‘Carbon-Free’ Cement Is Now A Step Closer

Major breakthroughs see emissions slashed from the world’s most destructive material

Concrete is the second-most used material on the planet (water takes top spot). Wherever humans are, you’re likely to find the dull, grey substance. Concrete is used to build everything from homes and hospitals, to airports and dams. Amongst other things, it has allowed humanity to conquer the underground, providing safe passage for trains, and routes for water, waste, electricity, and internet services. It would be hard to imagine the modern world without concrete.

It is also widely considered the most destructive material on Earth. The damage wrought by concrete is not just about – as Guardian journalist, Jonathan Watts once wrote, its ability to “entomb vast tracts of fertile soil, constipate rivers, choke habitats and – acting as a rock-hard second skin – desensitise us from what is happening outside our urban fortresses.” It’s about the emissions that are generated while making it.

Depending on who you ask, concrete production accounts for between 7 and 10% of all CO₂ emissions. But the vast majority of those emissions come from a single ingredient of modern concrete; Portland cement. To make it, you pulverize limestone, silica alumina and iron, mixing them together in specific proportions. You then heat your mixture in a kiln at temperatures of up to 1650 °C (~3000 °F), which releases CO₂ from the limestone, and produces a lumpy mixture of stable compounds termed clinker. You then let the clinker cool before pulverizing it again, and adding a small amount of gypsum. What you’re left with is Portland cement, a fine powder that effectively acts as the ‘glue’ in concrete. It’s also the target for most efforts to decarbonize the material, which aim to alter the carbon-rich chemistry without negatively impacting the cement’s performance.

Concrete’s other two ingredients also have an impact on the environment. Most of its volume is aggregate – sand and gravel or crushed stone quarried from mines – and about 7 % is water. Small quantities of other additives can be included to fine-tune the mix. Through a process called hydration, the cement powder and water react, hardening and binding the aggregates and additives into the tough, rocklike mass we’re all familiar with. As I’ve written about previously, this hardening process continues for years after the concrete has been poured, which means it gets stronger as it ages; at least for a while.

Given the ubiquity of concrete, and its ever-increasing carbon footprint, the race is on to find better, less damaging ways to make it. And in the past fortnight, two different approaches – reported in the scientific literature – caught my eye.

Coal waste

The first is from a research group at RMIT University in Melbourne, Australia. They’ve found a way to replace 80% of the cement in concrete with a waste product from coal-fired power plants.

Coal currently represents a large-but-decreasing proportion – 21 GW (gigawatts) – of Australia’s energy landscape. Burning coal produces not only staggering amounts of greenhouse gases and other pollutants such as heavy metals, it also leaves behind a fine powder called fly ash, which is mainly comprised of silicon, oxygen, aluminum, iron, and calcium. For every four tons of coal that is burnt, approximately one ton of fly ash is produced, which means that there is a lot of it around. According to RMIT, fly ash “accounts for nearly a fifth of all waste [in Australia] and will remain abundant for decades to come, even as we shift to renewables.”

The team, led by Dr Chamila Gunasekara, are not the first to propose using fly ash as a partial substitute for Portland cement. There are already numerous products on the market that use up to 40% fly ash in the mix, with some that achieve 60%, but at the cost of producing a weaker, less durable concrete.

By adding nano silica and hydrated lime to their fly ash, the RMIT team have successfully produced concrete with 65% and 80% replacement of cement. Their concrete increased its strength over a period of 450 days, and structures made with it could withstand exposure to sulfuric acid and sulphates – found in acid rain – for up to 24 months. Alongside this, they also developed a mathematical model to understand how various ingredients in ‘low-carbon’ concretes behave. Dr Yuguo Yu, one of the study’s authors, said that this “offers us opportunities to reverse engineer and optimise mixes from numerical insights.” For example, he says “The inclusion of ultra-fine nano additives significantly enhances the material by increasing density and compactness.”

All of this work has also allowed the researchers to utilize an even lower-grade waste material called ‘pond ash’, “potentially opening a whole new hugely underutilised resource for cement replacement,” Gunasekara said. “These ash ponds risk becoming an environmental hazard, and the ability to repurpose this ash in construction materials at scale would be a massive win.”

Of course, the RMIT approach still relies on the fossil fuel sector to operate. But a paper published just last week has found a path that could potentially do away with carbon emissions entirely.

Electric cement

At the heart of this is an approach that combines steel recycling and cement recycling, and it’s being led by engineers at the University of Cambridge.

The process starts with waste concrete from the demolition of old buildings. This is crushed and the aggregate separated out, leaving a cement paste (cement + water) behind. The recovered cement paste is then brought to an electric arc furnace, where it used instead of lime-flux in steel recycling.

In standard steel recycling, the role of lime-flux is to form a coating on the surface of steel as it melts in the electric furnace – this protects the liquid metal from oxygen in the air. Once the steel is released from the bottom of the furnace, the coating – now called slag – cools quicky and is crushed into a powder. After that, it is often sent to landfill. But by switching out the lime for recovered cement, the Cambridge technique changes the equation, “We found the combination of cement clinker and iron oxide is an excellent steelmaking slag because it foams and it flows well,” said Dr Cyrille Dunant who invented the process. In addition, the heat of the furnace reactivates the cement. “If you get the balance right and cool the slag quickly enough, you end up with reactivated cement, without adding any cost to the steelmaking process.”

Importantly, this means that they’re not producing a cement supplement or substitute; they’re taking old, deactivated cement and turning it into new, ready-to-use cement.

Dunant and his colleagues, Dr Pippa Horton and Professor Julian Allwood, have filed a patent application on their process, and founded a company called Cambridge Electrical Cement (CEC) to bring it to market. It’s still early days, but recent successful ‘trail melts’ carried out in a 7-tonne electric-arc furnace at the Materials Processing Institute show that the process is scalable.

Long-term, the furnace could also be powered by renewable energy. As CEC’s senior project manager Patricio Burdiles has been quoted as saying, that would effectively reduce the carbon footprint of the process to zero, “Our reactivated Portland cement is carbon-neutral since there are no process emissions, none from the burning of fuel and, potentially, not even any from the running of the EAF, beyond those already involved in recycling steel.”