By Melissa Reynolds

At a time when everyone is trying to be a little more environmentally conscious, even dull grey slabs of concrete have the possibility to go green.

Long known as one of the strongest building materials available, it's no wonder concrete is so widely used all over the world, from buildings, to sidewalks, to bridges and roads. With its use dating back to the Roman Empire, its durability has certainly stood the test of time and the elements. And its popularity has not waned, either. According to ConcreteIdeas.com, more than six billion tons of concrete are produced each year - enough for one ton for each person on Earth.

Because of its variety of uses, concrete has long been valued as an essential building material. It hardens quickly, is easily molded and is strong yet flexible, so is quite easy to work with. In addition to this, concrete has many more qualities that make it desirable.

Because concrete is generally mixed as needed, in most cases there is not much left for waste. Once it is made, it is made to last, and because of this, few repair materials are needed.

Concrete is also quite resistant to heating and thawing, making it ideal for almost any climate in the world. It can survive wild winds and rain and, unlike wood, bugs and animals cannot chip away at it. Concrete can also absorb a lot of heat, making homes cool in the summer months without the need for air conditioning.

ConcreteThinker.com, a site which provides information about cement-based applications for sustainable development, states that because it is primarily made of limestone, one of the most abundant resources around, concrete can often be made close to home, cutting down on shipping and transport costs. The materials used to manufacture concrete are usually taken from within 300 miles of the manufacturing plant.

Finally, once concrete has outlived its usefulness, it can be recycled by being crushed and used in roadways, sidewalks or as barriers in gardens or yards.

Despite all of its benefits, concrete has many qualities that make it undesirable in protecting the environment. The initial process of making concrete is not an entirely environmentally friendly one. The main ingredients are crushed stone or gravel, water and sand, with about 10% of the mixture consisting of cement. Cement accounts for 7% to 15% of concrete's total mass and is made by heating limestone and clay, and grinding the substance into a powder. Inhabitat.com, a weblog devoted to the future of design, tracking the innovations in technology, practices and materials that are pushing architecture and home design towards a smarter and more sustainable future, warns that the fuels used to heat and grind the limestone and clay create carbon dioxide and more is produced when the mixture is heated.

Despite being only one building material, concrete has a solid share of carbon dioxide emissions. Cement manufacturing produces about 5% of global carbon dioxide emissions, making it a big contributor to global warming, says Physorg.com, a web-based science news service. This is a significant amount, considering how widely the material is used. Although there are reports that state that as concrete ages, it carbonates and reabsorbs all of the carbon dioxide it creates, this can take hundreds of years, especially given concrete's durability.

Because concrete is one of the most widely used substances in the world, the problem stems from volume. According to the World Business Council for Sustainable Development, concrete production has nearly quadrupled since 1970, and may reach five billion metric tones by 2030, especially as some countries such as China and India continue to grow.

There is hope on the horizon, though. Steps are being made to reduce concrete's impact on the environment. During manufacturing, much of the carbon dioxide emission comes from the heat and the chemical reaction as limestone is broken down. By using what are called "supplementary cement materials" such as fly ash, slag and silica fume, manufacturers can reduce the need for cement and therefore reduce carbon dioxide emissions.

Many companies are looking at alternatives as well. According to Inhabitat.com, the English company Novacem has created cement that uses magnesium silicates which do not produce carbon dioxide. Because its production is done at a lower temperature, the cement absorbs more carbon dioxide as it hardens - enough to more than offset the amount generated during manufacturing. A California-based company, Calera, is using a new process as well. Calera's technique is to filter carbon dioxide emissions through seawater to create a carbonate byproduct. This cement substance is then mixed with aggregate and water to create concrete, taking away carbon emissions and avoiding the need to heat the ingredients.

The importance of concrete is obvious, but the importance of reducing its carbon footprint is becoming essential.

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