Life-Cycle Studies: Concrete
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Stepped on for more than 2,300 years, concrete rarely gets the respect it deserves. Perhaps the most ubiquitous building material, concrete strengthens our homes, schools, office buildings, and hospitals. Its manufacture is also among the most energy-consuming and polluting industrial processes. Analysts expect greenhouse gas emissions from global concrete production to become a larger contributor to climate change than the European Union in the next 20 years.
Concrete - a mixture of cement, sand, and water, heated in a kiln - became commonplace during the Roman Empire. Although visionary architect Vitruvius wrote that the sand should be free of any earthy impurities, Roman cement itself was sometimes mixed with fat, milk, and ox blood to increase its adhesive properties.
The spread of concrete ever since reflects global distributions of wealth and prosperity. Cement production boomed during recent decades, from 594 million tons in 1970 to 2.3 billion tons in 2005. As the growth of leading producers China and India continues, global cement production may reach 5 billion tons in 2030, the conservation group WWF estimates.
Modern concrete's main ingredient, cement, is most commonly produced using a method invented by Englishman Joseph Aspdin in 1824. His Portland cement (named after the limestone used in St. Paul's Cathedral and Buckingham Palace) mixes a powder of alumina, silica, lime, iron oxide, and magnesium oxide, which is then heated at temperatures up to 1,450 degrees Celsius.
Heating and grinding the cement materials consumes an average of 4-5 gigajoules of energy per cement ton. The industry as a whole uses at least 8 billion gigajoules each year. Cement production-through cement plants' fossil-based energy consumption, the CO2 burned off when limestone is heated, associated vehicle use, and other factors-accounts for about 6 percent of global anthropogenic greenhouse gas emissions, according to a recent WWF report.
In addition to its contribution to climate change, concrete production generates substantial amounts of waste. In China, it is responsible for more than 40 percent of industrial dust emissions. The dust can be recycled into the production process, but the U.S. Environmental Protection Agency warns that the highly acidic substance could pose "toxicological problems, human tissue burns...corrosion in pipes, and objectionable taste in drinking water" if released into the air or water.
Doing It Better
The cement industry's interest in reducing energy costs has led many countries to replace small-scale cement plants with larger, more efficient models. The most efficient kiln models accounted for about 6 percent of China's cement output in 1995, but these are expected to provide 80 percent of Chinese cement next year.
Yet state-of-the-art kilns still consume about 3 giga?joules of energy per cement ton. Renewable energy sources, such as sustainable biomass, could provide more than the current 5 percent of kiln fuel in most developing countries, or kilns may be updated with carbon capture and seques?tration technology. But even if that technology becomes affordable, the International Energy Agency still expects the cement industry to account for 9 percent of global carbon dioxide emissions in 2050.
Alternatives emerged this past year that may redefine the future of concrete. Competing U.S. and British inventors claim they have developed cement production methods that generate zero greenhouse gas emissions and capture emissions released as the cement hardens. If true, their discoveries could become the pillars of a sustainable future.
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