Concrete changes the world. Nanotechnology changes the concrete
world. The nano-engineered concrete can be intelligent, strong, durable,
easy to fabricate, recyclable and eco-friendly. Its potential benefits
include improved infrastructures reliability and longevity, enhanced
structural performance and durability, improved safety against natural
hazards and vibrations, reduced lifecycle costs in operating and
managing infrastructures, and reduced burdens on resources, energy and
environment.
Figure 1: Concrete price and usage; b) Energy
consumption for concrete production; c) The cumulative carbon
sequestration from 1930 to 2013; d) Elemental composition of the earth;
e) Cement demand prediction [1-5].
Concrete's excellent properties and low cost have made it tremendous
quantity of concrete (4 billion cubic meters per year) the world's most
widely used engineering material (Figure 1a). A has been consumed
worldwide for infrastructure construction.
China accounts for approximately 60% of the total concrete consumption
with the per capita amount of 2 cubic meters. The manufacturing of
cement, a key ingredient in concrete, has a significant impact on nature
source, energy and environment. In fact, concrete has lower energy
consumption and carbon emissions compared to other engineering materials
(Figure 1b). According to recent research, as carbon sequestration,
concrete can reabsorb a large fraction of CO2 released from cement
production. From 1930 to 2013, carbonating concrete absorbed 43% of the
cumulative CO2 emissions associated with the high-temperature
calcination of carbonate minerals during cement production (Figure 1c).
In addition, in terms of resource, it is almost impossible to find an
alternative construction material to concrete. This is because O, Si,
Al, Fe, Ca, Na, K and Mg comprise 98% of the crustal composition, which
are the main components of concrete (Figure 1d). In the long term, on
the basis of the urban development of the developing countries and the
world's population growth rate, concrete will continue to be massively
consumed as construction materials in the whole world. Taking the
developing countries such as China and India for example, concrete usage
converted by the total amount of cement will nearly double in the
coming several decades (Figure 1e). Therefore, concrete is the largest
material foundation bearing the civilization in today’s society and even
in future society. The production and utilization of concrete are
closely related to source, energy and environmental issues, thus having a
strong effect on the sustainable development of human society [1-5].
Figure 2: Multi-component, multi-phase and multi-scale nature [4].
Concrete has a multi-component, multi-phase and multi-scale nature
and is considered as the most complicated composite while fabricated
with the simplest production process (Figure 2). The feature of
thermodynamic metastability has an effect on the concrete volume
stability. Under deformation, shrinkage and loading, it is vulnerable to
interrupt or destroy homophase continuity and heterophase bonding. In
addition, concrete is known for its brittleness with low tensile
strength, poor deformation performance and high cracking tendency. The
presence of cracks tends to weaken the integrity and bearing capacity of
structures and severely affect their safety, serviceability and
durability, causing potential safety problems on construction.
Especially with the trend toward large-scale and complicated
infrastructures, extreme service environment, multi-factor coupling and
ever-enlarging application field, these problems are becoming more
serious and facing with a plenty of new challenges as well. In this
case, high- performance and smart/multifunctional concrete becomes the
only way to implement the sustainable development of concrete
structures. High-performance and smart/multifunctional concrete has
excellent mechanical properties, durability and processability needed
for structural material. Meanwhile, it also presents selfsensing,
self-healing and self-adjusting features. Making use of high-
performance and smart/multifunctional concrete can effectively enhance
the safety, comfort and durability of infrastructures and maintain a
coordinated relationship between infrastructure and environment.
As shown in Figure 2, concrete is a multi-scale complex system.
Generally, the normal aggregate in concrete has a particle
size ranging from millimeters to centimeters and the particle size of
ordinary cement itself is usually 7-200|im. However, cement hydrated
phases are primary nano structured materials mainly condensed by C-S-H
gel tens of nanometers in size. Therefore, due to its natural attribute,
concrete has the properties of nanomaterials. In addition, the
scientific community and industry are always spontaneous to manipulate
the nano-scale behavior inside concrete using nanotechnology to enhance
or modify concrete performance in the process of concrete development,
such as nano crystals, mineral admixtures and chemical admixture used
for concrete preparation. It should be recognized that nanotechnology in
concrete is not a new technique. It is just attributed to the rapid
development of nanotechnology in recent two decades improving the
understanding of the nano-scale behavior inside concrete and enriching
the methods for concrete reinforcement and modification via
nanotechnology. In this manner, research in the application of
nanotechnology in concrete reaches a very active period.
Awareness of nanotechnology applications in concrete starts at 2001. The addition of nano-SiO
2 to concrete was first used for concrete reinforcement. After that, nano-ZrO
2, nano-TiO
2
and nanocarbon material were applied one after another for the
enhancement and modification of concrete. Much work indicated that the
big gains in mechanical, durable and functional properties of concrete
were achieved by nano nonmetallic oxide and metallic oxide modification.
The addition of nano-SiO
2 increased the 3d/28d compressive
and flexural strengths by 48.1%/48.7% and 45.6%/16.0%, respectively.
Meanwhile, the addition of nano-SiO2 can increase the freeze-thaw
resistance, chloride penetration and permeability, abrasion resistance
and fire resistance of concrete [6]. The fracture toughness of concrete
can be enhanced by 400% when nano-ZrO
2 is used as fillers [7]. The flexural and compressive strengths of concrete with nano-TiO
2
at age of 28 d achieve increases of 87% /6.69 MPa and 12.26%/12.2 MPa
with respect to concrete without nano-TiO2, respectively. Nano-TiO
2 can also endow concrete with the photocatalytic effect to decompose both organic pollutants and oxides such as NO, NO
2 and SO
2
[8]. Moreover, extensive research endeavors demonstrated the potential
of various nano carbon materials including carbon nanotubes (CNTs),
carbon nanofibers (CNFs), and graphene for enhancing/modifying concrete
materials [9].
Figure 3: Graphene platelets acting like�filters�for chloride ions [11].
The observed best performance enhancement of concrete with CNTs or
CNFs include a relative/absolute enhancements of 79%/74MPa and
64.4%/5.6MPa in compressive and flexural strength [10], a 34.28%
increase in tensile strength, a 270% increase in fracture toughness, a
14% increase in fracture energy, an over 600% improvement in Vickers’s
hardness at the early ages of hydration, a 2200% increase in deflection,
a 130% increase in ductility, an over 430% improvement in resilience
and a 227% increase in Young’s modulus. Graphene can improve the
tensile, flexural and compressive strength of concrete by 78.6%, 60.7%
and 38.9%, respectively. The presence of CNTs obviously enhances the
transport property and durability of concrete materials. Graphene
significantly improves the moisture transport performance, the acid
resistance and the chloride ion penetration resistance (as listed in
Table 1 and Figure 3) of the concrete.
Table 1: Chloride migration coefficient of concrete with grapheme.
DRCM: Chloride migration coefficient from non-steady-state migration test
The electrical resistivity reduction extent of concrete materials
than that of concrete without CNTs. The damping capacity of with
CNTs/nano carbon black composite filler is 99.9%. The concrete with CNTs
is 1.6 times than that of concrete without CNTs. thermal conductivity
of CNTs concrete composites is 85% greater The addition of CNTs into
concrete materials can lead to a 27%decrease in electromagnetic wave
reflectivity at a frequency of 2.9 GHz. Additionally, the composites
with CNTs, CNFs or graphene feature smart self-sensing (e.g. sensing
stress, strain, crack, damage, temperature and smoke), self-heating and
steel cathodic protection performances. Nano fillers not only can
enhance/modify the also have strong impact on the rheology and
workability of fresh concrete [11]. Nano fillers have higher surface
energy compared with cement particle. Therefore, as shown in Figure 4,
the addition of nano fillers raises the system energy of cementitious
composites, thus importing negative entropy to the system of composites.
Figure 4: System of nano-engineered concrete [7].
The mechanisms of nano-core effect on the enhancement/ modification
are mainly due to two aspects: intrinsically excellent mechanical,
electrical, thermal and electromagnetic properties and morphology
features (high aspect ratio); and promoting cement hydration, optimizing
C-S-H gel structure and forming ultrafine and compact crystals,
improving interfacial transition zone and pore structure, controlling
nano-scale cracks, autogenous curing, improving early strength and
decreasing autogenous shrinkage through nucleating effect (Figure 5).
Figure 5: Schematic diagram of effect of nano fillers on the hydration products growth around cement particles [11].
As a new industrial revolution, nanotechnology infiltrating in the
field of civil engineering provides new impetus for developing high-
performance and smart/multifunctional concrete. To restructure or modify
material structural units in nanoscale via interpreting material
genetic code and drawing the blueprint of nanoscale properties provides
new theory and method to develop high- performance, durable,
smart/multifunctional, and environmentally friendly concrete (Figure 6).
The utilization of nanotechnology helps promote the understanding of
concrete behavior, manipulate and design concrete performance, lower the
concrete production and ecological cost, extend the service life of
engineering infrastructures and reduce the relative demand of concrete.
It is of profound significance to guide the sustainable development and
application of concrete material and infrastructures.
Figure 6: Nano-engineered concrete based on nano-core effect.
The authors thank the funding supported from the National Science Foundation of China (51578110 and 51428801).
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