Nanoparticles for high performance concrete (HPC) - Nanotechnology in Eco-efficient Construction - page 40

3.2.1 Mechanical properties
Porro et al. (2005) refers to the use of nano-silica particles as increasing the
compression strength of cement pastes. The same authors state that this
phenomenon is not due to pozzolanic reaction, as the calcium hydroxide
consumption is very low, but rather to the increase of silica compounds
which contribute to a denser micro-structure.
According to Lin et al. (2008), the use of nano-silica on sludge/fl y ash
mortars, compensates for the negative effects associated with sludge incor-
poration in terms of the setting time and initial strength. Sobolev et al.
(2008) reported that the addition of nano-silica produced an increase in
strength of 15-20%. Other authors (Gaitero, 2008; Gaitero et al. , 2009)
believe that nano-silica causes an increase in the C-S-H chain dimension
and stiffness. Nasibulin et al. (2009) reported a twofold increase in strength.
Chaipanich et al. (2010) records that 1% of carbon nano-fi bers (by binder
mass) can compensate for the strength reduction associated with the
replacement of 20% fl y ash. Konsta-Gdoutos et al. (2010a) also studied the
effect of carbon nano-fi bers on cement pastes (0.08% by binder mass) and
observed an increase in strength.
Nazari and Riahi (2011a) used ZrO 2 nanoparticles with an average par-
ticle size of 15 nm and reported an improvement in the fl exural strength of
self-compacting concrete up to 4 wt%. Increasing the nanoparticle content
caused a reduction in fl exural strength because of the inadequate dispersion
of nanoparticles within the concrete matrix. Givi et al. (2010) studied the
effects of different particle sizes of nano-SiO 2 (15 and 80 nm) and reported
that the optimal replacement level of nano-SiO 2 particles was gained
at 1.0% and 1.5%, respectively. The effect of nanoparticle addition is
threefold:

1. As the average diameter of C-S-H gel is approximately 10 nm, the
nanoparticles fi ll the voids in the CHS structure, so producing a denser
concrete.

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2. The nanoparticles act as nucleation centers, contributing to the develop-

ment of hydration in Portland cement.

3. Nanoparticles react with Ca(OH) 2 crystals and produce C-S-H gel. They

also act as kernels in the cement paste which reduces the size of Ca(OH) 2
crystals.

3.2.2 Durability

Investigations carried out by Ji (2005) showed that concrete containing
nano-silica particles has lower water permeability. This is due to the reduc-
tion of the amount of Ca(OH) 2 which produces a denser inter-facial transi-
tion zone (ITZ). A reduction of chloride ion permeability as a result of