Imagine a city that functions like a rainforest, soaking up greenhouse gases instead of emitting them. John Harrison, a Tasmanian inventor, says this can be achieved just by changing the mix of the cement we use.
Portland cement is ubiquitous. Last year a quarter of a tonne was manufactured for each human being on the planet. And every tonne of cement produces a tonne of carbon dioxide. Cement production accounts for more than 5% of all greenhouse gases emitted in Europe and the United States, and more than 10% in China.
Cement, essentially a mixture of chalk, clay and calcium, must be cooked in a kiln at 1 450ÞC. This causes chemical reactions that give the material its strength, but also release carbon dioxide. Substitute magnesite for calcium, says Harrison, and the result is ”eco-cement”, a product with all the structural qualities of Portland cement that produces almost no carbon dioxide.
Eco-cement offers advantages at every stage of its life, says Harrison. It can be made in a low-temperature kiln, saving energy. It absorbs carbon dioxide while setting, and for years afterwards. It is more resistant to salt, acid and chlorine corrosion than Portland cement, needs to be replaced much less frequently and at the end of its lifespan is easier to recycle into new cement products.
Eco-cement can be made using a higher proportion of waste materials such as fly ash from coal burning and slag from blast furnaces.
Eco-cement produces about a 10th as much carbon dioxide as Portland cement. When organic material is added, a concrete block can be built that is a net carbon sink. In other words, a building can be made to function like a tree. — Â