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model reactants. Also in this case, removal effi ciencies are almost 100% in
batch conditions and close to 60% in fl ow conditions. Experimental tests
performed in fl ow conditions are undoubtedly more relevant than batch
ones, as they are more representative of the actual working conditions of
these materials in service.
13.5.2 Self-cleaning
One of the fi rst studies on the self-cleaning attribute of photoactive materi-
als is that proposed by Cassar in 2004, describing the impregnation of white
cement disks with a yellow dye (phenanthroquinone) and the subsequent
restoration of the initial white colour in specimens containing TiO
2
. Similar
works on cement pastes and mortars were carried out with other organic
dyes, such as rhodamine B. Aqueous solutions of the dye with a concentra-
tion of 0.05 g/L were spread on the surface of mortars and allowed to dry,
then colour variations during irradiation were monitored with a spectro-
photometer (Ruot
et al.
, 2009).
This method allows an easy evaluation of the self-cleaning property. The
spectrophotometer measures the colour of the surface, and converts it into
a set of chromatic coordinates (usually the CIELab colour system, defi ned
by the Commission Internationale de l'Éclairage, where
L
* is brightness,
a
* varies from green to red, and
b
* from blue to yellow). In this space,
colour changes can be measured as geometric distance between two points,
which correspond to two different colours to be compared (for example, A
and B in Fig. 13.9). It is possible to calculate the overall colour change
(
Δ
col), or to analyse only the changes in hue (
Δ
hue), or to identify changes
in the saturation of a single hue (
sat). The latter parameter is exploited in
dye degradation measurements: since dyes lose their colour when the
molecular structure is degraded, colour saturation can be used as a repre-
sentative parameter of dye concentration. In the case of rhodamine B,
which exhibits a strong magenta colour, a decreased intensity of red com-
ponent of colour indicates that the dye is being degraded.
Yet, it is important to keep in mind that colour changes do not necessarily
mean complete degradation, and the organic molecule can still be partially
integer even after the colour has disappeared, since it is suffi cient to break
the molecule chromophore groups to induce the colour loss. This is why
these tests often refer to dye 'decolourization' rather than 'mineralization',
since the latter can only be measured by TOC (total organic carbon) or
spectrometric measurements.
Another important experimental path that can be chosen to test the self-
cleaning attribute of cement-based materials is their exposure to a polluted
environment and monitoring of surface colour, which is performed with the
same colour measurements described for dye decolourization. The polluted
Δ
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