How Teesside and Ukraine shared a fascination with slag

One of the North East’s most remarkable structures has withstood the ravages of the North Sea, and the explosives of the Royal Engineers, for 120 years and is now a world leader in an unusual, and green, construction technique that is shares with the Ukrainian city of Mariupol.

Tragically, in Mariupol, the Russians have mercilessly tested the strength of slag concrete to the point of annihilation.

The concrete constructions of Mariupol have been devastated by Russian bombardment

Cement is at the heart of concrete, which is one of the world’s most important building materials. Most concrete is made out of Portland cement, which is a mixture of clay and limestone heated in a kiln – which was fired in the old days by Durham coal – and then ground into a powder.

However, scientists have long sought an alternative, eyeing the slag at the bottom of blast furnaces which they knew had a very similar chemical composition to Portland cement.

Indeed, when the British Association for the Advancement of Science met in Newcastle in 1889, it recorded in its papers that “the dream of the northern cement industry (was) that out of the refuse blast furnace slag (of which Middlesbro' alone makes thousands of tons every week), Portland cement may some day be made”.

The scientists’ attention was drawn to an experiment that was going on down the east coast at Skinningrove, where a jetty had recently been built out of cement made from the slag from the bottom of the Loftus Iron Company’s clifftop blast furnaces.

Looking from an unusual shaped pillbox down onto the jetty at Skinningrove in the early 1960s

The iron company had been formed in 1874 to make use of the locally mined ironstone. The main market for its foundry iron was in Scotland, but as rail transport was expensive, in 1882 work on the jetty began so the iron could be sailed directly to the River Forth.

Because of Skinningrove’s location, getting raw materials to the site was difficult, so some bright spark – the credit is often given to the iron company’s managing director Thomas Hutchinson – made use of the waste slag from the blast furnaces, grinding it into a powder to make slag cement and then slag concrete.

It worked, in that the jetty was operational in 1888 when the company’s ship, the SS Runswick, made its first voyage, although because the jetty was so short, it could only be used for an hour or so at high tide.

Consequently, when the scientists gathered in Newcastle in 1889, the jetty was being extended by 40ft using more slag concrete.

The minutes of the conference conclude: “The success of this new departure would mean a veritable revolution in the cement industry.”

The lengthened jetty opened in 1891 and soon three steamships were plying their trade from it.

How the iron and steel works overlooked the terraces of Skinningrove in 1994

But it was not plain sailing for the company. The trouble was not with the jetty, but with the nature of the iron industry, which spluttered in and out of life. By the 1930s, the jetty was unused.

At the start of the Second World War, fearing invasion, the Royal Engineers attempted to blow a hole in the jetty to prevent the Germans from invading along it.

They failed. The jetty, and its slag concrete, could not be shifted, and so it remains to this day its full length.

The jetty jutting out in the North Sea. Picture by John Carter, of Stockton, a member of The Northern Echo Camera Club

The scientists in Newcastle never doubted the “extraordinary cementation value” of the powder made from slag. What worried them was that once the slag had been ground up, it quickly absorbed carbon dioxide from the atmosphere and so lost its stickiness. This did not matter if you had a blast furnace on hand, but it prevented the slag cement from travelling anywhere, and so the slag revolution never caught on. The Skinningrove jetty was just one of handful of buildings in the world to be made out of furnace waste.

Until the 1950s, when the Soviet Union was rebuilding after the war. It suffered a shortage of building materials, and Ukrainian scientist Victor Glukhovsky realised that the Victorians in Skinningrove had been onto something.

Mariupol – a city we’ve now learned a distressing amount about – had the second largest iron and steelworks in Ukraine which had been producing slag since 1897. Under Glukhovsky’s guidance, it was ground up so that in Mariupol the highest concentration of slag concrete buildings in the world grew up.

The Russian bombardment has devastated the concrete residential buildings of Mariupol

In recent years, these buildings have become even more important to environmentalists. The creation of conventional Portland cement causes the emission of carbon. According to a fascinating article in the Sunday Times by Sky News’ economics editor Ed Conway, the world produces five tonnes of conventional concrete for every person on the planet every year – so much that concrete is now responsible for more greenhouse emissions than aviation.

Slag cement doesn’t. It is green.

“Scientists from around the world would occasionally be found wandering the lanes of Mariupol, staring up in awe at its austere apartment blocks,” wrote Mr Conway. “It was one of the world’s most important sites of scientific interest.”

The scientists were checking to see if this low-carbon concrete could possibly work? “It is all very well mixing it in a lab, but we also need physical proof that the material can stand the test of time,” said Mr Conway. “That is, or was, the secret of Mariupol.”

The Russians have proved that even slag concrete has its breaking point, although they also proved that the Ukrainian people are made of even sterner stuff than slag concrete.

Should any scientists wish to test the concrete’s mettle, one of the few fingers of it still stretches out into the North Sea, although people are discouraged from walking along it.

The end of the jetty at Skinningrove and, below, looking from a Second World War pillbox towards the jetty

READ MORE: The 150th anniversary of the slag brick that is one of the defining features of the Tees Valley