Civil Engineering Reference
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Moreover, UV light is also considered harmful to humans, as it can provoke
skin and eye damage.
Possible alternative semiconductors that are susceptible to photoactiva-
tion are zinc oxide, ZnO (band gap 3.4 eV), mainly for the wide range of
morphologies and properties achievable depending on the synthetic routes
adopted, and tungsten oxide, WO
3
, whose band gap of 2.7 eV is promising
for applications in visible light conditions. Yet, few examples can be found
of their use in photocatalytic construction materials. Studies on ZnO-
modifi ed cements usually deal with the related modifi cation of mechanical
properties, and controversial results have been proposed: the material
strength was proved to increase by substituting a few percent of cement
with zinc oxide (Riahi and Nazari, 2011), while Lackhoff
et al.
(2003)
observed a set-retarding effect and a correlated decrease in mechanical
resistance due to water losses during the prolonged dormant phase before
setting. As for WO
3
, tests were performed by Linkous
et al.
(2000) by coating
a cement substrate with either WO
3
or TiO
2
, and results indicated that the
presence of WO
3
alone induced a lower photoactivity compared to TiO
2
.
On the other hand, good photoactivation effi ciencies were observed in
hybrid oxides (mainly WO
3
-TiO
2
and SiO
2
-TiO
2
): attention is now focusing
on these composite systems as a means to improve the properties of pho-
tocatalytic materials. WO
3
-TiO
2
hybrid nanoparticles were proved to exhibit
photoactivity also under visible light activation (Chai
et al.
, 2006). Yet, no
information is available on the interaction between the composite WO
3
-
TiO
2
nanoparticles and mortars or concrete, especially concerning possible
infl uences on the setting properties of the material, or on its fi nal mechani-
cal behaviour.
SiO
2
-supported TiO
2
materials have been extensively used as photocata-
lysts for a wide variety of reactions. The higher effi ciency observed in a few
works compared to pure titanium dioxide has been ascribed to different
physicochemical properties, which depend on the possible interactions
between the two oxides and on synthesis conditions (sol-gel, grafting of
TiO
2
on SiO
2
, coprecipitation, impregnation, chemical vapour deposition).
The most interesting point about silica-supported TiO
2
is that silica itself is
a common ingredient of cement-based materials, which makes the integra-
tion of these composite nanopowders easier (Bellardita
et al.
, 2010).
13.3 Applications of semiconductor photocatalysis
13.3.1 Photocatalytic degradation of pollutants
TiO
2
photocatalytic activity can be exploited to solve manifold purifi cation
issues arising not only from industrial uses or production of harmful sub-
stances, but also from heating and transportation. It exhibits good effi ciency
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