Concrete is the most widely used man-made material, and the manufacture of cement–the main ingredient of concrete–accounts for 5 to 10 percent of all anthropogenic emissions of carbon dioxide, a leading greenhouse gas involved in global warming. But now, researchers at MIT studying the nanostructure of concrete have made a discovery that could lead to lower carbon-dioxide emissions during cement production.
The researchers found that the building blocks of concrete are particles just a few nanometers in size, and that these nanoparticles are arranged in two distinct manners. They also found that the nanoparticles’ packing arrangement drives the properties of concrete, such as strength, stiffness, and durability. “The mineral [that makes the nanoparticle] is not the key to achieving those properties … rather, it’s the packing [of the particles],” says Franz-Josef Ulm, a civil- and environmental-engineering professor at MIT who led the work. “So can we not replace the original mineral with something else?” The goal is to formulate a replacement cement that maintains the nanoparticles’ packing arrangement but can be manufactured with lower carbon-dioxide emissions.
Cement manufacture gives rise to carbon-dioxide emissions because it involves burning fuel to heat a powdered mixture of limestone and clay at temperatures of 1,500 ºC. When cement is mixed with water, a paste is formed; sand and gravel are added to the paste to make concrete. But scientists do not fully understand the structure of cement, Ulm says.
The biggest mystery is the structure and properties of the elementary building block of the cement-water paste, calcium silicate hydrate, which acts as the glue holding together all the ingredients of concrete. “All of the macroscopic properties of concrete in some way are related to what this phase is like at the nanometer level,” says Jeffrey Thomas, a civil- and environmental-engineering professor at Northwestern University.
If this structure was better understood, researchers could then engineer cement on a nanoscale to tailor the properties of concrete, says Hamlin Jennings, a civil- and environmental-engineering and materials-science professor at Northwestern. Because researchers do not know the behavior of cement on a nanoscale, until now, “progress in concrete and cement research has largely been hit-and-miss,” Jennings says.
Jennings had predicted that calcium silicate hydrate is a particle with a size of about five nanometers. Ulm and his postdoctoral researcher Georgios Constantinides have confirmed this structure using a technique called nanoindentation, which involves probing cement pastes with an ultrathin diamond needle.
The researchers found that the calcium-silicate-hydrate nanoparticles were arranged either in a manner similar to oranges randomly jam-packed in a box or like the pyramidal arrangement of oranges in a grocery store. With these two arrangements, the particles fill, respectively, 63 and 74 percent of the volume of the cement paste, the rest being water and air. The relative volumes that the nanoparticles occupy in the paste control the mechanical properties of the cement paste, the researchers found.
Ulm and Constantinides are now planning to change the components in cement–one idea is to substitute magnesium for calcium–so that it takes less heat to make cement but the resulting nanoparticles still have the same packing arrangement as the calcium-silicate-hydrate nanoparticles.
They also plan to study the nanoparticle until they understand it down to the atomic level. This will give them even more freedom to nanoengineer cement, Ulm says. “We could add chemicals to it in order to improve the performance [of concrete] with maybe less cement, or the same cement that gives higher strength.”
With two billion tons of cement being produced in the world every year and concrete demand rising with the growth going on in China and India, Ulm says that it is imperative to reduce concrete-related carbon-dioxide emissions as part of the effort to combat global warming.