Nanoparticles for high performance concrete (HPC) - Nanotechnology in Eco-efficient Construction - page 45

to the mixture. Zhang and Li (2011) used a water-reducing agent (UNF-5,
a type of b-naphthalene sulfonic acid and formaldehyde condensates) to
help disperse the nanoparticles in the cement paste and to achieve a good
degree of workability in the concrete. A de-foamer (tributyl phosphate) was
also used to decrease the number of air bubbles. To prepare concrete con-
taining nanoparticles, a water-reducing agent was fi rst mixed with water in
a mortar mixer. The nanoparticles were then added and stirred at high
speed for fi ve minutes. A de-foamer was added during stirring. Following
this, the cement, sand and coarse aggregate were mixed at low speed for
two minutes in a centrifugal concrete blender. The mixture of water, water-
reducing agent, nanoparticles, and de-foamer was then slowly added and
stirred at low speed for a further two minutes to achieve good
workability.
Dispersion diffi culties also occur when carbon nanotubes or carbon
nanofi bers are used because of their strong Van der Waals self-attraction
(Xie et al. , 2005). Sanchez and Ince (2009) confi rmed that Van der Waals
forces hold the carbon nanofi bers together in clumps (Fig. 3.5).
These authors found that silica fume facilitated the dispersion of carbon
nanofi bers due to its small particle size when compared to that of anhydrous
cement particles (around 100 times smaller). Figure 3.6 shows silica fume
particles intermixed with carbon nanofi bers. The authors recorded that even
when carbon nanofi ber dispersion was facilitated by silica fume, a signifi cant
number of carbon nanofi ber pockets still remained. Konsta-Gdoutos et al.
(2010b) used an aqueous surfactant and ultrasonic energy to achieve a high
degree of carbon nanofi ber dispersion. They found that a constant surfac-
tant to carbon weight ratio of 4.0 achieved effective dispersion. Nochaiya
and Chaipanich (2011) also found that homogeneous dispersion can be
obtained if carbon nanotubes are mixed with water and then subjected to
ultrasound for one hour. Nasibulina et al. (2012) suggest that high-quality
dispersion of carbon nanotubes may be achieved by a two-step method:

￿ ￿ ￿ ￿ ￿ ￿

0 wt% CNF 0.5 wt% CNF 2 wt% CNF

CNF pockets

SF
agglomerates

SF agglomerates

3.5 Scanning electron micrographs of the fracture surface of hybrid
CNF/SF cement composites, revealing the presence and distribution of
SF agglomerates and CNF pockets at a magnifi cation of 40 × (Sanchez

and Ince, 2009).