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Can concrete be sustainable?

New research points the way to green concrete

Concrete lining
Projects like AlpTransit use custom mixtures of concrete, for fire safety and strength

The human world is shaped by concrete. It makes modern infrastructure possible, and is by far the most commonly used construction material. However, although it may look slick and uniform, its contributions to air pollution and climate change are enormous. This impact has long been assumed unavoidable, a necessary price of progress.

New research in Switzerland challenges this presumption...

Switzerland's Gotthard road tunnel experienced a deadly fire in October, 2001. Two trucks collided, and the resulting inferno killed 11 people, and injured scores more. Most of the deaths were caused by the build-up of toxic fumes from the burning fuel. The structural integrity of the tunnel was seriously damaged, causing collapse as the concrete lining failed.

The accident prompted investigations into how concrete can be improved, to make it safer and more fire-resistant. It has always been assumed, however, that admixtures which change characteristics, such as fire retardation and adsorption/absorption, would be paid for by a loss in strength, hardness or elasticity.

A team of researchers in southern Switzerland began experimenting with the innovative idea of including activated carbon in concrete. To their delight, it not only worked to improve fire resistance, but led to a stunning discovery: activated carbon in concrete can also suck pollutants out of the air!

In fact, their concrete can sequester so much CO2 and NOx, it provides a net sink for the pollutants. Employing concrete of this type not only reverses the environmental impacts of the manufacture of the cement itself, but can continue to cleanse the air of NOx and CO2 from other sources for decades. And the new concrete shows no loss in strength, durability or longevity.

Three Gorges Dam, China
China alone consumes 53% of the world's concrete production

The combination of these remarkable properties with its enhanced fire protection makes this new concrete particularly suitable for use in road tunnels, where the accumulation of vehicle exhausts, combined with fire danger, is an ever present hazard.

"We got the idea while developing custom-made concrete mixtures for the AlpTransit Rail Tunnel project", says lead engineer and researcher, Michel Di Tommaso, of IMM, the Materials and Mechanics Institute in Ticino, the Italian-speaking canton of Switzerland. "The Gotthard road tunnel fire was a wake-up call. The concrete they had used failed because of spalling - which is when water molecules locked into the concrete matrix evaporate in the heat, and, with nowhere to go, cause it to pop... like popcorn."

A standard anti-spalling technique in concrete manufacture is the inclusion of polypropylene (PP). PP melts when there is a fire, which leaves micro-fissures throughout the concrete, providing space and paths for the water vapour to escape and avoid catastrophic pressure build-up.

"Although PP alleviates the spalling problem, it does nothing for the pollutants issue, which is an even more serious problem." Di Tommaso and his team decided to investigate whether there were alternatives which could provide a two-in-one solution: even greater fire safety with pollutant cleansing.

The gases which are of most concern for environmentalists are the group of compounds produced by the burning of fossil fuels. These emissions include various combinations of nitrogen and oxygen, known collectively as NOx. The most common NOx resulting from car exhausts are NO and NO2. The latter is a photosynthetic gas, and reacts with sunlight to produce ozone, another major urban pollutant. It will also combine with water in clouds to form acid rain.

A second pollutant which is causing concern is carbon dixoide, the infamous greenhouse gas which countries around the world have made commitments to reduce. Commitments which innovations like green concrete could help to achieve.

"Finding a cheap, standardised method of producing resilient, safe and clean concrete, with the added bonus of being a net CO2 sink, would have a tremendous impact on the global warming problem as well," says the concrete specialist Di Tommaso enthusiastically.

Cement manufacturing plant
Concrete accounts for 6% of the world's anthropogenic greenhouse gas emissions (Photo credit: www.cemnet.com)

When organic matter is heated in the absence of oxygen, a process known as pyrolysis, it produces a substance which is around 70% pure carbon, but in a structure which is very porous, with an enormous surface area. Such a substance has the ability to draw molecules from the air and lock them more or less permanently onto its surface. This is is known as adsorption. The activated carbon, or biochar, produced through pyrolysis has long been used in filters to remove pollutants from liquid and gas streams. However, no-one knew that when it is included in porous concrete, the air which comes into contact with it will be cleansed of its dangerous pollutants.

Switzerland uses a lot of concrete in its enormous civil engineering projects, such as the new AlpTransit railway tunnel, the longest in the world, completed this year. But Europe's consumption is dwarfed by China, currently consuming 53% of the world's annual concrete production. Something of the order of 5.5% to 6.5%1 of anthropogenic greenhouse gases are the result of cement production for concrete alone. Turning the over 4 billion tonnes of cement produced annually into a net sink, would save more than 3 billion tonnes of CO2.

"The bio-char seems to offer a chance to make concrete sustainable, while retaining all the other qualities we need in concrete across a full range of applications," says Michel Di Tommaso. "The science has shown it works. All we need to do now is to convince the industry to change its ways - for the right reasons."

IMM SA is a corporate member of UmweltScience, and more details about its research may be read on its member page: Corporate Homepage: umwelt.science/IMM.

A profile about the researcher Michel Di Tommaso is also available: Profile: Michel Di Tommaso.

1 Cemnet.com: Harnessing CO2

Article by Andrew Bone, Wednesday, 10th August 2016