In construction, the term 'green concrete' usually refers to concrete that has set, but not yet hardened. But there's another definition for this term relating to its environmental impact, and it's one that builders should note.

Conventional concrete is made of sand, rock, water and a caustic material called Portland cement, which is created from hydraulic lime. This essential concrete component binds the finished product together.

Mining, manufacturing and transporting the materials for Portland cement is highly resource- and energy-intensive, releasing about a ton of carbon dioxide into the atmosphere for each ton of Portland cement created. Cement production produces about 5% of all greenhouse gases emitted in the United States, and that percentage will only grow as cement production rises from 2.55 billion tons produced annually in 2006 to an expected 3.7 to 4.4 billion tons by 2050.

But there are alternatives to Portland cement that are far more environmentally friendly. Fly ash left over from burning coal in power plants can replace up to half the Portland cement required to manufacture concrete. Not only does using this waste product in concrete produce a final product that's less porous than conventional mixes, making it resistant to corrosion and deterioration, it also helps solve the problem of disposal. Power companies typically send fly ash to landfills or store it in potentially dangerous ash ponds.

Another waste product known as blast-furnace slag has a similar effect when used in place of Portland cement. Slag is a byproduct of smelting ore in the metal purification process, during which impurities are separated from molten metal. Slag cement requires nearly 90 percent less energy to produce than an equivalent amount of Portland cement. Using 'blended cement' made with fly ash or slag could reduce CO2 emissions from concrete production by as much as 20%.

Some 'green concrete' even captures and permanently stores the CO2 that's released into the atmosphere from power plants and other sources. Companies like London-based Novacem are developing cement that absorbs more CO2 once in place than it released during the manufacturing process. When magnesium oxides are mixed with water, they create a solid-setting cement that reacts with atmospheric carbon dioxide as it hardens, strengthening the cement while trapping the gas.

Other ways to reduce the carbon footprint of concrete in construction include the use of recycled water, vegetable fiber or reclaimed concrete and masonry in the mix.

Concrete is the single most-used material in construction, so switching to a greener mix could have a significant environmental impact. In an industry that tends to be resistant to change, preferring to stick to time-proven methods and materials, this could be a challenge - but as we learn more about the benefits of green concrete, gaining insight on its long-term performance, it's likely to see much broader use around the world.

United Rentals has the tools and expertise you need to get to get your concrete and masonry job done. Got questions? Go to UnitedRentals.com or call 800.UR.RENTS.