Civil Engineering Reference
In-Depth Information
a consequence of observed reduction in setting time and fi nal porosity of
TiO
2
-containing concrete, in spite of Chen and Poon's observations (2009a)
rebutting any pozzolanic nature of TiO
2
. Their experimental tests showed
no TiO
2
mass change during hydration, which suggests an inert behaviour
of the nanopowders, whose maximum effect is that of providing a wide
surface for nucleation or clinging of hydration products. This issue is still
open for discussion.
The majority of works in this fi eld are related to hydraulic binder-based
materials, as they represent the larger volume of products, especially con-
cerning the realization of new structures; a few works are also available on
non-hydraulic binders, focusing on the conservation of historical and con-
temporary structures. In this specifi c context, Karatasios
et al.
(2010) inves-
tigated the ageing of lime mortars, and specifi cally their carbonation. They
attributed to the presence of TiO
2
an effect of CO
2
release, which acceler-
ated carbonation reactions. In fact, TiO
2
presence would have caused the
photocatalytic degradation of organic substances adsorbed on the material
surface, releasing CO
2
as a reaction product. The process was considered
benefi cial since surface carbonation of these materials decreases the risks
of calcium leaching by run-off water, one of the main causes of binder
degradation, due to the lower solubility of calcium carbonate compared to
calcium hydroxide (Hansen
et al.
, 2003).
13.4.2 Ageing of the material
Understanding the evolution of the material during its whole lifetime and
the possible onset of negative interferences between TiO
2
and its hosting
environment is just as important as studying its benefi cial effects.
The alkaline material undergoes carbonation in time, which decreases
capillary absorption and causes the precipitation of calcium carbonate
inducing a solid volume increase of the material higher than 10% (Ceuke-
laire and Nieuwenburg, 1993; Castellote
et al.
, 2009). These precipitates tend
to obstruct active sites, thus decreasing the photocatalytic effi ciency of TiO
2
mainly for a shielding effect (Chen and Poon, 2009b). This effect builds up
with the accumulation of contaminants on surfaces exposed to the environ-
ment, as noted in a report of the Hong Kong Environmental Protection
Department (Yu, 2003) (Fig. 13.8).
Although some data are already available, the infl uence of carbonation
on the photocatalytic and self-cleaning effi ciency of TiO
2
embedded in
construction materials has not been examined thoroughly yet. This can be
a vital aspect to defi ne the service life of such materials, as a careful design
of the mix must take into account a possible decrease, and eventually the
loss, of this functionality. This is true for any kind of element considered -
mortar panels for cladding systems, repair plasters, and even more impor-
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