Concrete, the second most important material in construction after water, is advantageous however; its low tensile strength requires reinforcement. Nanotubes are found to be promising natural materials for the reinforcement of concrete at nano scales. Different nanotubes suggested as a natural means for reinforcing concrete are carbon nanotubes, clay nanotubes, and cementitious nanotubes.
Why Do We Need Natural Reinforcing Materials?
Cement-based materials, e.g. concrete, are one of the most commonly used materials by mankind due to its inexpensive raw materials and its high compressive strength. The importance of concrete in the construction sector is undeniable due to its advantageous properties. One of the advantages of concrete over the other construction materials is that concrete can be formed on-site and cast into various shapes. Even though concrete is the backbone of the supporting system of a building, it exhibits brittle behavior under tension and is vulnerable to the uncontrolled propagation of cracks. The ultimate tensile strength of cement is in the order of 1-10 MPa which is significantly open for improvement and the reason for its weak mechanical properties. The most common reasons for cracking are moisture, temperature, ground movements, environmental stresses (e.g., corrosion assisted cracking), tensile loading, shrinkage, design flaws, and extensive chemical reactions. In order to aid these problems, concrete is traditionally reinforced through a number of methods. Uniformly incorporating steel rods through the concrete is the most conventional method for reinforcing concrete. However, steel rods only prevent macro-cracking. Another method for reinforcing concrete is the use of randomly oriented steel, glass, or synthetic fibers which prevents cracking on the micro scale. However, the traditional approaches to the design of reinforced concrete relate to scales that are too large to have much effect on the chemical and mechanical damage that begins at the nanoscale. In accordance with rapid development in the construction industry, the need for reinforcing methods on the nano-scale has emerged. Preventing mechanical and chemical damage at the nano-scale is much more effective since it prevents the diminishing problems at the source. Nanotubes are considered as the appropriate nanostructure for the reinforcement of concrete due to their exceptional mechanical strength. These properties first attracted attention when carbon nanotubes were discovered. Later, this newly found nanostructure was employed to several different materials with the hopes of improving mechanical properties. These materials include clay nanotubes, portlandite, and Calcium-Silicate-Hydrate (C-S-H). All of these nanotube structures are considered a natural means for reinforcing concrete. However; while clay nanotubes, portlandite, and calcium-silicate-hydrate are readily accepted as water-compatible, carbon nanotubes require surface functionalization to be accepted as compatible. We need natural reinforcing materials for the improvement of concrete’s mechanical properties and homogenous distribution at the mixing stage.
Functionalized Carbon Nanotubes for Reinforcing Concrete
Carbon nanotubes (CNTs) are the first that come to mind when we are talking about reinforcing concrete at the nanoscale. CNT is a novel one atom thick carbon allotrope rolled into a tube shape. CNTs attract attention with their good mechanical, thermal, and electrical properties but for the reinforcing applications, their mechanical strength is of importance. Commonly, CNTs are classified as either single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs), based on the number of concentric tubes. The dimensions of CNTs range from 0.4 to 100 nm while their length is generally around 1-100 μm. The tensile strength is considered as an important criterion for reinforcing concrete and therefore investigated for every reinforcing material. The tensile strength of SWCNTs can reach 500 GPa, while that of MWCNTs can only reach 10–63 GPa. Another important criterion for reinforcing materials is Young’s modulus which is determined as 1 TPa for SWCNTs and 0.2 TPa for MWCNTs. One of the most important issues with CNTs as reinforcing materials is their hydrophobic nature which decreases the water solubility of the material and makes it challenging to form homogenous suspensions. This problem is often tried to be resolved through surface functionalization. Adding polar groups such as hydroxyl (–OH), carboxyl (–COOH), and carbonyl (=O) to the surface of CNTs significantly improves the wettability and promotes bonding between CNTs and cement hydration products. Another method for surface functionalization of CNTs is growing polymer chains such as polystyrene and PEG on the surface. In addition to these methods, ultrasonification and surfactants are also utilized in combination to obtain a homogenous dispersion. The reinforcing effect on the mechanical properties of cementitious composites can be affected by many factors relating to the dispersion of CNTs in cement paste and the bonding that exists between CNTs and the paste, including CNT length, CNT concentration, defects on the CNT surface, the surfactants used and the w/c ratio. For example, increased CNT length enhances crack-bridging and interaction with neighboring hydration products, but a decreases degree of dispersion. On the other hand, increased CNT concentration causes re-agglomeration and limited cement hydration. This is why the effects of CNT properties and concentration must be assessed properly for practical applications.
Clay Nanotubes for Reinforcing Concrete
Recently, nanoclays have attracted attention in industrial and construction applications due to their unique physic-chemical properties. Even though nanoclays are not the first option that comes to mind for reinforcing concrete, studies on the use of nanoclays as a cement composite deserve attention. Several different nanoclays including metakaolin, calcined halloysite, and halloysite nanotubes have been studied for reinforcing concrete. Amongst all the nanoclays, halloysite nanotubes (Hal) have been the most promising candidate having a similar structure to multi-walled CNTs and similar chemical composition to kaolin. Furthermore, halloysite clays can be extracted from natural deposits and are much cheaper than CNTs. As concrete additives to improve mechanical properties, calcined and non-calcined halloysite nanotubes are both found to be effective in improving the compression and flexural strength up to 36% and 5%, respectively. Clay nanotubes not only improve the mechanical properties of concrete but also reduce CO2 emissions during the production process for cement. Therefore, nanoclays can be accounted as both natural means for reinforcing concrete and a way for sustainable cement production. However, studies on nanoclay containing concrete are still at their infancy and require detailed research and adaptation for widespread production.
Cementitious Nanotubes for Reinforcing Concrete
A brand new group of materials for reinforcing concrete is suggested as cementitious nanotubes by H. Manzano and his team. The idea is based on fine-tuning the nanostructure of cement-based materials which are mainly composed of portlandite (Calcium Hydroxide) and the Calcium-Silicate-Hydrate (C-S-H) gel. Using inorganic nanotubes of these cementitious materials provides an interesting alternative to concrete reinforcement materials. The inherent compatibility of calcium hydroxide and CSH nanotubes with the cement matrix as well as their high stiffness and strength means that they have the potential for widespread use as ultimate fibers for the reinforcement of concrete at the nanoscale. Furthermore, portlandite and CHS nanotubes have hydrophilic nature as opposed to CNTs. The Young’s moduli of cementitious nanotubes are considerably lower than that of CNTs but still considerably higher than cement’s Young’s modulus. Young’s modulus of portlandite nanotubes is of the same order of magnitude as those of other inorganic nanotubes, such as imogolite (∼250 GPa), MoS2 (∼230 GPa), GaS (∼270 GPa), and chrysotile nanotubes (∼159 GPa). The ultimate tensile strength of portlandite nanotubes is reported as 8.4 GPa which is considerably higher than the ultimate tensile strength of cement. On the other hand, Young’s modulus of CHS nanotubes are considerably lower and around 15-30 GPa. Nevertheless, this value is still higher than Young’s modulus of cement. Drawing from these investigations, H. Manzano and his team report that cementitious nanotubes have great potential as a natural reinforcing material for concrete. However, further investigations on the stability of cementitious nanotubes and the widespread utilization of these nanotubes must be conducted.
Concrete, being the second most used material after water, holds an important place in the construction sector. It is preferred due to its low cost, abundance, and compressive strength. However, the low tensile strength of concrete creates mechanical problems such as brittle behavior under tension and vulnerability to the uncontrolled propagation of cracks. The conventional solutions to the cracking of concrete include the use of steel rods or randomly oriented steel, glass, or synthetic fibers. However, these solutions only prevent cracking on the macro or micro scales instead of reinforcing the material at nanoscales. For the reinforcement solutions at nanoscales, nanotubes are found to be effective candidates. Tensile strength and Youn’s modulus of these materials are the most important criteria for reinforcing concrete. The first option that comes to mind is carbon nanotubes due to their good mechanical properties. CNTs provide considerable improvement in the tensile strength of concrete. However, the hydrophobic nature of CNTs requires surface functionalization to be used as an appropriate reinforcing material. Clay nanotubes, especially halloysite nanotubes, are also considered as a promising reinforcing material. Halloysite nanotubes enhance the compressive and flexural strength while also reducing CO2 emissions. The most recent studies on the natural reinforcing materials attract attention to the cementitious nanotubes including portlandite and CHS nanotubes. These materials are advantageous in terms of their compatibility with cement even though their tensile strength is considerably lower than CNTs. All of these options indicate that nanotubes are promising materials for the reinforcement of concrete.