January 31, 2007 2:27 PM PST
Unlocking the secrets of concrete to cut greenhouse gases
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Franz-Josef Ulm, professor of civil and environmental engineering, and post-doctoral researcher Georgios Constantinides have conducted a series of studies that indicate that the ubiquitous building material derives its strength from how the different particles inside concrete are arranged relative to each other, rather than the chemical composition of the particles themselves.
As a result, Ulm and Constantinides believe it may be possible to make concrete out of different materials and processes. Concrete is currently made by heating limestone (calcium, carbon and oxygen) and clay (silicon, aluminum and other materials) to high temperatures (1,500 degrees Celsius). The process generates significant amounts of carbon dioxide.
Replacing the calcium from limestone with another material such as magnesium could conceivably lead to lower processing temperatures, and thus lower greenhouse gas emissions. Indirectly, it could also ease recycling and disposal costs. Now, industrial concerns pay to get rid of magnesium.
The work is preliminary, and it may take five years to identify materials, but the large amounts of concrete used each year mean new developments could put a sizable dent in carbon dioxide emissions. Roughly 2.35 billion tons of concrete are manufactured annually; that works out to a cubic meter for each person on Earth a year, according to the researchers at the Massachusetts Institute of Technology. The Romans built an empire with it and some believe Egyptians relied on it, too.
When the limestone and clay are mixed together and heated until the mixture turns into a powder, a significant amount of energy gets stored in the powder. When the powder gets mixed with water, the energy is released to form strong calcium-silicate-hydrate bonds, which bind sand and gravel together to make concrete.
On the nano scale, the resulting structure looks like a pyramid of oranges, an incredibly dense way to pack materials. (Coming up with a mathematical proof for packing spheres in a box in that familiar way bedeviled several prominent mathematicians for years.)
For their research, Ulm and Constantinides produced cement pastes from other materials and tested all of them with an atomic force microscope, sort of a particle probe. They poked the materials and measured the indentations.
The results showed that the different pastes exhibited similar densities and had a consistent "nanosignature." This led to the conclusion that the structure, rather than the material itself, gives concrete its strength.
The results of the pair's work is featured in the January issue of the Journal of the Mechanics and Physics of Solids.
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