Nanoscience and nanoengineering of cement-based materials - Nanotechnology in Eco-efficient Construction - page 21

been argued that this ratio is ultimately related to the spacing achieved
within cement particles (Bentz and Aitcin, 2008) and as a result of the
amount of porosity and all related parameters. It is common knowledge in
the concrete community that the lower the w/c ratio the higher the elasticity,
strength, and durability. Of course the process cannot be continued indefi -
nitely: as the distance between particles is reduced, the particles tend to
agglomerate with a subsequent loss of workability. To circumvent this
problem, a higher particle packing can be achieved by playing with the
particle size distribution of the dry mix (Fig. 2.6). Furthermore, the develop-
ment of superplasticizers that could create workable mixes for low w/c
ratios resulted in high strength and performance materials. As for most
materials, the stronger they become, the more brittle they tend to be. The
development of fi ber reinforcements (steel, polymeric, etc.) added ductility
to the system and created a material as attractive as metallic systems.
Cementitious materials with strength as great as that of mild steel have
been reported in the literature (e.g., Richard and Cheyrezy, 1995). Other
empirical techniques that alleviated some of the problems associated with
these high performance systems, like heat treatment, etc., have been
proposed.

2.3.2 Cementitious nanocomposites

Nanoparticles have sizes with at least one of their dimensions in the
1-100 nm range. As the particle size decreases, a larger proportion of atoms
is situated on the free surface compared to those atoms in the bulk, resulting

￿ ￿ ￿ ￿ ￿ ￿

(a) (b)

2.6 (a) Packing of mono-sized spheres. (b) The packing density of the
system can be signifi cantly increased by using an initial mix with a
particle size distribution.