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| Fri, 14th May 2021 12:32:00 |
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Building's hard problem - making concrete green |
A time-travelling Victorian stumbling upon a modern building site could largely get right to work, says Chris Thompson, managing director of Citu, which specialises in building low-carbon homes.
That's because many of the materials and tools would be familiar to him.
The Victorian builder would certainly recognise concrete, which has been around for a long time.
The world's largest unreinforced concrete dome remains the one at Rome's Pantheon, which is almost 2,000 years old. The Colosseum is largely concrete too.
Today we use more concrete than any substance, other than water.
That means it accounts for about 8% of the carbon dioxide (CO2) we emit into the atmosphere. That is substantially more than the aviation industry, which makes up about 2.5% of emissions.
But some companies are developing concrete that has a much lower CO2 impact.
Citu is building its headquarters in Leeds from a new low-carbon concrete that it says cuts CO2 emissions by 50% compared to traditional concrete.
It has used 70 cubic metres of it for the building's foundations.
This concrete, released last year by Mexico's Cemex under the label Vertua, is one of a series of recent developments helping pave the way to greener concrete.
Making cement, which makes up 10-15% of concrete, is a carbon-intensive process. Limestone has to be heated to 1,450C, which normally requires energy from fossil fuels and accounts for 40% of concrete's CO2.
This separates calcium oxide (which you want) from carbon dioxide (which is the problem).
This calcium oxide reacts further to form cement. Grind some into powder, add some sand, gravel and water, and it forms interlocking crystals.
Voilà, concrete.
So how can you do all this without releasing so much CO2?
One way is by replacing much of the conventional cement with heated clay and unburnt limestone, says Karen Scrivener, a British academic and head of the construction materials laboratory at Switzerland's Ecole Polytechnique Fédérale de Lausanne.
For a long time, people (think, Romans) knew you could substitute some of the cement with ash from burning coal (or volcanoes). Or more recently, slag from blast furnaces. This even improved concrete's strength and durability.
Prof Scrivener was approached by Prof Fernando Martirena from Cuba, who thought it might be possible to use clay in the production of concrete.
So together they worked out a way to replace a really big chunk of conventional cement, and produce equally strong concrete.
Not only would that mean 40% less CO2, it also works with existing equipment, according to Prof Scrivener.
And that's crucial for a material that has to be competitively priced.
Two companies last year began commercially cooking up this product, called LC3 (for limestone calcined clay cement).
"I reckon next year about 10 plants are going into operation, and really we can see an exponential take-off after that," she says.
Read original full article
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