It is versatile, strong and the industry’s favourite material – but cement is also a huge contributor to construction’s carbon emissions. Can it be used differently to cause less harm to the environment?

It is the wonder material that is easily made, mouldable, versatile and strong enough to support 80 storeys or more. Portland cement is the product that built the Hoover Dam, the Humber Bridge and much of post-war Europe. It could be called the foundation of the modern world.

However, the go-to construction material comes with a huge price: its carbon. The cement industry is the third-biggest producer of carbon dioxide emissions globally, with the estimated 3.5bn tonnes of cement made annually accounting for 8 per cent of total CO2.

With the world looking to move to lower carbon emissions as soon as possible, is there a better way of doing things? Construction News examines what companies are doing about this, what the alternatives are, and asks why they are not yet more widely used.

The key component of ordinary Portland cement (OPC) is clinker, which is formed by baking limestone with clay in a rotating kiln. The resulting product becomes the component in cement or concrete that forms such a strong bond when mixed with sand and aggregate.

However, making clinker generates a huge amount of carbon. For every tonne of cement produced, an estimated 622kg of CO2 is emitted. The carbon produced to create a standard concrete mix is 72kg per tonne.

Cement manufacturing has actually reduced its CO2 output since 1990, due to the closure of inefficient plants and a switch to renewable energy for power. However, it remains a high-carbon sector at a time when the world is looking to become net-zero as fast as possible.

Cemex Europe is among the cement manufacturers looking to make a change in this respect. Head of sustainability Paul Fletcher says Cemex is investing in alternative fuels to bring down the carbon cost of producing cement, putting money into carbon capture at its plants, and reducing the clinker content in its mixes.

“At our Rugby plant last year, we used 55 per cent alternative fuels or waste biomass content, which has helped reduce the footprint,” he says. “You still have carbon dioxide produced from the process, but once you have made the cement clinker you can blend it and add in slag and GGBS [ground granulated blast-furnace slag] – in effect, the clinker you make now goes further.”

The Kingston University Town House, assembled from a precast kit

For companies looking to lower carbon emissions on projects today, using a concrete mix that includes GGBS is an immediate way to lower the clinker content – and hence the carbon emissions of the final product. GGBS is a waste by-product of the iron and steel industries.

When iron is smelted, the limestone, ore and coke fed into furnaces produce iron and slag, which, when quenched, forms a substance that’s able to replace the largely clinker-based Portland cement in certain quantities. In ready-mixed concrete, GGBS is usually used to reduce the proportion of Portland cement by 50 per cent.

Contractor Willmott Dixon used GGBS mixes to build the Stirling Prize-winning Kingston University Town House campus building in south-west London in 2020. Despite using a concrete structure, the project showed that design-and-build changes can bring down the carbon footprint. The university opted against building a basement, which significantly saved time, costs and the carbon associated with constructing retaining walls.

Willmott Dixon used reduced-carbon concrete with 50 per cent GGBS replacing OPC in the foundations, 36 per cent for the structural frame and 15 per cent GGBS in the floor-slab construction. This helped lower its carbon content significantly. Varying the amount of GGBS, which sets more slowly than OPC, enables its technical performance to be tailored to a particular use.

The above-ground structure was also built using precast concrete, reducing the programme impact of slower curing times, while also helping to lower the project’s carbon footprint by reducing waste. Willmott Dixon senior operations manager Tony Mingoia says: “We [told] the client we could cast it offsite and have the same look and consistency [as Portland stone, a planning requirement].

“Precast concrete helped save on waste compared to pouring concrete in-situ. We did pour the foundations in-situ, but we managed to use 50 per cent GGBS in that area. The structure was a kit of parts that was delivered and assembled on a tower crane.”

“A clearer picture could be presented to the construction sector if low-carbon cements were certificated for use in clearly defined areas”

According to Mingoia, standardisation can pave a way to lower-carbon concrete. “The Town House is obviously a very unique building but, while it’s a bespoke-looking structure, the columns are generally all 450mm square,” he says. “We got more consistency and are looking to develop a standard kit of parts that can be built offsite.”

According to Rupert Myers, senior lecturer in sustainable materials engineering at Imperial College London, precast concrete has the potential to bring further carbon dioxide savings. “With precast you can control the curing of the concrete so you can control carbonation [which reabsorbs some CO2 back into the concrete once poured] and use different mix designs, which are harder to do in-situ,” he says.

“The idea of precast concrete has been around for decades [...] It doesn’t directly lead to carbon dioxide savings, but it enables many things, such as using less-standardised cement mixes. That transition [to precast] can enable much lower-carbon concrete.”

Myers says that adding precast techniques to design innovations can play a part in reducing the volume of material used.

“If people take an extra day [to design] what is being built and make it a more optimal shape, it can use less concrete,” he says.

In July, Laing O’Rourke won £335,000 from the Department for Business, Energy and Industrial Strategy’s Industrial Energy Transformation Fund for a project exploring the use of lower-carbon materials and processes to reduce carbon in prefabricated concrete, in components such as columns, walls, floor slabs and facades.

Myers says the construction industry needs to look more closely at where and how it uses concrete. “The main measure we need to be doing is to substitute Portland cement clinker and substitute concrete – where it makes sense – for low-carbon materials,” he says. “Timber buildings are lower carbon but we produce so much concrete it would be hard to replace it entirely with enough timber. I envisage not completely concrete buildings, but a hybrid of concrete, timber and other materials in the future.”

Can construction look to other alternatives beyond OPC? There are many innovations being developed, though these remain a niche part of the market (see box).

M25 operator Connect Plus is using DB Group’s Cemfree to repair a section of the Woodford West Viaduct in East London

For those looking for a viable large-scale solution to the concrete problem, ancient Rome would not necessarily spring to mind. Yet, as well as sanitation, medicine, wine and roads, the Romans also left an intriguing legacy when it came to concrete. In the heart of modern Rome, the world’s largest unsupported concrete dome encloses the Pantheon, which itself stands as a 1,900-year-old testament to Roman material engineering. The ancient chemistry that has endured for two millennia could also hold an answer to the problem of clinker.

Adam Gittins, a commercial manager at specialist cement maker DB Group, highlights that the Pantheon is constructed using a non-Portland cement formed with an alkali-activated binder. “We have a lot of case history of [non-Portland cements] and the technology has been proven for a long time,” he says. “The Pantheon was built using a non-Portland cement concrete and there is nothing in these types of concrete that would promote a degree of degradation. It’s a very durable material.”

DB Group launched a modern alkali-activated binder product in 2015 called Cemfree. The non-Portland cement is made up of 95 per cent GGBS with an activator accounting for the other 5 per cent. DB says it emits about 133kg of carbon dioxide per tonne – up to 77 per cent lower than other forms of concrete.

The product has started to be used commercially, including by M25 operator Connect Plus to repair a section of the Woodford West Viaduct in East London. In May, Bam Nuttall also used Cemfree for a 300 cubic metre pour to support the foundations of step-free access at Chatham station in Kent.

Cemex used 55 per cent alternative fuels or waste biomass content at its Rugby plant in 2020

Other manufacturers are also introducing ultra-low-carbon products. Last year, Cemex launched a range called Vertua, which has a similar low-carbon result. However, alkali-activated cementitious materials do not currently meet the relevant British standards for cement and concrete manufacture, which mandate a minimum clinker content, and so their use is limited.

The cost of the material is also higher than Portland cement. But backers – who are pushing for changes to British standards – believe prices will come down as they scale up production.

Gittins says the way forward is not to replace Portland cement in all uses, but to understand where high-strength conventional concrete is unnecessary. “We are not looking to replace cement used in the Shard,” he says. “We can’t reach the very high strengths that are needed for structural purposes. There will always be a need for Portland cement. But what baffles me is that there’s lots of low-grade, low-hanging fruit out there where you can replace concrete with low-carbon alternatives and make significant savings.”

Low-carbon alternatives could be used with no detriment in foundations for new-build housing, for example, he says, saving hundreds of thousands of tonnes of carbon dioxide each year.

Commentators say that regulator or government action would help to make a real change at a time when there is so much need to develop low-carbon solutions.

“A clearer picture could be presented to the construction sector if low-carbon cements were certificated for use in clearly defined areas, such as footings for homes, pavements, paving, and areas where OPC is currently used but is unnecessarily robust,” Gittins says.

Cemex’s Fletcher says that the decarbonising of the cement-manufacture process could also be sped up with assistance. “We find that inspectors for the government are ex-industry and are quite amenable to new ideas, but their hands are tied by regulations,” he says. “Rather than a walk-in-the-park, ‘do it tomorrow’ attitude, we are often allowed to conduct a trial and no more.

“To use hydrogen on a permanent basis [to provide part of the power for the cement production process] took over a year.”

The world needs to decarbonise quickly, and although many low-carbon techniques are still niche, bringing them into the mainstream would go a long way to helping the construction industry meet its climate ambitions.

Willmott Dixon’s Mingoia notes: “It is the old saying, adapt or die. Willmott Dixon has been around since 1852 – you can’t stand still. You have to embrace what’s new, otherwise someone else will. If we don’t, then we go out of business.”