Bath engineers turn china clay waste into cement that cuts emissions by 70%

A cement made largely from the waste of Cornwall’s china clay industry produces 70% less carbon dioxide than conventional Portland cement and reaches the same strength within 28 days, engineers at the University of Bath said on Thursday, after two years of tests that included casting a footbridge on the campus.

Cement manufacture is responsible for about 8% of global carbon dioxide emissions, chiefly because limestone must be heated to 1,450C and gives off the gas as it decomposes. The Bath material instead uses finely ground micaceous residue, the sandy waste left when kaolin is washed out of decomposed granite, activated with a small amount of lime and a sodium-based catalyst, and cured at 80C.

About 20m tonnes of the residue sits in spoil heaps around St Austell, where it has accumulated over two centuries and forms the conical hills known as the Cornish Alps. The industry still produces about 1m tonnes of waste a year. The team calculates that the heaps could supply the whole of the south-west’s concrete demand for 40 years.

“We have been looking for a substitute for clinker that does not depend on a by-product that is itself disappearing,” said Prof Deirdre Kavanagh, who leads the Bath group, referring to the fly ash and blast furnace slag used in low-carbon cements, both of which are becoming scarce as coal and blast furnaces close. “This stuff is sitting in a heap, it is free, and Cornwall would be glad to see the back of it.”

Test cubes of the material reached a compressive strength of 48 megapascals at 28 days, above the 40 required for most structural work, and showed less shrinkage than Portland cement. The campus footbridge, a six-metre span cast in March, has been monitored with embedded sensors and shows no cracking. Results are published in the journal Cement and Concrete Research.

Industry reaction was cautiously positive. A technical director at a major UK cement maker, who asked not to be named because the company is evaluating the material, said the chemistry was “sound” but that the 80C curing step would need to be eliminated for use on most building sites. The Bath team says a room-temperature version is 18 months away.

The china clay producer whose heaps supplied the residue has signed an agreement with the university and a Plymouth-based concrete company to build a pilot plant at Par docks capable of producing 50,000 tonnes a year by 2028. Cornwall council has indicated that it would specify the material for a new secondary school at Bodmin if it gains the necessary certification.

“People drive past those heaps and see a landscape,” said Kavanagh. “We see a quarry that has already been dug.”