Definition:
Concrete, the most ubiquitous
material in the world, is a nanostructured, multi-phase, composite material
that ages over time. It is composed of an amorphous phase, nanometer to
micrometer size crystals, and bound water. The amorphous phase, calcium-silicate-hydrate
(C-S-H) is the ''glue" that holds concrete together and is
itself a nanomaterial. Viewed from the bottom-up, concrete at the
nanoscale is a composite of molecular assemblages, surfaces (aggregates,
fibres), and chemical bonds that interact through local chemical reactions,
intermolecular forces, and intraphase diffusion.
Properties characterizing this
scale are molecular structure; surface functional groups; and bond length,
strength (energy), and density. The structure of the amorphous and crystalline
phases and of the interphase boundaries originates from this scale. The
properties and processes at the nanoscale define the interactions that occur
between particles and phases at the microscale and the effects of working loads
and the surrounding environment at the macroscale. Processes occurring at the
nanoscale ultimately affect the engineering properties and performance of the
bulk material .
Nano-Fe 2 O 3 has been found
to provide concrete with self-sensing capabilities as well as to improve its
compressive and flexural strengths. Nano-Al 2 O 3 has been
shown to significantly increase the modulus of elasticity (up to 143% at a
dosage of 5%) but to have a limited effect on the compressive strength.
Nanosized cement particles and nanobinders have been proposed as a way to
improve cement performance while reducing carbon emissions.
Cement pastes made
with nanosized cement particles have shown faster setting times and an increase
in early compressive strength compared to pastes prepared with common . The
concept of a nanobinder involves mechano-chemical activation that is
obtained by inter-grinding cement with dry mineral additives in a ball
mill. Mechano-chemical modification of cement with high volumes of blast
furnace slag has been shown to increase the compressive strength by up
to 62%.
Nanoclay
particles have shown promise in enhancing the mechanical performance, the
resistance to chloride penetration, and the self-compacting properties of
concrete and in reducing permeability and shrinkage. Natural clay particles are
micron and sub-micron in size, and the base structure of clay is composed of
crystalline layers of aluminum phyllosilicates with thicknesses on the order of
1 nm. Chemical binding of PVA (polyvinyl alcohol) to exfoliated
clay particles recently has been proposed to create linked clay particle
chains that, when incorporated in cement, were shown to improve the
post-failure properties of the material.
Use
of Nanoreinforcements:
Carbon nanotubes/nanofibers
(CNTs/CNFs) are potential candidates for use as nanoreinforcements in
cement-based materials. CNTs/CNFs exhibit extraordinary strength with moduli of
elasticity on the order of TPa and tensile strength in the range of GPa, and they
have unique electronic and chemical properties.
CNTs/CNFs,
thus, appear to be among the most promising nanomaterials for enhancing the
mechanical properties of cement-based materials and their resistance to crack
propagation while providing such novel properties as electromagnetic field
shielding and self-sensing. Single-wall CNTs (SWCNTs), multi-wall CNTs
(MWCNTs), and CNFs are highly structured graphene ring-based materials with
very large aspect ratios (of 1000 or more) and very high surface areas. SWCNTs
are single graphene cylinders and MWCNTs are multiple, concentric graphene
cylinders coaxially arranged around a hollow core. Unlike CNTs, CNFs present
numerous exposed edge planes along the surface that constitute potential sites
for advantageous chemical or physical interaction.
Compared to CNTs, vapor
grown CNFs have a lower production cost (about 100 times lower than SWCNTs )
and are suitable for mass production. While CNTs/CNFs have been extensively
studied in polymeric composites, their use in cement has, to date, remained
limited. Most research efforts have focused on CNTs compared to CNFs and have
been performed on cement pastes.
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