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How to trap carbon dioxide in concrete

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November 23, 2021 3:29 PM

The concrete was developed by the Romans, and their legacy is still visible in the magnificent roof of the Pantheon, the largest dome in the world made of unreinforced concrete. Since it was completed around 125 BC by the Emperor Hadrian, a great deal of concrete has been poured – currently around thirty billion tons a year – to build buildings, roads, bridges, dams and other structures. Today it is the most popular building material on the planet and the demand is growing.

This is bad news for global warming. The problem is that the key ingredient in concrete, cement (silicate powder to which sand, gravel and water is added to make concrete), is responsible for an enormous amount of greenhouse gas emissions: the five billion tons of Cement produced annually account for eight percent of the world’s carbon dioxide (CO2) emissions from human activities. If the cement industry were a country, it would be the third largest in the world in terms of emissions after China and the United States.

There are currently few practical alternatives to concrete. Interest is growing in cross-laminated timber (Clt), which is produced from trees and therefore can be a renewable resource, and is also used in the construction of some skyscrapers. But compared to concrete, for now Clt remains a not very widespread material. The main users of concrete, in particular China which produces more than half of the world’s cement, will not be without it in the short term. So the idea of ​​making this sector more sustainable might seem like a desperate undertaking. But it is not, because new technologies are being developed to make concrete greener. Perhaps green enough for it to absorb carbon dioxide rather than release it into the atmosphere.

Hot stuff
The starting point is where the emissions are greatest. The first step in producing cement is the extraction of limestone, which has calcium carbonate (CaCO3) as its main component. The limestone is mixed with the clay and passed in a rotary kiln at more than 1,400 degrees centigrade, through a process called “calcination”. The heat extracts the carbon and part of the oxygen, which when combined together form CO2. The remaining agglomerations, called clinker, are made up of molecular complexes of calcium oxide and silica, known as calcium silicates. The clinker is then cooled and ground to make concrete. Over half of the emissions related to cement production are a consequence of calcination, and most of the remaining ones come from the combustion of coal and other fossil sources used to power the process. Overall, almost one CO2 is released for every ton of fresh cement.

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Companies are developing equipment that can capture carbon dioxide directly from cement kilns

According to a study by Paul Fennell with colleagues at Imperial College London and published earlier this year in the journal JouleSince liming inevitably generates CO2, the most effective approach to decarbonising the cement industry would be to capture and store carbon dioxide before it is released into the atmosphere. Carbon dioxide could be stored underground or used in other industrial sectors, for example to produce synthetic fuels. But it could also be injected into concrete when mixed with water to promote the chemical reactions that cause the cement to harden (polymerization). CO2 has a similar effect, and stops at the calcium carbonate stage.

In fact, reversing the liming process in this way makes the concrete more durable than when using just water. Thus it is possible to reduce the emissions associated with the construction of a specific work and use even less cement, further reducing overall emissions. According to consultancy McKinsey, reverse liming could capture up to 5 percent of the emissions from cement, and as technology evolves, this could be as high as 30 percent.

Several companies are taking this path. Canadian CarbonCure has installed equipment that injects CO2 into premixed concrete in over 400 of its own plants around the world. This method was used to construct several buildings, including a new Amazon campus (which is a CarbonCure shareholder) in Virginia and a General Motors electric vehicle assembly plant in Spring Hill, Tennessee.

So far the CO2 used by CarbonCure has been captured by companies that produce industrial gases. But companies are developing equipment that can capture carbon dioxide directly from cement kilns. And Calix, which is headquartered in Sydney, Australia, is developing an electrically powered system that heats the limestone indirectly, from the outside of the kiln rather than from the inside. This allows you to capture pure CO2 without having to clean up the combustion gases from the fossil fuels burned inside the furnace: therefore, if the electricity used came from non-fossil sources, the cement produced would be completely green.

A pilot plant using this system has been successfully launched as part of a European Union research project at a site in Belgium run by Germany’s Heidelberg cement, one of the largest cement producers in the world. A larger demonstration plant is expected to open in 2023 in Hanover to help improve the technology.

Energy garbage
Another approach – less ecological, but still more sustainable than the use of fossil fuels – is to replace part of the coal burned in rotary kilns with municipal and industrial waste. Several companies are already doing this. For example, Cemex, a Mexican giant of building materials, which produces Climafuel, a fuel obtained from urban waste deprived of recyclable substances, rich in carbon of vegetable origin (biomass) present in the atmosphere until recently and which there it is simply returning, instead of being extracted from underground in the form of fossil fuel. Cemex has replaced up to 60 percent of the coal used in some of its British cement factories with Climafuel.

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The solution is to improve the technology for making concrete so that less is used for certain jobs

Companies are also looking for ways to replace some of the cement in the concrete with other materials. Many add “fly ash,” a by-product of coal-fired power plants, or crushed slag from iron blast furnaces. But none of these solutions are sustainable in the long term. As Peter Harrop, head of UK market research firm idTechEx and co-author of a new report on the future of concrete and cement, notes, coal use is decreasing and steelmakers are targeting new technologies and cleaner.

According to Harrop, an important part of the solution is to improve the technology for making concrete so that less is used for certain jobs. This means adding other substances, for example synthetic and natural fibers or even graphene, a material more resistant than steel made up of carbon sheets as thick as an atom: to obtain satisfactory results only small quantities are needed.

Graphene and other reinforcements will give life to new high-performance concretes, which according to Harrop will be particularly suitable for 3D printing, thanks to which it is possible to create layered materials with precision using robots and significantly reducing waste. “Using much less cement is essential,” he reiterates, especially since cement production looks set to double over the next twenty years.

Additives can also extend the life of concrete and reduce maintenance. At the University of Michigan, Victor Li and his colleagues use synthetic and natural fibers, along with CO2 injection, to make a collapsible concrete they call Engineered cementitious composite. The internal structure of the Ecc is inspired by mother of pearl, a flexible material that covers the inside of the shells of some molluscs, such as abalones and oysters.

Thanks to the flexibility of this concrete, bridges and roads withstand heavy traffic more easily and also improves the earthquake resistance of taller buildings. As it gets older, the Ecc only develops small superficial cracks. Victor Li states that in this way it is possible to dispense with water and avoid corrosion of the steel reinforcing bars present within the concrete, which can cause the reinforced concrete to crumble within a few years, sometimes leading to to the collapse of the structure.

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Towards zero emissions and beyond
The substitution of materials could go even further. Solidia, a company in New Jersey, USA, produces a type of cement that contains calcium silicates with a higher ratio of silica to calcium oxide than Portland, the standard variety. This has two consequences: the first is that the Solidia process requires less heat (therefore less fossil fuel) than conventional calcination and releases less CO2. The second is that, once mixed into concrete, Solidia’s silica-rich silicates can be cured faster than normal cement, using CO2 instead of water. Solidia is studying uses for its cement with one of its investors, LafargeHolcim, the Swiss building materials giant.

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With all these developments in mind, how eco-friendly could concrete become? Fennell argues that with better use of energy and the replacement of some materials, it would be quite easy to reduce CO2 emissions per tonne of concrete currently produced by about 80 percent. But companies could do much more by switching to ovens that are mostly or wholly fueled by biomass, such as wood. The carbon contained in the wood would be released into the atmosphere in the form of CO2, but if the carbon dioxide obtained by burning the wood were stored and not released, as new trees grow to replace the burned ones there would be a net output of carbon from the atmosphere.

Climate modeling scientists believe this track, called bioenergy with carbon capture and storage, (bioenergy with carbon dioxide capture and storage, Beccs), and among those to be pursued in order to reach the “negative emissions” necessary to achieve the goal of net zero emissions or net negative emissions. There is often talk of producing electricity based on Beccs, but this technology may be more suitable for concrete, because in a world attentive to environmental sustainability, the tools for capturing CO2 will be a reality, and will be used to manage the effects of liming. If this were to happen, one of the pariahs of global warming could redeem himself by helping to reduce the damage done to the planet and leaving behind a legacy in its own way as impressive as that of the Romans.

(Translation by Davide Musso)

This article appeared in the British weekly The Economist.

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