NEWS
Building for the future
COMPOSITES
A team of scientists from the United States,
South Korea and Australia have become the first
to investigate the nanostructure and behavior of
Portland cement and hydrated minerals under
high pressure. They assessed ways of making
cement stronger and with less carbon emissions
by examining its structure on the nanoscale at
the California High-Pressure Science Observatory
(Calipso).
Around 17 billion tons of the cement are used
every year. It is versatile and relatively cheap
to produce; on the downside, it releases huge
amounts of carbon dioxide into the atmosphere,
accounting for over 5 % of the total CO2 emissions
worldwide. The cement is made by baking
limestone (calcium carbonate) and clay (silicates)
at over 1400 °C, producing clinker, which is then
ground into a powder. Mixing the powder with
water forms calcium-silicate-hydrate (C–S–H),
a crucial binder for the cement paste. For their
study, which was published in Cement and
Concrete Research [Oh et al., Cement Concr Res
(2011) doi: 10.1016/j.cemconres.2011.11.004],
the team used the mineral tobermorite (a calcium
silicate hydrate), since one of its structures, 14 Å
tobermorite, is an ideal substitute for C–S–H in
nanoscale studies.
At Calipso, the researchers squeezed tiny amounts
of finely ground tobermorite dust at huge
hydrostatic pressures between the faces of two
diamonds in a diamond anvil cell. The flattened
points of the diamonds were gradually tightened,
increasing the pressure on the contents of the
sample chamber, with x-ray diffraction patterns
showing any changes in the arrangement of atoms
in the crystal structure. The resultant patterns
helped to determine the bulk modulus/stiffness of
the tobermorite due to changes in pressure.
With the data on the bulk modulus being integral
to mechanical modeling, they were able to
confirm that the compression behavior of the
a and b lattice parameters of 14 Å tobermorite
and C–S–H(I) are similar, implying that they both
could have very similar Ca–O layers.
As researcher Paulo Monteiro, from the University
of California at Berkeley, points out “It’s the
interlayers that compress, and only along the
c-axis. 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 ab-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.”
The team now hopes to develop total scattering
methods to analyze poorly crystallized complex
hydrated materials under high pressure, including
calcium silicate hydrates, as well as to validate
molecular dynamics models with experimental
results for other hydrated phases. In addition,
they may also explore how polymers can be used
to modify the mechanical properties of calcium
silicate hydrates.
Laurie Donaldson
Imaging with graphene
CARBON
A team of scientists in the UK have developed
a method of resolving structures of self-
scanning electron and atomic force
microscopy (AFM). Graphene oxide has
assembled block copolymers using graphene
oxide; a breakthrough that offers an effective
way of improving the real-space analysis of
nanoscale solution assemblies using multiple
complementary techniques. The approach
transforms the information on carbon-based
the advantage of being available in large
quantities, is robust, water dispersible and
nearly electron transparent, and much
cheaper than graphene itself.
However, the low contrast of the
predominantly carbon nanostructures makes
nanostructures using phase contrast TEM
it difficult to discern intrinsic structural
imaging.
features from the background, so heavy
The study, published in Soft Matter [Patterson
metal staining is generally used to reveal the
TEM of polymersomes and multilamellar structures on graphene oxide.
et al., Soft Matter (2012) doi: 10.1039/
details, although this does need subjective
Courtesy of Rachel K. O’Reilly.
c2sm07040e], reveals how graphene-based
interpretation, leads to artifacts, offers
stained to ensure an image contrast, adding
supports can allow for the stain-free imaging
reduced resolution, and precludes analysis
complexity to the sample preparation and image
of diblock copolymer nanostructures, and offer higher
that uses complementary techniques, such as AFM.
interpretation, but also preventing complementary
resolution, greater contrast, less ambiguous results, and
The technique could become central to the detailed
imaging and analysis techniques being applied. The
the ability to analyze the same specimen with a range of
analysis and correlation of individual nanoscale
team therefore used graphene oxide as a support, as
different techniques, thereby reducing the limitations of
assemblies, as well as enhance our knowledge of detailed
it doesn’t need staining and the specimens remain
each. It is hoped that the ability to enhance block copolymer
structures, properties and processing relationships in
stable under the electron beam for a long time. This
structure determination with graphene oxide could help
solution assemblies. It is hoped this new technique will
latter aspect means sample analysis by a range of
progress new drug and gene delivery systems, as well as
also help in other developments, such as in the study
electron microscopy techniques is achievable. In
nanoreactors in separation science and in nanoelectronics.
of biological specimens including viruses, proteins, and
addition, graphene oxide supports are used for
Transmission electron microscopy (TEM) usually
DNA constructs.
characterization of the same assemblies through
requires that polymers are chemically fixed and
Laurie Donaldson
JAN-FEB 2012 | VOLUME 15 | NUMBER 1-2
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