April 3 | Research
MEDIA CONTACT: Megan Fellman at 847-491-3115 or fellman@northwestern.edu
Novel Experiments on Cement Yield Concrete Results
|
|
Hamlin Jennings |
EVANSTON, Ill. --- Using some of the most modern tools of materials research, a team from Northwestern University and the National Institute of Standards and Technology (NIST) has shed new light on one of the world's most familiar construction materials -- Portland concrete. The researchers' insights into cement's nanostructure and how the material actually works could lead to greener, more economical and more durable concrete.
Concrete is a mixture of cement and water, together forming a cement paste, and sand and gravel. The cement paste, a gel of calcium silicate hydrate (C-S-H), binds everything together to form concrete. While cement and concrete were known to the Romans, who used it to good effect in structures such as the Colosseum and Pantheon, questions still remain as to just how the chemical reactions between cement and water result in the strong and durable material that is the basis of the world's infrastructure.
The Northwestern/NIST researchers now have uncovered some of cement paste's secrets. They are the first to precisely determine the composition and mass density of the tiny particles in the C-S-H gel -- a delicate network of solid nanoparticles surrounded by water molecules. The team also discovered important information about the distribution of water within the gel.
Their results, reported in a paper titled “Composition and density of nanoscale calcium-silicate-hydrate in cement,” appear in the April issue of the journal Nature Materials and were published online Sunday (March 25).
These new findings ultimately may make possible improvements in the properties of concrete made from Portland cement that could save hundreds of millions of dollars in annual maintenance and repair costs for concrete structures. In addition, such refinements could help reduce the amount of carbon-dioxide (CO2) emissions produced during the manufacture of cement.
More than 11 billion metric tons of concrete are consumed each year, making it the world's most widely used manufactured material. “Concrete is an incredibly versatile and useful material, but unfortunately the production of its active ingredient -- Portland cement -- is responsible for up to 10 percent of all manmade CO2 emissions,” said Jeffrey J. Thomas, research associate professor of civil and environmental engineering at Northwestern and one of the study's authors.
“Concrete usage around the world is increasing, and for most applications such as roads and buildings there are no viable alternatives on the horizon,” said Thomas. “However, it is possible to make concrete that incorporates significant amounts of recycled materials, such as slag and fly ash, reducing the amount of Portland cement that is needed. That is the kind of strategy that will help cut CO2 emissions in the future. But it is hard to make great strides when you don't fully understand the material you are working with.”
The researchers' findings with respect to cement may help create a new generation of concrete materials that require less energy and CO2 emissions to produce and yet have even better properties than today's concrete.
“The key to improving a material is understanding how its properties are related to its internal structure -- that is the basis of materials science,“ said Hamlin M. Jennings, professor of civil and environmental engineering and materials science and engineering at Northwestern and an author of the study.
“This approach, or philosophy, has resulted in huge improvements in the properties of metals, particularly steel,” said Jennings. “But so far we haven't had similar success with cement because we haven't understood its structure at the nanometer level. Now, after a very sophisticated set of experiments, we are beginning to get the critical information we need. This gives us a baseline for creating quantitative models that may eventually allow us to design cement and concrete to have just the properties we want.”
One key to the team's success is that they made their measurements without drying the cement, which affects the C-S-H structure. To date, attempts to pinpoint the amounts and different roles of water within the C-S-H have required taking the water out, either by drying or chemical methods, to determine how tightly it is bound in the cement. Instead, the research team, which also includes Andrew J. Allen, a physicist in the ceramics division of the National Institute of Standards and Technology and lead author of the study, combined structural data from small-angle neutron scattering experiments at the NIST Center for Neutron Research and from an ultrasmall-angle X-ray scattering instrument built by NIST at the Advanced Photon Source at Argonne National Laboratory.
The researchers were able to distinguish and measure the difference between water physically bound within the internal structure of the nanoscale C-S-H particles and adsorbed or liquid water between the particles. The new data imply significantly different values for the formula and density of the solid C-S-H than previously supposed.
The study was supported by the National Science Foundation.