Imagine a city that functions like a rainforest, soaking up greenhouse gases instead of emitting them. It is an unlikely scenario in a world that seems powerless to halt the spread of carbon dioxide in the atmosphere. But John Harrison, a Tasmanian inventor, says this can be achieved just by changing the mix of the cement we use.
Portland cement, invented nearly two centuries ago by a Yorkshire stonemason, is ubiquitous. Last year, a quarter of a ton was manufactured for each human being on the planet. And every ton of cement produces a ton of carbon dioxide. Cement production accounts for more than 5% of all greenhouse gasses 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 145ºC. This provokes 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’â€, 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 it 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. Governments are starting to insist that a proportion of fly ash is used in building contracts, but Portland cement can only take a limited amount of waste material. ‘We have made bricks of eco-cement that are 90% waste,†says Harrison.
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. In other words, a building can be made to function like a tree.
The idea of sequestering carbon dioxide from the atmosphere is gaining currency as governments show themselves unwilling or unable to reduce emissions. But schemes for pumping carbon dioxide into the sea or growing giant algal blooms are inherently risky, Harrison argues.
‘Any materials that take little energy to make, are cheap, strong, set by absorbing CO2, and can include a large proportion of wastes have to be an important part of the future of our existence. Green building technologies offer a way for all countries to reduce greenhouse gas,†he says.
An independent scientific appraisal by Fredrik Glasser, a materials expert at the University of Aberdeen, Scotland, found that eco-cement was strong, workable, cheap, resistant to corrosion and more environmentally friendly than Portland cement. It can be made using the the industry’s existing facilities.
Yet Harrison is finding it hard to make headway in the commercial market. ‘The building materials industry is conservative,†says Glasser. ‘Innovation is the exception, not the rule.â€
The greatest obstacle is that eco-cement’s key ingredient, magnesite, while abundant in the Earth’s crust, is rarely mined – until now it had few obvious uses. Given economies of scale, eco-cement should be cheaper than Portland cement because less energy is required for production.
The industry is just becoming aware of eco-cement. Britain’s Building Research Establishment is appraising the product, thanks to an industry-funded research grant. Harrison hopes the bottom-line benefits will persuade businessmen who are impervious to ecological arguments. ‘I think the extra durability of eco-cement, rather than its carbon-sequestering ability, is what will finally make the industry sit up and take notice,†he says. –