Cracking Concrete's Code

Where can you go to see a cool example of nanotechnology? Well, just about everywhere. A new study reveals that plain old concrete, the most common human-made material on the planet, owes its properties to its nanoscale structure. Down the road, this new understanding could lead to novel forms of concrete that require less energy and CO2 to generate and might allow researchers to engineer its properties much as they have done with advanced steel alloys.

Concrete isn't what comes to mind when most people think of nanomaterials. It's the oldest engineered construction material and was first used by the Romans. Today, some 2.5 billion tons of the stuff is made every year, but at considerable environmental cost. Concrete starts out as a combination of limestone, clay, and gypsum that is heated to 1500 degrees Celsius to form cement. When combined with water, cement glues particles of sand and gravel together to create concrete. Estimates indicate that the energy used to create concrete accounts for 5 to 10 percent of the world's CO2 emissions.

As concrete sets and begins to dry, it forms a network made up primarily of calcium-silicate-hydrate, or C-S-H. The organization of that network has long remained a bone of contention among materials experts, as concrete's structure has proven challenging to confirm with traditional tools, such as x-ray, electron, and neutron scattering. To gain better insight, Franz-Josef Ulm, a civil engineer at the Massachusetts Institute of Technology in Cambridge, and his postdoc Georgios Constantinides used a needlelike "nanoindentation" device to prod different types of hardened cement pastes--the part of concrete that holds everything together--from around the world. An atomic force microscope allowed them to see the nanostructure and judge the strength of each paste by measuring the effects of the needle. They found that each type of cement consisted of myriad 4-to-5-nanometer-wide C-S-H particles that were either randomly arranged or ordered like oranges on a store shelf. The combination of those nanostructures largely accounted for the material's strength and durability.

"It's a great paper," says Hamlin Jennings, a civil engineer at Northwestern University in Evanston, Illinois. Jennings adds that if researchers can learn to control the packing of C-S-H nanoparticles in concrete they might be able to engineer concrete like alloyed steel. Ulm says that the findings could also help researchers find novel starting materials that create the tightly packed nanostructures without vast energy inputs, thereby helping people tread a bit more lightly on the planet.

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