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It’s a brave man who challenges the world-wide cement industry, which produces getting on for two billion tonnes of the stuff every year.

All of it is Portland cement, invented by a Leeds stonemason named Joseph Aspdin two centuries ago (it was called that because its finish was thought to resemble stone from quarries at Portland in Dorset). Portland cement is made by cooking a mixture of chalk or limestone with clay in a kiln at high temperatures, a process that gives off large amounts of carbon dioxide.

Now John Harrison, an inventor from Tasmania, has found a way to make a cement that’s more ecologically acceptable. He replaces the calcium-based lime with reactive magnesia, a form of magnesium oxide. This can be kilned at a much lower temperature, so needing less fuel; more importantly it rapidly absorbs large amounts of carbon dioxide from the air when it sets and cures (Portland cement does this, too, but much more slowly).

Also, the new eco-cement permits large amounts of organic waste material to be incorporated. The result, Mr Harrison claims, is a cement that can act as a net carbon dioxide absorber; in other words, putting up a building using his cement would be much like planting a grove of trees.

And if eco-cements gained a foothold in our cities, they could immediately reduce the cement industry’s contribution to global warming, reabsorbing much of what was emitted in their creation.

Toronto Star, 27 Jul. 2002

Basic eco-cement produces about a tenth as much carbon dioxide as regular Portland cement. When organic material such as hemp fibre is added, a concrete block can be built that is a net carbon sink.

Guardian, 28 May 2003

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Last modified: 5 July 2003.