In nanoscale studies of calcium-silicate-hydrate, a binder critical to the strength and durability of Portland cement, the mineral tobermorite is a perfect stand-in for determining the crystal structure of this extraordinarily complex material. Highly structured layers of calcium and oxygen atoms alternate with “interlayers” of silicon, oxygen, calcium, and water molecules, where disorder may occur and adversely affect the material’s properties. Image: Crystal structure of 14Å tobermorite as refined by Bonaccorsi et al |
In
nanoscale studies of calcium-silicate-hydrate, a binder critical to the
strength and durability of Portland cement, the mineral tobermorite is a
perfect stand-in for determining the crystal structure of this
extraordinarily complex material. Highly structured layers of calcium
and oxygen atoms alternate with “interlayers” of silicon, oxygen,
calcium, and water molecules, where disorder may occur and adversely
affect the material’s properties. (Crystal structure of 14 Å tobermorite
as refined by Bonaccorsi et al).
It’s
no surprise that humans the world over use more water, by volume, than
any other material. But in second place, at over 17 billion tons
consumed each year, comes concrete made with Portland cement. Portland
cement provides the essential binder for strong, versatile concrete; its
basic materials are found in many places around the globe; and, at
about $100 a ton, it’s relatively cheap. Making it, however, releases
massive amounts of carbon dioxide, accounting for more than 5%
of the total carbon dioxide emissions from human activity.
“Portland
cement is the most important building material in the world,” says
Paulo Monteiro, a professor of civil and environmental engineering at
the University of California at Berkeley, “but if we are going to find
ways to use it more efficiently—or just as important, search for
practical alternatives—we need a full understanding of its structure on
the nanoscale.” To this end Monteiro has teamed with researchers at the
U.S. Department of Energy’s Advanced Light Source (ALS) at Lawrence
Berkeley National Laboratory.
Most
recently, at ALS beamline 12.2.2, Monteiro and his colleagues gradually
squeezed specks of fine dust of the mineral tobermorite between faces
of two diamonds in a diamond anvil cell, until they achieved pressures
like those 100 miles below the surface of Earth. This was the first
experiment to determine tobermorite’s bulk modulus—its “stiffness”—from
diffraction patterns obtained by sending a bright beam of X?rays through
the sample, revealing how its structure changed as the pressure
increased.
The results, which will appear in Cement and Concrete Research
and are now available online to subscribers, led to new insights into
calcium-silicate-hydrate (C?S?H), the material primarily responsible for
the strength and durability of concrete made with Portland cement.
Cement on the nanoscale
Portland
cement is made by baking limestone (calcium carbonate) and clay
(silicates) in a kiln at over 1400 C to make “clinker,” which is then
ground to a powder. When the powder is mixed with water,
calcium-silicate-hydrate (C-S-H) is formed, which, although poorly
crystallized, is a binder critical to the strength and durability of the
cement paste.
“We
and many other groups have developed sophisticated computer models to
understand the crystal structure and mechanical behavior of C?S?H, based
on observations of how it performs,” says Monteiro. “But we’re the only
group that uses minerals to validate the results of our models with
experimental results.”
Despite
the many studies and vast literature on cements and their components,
the atomic scale structure of C-S-H, owing to its high complexity, is
still imperfectly known. While the mineral tobermorite, a calcium
silicate hydrate named for a quaint village on the Scottish Isle of
Mull, is far less common than the makings of Portland cement, one of its
structures, designated 14 Å tobermorite, is a perfect stand?in for C-S-H
in nanoscale studies.
At Calipso, the California High-Pressure Science Observatory at beamline 12.2.2 of the Advanced Light Source, materials can be squeezed to tremendous pressures in diamond anvil cells, where they are trapped between the two diamonds in a small central chamber. The X-rays from the beamline pass through the diamonds and the sample, throwing diffraction patterns on a CCD detector that reveal the material’s structure. (Signals from diamond and corundum in the anvil cell mechanism must be subtracted from the diffraction patterns.) Image: Beamline photo by Roy Kaltschmidt |
The
studies were performed at beamline 12.2.2, the California High-Pressure
Science Observatory (Calipso), which is supported by the National
Science Foundation. Calipso is equipped with a choice of diamond anvil
cells, arranged so the X?ray beam passes through the diamonds and the
sample chamber between them. The diffracted X-rays fall on a CCD
detector, and the diffraction patterns can be used to determine the
structure of the material in the cells.
At
Calipso, the California High-Pressure Science Observatory at beamline
12.2.2 of the Advanced Light Source, materials can be squeezed to
tremendous pressures in diamond anvil cells, where they are trapped
between the two diamonds in a small central chamber. The X-rays from the
beamline pass through the diamonds and the sample, throwing diffraction
patterns on a CCD detector that reveal the material’s structure.
(Signals from diamond and corundum in the anvil cell mechanism must be
subtracted from the diffraction patterns.) (Beamline photo by Roy
Kaltschmidt)
The
tiny sample of tobermorite that Monteiro’s team used at the ALS
originally came from Southern California and was obtained from the Los
Angeles County Museum. The researchers ground it to a fine powder and
suspended it in liquid so that the diamond anvil cell would apply even
hydrostatic pressure to every grain in the sample chamber—an opening in a
metal gasket only 180 millionths of a meter in diameter.
“While
it’s possible to do X-ray diffraction with diamond anvil cells on a
laboratory bench,” says ALS beamline scientist Simon Clark, a co-author
of the research, “you can’t deal with samples this small without the
brightness of a synchrotron light source. Even if you could, what takes
eight hours in the lab we can do in half a minute—although we usually
take at least a minute so the researchers can write everything down in
their notebooks.”
Putting on the squeeze
As
the experiments proceeded, the flattened points of the cell’s two
diamonds were slowly tightened, concentrating pressure on the gasket and
the contents of the sample chamber. The X-ray diffraction patterns
revealed any changes in the arrangement of atoms in the crystal
structure.
Says
Monteiro, “The diffraction patterns give us the lattice parameters of
the tobermorite structure.” Lattice parameters allow the volume of the
unit cells, the material’s fundamental atomic arrangements, to be
calculated in three directions. “We watch how the lattice parameters
change as the pressure changes, using them as a strain gauge. By knowing
the applied pressure in the anvil cell, we can compute the bulk
modulus.”
In
C-S-H the calcium, silicon, and oxygen atoms are arranged in a stack of
flat layers. Highly structured layers of calcium and oxygen atoms
alternate with “interlayers” of silicon, oxygen, calcium, and water
molecules. In the plane of the layers (the a and b directions of the
lattice parameters), tobermorite is very stiff indeed, changing very
little as pressure increases.
Perpendicular to the plane, along the c-axis, tobermorite is more compressible, but not by much.
Even
in the c direction, pure tobermorite is stiffer than a synthetic
version of C-S-H the Monteiro team also tested, and to which they
compared it. The calcium-oxygen layers in the synthetic C-S-H were
similar to those in the tobermorite, so when altered silicon chains were
deliberately introduced into the synthetic in order to mimic the
disorder of natural C?S?H, it still retained its stiffness in the a-b
plane. But along the c-axis, the disordered synthetic C-S-H grew
significantly more squeezable.
“It’s
the interlayers that compress, and only along the c-axis,” says
Monteiro. “Differences in interlayer spacing, degrees of disorder in the
silicon chains, additional calcium ions, and water molecules all make
the bulk modulus of the two materials virtually the same in the a-b
plane, but different along the c?axis. The discovery suggests a number
of possibilities for improving the performance of cement—for example,
one might introduce special polymers into the C-S-H interlayers to shape
its behavior. This will certainly be an area for our future research.”
This
work was supported in part by the King Abdullah University of Science
and Technology and by the National Institute of Standards and
Technology. The Advanced Light Source is supported by the U.S.
Department of Energy’s Office of Science.