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Schematic drawing of nanoscale
calcium silicate hydrate (C-S-H) particles in cement showing the
multiple roles played by water as defined in the NIST/Northwestern
experiments. Solid red areas are calcium silicate, pebbled areas
in between show the water physically bound between the layers to
form solid C-S-H. Dark blue halos around the C-S-H particles are
water adsorbed on the surface; pale blue areas represent liquid
water caught in nanopores.
Image by NIST |
Cement may be the world’s most widely used
manufactured material - more than 11 billion metric tons are consumed
each year - but it also is one of the more complex. And while it was
known to the Romans, who used it to good effect in the Colosseum and
Pantheon, questions still remain as to just how it works, in
particular how it is structured at the nano- and microscale, and how
this structure affects its performance.
Cement is something of a paradox. It requires just
the right amount of water to form properly - technically it’s held
together by a gel, a complex network of nanoparticles called calcium
silicate hydrate (C-S-H) that binds a significant amount of water
within its structure. But once the cement has set, the C-S-H structure
retains a tough, unchanging integrity for centuries, even in contact
with water. To date, attempts to pinpoint the amounts and different
roles of water within the C-S-H in cement paste have required taking
the water out, either by drying or chemical methods. The
NIST/Northwestern researchers instead 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. Their experiments are the first to classify water
by its location in the cured cement.
As a result, the researchers were able to
distinguish - and measure - the difference between water physically
bound within the internal structure of the solid C-S-H nanoparticles
and adsorbed or liquid water between the nanoparticles. They also
measured a nanoscale calcium hydroxide structure that co-exists with
the C-S-H gel. The new data, which imply significantly different
values for the formula and density of the C-S-H gel than previously
supposed, have implications for defining the chemically active surface
area within cement, and for predicting concrete properties. They also
may lead to a better understanding of the contribution of the
nanoscale structure of cement to its durability, and how to improve
it. |
