The realist’s road to zero: #1 Concrete and steel

You might be trying to lose weight, cut costs or finish a city marathon — but there is no doubt that with many things, the closer you get to your goal the harder progress becomes. So when you have picked the low-hanging fruit, applied the 80-20 rule, and experienced diminishing returns, you realize that it is not for nothing that there are so many ways of describing the situation you are in. As when you try to reach the speed of light, each additional metre per second needs more energy than the previous one until eventually the requirement becomes infinite — and apparently impossible.

Now let’s talk about net-zero carbon. Much of the world plans to reach this target by 2050 and, on the face of it, we seem to be heading in the right direction. Your home, your commute, your business, your light bulb, pretty much everything in fact, almost certainly does what it does at the expense of much less CO₂ than twenty years ago. We are more efficient, and we have changed the way we do things. So if we just continue along the same path, we should get to net zero, right?

Unfortunately it’s not that simple. It is probably embodied, rather than operational carbon, that will represent the highest-hanging fruit, or the hardest 20% to cut. It is relatively easy to at least imagine a world from which our operational use of coal, gas and oil have been eliminated. In this fossil-free future, all vehicles are electric, powered by batteries charged from renewable energy sources. Buildings too, are electrically powered and heated. Electrifying railways is nothing new, and the first battery-powered ships are already in operation. Even aeroplanes can be powered by hydrogen or biofuel.

Generating enough renewable energy to meet all this new demand already represents an unprecedented challenge for the nations of the earth. The amount of fossil power that must be replaced is colossal. But with sufficient investment in renewables such as wind turbines, solar farms or, more controversially, nuclear power stations, it is at least theoretically possible.

Addressing embodied carbon is more complex. Around one quarter of all non-agricultural man-made CO₂ emissions result from the manufacture of materials and things. Paper, plastic and aluminium together account for around 4%, but steel and cement are by far the biggest contributors — each responsible for some 7-8%.

It is much harder to imagine a world in which steel and cement are no longer produced. You cannot make railway tracks out of anything but steel. Ditto container ships. Most white goods, industrial machinery, trains, planes and road vehicles all require at least some steel. Cement is the vital ingredient in concrete, which if anything is even more ubiquitous. While it is easy to construct a small building from timber, large buildings require concrete foundations, mortar and a structure made strong by either steel or steel-reinforced concrete. Bridges, sea defences, utility infrastructures — including the wind turbines, hydroelectric dams and nuclear power stations that will decarbonize the energy supply — are impossible to make without steel or concrete. There is no other material that matches steel’s low-cost combination of strength, stiffness and hardness, nothing to rival concrete’s mix of cheapness, strength, mass, fire resistance and durability.

The problem of process emissions

At first glance, this seems to present a similar problem to operational carbon. The production of cement and steel requires large amounts of energy to heat and process the raw materials of limestone and iron ore respectively. This heat is currently provided by gas or coke — so couldn’t we decarbonize both by using electrically generated heat from renewable sources instead? Not completely. The creation of both steel and cement involves chemical processes that release carbon gases, and these process emissions account for the greater part of their embodied carbon.

To illustrate this, compare the carbon bill for “virgin” as opposed to recycled steel. “At the moment, steel production globally produces on average around 1.9 tonnes of CO₂ per tonne of steel,” says Matthew Wenban-Smith, policy director at ResponsibleSteel, a global not-for-profit organization representing the steel supply chain. “If you recycle scrap steel using an electric furnace, the figure falls to roughly 0.3 tonnes of CO₂, which comes mainly from electricity production. If the source of the electricity is renewable, it can be as low as 0.1 tonnes of CO₂.”

But the world demands more steel than scrap can supply. “Most steel is recycled, but this still only supplies about 30% of global demand,” points out Wenban-Smith. “To give you an idea of the challenge the world faces, consider that in the US there exists about 10.5 tonnes of steel per person. In India the figure is 0.4 tonnes. If developing countries continue on anything like the same trajectory as the US, then there remains a requirement for a lot of virgin steel. A circular economy of using only scrap steel looks impossible without severely curtailing the ability of poorer nations to develop.”

This is not about reproducing the same patterns of excessive consumption, it’s about providing essential infrastructure to billions of people still living without. The world is some way off meeting the UN’s Sustainable Development Goals which include universal access to clean water and sanitation, networks for energy, transport and communications, and good-quality housing, education and healthcare by 2030. Delivering all this will require significant quantities of steel and concrete.

The limits of efficiency

It is possible to use steel more efficiently or more sparingly, but this will not be enough, says Wenban-Smith. “We can share cars, use more timber in buildings, design slimmer so that steel beams are only as big as they need to be, and we must do all of that, but it will only ever bring the numbers down. Efficiencies are not enough to change the story over the next 30-40 years. They won’t get us to zero.”

So the solution will have to involve making virgin steel in another way. According to Wenban-Smith, right now there are two main alternatives. One involves using hydrogen instead of coke to remove the oxygen from the iron ore, in a chemical reaction that produces water rather than CO₂. In the other, a biofuel such as charcoal replaces coke as both heat source and reduction agent — so carbon gas is still produced, but mainly only that which has been absorbed by growing the biomass fuel in the first place. There is also potential for direct electrolysis of iron ore, currently at a pilot stage of development.

While all these processes have potential, they are far from being easily or speedily scalable and, adds Wenban-Smith, nor should they be considered purely in terms of steel production. “You have to look at the wider picture. The cleanest hydrogen is produced from water using large amounts of renewable electricity — but will there be enough renewable energy given the fast-rising demands from other users? And hydrogen will also be needed for vehicle fuel cells or aviation fuel. As for biofuel, where does it come from? What are the land-use impacts? Would the land be better used to grow food, or for long-term afforestation?”

The future for cement and concrete is similarly opaque. Like steel, cement is currently made using heat from fossil fuels (in this case to heat limestone to 1450^°C) and, like steel, the chemical reaction involved produces large amounts of carbon gas. These process emissions are estimated to account for around half of the embodied carbon of a standard concrete mix. According to Jeremy Gregory, executive director of MIT’s Climate & Sustainability Consortium, the procedure is not easily changed. “At the moment there is no electrically powered cement maker anywhere in the world,” he says. “You have to use fossil fuels because of the temperatures involved. There’s a study in Sweden looking at what it would take, but practical viability is decades away.”

The world’s concrete carbon footprint can be reduced by efficient design or by substituting other materials — but again, there are many competing uses for these finite resources. “You can use timber for some things — but where does that timber come from? In India they have had to restrict the use of timber for building to stop deforestation. We already reduce the cement content of concrete by using cementitious substitutes such as fly ash or GGBS (slag), but supplies of these will become limited as the world decarbonizes.” And once again, such measures only reduce the carbon bill, rather than taking it to zero.

Gregory is still optimistic about the future of concrete, however: “I don’t think the world can do without ordinary cement, and without carbon capture, I don’t think it can be made to be zero-carbon. What is feasible is zero-carbon concrete.”

This, he explains, is because aggregates typically account for 80% of concrete by volume, and developments in the production of carbon-negative aggregate mean it is possible to make carbon-neutral or even carbon-negative concrete. These aggregates are formed from calcium and captured waste CO₂ from other processes, which could include cement or steel production. A process of mineralization combines the two to make carbon-negative aggregate. Companies in the US and the UK are already producing these products, along with carbon-neutral concrete blocks and pavers. “We do need to be careful about auditing this process,” he cautions. “We need to look at how the calcium is sourced, and the energy the processes use. But there’s certainly potential. And whereas there is some doubt about sequestering captured carbon underground — will it escape? — mineralization locks CO₂ away chemically for good.”

Understanding the scale of the challenge

Such innovation will help, but the profound nature of the zero-carbon challenge has yet to be fully appreciated, believes Will Hawkins, a lecturer in structural engineering at the University of Bath and a research associate with UK FIRES, a government-funded collaboration between six of the UK’s top universities focusing on zero-carbon solutions for industry. “There are many things you can do to reduce the carbon footprint of steel and concrete, but there are limits,” he explains. “Take cement substitutes in concrete. There is already limited GGBS and fly ash, with supplies expected to reduce as the wider economies decarbonize. You can use limestone, but only to an extent. Calcined clay is an attractive option, but still takes energy to produce and can only replace around half the ordinary cement. So we think that a 40% reduction in carbon is the best we can realistically expect.”

It’s a similar picture with steel, continues Hawkins. “Steelmaking from iron ore is always going to release carbon. Only carbon capture can make it zero-carbon. Can we be sure this will be operating at scale by 2050? I don’t think hydrogen is the answer because producing green hydrogen from electricity is a very energy-intensive process — and if we have already electrified home heating and transportation, will there be enough green electricity to deal with steel as well?”

The prospects, he says, are not encouraging. “One of our findings is that it is possible, and necessary, to have a 100% renewable electric grid. But even if we use the fastest historical rates of deploying renewable technology, there will still be a shortfall by 2050 after we have electrified heating and transport, for example. This means we need to be more energy efficient in everything we do, including the production of materials.” This is based on figures from the UK, he notes, where grid decarbonization is proceeding relatively quickly.

He paints a troubling picture, where the supply of alternative resources is woefully insufficient to meet rocketing demand. Electricity, already the go-to solution to replace fossil fuels in transport and heating, will also be needed in huge quantities for the production of essential materials — either directly as heat, or indirectly via the creation of green hydrogen.

Another go-to alternative, timber, will find itself under similar pressure. It can already be used in place of steel or concrete for most buildings, and as foundation piles. But with wood increasingly being used as an arguably carbon-neutral biofuel in heating systems and power stations — and being touted as an alternative to coke in steel production — it seems unlikely that supply could also meet the demand created by a large-scale switch to timber construction.

Couldn’t we just plant more trees? This is how many corporations and government departments have already achieved carbon neutrality, by lowering energy consumption and then buying offsets to account for the rest — very often in the form of carbon sequestering tree-planting schemes. “It works for a few corporations,” says Hawkins, “but if everyone were to do the same thing, there simply isn’t enough room on the planet to plant enough trees to offset enough carbon. It also takes decades for newly planted trees to start absorbing carbon in significant quantities.” Long-lived timber products are a great way to take carbon out of the air and store it, he says, but like every other innovation or strategy mentioned so far, they will only to be able to take us so far. We are still not at zero.

Carbon capture: the final step

There is one last card to play: carbon capture and storage. This has yet to be deployed at-scale, despite being relied upon by many governments and industries to get to carbon neutrality, says Hawkins, adding that “the capture technology is proven, at least, but the storage part of it less so”. Essentially the jury remains out on where or whether carbon can be stored permanently or economically. Underground sites such as depleted oil reservoirs might work, but many remain sceptical that they offer a genuinely permanent solution.

Carbon capture does offer some neat symmetries. If plants can take CO₂ out of the atmosphere, and then be burnt to fuel material manufacture, and the exhaust CO₂ captured and put to use in, for example, aggregate mineralization, then the whole process becomes carbon-negative. Such virtuous circles offer a glimpse of how industry might operate in the future — but would require wholesale changes to the way the global supply chain functions.

Creating market demand for net-zero materials will be important: this is the goal of the SteelZero initiative, run by global non-profit the Climate Group and ResponsibleSteel, of which WSP is a member. But policymakers will also have to lead the way, and perhaps make some tough choices. “Manufacturers and users have a huge role to play in encouraging waste reduction, recycling and looking for alternatives, but we can’t do it by ourselves,” says Wenban-Smith. “Governments need to be asking: what will the hydrogen economy look like? Where are the offsets coming from? And, vitally, do we have enough green electricity?”

This the magic of the zero-carbon target, adds Hawkins. “What is so effective is that you have to think about everything. We are now having much more profound and wide-ranging conversations than when it was just an 80% reduction. In a sense, every engineer is now an ecologist.” But the hard truth is that it won’t be possible to get to net zero without some difficult and unwelcome choices, he adds. “So perhaps we can’t have skyscrapers anymore. Maybe we have to prioritize concrete for essentials like sea defences, for example. And we have to decide on the best uses for steel and timber. They might not be what we think they are now.”

None of this will be easy — and the closer we get to our target, the harder it will become.

This article is the first in a series focusing on the toughest targets for decarbonizing the built environment, and the tough choices facing global policymakers on the most sustainable way to deploy finite resources. Over the next four articles, we explore the critical pinch-points and the highlight areas where innovation is most urgently needed, from the prospects for a hydrogen economy to the potential of nature-based offsets and carbon capture and storage. But next, we’ll consider renewable electricity: building the infrastructure to decarbonize the world’s power supply will inevitably involve “spending” a lot of carbon. How can we reduce the size of the bill?