Thursday, July 06, 2006

concrete proposals for reducing carbon emissions

Materials science is the neglected younger brother of the physical sciences, lacking the imagination tickling of either the macroscale or the microscale. But amazing insights into how to build things better continue to arise even in our so-called post-industrial society.

One material with humble origins that is being transformed is concrete, the combination of sand, limestone and water which is usually despised for its lack of imagination. Not all of us can reside in post-modern curvelinear Gehry shacks with laser-cut titanium roofing, so maybe bettering the ancient method of pourable stone would have benefits for developing nations. It would also reduce the need for metals like aluminum and hydrocarbon-based plastics. To produce one cubic meter of aluminum requires 1000 GJoules of energy, while a cubic meter of plastic requires 100 GJoules and a cubic meter of cement only 10. No high temperatures are needed for cement manufacture, only water under normal air temperature. But current cement suffers from a low tensile strength, an inability to bend and a tendency to fracture, compared to metals and plastics. This is caused by air pockets trapped during mixing and the pores and capillaries formed as water dries out of the cement after setting.

To overcome these problems, researchers tried a variety of techniques, from water-soluble polymers that glue the pores shut to squeezing the air bubbles to adding fine silica to reduce particle size or Kevlar fibers for strength. By combining all of these factors plus some high heat and pressure, it is possible to produce fantastically thin concrete works with the strength of steel, like the Footbridge of Peace over the Han River in Seoul.

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