Safety issues relating to nanomaterials for construction applications - Nanotechnology in Eco-efficient Construction - page 129

Carbon-based MNMs, such as carbon nanotubes, are some of the strong-
est materials currently known (Hayashi et al. , 2007). For this reason,
they are often used in concrete and ceramics to improve the mechanical
strength and durability (Becher, 1991; Luo et al. , 2004; Sobolev and
Gutierrez, 2005; de Ibarra et al. , 2006; Ge and Gao, 2008; Raki et al. , 2010).
In addition, carbon nanotubes help to prevent cracks by strongly binding
together cement and aggregates. Similarly, these materials are incorporated
into ceramics to also prevent crack propagation, and improve strength
and thermal properties. Alternate uses include nano- and micro-sensors
and actuators implanted into the structure to monitor real-time health and
environmental conditions such as overall wear, moisture content, and tem-
perature (Zhang et al. , 2006). These devices are known as nano- or micro-
electro-mechanical systems (NEMS/MEMS). Carbon-based MNMs are
capable of enhanced electron shuttling and can be used to harvest renew-
able energy in solar cells (Girishkumar et al. , 2005; Brown and Kamat,
2008).
In addition to carbon-based nanomaterials, metallic, non-metallic, and
alloyed MNMs also have benefi cial uses in the construction industry. Metal
oxide NPs are used to reinforce the mechanical and compressive strength
of concrete, generate non-utility electricity in solar cells, provide fl ame
resistance to ceramics and windows, and increase the hydration ability of
cement. Common metal oxide NPs used in construction are TiO 2 , SiO 2 , and
Fe 2 O 3 . Often, SiO 2 and Fe 2 O 3 are used as fi lling materials in the pores of
concrete to prevent weakening from road deicers, such as salt, that react
with the concrete constituents. When incorporated into concrete, these NPs
also enhance the mechanical strength.
Nano-scale TiO 2 and SiO 2 are also utilized in windows, pavements, walls,
and roofs to gain several useful benefi ts. Layers of nano-silica between glass
window panels can provide fi reproofi ng, where antirefl ective coatings of
SiO 2 nanoparticles will control exterior light to improve energy conserva-
tion via reduction of air conditioning usage (Mann, 2006; Rana et al. , 2009).
Reactive oxygen species (ROS) can be generated through reactions between
TiO 2 and UV wavelengths from artifi cial or natural light, making TiO 2 an
excellent antimicrobial agent and dirt-repellent (Paz et al. , 1995; Irie et al. ,
2004). By coating windows with TiO 2 , bacterial fi lms and dirt buildup will
be eliminated by these 'self-cleaning' windows. Titanium dioxide is also
superhydrophilic, which aids in the prevention of hydrophobic dust accu-
mulation. Similar results can be obtained on pavements, walls, and roofs, as
TiO 2 will also act as an antifouling agent under solar irradiation. Addition-
ally, light-mediated TiO 2 surface hydroxylation provides glass windows with
antifogging properties (Irie et al. , 2004; Kontos et al. , 2007). Electricity gen-
eration is possible through the use of TiO 2 and silicon-based fl exible solar
cells applied to roofs and windows (Zhu et al. , 2004).

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