The journey to circular cities: the current problem with materials



Can recycling cement, concrete and steel make construction more sustainable and if so, how do we do it to create circular cities, asks Andrea Lockerbie.

The materials at the heart of our cities, roads and infrastructure cause lots of environmental damage.

“The construction sector uses more raw materials than any other in the UK, produces the most waste, and is responsible for a quarter of the country’s carbon emissions. It is due an overhaul.”

This statement, from the Green Alliance’s recent report Circular Construction: building for a greener UK economy, neatly sums up why the pressure is on the construction sector to embrace circularity.

Green Alliance is calling on the government to introduce a sector-specific resource-reduction target for construction and to prioritise interventions: financial incentives, circular design, retrofitting, and use of better data.

As the report outlines, changes are coming regarding energy efficiency: from 2025, new buildings need to generate 70 to 80% less carbon after they are built. “But the focus on energy efficiency will not address all the sector’s environmental impacts,” the report states. “A significant proportion of the emissions associated with construction come from its use of materials.”

Circular cities: the journey towards net zero

Circular cities

UK Fires, a research programme aimed at enabling a rapid transition to zero emissions based on using today’s technologies differently, says “the impacts of construction are primarily about the use of materials: primarily steel and cement”.

Its Absolute Zero report states that all existing forms of cement production are incompatible with zero emissions – the same for primary steel production using blast furnaces.

Cement is the key ingredient in concrete, which, second to water, is the most consumed material in the world. According to the Global Cement and Concrete Association, cement production accounts for around 7% of global CO2 emissions. The Mineral Products Association believes the UK concrete and cement sector was responsible for around 1.5% of UK greenhouse gas (GHG) emissions in 2018.

When you produce cement, there are two different types of emissions associated with it.

Ways to reduce or eliminate emissions associated with cement and concrete are emerging, however. One exciting solution is Cambridge Electric Cement, discovered by engineers at the University of Cambridge. They are now working to scale up their emissions-free method of recycling cement. 

According to UK Fires, around 60% of emissions from cement production arise from the chemical reaction in which limestone is converted to clinker – the basis of cement. The remaining emissions are from the combustion of fossil fuels and waste materials, used to heat the kilns.

Circular citiesThere are ways to make lower-emission cement by mixing clinker with other supplementary materials, mostly by-products. The engineers at Cambridge, however, have invented a way of making the clinker itself without any emissions. 

The discovery was made by Dr Cyrille Dunant when he noticed that the chemistry of used cement is virtually identical to that of the lime-flux used in conventional steel recycling processes.

The team found that, if you replace this lime-flux with old cement powder derived from demolition sites, then, as the steel melts, the flux forms a slag that floats on the liquid steel. When cooled rapidly and ground into a powder, this slag is virtually identical to the clinker that is the basis of new Portland cement.

Dr Philippa Horton, business manager at the University of Cambridge engineering department, explains: “When you produce cement, there are two different types of emissions associated with it. You are performing a chemical reaction that releases the carbon dioxide. You also need to heat raw materials to a high temperature in a kiln – currently, that is done using fossil fuels and gives off emissions. 

“We eliminate the emissions that come from the process because we are using old cement paste. That cement, the first time it was manufactured, released carbon dioxide. So, when we reheat it, to “re-clinker” it, it doesn’t release carbon dioxide again because it has already been through that chemical reaction.

We eliminate the emissions that come from the process because we are using old cement paste.

“You could do that in a kiln, but then you would have the emissions from having to heat it up. This is the clever step – we are using the electric-arc furnace in the steel-making process, essentially, instead of a kiln. 

“The idea is that it shares the energy required to heat up the steel to also heat our clinker, raising it to a temperature that will re-clinker it. That is brilliant, because electric-arc furnaces are powered by electricity and, at some point in the future, the grid will hopefully be decarbonised.

“Then you have a way of producing cement with green electricity, and you don’t have the process emissions because you are using a waste product that has already given off the CO2 the first time around.”

Circular citiesThe team has already performed lab-scale experiments and slightly larger-than-lab-scale tests using 80kg furnaces and 100kg furnaces. Horton says: “We are now looking to scale that up, to see if those trends continue to be successful when we are in a much larger-scale furnace. We have plans to do that over the next year and a half. Hopefully, towards the end of next year, we will have a full-scale demonstrator.”

The team will also be investigating the use of “real world” input materials: the experiments so far have used “good” old cement, and steel of well-controlled quality. Horton explains: “The next step is to ask: ‘what is the operating window in which this function works, when is the steel too dirty or has too many contaminants? How good does your cement paste have to be?’

“The difficult bit is that we are completely reconfiguring a supply chain, so there is a different set of connections to have to form with different industries.”

A parallel project, called Cement 2 Zero and led by the Materials Processing Institute, with support from the University of Cambridge, is seeing how this could work with key players in the supply chain: Atkins, Balfour Beatty, Celsa, Day Aggregates and Tarmac.

Horton says the team is optimistic, but warns that it’s still early days: “I get regular emails from big construction projects saying: ‘Can we have some of your cement?’ But we are still testing it in a lab.” 

The next steps

German construction site

The aim is to learn more about the process and its potential during the next couple of years and look to commercialise it. The fact that the supply chain has come together and committed a large amount of money to the project shows the desire for such solutions.

There are some limitations. The team calculates that around a quarter to a half of current cement needs, or projected cement needs, in 2050 could be met by the new process, so there would still be a need for other cement solutions, or to use cement more efficiently. There are also constraints: the UK has limited domestic steel recycling capacity, with much of the country’s steel scrap exported for recycling.

Making steel from iron ore and coke in blast furnaces is another big emitter of emissions from construction. According to the British Constructional Steelwork Association, steel production accounts for around 8% of all global GHG emissions.

As the Absolute Zero report sets out, in a zero-emissions economy, there can be no new steel because the blast-furnace chemical reactions that extract pure iron from iron ore use carbon from coal or coke and emit GHGs. Recycling more steel, however, in electric-arc furnaces powered by renewable energy, could help resolve this, as well as increase steel reuse.

We are trying to establish an inventory of reuse steel to sell to anybody and everybody that wants it.

Roy Fishwick is managing director of Cleveland Steel & Tubes, a business that has been re-testing and repurposing surplus oil pipe into construction for decades. He says there has been a steadily increasing focus on steel reuse in the past 10 years, which has “gone bananas” during the past 18 months, with demand exceeding what is available. 

This interest is mostly in London, where Whole Life Carbon Assessments and Circular Economy Statements are now mandatory for large-scale London developments under the London Plan 2021. Demand is also being driven by the public, investors, clients, asset owners, and those letting properties who want to have low-carbon or net-zero buildings.

Cleveland Steel & Tubes was involved with the exemplar Holbein Gardens development, which reused 13.5 tonnes of steel. This was property developer Grosvenor’s first net-zero workplace in London, and the developer reports high demand for sustainable spaces. A green lease has already been signed for the entirety of Holbein Gardens by private equity investor Oakley Capital. 

construction-wasteFishwick says that some businesses in London are prepared to pay more and go to a lot of trouble for green “firsts”. But there is a challenge around getting people to think outside their own projects and consider the bigger picture. “We are trying to establish an inventory of reuse steel to sell to anybody and everybody that wants it,” he says.

“At the moment, people say: ‘Can I use it?’ – and if they can’t, they lose interest. What they should be doing is asking ‘Can somebody use it?’ and then trying to ensure the release of those products into the circular economy. If they can do this – and not just for steel – it will ultimately benefit the planet.

“Whereas, if you are only looking at your own job, you might have some benefit, but what you really get is a lot of PR and not that much benefit. If you try to release all the materials you can, you are making the most difference.”

He adds that the current design, specification, fabrication and redesign process does cause issues. “My biggest challenge as a business is that I’m trying to secure material that people won’t need for 6-18 months. They may change their minds along the way because the designs will change, the client will decide to move the floor around, or whatever – and then, suddenly, none of it works.”

My biggest challenge as a business is that I’m trying to secure material that people won’t need for 6-18 months.

There are physical and economic challenges, too. Fishwick recently went to look at a job in London: “The steelwork was fantastic, but there was no way of getting it out of that building and onto a lorry without the project spending about another £300,000, and there just wasn’t enough steel in there to make that sensible. You might be looking at £1,000 per tonne to recover the steel – and that’s what it’s worth new from the mill.”

But should carbon win over cost? “I tend to be a pragmatist, so, yes, the carbon wins over the cost. But if you make it hurt so badly for cost, people are just going to find every way possible not to do it. So, on that particular job, my suggestion was more basic. The building was seven floors: if we can’t do it all, let’s talk about the last two or three floors and see what we can get from that.”

Getting as much reusable steel out of old buildings and available to the market should be the priority: “From the demand we are seeing, interest is huge. The biggest danger is a lack of materials to satisfy the demand; then people start to switch off.”

constructionFishwick believes tenders should require optimum material reuse to drive this, so people can extract as much material from buildings as is sensible and divert it into a proper reuse marketplace. Steel reuse is still a very marginal activity; indeed, not everything can be reused.

From Fishwick’s experience, 30-50% of steel for a project would currently still need to be new. But the important next step is to support this demand for reuse and get a proper market up and running and growing. 

A focus on reducing raw-material use and emissions is starting to happen in construction, but there is “still plenty of low-hanging fruit in the improvement of resource use and climate impact”, with untapped potential for better performance, says Green Alliance.

It believes raw-material use in construction could be cut by 35% by 2035, just by using current technology and best practice, and it wants the government to “trigger action now, to ensure the industry is fit for the future”.

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