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In the
present study, nanoparticles of graphene and graphene oxide without
functionalizing the surface were added to the concrete blend and the effects of
these particles on morphology and mechanical properties were investigated.
Three samples containing 0.28, 0.56 and 1.1 weight percent graphene and 3 samples
with these weight percents containing graphene oxide were prepared and the
properties of these samples were investigated with those of pure cement
mortars. The results show that the compressive strength of the samples
containing both graphene and graphene oxide increase significantly.
Keywords: Nanoparticles, Nanocomposites, Graphene nanoplatelets, Electrical
properties
INTRODUCTION
In recent years, nanotechnology applications
have been growing in different domains. Because of their unique properties,
nanoparticles have been gaining increasing attention and been applied in many
fields to fabricate new materials with novelty functions. Many scientific
articles have been published and within the next few years large investments
will be accomplished in this area [1,2]. In Civil Engineering, major
development has been achieved through the production of materials with new
functions or by improving the performance of existing ones [3]. If nanoparticles are integrated with
traditional building materials, the new materials might possess outstanding or
smart properties for the construction of super high-rise, long-span or
intelligent civil infrastructure systems [4].
Graphene nanoplatelets, a two-dimensional monolayer of sp2-bonded carbon atoms, have unique mechanical, thermal and electrical properties, large specific surface area and low manufacturing cost that make them ideal reinforcing materials. However, very little work has been reported on using graphene and grapehe oxide, synthesized from chemical oxidation of graphite, in manufacturing multifunctional cement based nanocomposites [5-9].
EXPERIMENTAL
Materials
Portland cement (type ΙΙ) was used as a binder
with a specific area of 0.32 m2/g (blaine fineness). The sand used
aggregate had the average particle size of 0.3 mm. The chemical composition of
this cement is given in Table 1.
Pure nanoparticles of graphene and graphene oxide (GraphenExpert, Turkey) were
used as the reinforcing materials of cement. The properties of these two
nanoparticles are shown in Table 2.
Mortars formulation
The
mixture components of the mortars are shown in Table 3. Mortars were prepared with binder/sand weight ratio (B/S)
of 0.36 and water/binder (W/B) of 0.48 according to ASTM-C109. The
nanoparticles of graphene (G) and graphene oxide (OG) were replaced by cement
and the samples were produced with 0, 0.28, 0.56 and 1.1 wt% G and 0.28, 0.56
and 1.1 wt% percent OG.
Testing
procedures
The
components were mixed according to Schmidt et al. [10]. On the basis of Table 3 nanoparticles of G and OG were
added to the water separately and the solution was stirred for 15 min. Then the
cement was added to the solution and they were mixed at low speed for 30 s. The
sand was added to the mixture afterward mixing resumed at high speed for about
30 s. After stopping the mixing process for 2 min, blending the mixture was
started again for another 60 s.
The produced mortars were poured into cubic
molds with the size of 5 × 5 × 5 mm. An external vibrator was used to
facilitate compaction and decrease the amount of air bubbles. The samples were
demolded after 24 h and then cured in water at 23 ± 2° for 7 days.
The density of the samples after curing in
water for 7 days was measured by Archimedes technique. In order to determine
the compressive strength of the samples, compressive test was applied at the
speed of 0.9 KN/S.
The morphology of the fracture surface of the
samples after compressive test was observed in a scanning electron microscope
(SEM, EM-3200).
RESULTS AND
DISCUSSION
Mechanical
properties
The
densities of the samples after 7 days curing in water are given in Table 4. The results of the density for
both samples containing G and GO particles are close to each other and changing
the weight percent of the nanoparticles does not change the density remarkably.
But the results of the compressive strength shown in Figure 1 indicate that a little enhancement in the contents of
nanoparticles lead to a great change in the compressive strength of the
samples. Unexpectedly, both G and OG had an inverse effect on the compressive
strength of the samples. According to rule of mixtures for the composites, it was
predicted that the used nanoparticles would increase the compressive strength
of the prepared cement mortars. It can be seen that the compressive strength of
the OG samples are higher than G samples in all of the cases. This is another
unanticipated result that was taken from this experiment.
Microstructure
The
microstructure of the samples containing G and OG nanoparticles is shown in Figure 2. It can be seen that in all
the samples, although the particles have been well distributed in the matrix,
there is not a good adhesion between the nanoparticles and the matrix. As
reported earlier by Pellenq and Van Damme [11] strength of the concrete is originated
from hydration process. The main product of the hydration process is a rigid
gel called Calcium Silicate Hydrates (C-S-H). This gel is responsible for
cohesion and strength of the concrete structures. When nanoparticles of
graphene are used in the concrete without functionalizing the surface of the
particles, the interfacial strength between C-S-H gel and graphene particles is
very low. Alkhateb et al [5] calculated this interfacial strength and they came
to conclusion that is it much lower compared to the status of using functional
groups on the surface of the graphene nanoparticles. The low compressive
strength of the prepared specimens can be directly attributed to the weak
connection of cement mortars and dispersed particles. Using suitable functional
groups may solve this problem and lead to higher strength. So when using
mixture rules in nanocomposites, this unforeseen issue must be taken into
account in order to reach to the theoretical results.
CONCLUSION
According
to the experiments, using nano graphene and nano graphene oxide in the cement
mortars matrix without applying appropriate functional groups on the surface of
the platelets will not increase the compressive strength of the concrete
because the interfacial strength between nanoparticles and the matrix will be
very low. So the failure of the composite will start from the interfacial
surface, causing the reduction of the compressive strength of the concrete.
Therefore, using suitable functional groups for having higher cohesion is
necessary.
1. http://www.nsf.gov/
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revolutionary creation of new advanced materials-carbon nanotube composites.
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(2008) Nanomaterials in architecture, interior architecture and design.
Birkhäuser, Berlin.
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Microstructure of cement mortar with nanoparticles. Composites Part B 35:
185-189.
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Fröhlich S, Piotrowski S (2012) Ultra-high performance concrete and
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