Concrete manufacturers and users have been working on new carbon-reducing recipes for the building material. One day, these may allow for fully carbon-free concrete. But first, they must show that these mixes are as reliable as more traditional ones.

When manufacturers produce concrete, they contribute to carbon emissions and climate change in two ways. The kilns used to convert limestone into clinker require huge amounts power, burning natural gas continuously at temperatures over 1,000C. And in the process of turning limestone (CaCO3) into cement clinker (CaO), vast clouds of carbon dioxide are produced. Quite simply, CaCO3 – CaO → CO2.

In a recent article and podcast episode, Engineering Matters looked at how industry is working with government in the UK, to eliminate the energy-related emissions, by trialling the use of hydrogen in place of natural gas.

But how can the industry reduce or eliminate those process emissions, caused by the nature of the lime-into-clinker reaction? 

One way has been to use alternative materials, generated as a by-product of other industries, in place of some of the cement clinker.

“When we’re making concrete, we’re setting about our stall to actually reduce the amount of clinker we use in our concrete by using cement replacers,” says Robert Gossling, head of engineering solutions at Tarmac. “Traditionally, that’s been fly ash from power stations from burning coal, and it’s been ground granulated blast furnace slag, which is the by-product of the iron and steelmaking process.”

Both of these materials are in short supply in the UK. This is a result of broad economic trends—the deindustrialisation of many countries in the Global North—and of government’s emissions-cutting goals. The UK has shut down coal fired power plants to protect the planet. And iron and steel production has moved to countries with lower labour costs.

The result has been that fly ash is barely available at all, and GGBS has become an expensive import, with all the carbon costs of long distance shipping. A new approach is needed.

Working with nature

One way to reduce carbon footprints is by looking at civil engineering in new ways. We can see many examples of this in work to improve flood defences, which are themselves needed due to the impact of climate change.

Those innovations are being explored by contractors, like BAM, as national frameworks manager Ruth Young explains.

“By bringing in the circular economy, bringing in nature based solutions, looking at design and at  lean engineering principles, there are many different aspects that can contribute to that net zero picture. So it’s not necessarily just looking at materials and the carbon content of materials.”

That too has been the approach of clients, like the UK Environment Agency, as it works on building flood defences, that are themselves needed because of the impact of climate change,

“In some cases, there are quite a lot of concrete elements in coastal defences,” says Andy Powell, innovation manager with the EA. “And in other cases, we’re using soft engineered solutions, such as mud embankments using earth materials.

“It could be some upstream, up catchment, solutions, slowing the water down. Flood alleviation schemes that will be recreating the floodplain to an extent. There’s a big focus on increasing the use of natural solutions.”

Cutting carbon with concrete

In other places, these soft engineering solutions just won’t work. We can’t turn busy towns into floodplains.  And if we do nothing, there are carbon costs associated with the damage caused by flooding.

“We’ve done some work looking at the impact from our work in terms of how we’re reducing flood risk,” says Powell. “And that has a benefit both in terms of reducing flood risks directly, but also in terms of carbon reduction as well. There’s a carbon impact from drying out flooded properties, and from replacing goods and vehicles. We’ve worked out that our current capital programme would deliver about 268,000 tonnes of carbon equivalent reduction [through limiting the impact of flooding].”

Major civil works like this will always require steel and concrete, or very similar materials. One step on the path to Net Zero will be reducing the embodied carbon of these materials. 

Concrete used in the UK is typically specified using the standard BS 8500. This is essentially a recipe book. It lists different ways of mixing concrete, with performance data for each recipe.

Traditional concrete is known as CEM1. One of the cleanest forms of concrete listed in BS 8500 is a ternary composite cement. In materials like this, part of the cement clinker is replaced with a limestone filler.

“It’s cement clinker, it’s filler, and the balance is fly ash or slag,” says Gossling. “The lowest carbon version of these ternary composite cements is a C6, which has a minimum 35% clinker. It can have up to 20% filler and therefore 45% slag.”

In the latest version of the standard, recipes using even less cement clinker are listed. But all of the mixes available in the standard must use some minimum amount of clinker. And this precludes the use of some recently-commercialised materials, which use no clinker at all. These are known as alkali-activated cementitious materials, or AACMs.

“All low carbon concrete has minimal clinker and maximum replacer,” says Gossling. “The AACMs go a step further and totally remove clinker from the equation. You’ve got a very alkaline chemical as an admixture and you’re activating fly ash or slag today with this, and there is zero cement. And that’s why it’s getting to the very lowest carbon footprint.”

Today the alkali is used to activate—to make reactive in the same way as cement—the replacement materials, such as fly ash and ground granulated blast slag. But even more gains could be achieved by replacing these with something new.

“We’ve been looking at what is the solution to replace GGBS,” says Gossling. “And that solution is going to be calcine clay. There’s a little bit of industry work going on at the minute, government funded with MPA, and some of the key concrete producers, just checking the durability numbers out. And most people—Tarmac included—are looking at how and when do we actually start producing calcine clay in the UK. We know we’ve got suitable clays for this. 

“In the future, the activation technology that we’re using today in this AACM product, that technology probably works with calcine clay as well. So it could be with calcine clay, there’s a future with zero cement” 

Calcine clay can be processed in kilns, like traditional clinker. But it doesn’t emit process carbon. That means the only source of emissions comes from energy used to fire the kiln.

Testing time

The recipe book for concrete, BS 8500, lists a variety of ways to mix cement with other materials to produce different types of concrete. But it doesn’t include novel materials using no clinker at all. The normal standards revision process would take too long, given the urgent need to cut carbon emissions.

The BSI has produced a new standard, BSI Flex Standard 350. Rather than specifying recipes, this standard describes steps users and materials suppliers can take to prove the performance of new materials.

Now concrete suppliers can identify a novel material, perform small scale tests, and then—with the support of the right client and contractor—move to real world testing.

And that is what Powell, Young and Gossling have been doing at Hexham, in the North East of England. 

In 2015, Storm Desmond hit north east England. The storm formed an ‘atmospheric river’ of moist air, which soaked the region. This fed and filled the Tyne, which swelled, broke its banks, turned fields into lakes, and submerged homes and community assets.

Three people were killed, and around £870m of damage was caused. The recovery from that damage and loss created hundreds of thousands of tonnes of carbon dioxide to be released into the atmosphere.

The Environment Agency set about planning how to alleviate flooding in the region. 

“The Hexham scheme was designed to provide flood risk management and flood protection to the town,” says Powell. The agency’s response made use of soft engineering, of flood plains, and of earthen embankments. But it would also—where the Tyne flows through the town—require some use of concrete. And this would be an ideal testing ground for novel materials.

Young, and colleagues at BAM, proposed making use of the project to test different concrete types. As these would be used in a flood wall, on EA-owned land, they would be exposed to the elements, but be accessible for testing. 

“It basically went from there,” says Young. “It opened up the doors in terms of what was actually out on the market and how it could be included within the permanent structure. We didn’t want to go down the route of doing something within an uncontrolled low risk environment. We wanted it to be included within the flood wall itself.”

That fitted perfectly with the Environment Agency’s approach to promoting novel materials—an approach that will, over decades, help to reduce the carbon emissions that produce the need for flood defences.

“As part of our net zero carbon target, we’ve we’ve got different work streams to help us achieve that, one of which is an Innovation Pilot Fund, which I coordinate,” says Powell, “And that’s very much the Environment Agency working with our delivery partners, designers, and contractors, looking at the projects that we’ve got in the programme for those opportunities to trial some emerging and new construction materials and products.”

For the trial, BAM poured three concrete types. 

The first was a C3A, already covered by the standards. This uses 50% Portland cement, with 50% GGBS. This was used for most of the work.Then, two short sections were poured using novel mixes.

One was a Portland limestone fines concrete. That consists of 20% crushed limestone, with  35% Portland cement, and then 45% GGBS. And the last was an AACM, of a type that Tarmac has developed with the help of an Australian partner, Wagners, and is just now bringing to market.

“What we’ve recently  launched in London is an AACM system using the Wagner Earth Friendly Concrete activation system that they developed in Australia.  It’s a liquid system. We’ve got one concrete plant running in London now, at Silvertown, and we’re looking at setting up a second so that we’ve got a backup plant.”

For the Hexham trial, the third section of the flood wall defences were poured using 95% GGBS and 5% of the alkaline activation solution.

It is vital that any novel material can be used with the same ease as those it replaces, that it is equally durable, as well as achieving its carbon cutting goals. And the Hexham trials showed that this was the case for the new concrete types.

“This was constructed back in February 2023,” says Gossling. “There’s a lot of testing being done for durability. And some of those answers still aren’t out yet. But we did learn how to make both types of concrete in our plants. Both survived three hours shelf life to getto site, it was placed very well, it had an excellent finish.”

Young adds, “Once that low carbon concrete reached the site, we were able to pour it as usual.”

And the carbon emissions calculations were equally encouraging. The partners compared the C3A used for the main sections against CEM1: the traditional ‘standard’ cement. Then the two shorter sections, using the composite ternary mix and alkali activated mix, were compared in the same way.

“The C3A was roughly a 50% reduction on a CEM1 mix; the composite ternary mix was a 64% reduction against a CEM1; and the AACM one was actually a 70% reduction,” says Gossling, “And actually if we made that same project today in the AACM, that would be a 79% reduction against a CEM1”

The tests at Hexham showed that novel materials—even those like the AACM, with no carbon emitting cement clinker—can be poured like traditional materials, and offer considerable carbon gains. With access to the sea wall, ongoing performance tests will be possible.

And with the adoption of BSI Flex Standard 350, tests like this will give industry and standards experts vital information on how these new materials can be used safely, quickly enough that they can have a real impact on climate change.