How nanomaterials are driving the new generation of cement products. Nanomaterials are promising candidates for the next generation of high-performance structural and multi-functional composite materials.
Cement is one of the most important building materials, and world production has increased significantly in recent years. There is also a need to create new strong concrete products by using new composite materials with superior properties to existing materials. Carbon nanotubes (CNTs), Carbon Nanofibers (CNFs) and Graphene have all been investigated for use in the construction industry. However, widespread implementation has yet to occur with, technically, one of the biggest problems in the use of nanomaterials being dispersion in the matrix material. Due to their high agglomeration and bundling tendency, carbon nanomaterials cannot be easily and homogeneously dispersed in cement by a simple mixing procedure . Usually, multi-step, time-consuming processes are required.
Nanocomposites
Nanocomposites are materials that incorporate nano-sized particles into a matrix of standard material such as polymers. Adding nanoparticles can generate a drastic improvement in properties that include mechanical strength, toughness and electrical or thermal conductivity, which wil have major implications for the construction industry. For example, these properties could one day lead to a building that is highly resistant to the effects of an earthquake. The effectiveness of the nanoparticles is such that the amount of material added is normally only 0.5-5.0% by weight. They have properties that are superior to conventional microscale composites and can be synthesized using simple and inexpensive techniques.
They are leading to new possibilities for engineered materials, improving existing properties by orders of magnitude within the starting materials, as well as allowing for a huge property enhancement of properties in the composite structures. The properties including mechanical, electrical and thermal may differ depending upon the composition of the materials used for the synthesis of the composites. The transition from microparticles to nanoparticles results in dramatic changes in physical properties. Nanoscale materials have a large surface area for a given volume. Since many important chemical and physical interactions are governed by surfaces and surface properties, a nanostructured material can have substantially different properties from a larger-dimensional material of the same composition. Nanocomposites contain relatively small amounts (<10%) of nanometer-sized clay particles. These materials significantly enhance the mechanical and thermal properties of the base resin, as well as, for example, improve barrier performance and flame retardancy. All of these performance benefits are available without increasing the density or affecting any properties of the base polymer.
Carbon Nanotubes
Carbon nanotubes are ideal constituents of specialty polymers, copolymers, polymer composites, electronic materials and biological structures where their outstanding physical properties, such as high strength, exceptional thermal conductivity, and singular electronic properties distinguish them from all other nanomaterials.
Single-walled nanotubes (SWNTs) are made up of a single cylinder, approximately 1 nm in diameter. In multi-walled nanotubes (MWNTs), cylinders are nested, with the total diameter ranging from 5 nm to 100 nm, while double-walled nanotubes (DWNTs) are MWNTs with just two layers.
In addition to being single- or multi-walled, CNTs can be long or short, have open or closed ends, and SWNTs can have different types of spiral structures or “chiralities,” all of which influence their electrical properties-whether they’re insulators, conductors, or semiconductors. The most popular production techniques for carbon nanotubes are carbon-arc discharge, laser ablation of carbon, or chemical vapour deposition. The CVD technique is the most commonly used for making nanotubes. Some of the most frequently utilized techniques to prepare polymer/CNT and/or polymer/clay nanocomposites include melt mixing, solution casting, electrospinning and solid-state shear pulverization. When properly dispersed within the matrixes, CNTs have the ability to improve the properties of the resultant materials several orders of magnitude relative to the unfilled polymers. The enhanced properties may include tensile behaviour, strength, toughness, stiffness, electrical and thermal conductivity and crystallization kinetics.
Current and potential applications of polymer/CNT nanocomposites include photovoltaic devices, optical switches, electromagnetic interference (EMI) shielding, aerospace and automotive materials, bicycle and tennis racquet frames, packaging, adhesive and coatings. Due to their superiority to traditional reinforcing materials such as glass fibers or carbon fibers, CNTs have been used as significantly stronger and tougher fiber-reinforcing material in concrete. They exhibit greatly enhanced mechanical properties along with extremely high aspect ratios (length-to-diameter ratio) ranging from 30 to more than many thousands. Mechanically, CNTs have a Young’s modulus of 1054 GPa, a tensile strength of 150 GPa and a density of 1.4 g-cm-3. Thus a carbon nanotube has strength of 150 times that of steel and is approximately six times more lighter. Carbon nanotubes can also bear torsion and bending without breaking.
Due to their size (ranging from 1 nm to tens of nm) and aspect ratios, CNTs can be distributed in a much finer scale than common fibers, giving as a result a more efficient crack bridging at the very preliminary stage of crack propagation within composites.
Carbon nanofibers
Carbon nanofibers (CNF) are a unique form of vapor-grown carbon fibers that fill the gap in physical properties between conventional carbon fibers (5–10 mm) and carbon nanotubes (1–10 nm). The main difference between carbon nanotubes and carbon nanofibers lies in the configuration of the underlying planes that are created by the alignment of carbon atoms. While nanotubes display an axial alignment of concentric cylindrical planes mainly composed of hexagonal substructures, nanofibers are characterized by a parallel and homogeneous alignment of nanoscopic graphene layers along the axis. Carbon nanofibers are produced from the catalytic decomposition of hydrocarbon gases or carbon monoxide over selected metal particles. Carbon nanofibers owe their excellent properties to its longitudinal geometrical appearance and its above-average ratios of length and diameter (L/D-ratio).
Industrial applications for carbon nanofibers include polymer and elastomer fillers, commercial hydrogen storage systems, radiowave-absorbing composites, lithium battery electrodes, construction composites, oil additives, gas-distribution layers for fuel cells, filters and absorbents.
CNFs have several distinct advantages as a reinforcing material for cement based materials in comparison to traditional fibers. As with CNTs, they exhibit significantly greater strength and stiffness than conventional fibers, their higher aspect ratio has been demonstrated to arrest nanocracks and demand significantly higher energy for crack propagation. Nanofibers also have the potential to control the formation of nanocracks in the matrix, producing a high-performance cementitious nanocomposite. CNTs also exhibit electromechanical properties that when subjected to stress/strain, the electrical properties of CNTs change, expressing a linear and reversible piezoresistive response, leading to potential use as nanocomposites that could be used as stress/strain sensors.
Graphene
Graphene is a flat one-atom thick sheet of sp2 carbon atoms densely packed in a honeycomb crystal lattice structure. It is the basic structural element for graphite, carbon nanotubes, and fullerenes. Graphene samples are available as nanoflakes on Si/SiO2 substrate wafers. Each layer is monoatomically thin with a thickness of ~0.34nm, though it is possible to produce multi-layered flakes. Using microscopic imagery, one can easily find the flakes and process them using microelectronic fabrications techniques.
Because of its unique mechanical, thermal and electrical properties graphene is considered an ideal candidate for reinforcing materials in composites. The addition of small amounts of graphene nano-platelets to cement may lead to an increase in the composite material toughness and tensile, flexural, and impact strength. Graphene is a possible replacement material where carbon nanotubes are presently used. Although graphene and carbon nanotubes are nearly identical in their chemical makeup and mechanical properties, graphene is better than carbon nanotubes at lending its attributes to a material with which it’s mixed, making it potentially more suitable for cement applications.
The potential for nanomaterials to greatly improve the performance of construction composites and concrete is clear. These materials will lead to the development of novel, sustainable, advanced materials with unique mechanical and electrical properties. However, as the construction industry is traditionally conservative, cost sensitive and low-tech there are a number of challenges, such as dispersion and functionalization of nanomaterials, processing, handling issues, scale-up and materials cost that need to be overcome before their widespread adoption can become a reality.
