990s.
Both the aggregate and the sand by-products of the clay production process are also used in concrete. Cornish Concrete Products (CCP) started producing its ‘ecocrete’, containing 95% of secondary sourced aggregates, 15 years ago.
London building materials supplier S Walsh started bringing secondary aggregate produced from stent to London back in 2014. More recently, it has upped the environmental credentials of its supply, shipping the secondary aggregate from Cornwall to Essex via the sea. In 2021 it signed a 20-year agreement with the Port of Tilbury to create two new freight hubs which will allow it to ship in 400,000 tonnes a year of the secondary aggregate.
Students of the circular economy might be interested to look at how the cost of the secondary aggregate produced from stent has changed over the years. And – as the cost of offsetting carbon rises – it will be interesting to see how its cost changes in the future too.
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