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In the 2,000 or so years since the Roman Empire employed a naturally occurring form of cement to build a vast system of concrete aqueducts and other large edifices, researchers have analyzed the molecular structure of natural materials and created entirely new building materials such as steel, which has a well-documented crystalline structure at the atomic scale.

Oddly enough, the three-dimensional crystalline structure of cement hydrate - the paste that forms and quickly hardens when cement powder is mixed with water - has eluded scientific attempts at decoding, despite the fact that concrete is the most prevalent man-made material on earth and the focus of a multibillion-dollar industry that is under pressure to clean up its act. The manufacture of cement is responsible for about 5 percent of all carbon dioxide emissions worldwide, and new emission standards proposed by the U.S. Environmental Protection Agency could push the cement industry to the developing world.

"Cement is so widely used as a building material that nobody is going to replace it anytime soon. But it has a carbon dioxide problem, so a basic understanding of this material could be very timely," said MIT Professor Sidney Yip, co-author of a paper published online in the Proceedings of the National Academy of Sciences (PNAS) during the week of Sept. 7 that announces the decoding of the three-dimensional structure of the basic unit of cement hydrate by a group of MIT researchers who have adopted the team name of Liquid Stone.

"We believe this work is a first step toward a consistent model of the molecular structure of cement hydrate, and we hope the scientific community will work with it," said Yip, who is in MIT's Department of Nuclear Science and Engineering (NSE). "In every field there are breakthroughs that help the research frontier moving forward. One example is Watson and Crick's discovery of the basic structure of DNA. That structural model put biology on very sound footing."

Scientists have long believed that at the atomic level, cement hydrate (or calcium-silica-hydrate) closely resembles the rare mineral tobermorite, which has an ordered geometry consisting of layers of infinitely long chains of three-armed silica molecules (called silica tetrahedra) interspersed with neat layers of calcium oxide.

But the MIT team found that the calcium-silica-hydrate in cement isn't really a crystal. It's a hybrid that shares some characteristics with crystalline structures and some with the amorphous structure of frozen liquids, such as glass or ice.

At the atomic scale, tobermorite and other minerals resemble the regular, layered geometric patterns of kilim rugs, with horizontal layers of triangles interspersed with layers of colored stripes. But a two-dimensional look at a unit of cement hydrate would show layers of triangles (the silica tetrahedra) with every second, fifth or eighth triangle turned up or down along the horizontal axis, reaching into the layer of calcium oxide above or below.

And it is in these messy areas - where breaks in the silica tetrahedra create small voids in the corresponding layers of calcium oxide - that water molecules attach, giving cement its robust quality. Those erstwhile "flaws" in the otherwise regular geometric structure provide some give to the building material at the atomic scale that transfers up to the macro scale. When under stress, the cement hydrate has the flexibility to stretch or compress just a little, rather than snapping.

"We've known for several years that at the nano scale, cement hydrates pack together tightly like oranges in a grocer's pyramid. Now, we've finally been able to look inside the orange to find its fundamental signature. I call it the DNA of concrete," said Franz-Josef Ulm, the Macomber Professor in the Department of Civil and Environmental Engineering (CEE), a co-author of the paper. "Whereas water weakens a material like tobermorite or jennite, it strengthens the cement hydrate. The 'disorder' or complexity of its chemistry creates a heterogenic, robust structure.

"Now that we have a validated molecular model, we can manipulate the chemical structure to design concrete for strength and environmental qualities, such as the ability to withstand higher pressure or temperature," said Ulm.

CEE Visiting Professor Roland Pellenq, director of research at the Interdisciplinary Center of Nanosciences at Marseille, which is part of the French National Center of Scientific Research and Marseille University, pinned down the exact chemical shape and structure of C-S-H using atomistic modeling on 260 co-processors and a statistical method called the grand canonical Monte Carlo simulation.

Like its name, the simulation requires a bit of gambling to find the answer. Pellenq first removed all water molecules from the basic unit of tobermorite, watched the geometry collapse, then returned the water molecules singly, then doubly and so on, removing them each time to allow the geometry to reshape as it would naturally. After he added the 104th water molecule, the correct atomic weight of C-S-H was reached, and Pellenq knew he had an accurate model for the geometric structure of the basic unit of cement hydrate.

The team then used that atomistic model to perform six tests that validated its accuracy.

"This gives us a starting point for experiments to improve the mechanical properties and durability of concrete. For instance, we can now start replacing silica in our model with other materials," said Pellenq.

Other team members are graduate student Rouzbeh Shahsavari of CEE and Markus Buehler, MIT's Esther and Harold E. Edgerton Career Development Associate Professor of Civil and Environmental Engineering; Krystyn Van Vliet, MIT's Thomas Lord Associate Professor of Materials Science and Engineering; and NSE postdoctoral associate Akihiro Kushima.

This research was funded by the Portuguese cement manufacturer, Cimpor Corp., enabled through the MIT-Portugal Program.

About The Researchers :

1. Sidney Yip

Professor of Nuclear Engineering, and Materials Science and Engineering

Born in Beijing, Sidney Yip lived in China until 1950 when he immigrated to the U.S. with his parents and two brothers. He attended high schools in San Francisco and New Jersey, graduating in 1953. All his college degrees were awarded by the University of Michigan, a B.S.(1958) in Mechanical Enigneering, and M.S.(1959) and Ph.D.(1962) in Nuclear Engineering. After a postdoctoral year at Michigan, and two years as a Research Associate in the Department of Applied Physics at Cornell University, he joined the Nuclear Engineering Faculty at MIT, becoming a Professor in 1973. Since 2000 he holds a secondary appointment in the Department of Materials Science and Engineering.

In the Nuclear Science and Engineering Department his recent teaching duties have been in nuclear physics at the graduate and undergraduate levels, graduate subjects in neutron physics and in nuclear and atomic collision phenomena, and an advanced subject on statistical processes and atomistic simulations. Every fall term he co-leads a Freshman Advisor Seminar on career options in biomedical research. In Spring 2002 he introduced an Institute-wide undergraduate subject on introduction to modeling and simulation along with 9 other faculty colleagues spread over 6 departments. Current departmental duties include serving as the Financial Aid Officer, Undergraduate Registration Officer, and the Coordinator of the Nuclear Science and Technology Program.

As his academic disciplines he lists particle and radiation transport theory, molecular modeling of materials, and computational statistical physics. His research on transport phenomena and liquid-state dynamics spanned about 15 years during which he co-authored the monographs Foundations of Neutron Transport Theory (1967), Neutron Molecular Spectropscopy (1968), and Molecular Hydrodynamics (1980). Since 1975 he has focused on the theory and atomistic simulation of fundamental materials properties and behavior, specializing in developing molecular models to elucidate the atomic-level mechanisms governing such phenomena as melting, elastic instabilities, crack-tip plasticity, solid-state amorphization, and grain-boundary structure and dynamics. Currently his interests are in mechanical and thermal behavior of structural and functional materials, specifically problems of strength, deformation and toughness of metals and ceramics, dislocation core structure, interactions and mobility, environmental effects of hydrolytic weakening, and studies of electronic and thermal conductivities in nanostructures.

Among professional activities he has been active in organizing a series of workshops on multiscale materials modeling. He is the Principal Editor of Journal of Computer-Aided Materials Design, and an editorial board member of Modelling and Simulation in Materials Science and Engineering. His honors include a Guggenheim fellowship, fellowship in the American Physical Society, Alexander von Humboldt Foundation Senior Scientist Award, and University of Michigan Alumni Merit Award.

2.

5.  Krystyn J. Van Vliet

Thomas Lord Associate Professor of Materials Science
and Engineering

ScB, Materials Engineering, Brown University, 1998
PhD, Materials Engineering, MIT, 2002