While hundreds of scientists are gathered in Paris to draw plans about how to fight global warming, MIT engineers are also working on the reduction of world emissions of carbon dioxide. As cement, which is the primary component of concrete, accounts for 5 to 10 percent of the world's total carbon dioxide emissions, they're looking at how nanoengineered concrete could cut CO2 emissions. They found that the source of concrete's strength and durability lies in the organization of its nanoparticles and that it would be possible to cut world carbon dioxide emissions by up to 10 percent within five years.
This project has been led by Franz-Josef Ulm, Professor of Civil and Environmental Engineering, with the help of Georgios Constantinides, a postdoctoral researcher in materials science and engineering. Here are the basis for their research.
Cement, the oldest engineered construction material, dating back to the Roman Empire, starts out as limestone and clay that are crushed to a powder and heated to a very high temperature (1500 degrees Celsius) in a kiln. At this high temperature, the mineral undergoes a transformation, storing energy in the powder.
When the powder is mixed with water, the energy is released into chemical bonds to form the elementary building block of cement, calcium-silicate-hydrate (C-S-H). At the micro level, C-S-H acts as a glue to bind sand and gravel together to create concrete. Most of the carbon dioxide emissions in this manufacturing process result from heating the kiln to a temperature high enough to transfer energy into the powder.
And while the researchers were studying a wide variety of cement pastes using a new technique named nano-indentation, they were surprised to discover that "the C-S-H behavior in all of the different cement pastes consistently displays a unique nanosignature, which they call the material's genomic code."
Below is a view of contour plots of the indentation modulus for one series of tests shown on the left. An SEM image of a cement paste is also shown on the right, and "demonstrates the qualitative resemblance of the mechanical maps with electron microscopy images." (Credit: MIT and K. Scrivener for the SEM image)
The C-S-H particles (each about five nanometers, or billionths of a meter, in diameter) have only two packing densities, one for particles placed randomly, say in a box, and another for those stacked symmetrically in a pyramid shape (like a grocer's pile of fruit). These correspond exactly to the mathematically proved highest packing densities allowed by nature for spherical objects: 63 and 74 percent, respectively. In other words, the MIT research shows that materials pack similarly even at the nano scale.
Now, the researchers want to find "a different mineral to use in cement paste, one that has the same packing density but does not require the high temperatures during production." If they reach this goal, it would be possible to cut world carbon dioxide emissions by up to 10 percent according to their estimations.
This research work has been published by the Journal of the Mechanics and Physics of Solids under the name "The nanogranular nature of C–S–H" (Volume 55, Issue 1, Pages 64-90, January 2007). Here is a link to the abstract. And here is another link to the whole long technical paper, even if I cannot guarantee that this link will continue to be available. The above illustration has been extracted from this paper.
Finally, even if these two MIT researchers are extremely bright, I doubt that they can reach such an ambitious goal within five years. But who knows?
Sources: Denise Brehm, Massachusetts Institute of Technology, January 29, 2007; and various other websites
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