The quest for smarter, and greener, cement

This is because there is no real alternative to the material that underpins everything from Hadrian’s Wall to the Burj Khalifa. Concrete – and the cement that plays a critical role in its performance – is here to stay.

Long experience with cement and concrete, however, has led to constant improvement. This, coupled with a natural industry conservatism, makes technology advances hard to come by.

“Portland cement is an amazing material,” says Theo Hanein, a researcher in cement materials at the Department of Materials Science and Engineering at Sheffield University. “There’s no reason to move away from it, other than its effect on the planet.”

Cement’s lack of sustainability is well known. When producing clinker – a precursor to cement – the calcining process converts calcium carbonate to calcium oxide, releasing carbon dioxide. In addition, the reaction itself takes place at close to 1,400°C, which involves burning large quantities of fossil fuel.

The maturity of cement and concrete has not stopped researchers looking into further refinements. These include ways to improve strength, simplify production and incorporate diagnostic capability. However, each of these goes hand in hand with a need to improve sustainability at the same time.

Hanein recently co-wrote a paper in Energy & Environmental Science proposing a chemical alternative to calcining. Rather than wrenching CO2 away from calcium carbonate at high temperature, he proposes reacting the mineral with sodium hydroxide at room temperature and pressure. This produces calcium hydroxide, which can be used to make clinker, and sodium carbonate – which ‘locks in’ the CO2, rather than releasing it.

At the moment, the technique is not as ‘green’ as it appears, because sodium hydroxide production is very energy-intensive. “That would change if we used green electricity,” says Hanein.

A similar process – though carried out ‘mechanically’ – has been proposed by researchers at Johannes Gutenberg University (JGU) in Germany.

In a paper in Advanced Functional Materials, researchers have proposed combining calcium carbonate and sodium silicate in a high-impact ball mill. This avoids the need for calcining, with its high energy input, and produces an intermediate compound that can be used in cement production. Again, the CO2 that is released during calcining is ‘trapped’ within an inorganic mineral.

‘Solid state’ reactions like this can be confusing to non-chemists, but Wolfgang Tremel, professor of chemistry at JGU, explains that it is achieved by reducing particle size and increasing surface area. This, combined with heat produced in the mill, drives the chemical reaction forwards. “So far, we’ve only done this at small scale,” says Tremel. The challenge now is to make it work at a larger scale.

“I think any patent that succeeds will have to set up the process to make cement at scale in this way,” he says.

Although the cement industry uses ball mills widely, he says they typically do not reduce particle size enough to allow the type of reaction he describes. “The mills don’t have a high enough impact.”

Tremel says that milling is gaining increasing use as a ‘green’ technique in the chemical industry, as it can help to avoid the use of solvents. This, he says, may help to drive its adoption in other industries, such as cement – but it will take time.

“Doing this on a technical scale would take many years and would not provide a short- or medium-term remedy for CO2 emissions,” he points out.

Back at Sheffield University, Hanein is looking into other ways to improve cement performance. He leads a project called FeRICH, which aims to use steel-industry waste as a raw material in the cement-making process.

“We’re looking to make iron-rich cements, and use less calcium,” he says. These types of cement would have less need for calcining. Moreover, using steel slag as a raw material would cut the need for virgin limestone (calcium carbonate).

The iron-containing ferrite phase also helps to lower the temperature at which clinker is formed, from around 1,500°C to 1,350°C. “In the kiln, it creates a flux that speeds up reaction kinetics,” he explains. “This helps the clinker phase form more easily.”

Clinker has several ‘phases’, each based on a different metal. One of them is the ferrite phase, but it is relatively minor. “It’s the least studied phase, because it’s the smallest,” says Hanein. “We need a better understanding of the ferrite phase.”

Part of FeRICH’s aim is to understand the fundamentals of the ferrite phase, such as how it is hydrated, in order to produce iron-rich cements. The project begins later this year and will run for three years. Hanein believes that ferrite-rich cement mixtures could be developed in as little as five years. He estimates that full commercialisation – following various approvals – could happen in 7-10 years, though he notes that the current technology readiness level (a measure of a technology’s maturity) “is quite low”.

Other than the sustainability angle, another potential benefit of raising ferrite content could be to imbue concrete with electrical and magnetic properties. Though only an idea at this stage, Hanein says this could allow the creation of “intelligent infrastructure”. This will be looked at in the final year of the project. “It could identify which areas need repair, where there are cracks,” he says. “It might even be used to make a car park that recharges cars.”

Adding electrical properties to cement has been considered elsewhere – including at Chalmers University in Sweden, where researchers have developed a concept to create a cement-based rechargeable battery.

It could allow a 20-storey building to store energy, by adding carbon fibres to the cement and electrodes to the building structure.

“Earlier studies of concrete battery technology showed very low performance, so we had to come up with another way to produce the electrode,” says Emma Zhang, co-author of a paper in Buildings on the concept.

The paper explains the lab-scale proof of concept. The cement mixture incorporates around 0.5 per cent by weight of short carbon fibres, which adds conductivity and flexural toughness. A metal-coated carbon fibre mesh  with an iron anode and nickel cathode is then embedded into the mixture.

The average energy density is 7Wh/m2  over six charge and discharge cycles, which Zhang says is about 10 times that of other concrete battery concepts. While the energy density is low compared with commercial batteries, this limitation could be overcome because of its huge volume when used as part of a building.

Zhang sees several potential uses for such a battery, including energy storage, monitoring and applications such as providing 4G connections in remote areas, or protecting against corrosion in concrete infrastructure.

“It could also be coupled with solar panels to provide electricity and become the energy source for monitoring systems in highways or bridges, where sensors operated by a concrete battery could detect cracking or corrosion,” she says.

The idea is still a long way from commercialisation, with many technical issues still to be solved – such as extending its service life and developing recycling techniques.

“As concrete infrastructure is usually built to last 50 or even 100 years, the batteries would need to be refined to match this – or be easier to exchange and recycle,” she explains.

UK-based researchers have developed a way to reduce the carbon footprint of concrete – while also strengthening it.

The sequestration process developed by Concrete4Change (C4C) was showcased at last year’s COP26 conference in Glasgow. It relies on injecting CO2 gas – captured from various sources – into special ‘carriers’ that are added to cement. Over a relatively short period, the absorbed CO2 will cause chemical reactions that create stronger concrete.

“This process, called carbonation, happens naturally in concrete, but takes a very long time,” explains Sid Pourfalah, CEO of C4C.

When cement is hydrated, one of the many chemicals formed is calcium hydroxide. This can further react with carbon dioxide to form calcium carbonate – which helps form a stronger structure.

Although concrete can absorb up to 30 per cent of its weight in CO2, this process can take up to 100 years, says Pourfalah. “This would require a specific temperature and humidity – and that’s something that can’t be controlled,” he says. “In addition, absorption only happens at the surface.”

C4C has developed a solid-state ‘carrier’ – a polymeric additive that can take CO2 and transfer it into the cement mix. This distributes the CO2 throughout the cement, allowing carbonation to take place at any depth.

“We shrink the carbonation process from 100 years to 100 hours,” says Pourfalah.

A common plan for reducing atmospheric carbon dioxide is carbon capture and storage, in which CO2 is caught from industrial process and buried. A recent refinement to this is carbon capture and utilisation. Here, rather than simply burying CO2, it is put to use in some way. In C4C’s case, it is absorbed into cement and “permanently mineralised”.

The carrier itself is “polymer based” – but it is not envisaged as a single substance. Pourfalah says the idea is to make different carriers from materials that are available in each local market, but they will all do the same job.

One benefit of stronger cement is that less of it is required to make concrete. This is an effective way to reduce carbon footprint, because the cement has far more embedded carbon than the aggregate it is mixed with to form concrete. Pourfalah says this is a more effective long-term solution than other approaches such as using fly ash as an additive. This is because fly ash comes from coal-fired power stations, which are declining in number.

Any modifications to cement must be carefully assessed, in order to ensure that the changes do not affect concrete quality. For this reason, C4C is working with the University of Warwick on an extensive testing programme.

“Every time you mess with the chemistry of concrete, you want to know how it may affect properties,” says Reyes Garcia, assistant professor in structural engineering at the University of Warwick.

For instance, carbonation – the process that C4C is exploiting – can change the pH of cement and make it slightly more acidic. When carbonation happens naturally, this can lead to faster corrosion of internal steel structures.

Results to date are promising, he says, regarding key attributes such as strength and durability.

“We’re using a very small amount of the carrier and can increase strength by 20 per cent,” he says. “That would allow us to reduce the amount of cement used in concrete.”

However, because the industry is very conservative, the team needs to carry out many tests that allow comparisons with ‘standard’ concrete. This includes casting and testing small specimens and analysing the microstructure.

“We’ve found that the additives mix uniformly and don’t form lumps, which would affect long-term behaviour,” Garcia explains.

C4C is aiming to develop a range of products, once testing is complete. The first generation – which could be on the market in two years – should be able to reduce emissions by 20 per cent, and cost by around 10 per cent, says Pourfalah.

The construction sector is under pressure to reduce carbon emissions. Ongoing refinements in cement and concrete technology – carried out with a nod towards the environment – could put it on the road to improved sustainability.