Ecomaterials Concrete

As possibly the most extensively used building material, concrete attracts criticism from environmentalists on account of its carbon intensive production techniques and its use of a once-only natural resource, limestone. Cement is formed by heating clay and lime in a rotary kiln to a temperature of about 1450°C which produces some 3000 kg per tonne of carbon dioxide (CO2). In addition the heating process produces a chemical reaction through the conversion of calcium carbonate into calcium oxide which releases about 2200 kg of CO2. Add to this the carbon miles in transportation, the impacts caused by mining etc. and concrete gains few points on the sustainability scale.

The development of the technology of geopolymers offers the prospect of a more eco-friendly concrete. Geopolymerisation is a geosynthesis which is a reaction that chemically bonds minerals to form molecules that are structurally comparable to the molecules which provide the strength to rocks. In the opinion of Jean Davidovits of the French Geopolymer Institute at St Quentin these 'geopolymeric' concretes would reduce CO2 emissions associated with conventional concrete by 80-90 per cent. This is said to be due to the avoidance of calcination from calcium carbonate and the lower kiln temperature of 750°C. The market availability of this material is said to be at least five years away (see

Saving energy is one thing; buildings as carbon sinks is another, yet this is the destiny of buildings according to John Harrison, a technologist from Hobart, Tasmania. He has produced a magnesium carbonate-based 'eco-cement'. In the first place it only uses half the energy for process heating required by calcium carbonate (Portland) cement. The roasting process produces CO2 but most of this is reabsorbed by a process of carbonation as the cement hardens. Using eco-cement for such items as concrete blocks means that nearly all the material will eventually carbonate resulting in an absorption rate of 0.4 tonnes of CO2 for every tonne of concrete. The ultimate eco-credential of this material is the rate of carbon sequestration. According to Harrison 'The opportunities to use carbonation processes to sequester carbon from the air are just huge. It can take conventional cements centuries or even millennia to absorb as much as eco-cements can absorb in months'

('Green Foundations', New Scientist, 13 July 2002, p. 40). This means that an eco-concrete tower block can perform the same function as growing trees as it steadily fixes carbon. Harrison estimates that a shift to eco-cement could ultimately cut CO2 emissions by over 1 billion tonnes since it could replace 80 per cent of uses currently served by Portland cement.

There is one further attribute to this material. Being less alkaline than Portland cement it can incorporate up to four times more waste in the mix than conventional cement to provide bulk without losing strength. This could include organic waste which would otherwise be burnt or added to landfill, sawdust, plastics, rubber and fly ash.

Eco-cement is not unique in its pollution absorbing properties. Mitsubishi is producing paving slabs coated with titanium dioxide which remove most pollutants from the air. In Japan 50 towns are already using these photocatalytic cements and in Hong Kong it is estimated that they remove up to 90 per cent of the nitrogen oxides or NOx gases that create smog. Magnesium-based concrete coated with titanium dioxide could be the basis for eco-cities of the future.

Electric Car Craze

Electric Car Craze

Electric Cars Are A Savior From Pollution And Can Be A Laudable Investment For You. Find Out How Electric Car Conversion Shrinks Your Driving Expenses.

Get My Free Ebook

Post a comment