The global concrete and cement market was valued US$457.2 billion in 2011. Cement is one of the most important building materials, and world production has increased significantly in recent years especially in developing countries. As the scale of structures created with concrete has increased dramatically, the need for advanced materials has too. There is also a need to create high strength and strain concrete and cement products by using new composite materials with superior properties to existing materials. The emergence of nanotechnology applications in concrete and cement is relatively recent and most developments are still in the early commercialization process.
Nanomaterials affect cement and concrete in different ways including their processing conditions, released CO2 emissions, service life and functionalities. Addition of nanoparticles is leading to stronger, more durable, self-healing, air purifying, fire resistant, easy to clean and quick compacting concrete. Nanomaterials currently being developed are nano silica (silica fume), nano Alumina (Al2O3), nanostructured metals, carbon nanotubes (CNTs), graphene and carbon nanofibers (CNFs). Concrete structures are also benefitting from nano-enhanced coatings that prevent graffiti and other unwanted stains form adhering to it. Photocatalytic nano-enhanced cement has found application in the construction sector, on a relatively limited scale. The use of titanium dioxide (TiO2) as a photocatalyst in concrete pavements has received considerable attention in recent years due to its ability to decontaminate via photocatalytic processes.
Widespread implementation of nanomaterials has yet to occur in the construction industry with, technically, one of the biggest problems in their use 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.
Figure 1: Nano-TiO2 spray effect.
Nanoparticles
The most widely investigated nanoparticles in cement and concrete are nano-oxides, especially SiO2 and Fe2O3. The addition of these nanoparticles to cement paste containing high volumes of fly ash and to sludge ash concrete mortars resulted in an increase in compressive strength, resulting in high-performance and self–compacting concrete with improved workability and strength. Nano-Fe2O3 and nano-SiO2 were also used to increase the abrasion resistance of concrete for pavement. Nanoscale TiO2 have also been developed to remove organic pollutants from surfaces directly exposed to ultraviolet radiations such as road pavements and cement-based façade finishing products. Adding silica nanoparticles to cement paste has been demonstrated to effectively reduce the degradation rate as well as its negative consequences.
It has been found to be more effective than micron sized silica for improving permeability, and durability. 15 to 20 kg of nano silica provides the same strength as 60 kg of regular or micro silica.
Carbon Nanotubes
CNTs have been developed as a reinforcing material with extremely high strength and Young’s modulus, improved elastic behaviour as well as electronic and thermal properties. They act as fillers, producing denser materials; inhibit crack growth at very early times, preventing propagation; and enhance quality of the paste-aggregate interface, resulting in much stronger and tougher concrete. Carbon Nanotube Engineered Surfaces LLC, is a new start-up from the University of Alabama, producing concrete products incorporating nanotubes. However, cost of carbon nanotubes in cement compostite materials hinders commercialization at present. Even with significant reduction in price, these materials will likely remain in niche construction applications.
Figure 2: Mechanism of photocatalytic NOx oxidation on active concrete road.
Figure 3: Jubilee Church in Rome, the outside coated with nanophotocatlytic TiO2 coatings. Photocatalytic titanium dioxide is energized by UV and accelerates the decomposition of organic particulates and airborne pollutants such as nitrous oxide (NOx).
Nanofibers
Ruptures in cement or concrete may start at high loads or via incorrect processing technology. The cracks that form are initially at the nanoscale, but gradually get larger and agglomerate together to form micro-cracks resulting in is material impairment that can lead to building element damage. Ordinary Portland cement (OPC) is the most widely used material in the construction industry. However, as it has poor tensile stress and strain capacity it must be reinforced with steel bars. In addition, fibres of various types may be added to delay the development of micro-cracks and improve resistance to tensile stress.
Nanofibers have been developed for improving the tensile strength, stiffness and durability of cementitious materials. They allow for an enormous increase of Young’s modulus of elasticity and overall strength. This results in an essentially crack free material ready to use in a variety of structural applications. Carbon nanofibers act as a bridge between emerging cracks, so they do not become enlarged. Nycon (www.nycon.com/ncwp/product/nycon-g-nano/) has produced Nycon-G-Nano, a nanofiber incorporated into cement products.
Graphene
Graphene nano-platelets have unique mechanical, thermal and electrical properties that make them ideal reinforcing materials in multifunctional cement based composites. Results have shown that graphene improves interfacial strength of cementitious nanocomposites. Researchers at Monash University (contact Dr Ina Sambor, ina.sambor@monash.edu) have developed a cementitious matrix containing graphene oxide to enhance the strength and durability of concrete. The newly formulated cement composite contains graphene oxide. This matrix formulation does not require a dispersing agent, a surfactant, or a stabilizing agent and can be prepared in different weight aspect ratios using water as a dispersing medium. The mechanical properties have been tested in the laboratory and measurements show that only 0.05% of graphene oxide (GO) is needed to improve flexural strength of an ordinary Portland cement matrix by from 41% to 59% and compressive strength by from 15% to 33%.
Nanocoatings
Concrete structures are subject to dynamic environments, such as dynamic loads, ultra violet from direct sun-light, continuous expansion and contraction by heat, rain or water splash, impacts from debris, erosion, etc. In this condition, most resin coatings deteriorate in a short period of time in the form of cracking, blistering, disbanding, or chalking. Nanocoatings have been applied to cement products to reduce corrosion and ingress of harmful chemicals. Nanokote (www.nanokote.com.au) has produced NK-TC 01, a coating material that exhibits very high abrasion resistance, high chemical resistance and easy to clean surface properties. The coating material is applied to concrete and cement based substrates to give an opaque satin-gloss or matt finish. German company Nano-Care AG (www.nanocare-ag.com) is manufacturing oltra-thin easy-clean nano-coatings, based on silicon dioxide (SiO2) for cement products. They are used as an invisible, water- and contamination-resistant, UV-stable coating of porous substrate surfaces, regardless of whether the surface is a natural stone such as sandstone, concrete, terracotta, clay brick or stone panelling. Hi-Proguard (http://mui-int.com/Hi-Proguard-for-Concrete.php) is another nano-based product for concrete protection.
Self-healing concrete
Nanomaterials are also playing an important role in self-healing concrete. Nano-polymers are utilized for superior performance of bulk matrix, steel/cement paste interface and steel surface When self-healing concrete cracks, embedded microcapsules rupture and release a healing agent into the damaged region through capillary action. The released healing agent contacts an embedded catalyst, polymerizing to bond the crack face closed. In fracture tests, self-healed composites recovered as much as 75 percent of their original strength. They could increase the life of structural components by as much as two or three times. Self-healing concrete based on nanomaterials has been developed at Delft University of Technology.
Figure 4: Self-healing concrete (Source: TU Delft).
