Cement’s basic molecular structure finally decoded

Robustness comes from messiness, not a clean geometric arrangement


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.


Topics: Environment, History, Civil and environmental engineering, Innovation and Entrepreneurship (I&E), Nuclear science and engineering

Comments

Very interesting article. The methodology used was fascinating. In the write-up that appears herewith, in paragraph 5 that starts with the word, , the author of this story stated that the silica tetrahedra are "...three-armed silica molecules..." The silicon cation in the center of each silica tetrahedron is charged 4+ and thus has four so-called (+1)bond posts that under ideal circumstances are each shared with one (-1) bond of an oxygen ion to form the Si-O tetrahedron. Therefore, I think the author of the article would have been better served to have called the the silica tetrahedra..."four-armed silica molecules..." and thus the derivation and use of the word, TETRAhedron.
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