Nanoscience and nanoengineering of cement-based materials - Nanotechnology in Eco-efficient Construction - page 16

absorption and scattering cross section of neutrons across the periodic table.
SAS is particularly suitable for cement-based materials as it does not
require drying and can be conducted in situ during hydration. Furthermore,
it circumvents the need for specimen preparation and any associated inter-
ference that might be incorporated, as with microscopy techniques.
The technique involves measuring the intensity of neutron or X-ray scat-
tered (due to heterogeneities) through small angles, usually less than 1°. By
employing a suitable model for data interpretation, one can determine
information on the geometrical characteristics like size distribution, volume
fractions, surface area, fractal characteristics, etc. In general, and owing to
their neutrality, neutrons can penetrate the specimen far better than X-ray
scattering techniques, thus probing the material on higher length-scales. In
fact, recent instrument modifi cations, namely ultrasmall-angle scattering
(USANS and USAXS), allow through crystal diffraction optics material
data to be received from much lower scattering vectors, thus enabling
microstructure characterization to extend to larger domains, i.e. > 1 μ m for

USAXS and m for USANS. The combination of all these techniques

suggest that C-S-H is a nanogranular material with a fundamental unit
on the order of 5 nm in the vertical direction, a chemical formula of
(CaO) 1.7 (SiO 2 )(H 2 O) 1.8 and a density of 2604 kg/m 3 (Allen et al. , 2007;
Jennings et al. , 2007). This unit appears to agglomerate, possibly with fractal
characteristics, into larger domains with local spatial variability in particle
packing and therefore densities. Information on the larger CH crystals can
be obtained through inelastic neutron scattering (INS), whereas the differ-
ent states of free and bound water found in cement systems can be probed
when employing quasielastic neutron scattering techniques (QENS). Apart
from a fundamental understanding of the C-S-H phase, the techniques have
been employed for monitoring the hydration mechanism and its alteration
through accelerators or decelerators, the effect of cement additives and
cement replacement materials, and the effect of calcium leaching.

> 10 μ

￿ ￿ ￿ ￿ ￿ ￿

The microstructure of concrete revisited

It is now clear that at the centimeter scale, concrete or mortar can be con-
sidered as a two-phase composite with a (usually) weak interface generated
by the local packing of the cement particles during hydration that generates
what is known as an interfacial transition zone (ITZ). While this macro-
scopic visualization is quite accurate at that level, it cannot capture all the
peculiarities that cement-based materials exhibit, e.g., access to aggressive
ions, time-dependent deformation, sensitivity to humidity, response to low
and high temperatures, pressure sensitivity of strength, and many more. All
these require understanding of the percolated C-S-H phase which controls
the macroscopic response. The very nature of C-S-H has been the subject