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here is a mistaken assumption that concrete and cement are the same thing. While cement is used to bind concrete, as much as 70% of concrete is not actually cement. Yet cement production is responsible for possibly as high as 80%-90% of all carbon emissions from concrete. As a result, concrete cannot ever be eco-friendly without also cleaning up cement.

Portland cement is the most common variety of the material in the world. Every year, an estimated 3.5 billion tonnes of Portland cement is produced globally, emitting 622kg of CO₂ per tonne.

The cement industry is responsible for more than 7% of all man-made carbon emissions per year worldwide. However, there are predictions this figure could jump to 25% by 2050 if current trends continue. If the world is to prevent the worst effects of climate change, then these industrial rises need preventing and greener production processes are required.

A complete reduction in CO₂ emissions from cement is not going to be possible immediately. What is more realistic is savings over various areas that contribute to an overall reduction in carbon emissions. But the cement industry will have to do its bit rather than wait for clean technology from other sectors.

Félicie Krikler, director at Assael Architecture

Félicie Krikler, director at Assael Architecture

The CO₂ problem

Making cement requires rock to be heated to temperatures of up to 1,450^oC in a kiln to make clinker, which is milled down to a powder. This chemical reaction to heat the rock results in the release of CO₂.

While greener energy supplies for the kilns can make a difference, it is not possible to stop the release of CO₂ from heating the rocks. Furthermore, there are few green energy sources capable of achieving the high temperatures required.

Alternative fuels such as hydrogen have the potential to provide the necessary energy to produce cement. Hydrogen emits no carbon, and water is the only by-product. Yet hydrogen would need to be made using green energy via electrolysis to be fully carbon-free.

A partnership between Hanson UK and the University of Swansea is currently assessing the potential of hydrogen as a viable energy source for cement production. A hydrogen demonstration unit is now testing the viability of the fuel at Hanson UK’s Regen ground granulated blast slag (GGBS) plant in Port Talbot, South Wales.

Regen GGBS replaces as much as 80% of the cement in concrete but requires high volumes of electricity and natural gas. Yet its production has roughly 10% of the carbon footprint of Portland cement. If green energy is used throughout production, then the carbon impact is further slashed.

The project has received £9.2m from the European Regional Development Fund (ERDF) as part of the Reducing Industrial Carbon Emissions (RICE) initiative.

Bethel secondary school in Burkina Faso is one of Article 25’s school projects in West Africa.

Alternative materials

Another option is to change the materials used to make cement and concrete. Waste products from other industries are already used in the mix such as GBBS from the steel industry or fly ash from coal-fired power stations. However, an issue persists with the supply.

At present, slag accounts for no more than 10% of cement production globally. And by-products from heavy industries may well be radically different or even obsolete in a low-carbon future.

Using as much as 20% of ground limestone is expected to be more common in UK cement within the next five years, and is already used in European countries such as Ireland and Sweden.

Meanwhile, LC³ is a new variety of cement that contains limestone calcined clay. While 50% of it is clinker, it also comprises 30% calcined clay, 15% limestone, and 5% gypsum. Its manufacturers claim that using low-grade clays and limestone cuts CO₂ emissions by up to 40%. Furthermore, existing cement production facilities require no large-scale modifications. The material is being developed in Switzerland and has already been used to build an entire house in India.

One downside is that low-carbon concrete can take longer to achieve its full strength than traditional varieties. While concrete made from Portland cement can take around 28 days to achieve full strength, low-carbon varieties can sometimes take double this amount of time before they are strong enough to build on. However, this issue can be avoided by increased levels of planning in the design stages.

Carbon-neutral cement

Two PhD students at Imperial College London may have cracked the formula to make carbon-neutral cement on an industrial scale. Barney Shanks and Sam Draper have developed a means to capture carbon from flue gases used in existing industrial processes. The captured carbon is then turned into solid matter – pozzolana, which contains silica in reactive form – that can be blended with existing Portland cement.

The aim is to use the technology – loosely described as a modified pipe – at existing cement plants. The project is still in the early stages, but the signs are encouraging so far.

“We wanted to take this mentality that there is going to need to be some sort of radical shift here,” says Draper. “Rather than using standard waste materials, can we engineer something that is in itself carbon negative?”

And the results are impressive. In the formula, one tonne of silica sequesters approximately 1,470kg of CO₂.

“We can take any flue gas, which is just waste gas stream that's got a CO₂ percentage of between 15% and 25%,” adds Shanks. “That gas stream goes straight into the process. It doesn't need any pre-processing or compressing, purification, anything like that. It just goes into the system and the carbonate mineral drops out.”

Historically, the construction industry has been slow to embrace change, instead preferring to stick with tried-and-tested methods and materials. Shanks explains the challenge has been to develop materials that are as similar as possible to what those in the industry are familiar with. Nevertheless, even if the product is chemically identical to what is already on the market, the material will still need approval from regulatory bodies.

“That's such an important aspect of this. Because I think that's a reason why a lot of these projects fail is because people in the industry are a bit like: 'It's so different. We already have all the infrastructure; we already know how we work. We've been using Portland cement for 2,000 years. And we really know how it works',” says Shanks.

“To bring in something very new would be difficult. So, we're trying to get it as similar, not only in mechanical properties. But also, in how it looks, how it feels, how it acts in a cement mixer, so that you can just swap it out.”

At present, the processes have produced small cubes that fit on the palm of the hand. But there are plans to scale up. The pair are currently having encouraging discussions with cement manufacturers to set up a pilot plant on the side of their factory to take in the gases and produce a cement replacement. Shanks and Draper are also speaking to parties working on projects who are willing to use the material on non-structural components.

“The first structural pour could be within the next 18 months to two years, then it's just a matter of scaling up,” adds Draper.

Greater efficiency of materials

In the not-too-distant future, structural engineers will need to be brought in earlier in the design stages as building projects may well have carbon limits. Will Arnold, head of climate action at the UK’s Institution of Structural Engineers, has even called for greater investment in R&D for construction on a similar scale to wartime to meet the challenges faced with climate change.

While new processes and technologies are in development, the fastest way to save carbon from concrete is using what already is in place. For example, building on existing structures rather than knocking down old buildings.

“With all construction, the biggest carbon savings come out of reusing what you've already got. That goes beyond just construction. Most carbon emissions in the world come from creating something new,” says Arnold.

“If you can reuse the foundations of existing buildings. Or even better, use foundations, basement and superstructure of the existing building and build on top of that. That goes a really long way because then you're knocking two-thirds of the carbon footprint of your new buildings by reusing everything that's already there.”