'Green' Concrete Could Reduce CO2

Five to 10 percent of the world's total CO2 emissions come, not from automobiles or forest fires, but from manufacturing cement. The global warming gas is released when limestone and clays are crushed and heated to high temperatures.

But a new understanding about the fundamental properties of nano-sized particles in concrete could put an end to burning.

"We are looking for materials that allow us to reduce the high temperatures required for the production of cement. Any modification we do at that stage can have an enormous impact," said Franz-Josef Ulm, professor of civil and environmental engineering at the Massachusetts Institute of Technology in Cambridge, Mass.

Ulm and postdoctoral researcher Georgios Constantinides reported their findings in a recent issue of the Journal of the Mechanics and Physics of Solids.

The team showed that concrete's strength and durability — a characteristic that scientists have been hard-pressed to describe — lies in the organization of spherical nanoparticles, namely calcium silicate hydrates.

For an analogy, Ulm referred to oranges. If a bunch are dumped randomly into a box, they fall together randomly in a structure that is not particularly dense. Some cement that is not very strong has particles arranged in a similar configuration.

But, oranges can be arranged in a more dense formation—by stacking them in a pyramid.

It turns out that nature seems to pack such particles into one or the other density, and in no density in between. And the strongest concrete materials have particles with this arrangement.

Unfortunately, it's the heating, at temperatures around 1,500 degrees Celsius, that gives the particles their pyramid shape.

But apples can also be stacked in a pyramid.

"If you replaced oranges with apples, it wouldn't change much the overall behavior, and here is the potential impact," said Ulm.

If they can find particles other than calcium silicate hydrates that will pack in high densities without the burning, they could reduce the carbon dioxide emissions.

Understanding the fundamental properties of concrete's nanostructure may not sound exciting, but it's right up there with the realization that turned cast iron into steel, said Hamlin Jennings, professor of materials science and civil and environmental engineering at Northwestern University in Evanston, Ill.

"All of sudden we went from cast iron to steel and the whole industrial revolution took place," said Jennings.