Civil Engineering Reference
In-Depth Information
maintenance costs during the lifetime of the structure), and possibly an
improvement of the surrounding air quality. The latter effect is actually
connected with the extension of photocatalytic surfaces in relation with the
volume of air to be depolluted: a limited space such as an indoor environ-
ment can benefi t from this property, while a single building in a polluted
city area will not be able to have a positive effect on the huge volume of
air that comes into contact with it, this contact in most cases being very
short due to wind.
Yet, the interaction of TiO
2
with such a complex hosting environment
may also have negative consequences. In fact, materials based on either
hydraulic (cement, hydraulic lime) and non-hydraulic (gypsum, lime)
binders consist of a mixture of calcium-based inorganic compounds, mainly
calcium oxide/hydroxide, carbonate, silicate and sulphate. These constitu-
ents do not occupy the whole volume of the material, and materials are
typically porous at the micro- and also nano-scale: this porosity is where
TiO
2
usually fi nds its collocation, acting as a further aggregate, or nano-fi ller.
Therefore, all reaction products of cement hydration that remain unbounded
in the material porosities can adsorb on the TiO
2
surface, if small (e.g.,
impurities, germs of calcium hydroxide crystals, etc.), thus 'stealing' active
sites to external polluting substances that could be degraded in a mecha-
nism of competitive adsorption. Furthermore, a side effect of increased
electron-hole couple recombination can also occur on adsorbed species
(Lackhoff
et al.
, 2003; Kwon
et al.
, 2006).
This is the most evident infl uence of the alkaline hosting material on TiO
2
photoactivated properties; it is then immediate to wonder whether TiO
2
itself may lead to changes in the material characteristics. Concerning the
fresh state, the key modifi cation induced by TiO
2
nanopowders is a decrease
in workability: in fact, this is not correlated to the chemical nature of the
particles, but to their nanometric size, which produces a drastic change in
the rheological behaviour of the mix. This change is quite pronounced, and
must be considered in order to assure a determined workability.
On the other hand, hardening properties are just slightly affected by TiO
2
.
Attention has been focused on the observed increase in the compressive
resistance of the material: this was mainly ascribed to the already cited
fi lling effect, which was addressed by several works and summarized in the
review by Sanchez and Sobolev (2010). Yet, controversies arise when a pos-
sible active behaviour of TiO
2
is considered. Lackhoff
et al.
(2003) and Li
et al.
(2007) hypothesize a pozzolanic activity of TiO
2
; while the former
justify this as an indirect consequence of an accelerated cement hydration
observed with NMR relaxometry, in the latter case no experimental valida-
tion is provided, and the assumption is probably made in the wake of the
pozzolanic activity of silica nanopowders proved in previous works (Ji, 2005;
Jo
et al.
, 2007). This was further supported by Nazari and Riahi in 2010, as
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