The exceptional chemical and physical attributes of nanoscale materials offer exciting prospects for a diverse range of applications, including reinforcement of structural integrity, energy conservation, antimicrobial efficacy, and self-cleaning surfaces. As a result, engineered nanomaterials and nanocomposites have emerged as compelling options for deployment across the construction and associated infrastructure domains.
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Cutting-edge nanomaterials such as carbon nanotubes, graphene, and metal oxides have captivated the interest of researchers in the pursuit of advanced construction materials.
This article delves into the latest developments in nanomaterial applications, enhancing conventional construction materials, anticipating plausible environmental release patterns, and outlining potential detrimental biological and toxicological effects while also exploring strategies for prevention.
Mechanical enhancement, energy conservation, antimicrobial potency, and self-cleaning capabilities are among the promising applications of nanomaterials in the construction industry.
What are Nanomaterials?
Nanomaterials are categorized into four groups based on their dimensional properties: zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D).
Nanoparticles are representative of 0D nanomaterials, where all dimensions fall within the nanoscale (measuring between 0.1 and 100 nm). 1D nanomaterials like nanotubes, nanorods, and nanowires, have one dimension outside the nanoscale. 2D nanomaterials display a platelike form and include nanofilms, nanolayers, and nanocoatings. 3D nanomaterials or bulk nanomaterials possess three dimensions exceeding 100 nm and can include nanoparticle dispersions, nanowire bundles, and multi-nanolayers.
The fabrication of nanomaterials involves two primary approaches: "top-down" and "bottom-up." In the top-down approach, external forces are utilized to break down a bulk material into smaller fragments until the desired nanoscale size is achieved. The bottom-up approach involves synthesizing individual atoms or molecules via chemical reactions or self-assembly processes to create the final nanostructure.
Nanomaterials in Construction
The construction industry can incorporate various nanomaterials to strengthen materials, develop functional paints and coatings, and design high-resolution sensing and actuating devices with exceptional structural properties.
The use of carbon-based nanomaterials in the construction industry has gained significant attention from researchers. Carbon is an abundant element, and organic materials are constructed from chains of carbon atoms connected by covalent bonds, resulting in the formation of hundreds of chemical compounds with the same number of carbon atoms. Carbon-based nanomaterials are considered less toxic than those based on inorganic metals, making them attractive for use in construction.
Carbon nanotubes (CNTs) are 2D nanomaterials that exhibit interesting and useful properties.
There are two types of CNT: single-wall CNT (SWCNT) and multi-wall CNT (MWSNT). SWCNTs can be imagined as a perfect graphene sheet rolled into a cylinder with the hexagonal rings placed in contact, forming a tube sealed by two caps, each of which is a hemi-fullerene of the appropriate diameter.
CNTs possess excellent mechanical properties, making them suitable candidates for improving the volume stability of cement-based materials. MWCNTs have a Young's modulus on the order of 270-950 GPa and tensile strength of 11-63 GPa.
CNTs act as polymeric chemical admixtures, enhancing the mechanical durability of concrete mixtures and preventing crack propagation. The use of CNTs as crack bridging agents in non-decorative ceramics improves their mechanical strength and thermal properties while reducing their fragility.
CNTs are also incorporated into devices implanted in construction structures for real-time monitoring of material damage and health and environmental conditions.
The addition of a small amount of CNT in cementitious materials results in significant improvements in their mechanical properties. For example, the addition of 0.5 wt% MWCNT into a cement matrix increases the flexural strength and compressive strength by 25% and 19%, respectively. Similar results were observed with the addition of CNT or tungsten di-sulfide nanotubes to cement.
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However, effective dispersion of the as-produced aggregated nanotubes in a matrix is a major challenge. Sonication or dispersants (surfactants) can facilitate the integration of individual nanotubes into a cement paste matrix.
Graphene is a flat monolayer of carbon atoms packed tightly into a two-dimensional honeycomb lattice. This material has received much attention due to its unique properties, such as high surface area, high mechanical strength, easy functionalization, excellent conductivity, and possible mass production.
By oxidizing graphite using strong oxidizing agents, oxygenated functionalities are introduced into the graphite structure, expanding the layer separation and making the material hydrophilic, enabling it to be dispersed in water. This property allows graphite oxide to be exfoliated in water using sonication, ultimately producing single or few-layer graphene, known as graphene oxide (GO).
Compared with CNT, graphene oxide is readily dispersible in water using moderate sonication, exhibiting a very low percolation threshold and significantly limiting the addition level required.
Metal Oxide Nanomaterials
Metal oxide nanoparticles are a popular choice among nanomaterials and are especially useful in the construction industry.
Concrete structures can be weakened by de-icers such as CaCl2 and MgCl2, which can penetrate through the micropores that develop during cement hydration. Fortunately, fillers like silica (SiO2) and iron oxide (Fe2O3) can be used to reinforce concrete and pack the pores, thus preventing weakening. Incorporating these nanoparticles into fly ash as a cement replacement can also improve the mechanical properties of the concrete.
Metal oxide nanoparticles also have interesting applications in window coatings. Silica nanoparticles can be used as anti-reflective coatings to control exterior light and save energy, while TiO2 coatings can absorb UV rays to create reactive sites that remove bacteria and dirt on windows. This makes them ideal for creating self-cleaning surfaces, as hydrophobic dust has difficulty accumulating on highly hydrophilic surfaces created by photoinduced species.
Additionally, metal oxide nanoparticles can be incorporated into dye-sensitized TiO2 solar cells, which can be coated onto the outside surfaces of buildings to produce sustainable and green energy.
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References and Further Reading
Huynh, T.V., et al (2018). Nanomaterials in Construction: An Overview. [Online] Research Gate. Available at: https://www.researchgate.net/publication/341025094_NANOMATERIALS_IN_CONSTRUCTION_AN_OVERVIEW (Accessed on 7 April 2023).
Lee, J., et al (2010). Nanomaterials in the Construction Industry: A Review of Their Applications and Environmental Health and Safety Considerations. ACS Nano. doi.org/10.1021/nn100866w.
Moradiya, M.A. (2019). Nanomaterials and Their Use as Construction Materials. [Online] AZO Build. Available at: https://www.azobuild.com/article.aspx?ArticleID=8287.