• The first trace of scientific works on this subject is a paper published by Keidel in 1929, suggesting an active role of TiO₂ in the fading of paints.

• In 1938 the photobleaching of dyes caused by UV-irradiated TiO₂ was investigated by Goodeve and Kitchener, and ascribed to the presence of active oxygen species detected on TiO₂ surfaces.

• In 1964 Doerffler and Hauffe first proposed a research paper whose title contained the term ‘heterogeneous photocatalysis’: zinc oxide was used as photocatalyst.

• In the same year, Kato and Masuo reported the use of a TiO₂ suspension to photocatalyse the oxidation of tetralin, opening the way to similar processes proposed by McLintock and Ritchie on ethylene and propylene in 1965.

• In 1972 Fujishima and Honda for the first time reported the production of a TiO₂-based electrochemical cell for water splitting, which consisted of a single crystal rutile photoanode and a platinum counter electrode. This is now known as the ‘Honda–Fujishima effect’.

• Frank and Bard (1977a, 1977b) first used TiO₂ as remedy to environmental pollution issues by studying the liquid phase reduction of cyanide ions in the presence of TiO₂ powders.

• This publication was followed in the same year by the development of nitrogen reducing solar cells – photocatalytic reduction of N₂ to ammonia – performed on pure and Fe-doped TiO₂ (Schrauzer and Guth, 1977).

• In 1978 TiO₂-catalysed organic synthesis was introduced by Kreutler and Bard (photosynthesis of methane from acetic acid).

• In 1985 TiO₂ was first applied successfully in biocide bacteria photokilling (Matsunaga et al., 1985); this concept was then applied to tumour cells (Fujishima et al., 1986).

• Matthews (1987) was the first to use TiO₂ powders immobilized on a substrate for liquid phase organic photocatalysis, to avoid filtration and resuspension.

• In 1991 O’Regan and Gratzel proposed the use of TiO₂ as anode in dye-sensitized photovoltaic cells.

• In 1995 the superhydrophilic effect was first noticed by Fujishima’s research group, leading to the development of self-cleaning and antifogging surfaces (Wang et al., 1997).

• The first photocatalytic TiO₂ products were delivered commercially in the late 1990s in the form of self-cleaning tiles and glass: in 1995, the Japanese company TOTO Ltd. started the manufacture of antibacterial Cu- or Ag-containing TiO₂ tiles, mainly for operating rooms (Watanabe et al., 1995).

• The Marunouchi Building (or Marubiru), in Tokyo, opened in 2002, one of the first buildings featuring self-cleaning windows (Ohtani 2010).

• Photoactivation was proved to induce a hardness modification in the surface of TiO₂, due to compressive stresses that are induced by volume increases connected with the onset of the superhydrophilic state (Shibata et al., 2003).

• The church Dives in Misericordia (Rome, Italy), designed by Richard Meier for the Jubilee and finished in 2003, is one of the first examples of photocatalytic technology in Europe: it was built with TiO₂-modified cement (former constructions: a school in Mortara, Italy, in 1999, and the Cité de la Musique in Chambéry, France, in 2000).

• 20,000 m² of self-cleaning windows were installed in the terminal building of Chubu International Airport (Japan), completed in 2005 (Fujishima and Zhang, 2006).

• The application of TiO₂ as cool material in energy-saving technologies for building was proposed: it exploits the latent heat flux generated by the evaporation of thin films of water on superhydrophilic surfaces (Irie et al., 2004).

13.2.1 Semiconductor activation

The initial process for semiconductor-supported heterogeneous photocatalysis of organic and inorganic compounds is the generation of electron-hole pairs, initiated by light absorption with energy equal to or greater than the band gap (Fig. 13.1). The process under irradiation can therefore be divided in four steps, as described by Schiavello (1988):

Photoinduced charge transfer to other species (organic or inorganic molecules, water) relies on molecules migration and adsorption onto the semiconductor surface.

At its surface, the semiconductor can donate electrons to reduce an electron acceptor, given that the semiconductor CB bottom must be higher than the acceptor reduction energy, as a necessary condition. The acceptor in most cases is represented by oxygen, which forms a superoxide ion, O₂^•–, or hydrogen peroxide, H₂O₂: they both have excellent reactivity and play important roles in photocatalytic reactions.

On the other side of the band gap, holes can combine with electrons provided by donor species adsorbed on the semiconductor surface, thus oxidizing the donor itself: in this case, the necessary condition is that the top of the semiconductor valence band must be lower than the donor oxidation energy. Typical oxidation reactions take place between semiconductor and water molecules to form hydroxyl radicals OH^•, which are extremely active and tend to react easily due to their strong oxidizing power.

In both cases, the rate of charge transfer is strongly influenced by the respective positions of conduction and valence band edges and the redox potential levels of adsorbate species: the greater the difference between semiconductor and adsorbate energy levels, the faster the redox reactions (Linsebigler et al., 1995).

Charge transfer must compete with charge carrier recombination, which can take place in the volume of the semiconductor or at its surface and strongly decreases photocatalytic yield, i.e., the number of events occurring per absorbed photon. Charge carrier trapping can help increase the photogenerated species’ lifetime. Localized energy surface states ascribed to irregularities and surface defects can act as charge traps, and thus reduce recombination effects; adsorbed oxygen usually acts as an electron scavenger for the trapped electrons, while trapped holes can react with oxygen ions or hydroxyl groups.

13.2.2 The most common photocatalyst: titanium dioxide

Titanium dioxide (TiO₂) is the most common among titanium minerals, and is extensively used in everyday life, specially as white pigment in painting, food and cosmetic industries, thanks to its high refractive index. It is found in nature in four polymorphs: anatase, which presents a distorted tetragonal crystal structure; rutile, which is also tetragonal; brookite, with orthorhombic crystal structure; and TiO₂(B), with monoclinic structure. Only two of the phases, anatase and rutile, are interesting for practical applications, as they are wide band gap semiconductors.

The E_g value for anatase is 3.20 eV, which corresponds to a wavelength absorption threshold of 384 nm. This means that its activation requires an irradiating source with wavelength lower than that indicated, that is, in the near-UV region, while visible light is not sufficiently energetic to induce photoactivity in this material (Fig. 13.2).

The parameters which mostly concur to determine photoactivation efficiency, and specifically photocatalysis, are the following, as defined by Carp et al. (2004):

In Section 13.3 the main application fields of titanium dioxide will be described, as summarized in the well-known scheme of Fig. 13.3, together with the reasons for its success in such different environments. Before that, a brief look at other semiconductors that can be used as photocatalysts is required for a comprehensive treatise.

13.2.3 Other photocatalysts

Titanium dioxide is by far the most studied photocatalyst, but several works are also dedicated to the study of other transition metal oxides and to the evaluation of their semiconducting nature and consequent photocatalytic activity. Probably the most important reason for this broadening of the sector is the fact that titanium dioxide can only be activated by UV light, which occupies a very narrow percentage of natural light: the choice of a photocatalyst which is active under visible light would allow a wider slice of sunlight to be exploited, and therefore increase the material efficiency. Moreover, UV light is also considered harmful to humans, as it can provoke skin and eye damage.

Possible alternative semiconductors that are susceptible to photoactivation 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, WO₃, 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-modified cements usually deal with the related modification 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 WO₃, tests were performed by Linkous et al. (2000) by coating a cement substrate with either WO₃ or TiO₂, and results indicated that the presence of WO₃ alone induced a lower photoactivity compared to TiO₂.

On the other hand, good photoactivation efficiencies were observed in hybrid oxides (mainly WO₃-TiO₂ and SiO₂-TiO₂): attention is now focusing on these composite systems as a means to improve the properties of photocatalytic materials. WO₃-TiO₂ 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 WO₃-TiO₂ nanoparticles and mortars or concrete, especially concerning possible influences on the setting properties of the material, or on its final mechanical behaviour.

SiO₂-supported TiO₂ materials have been extensively used as photocatalysts for a wide variety of reactions. The higher efficiency 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 TiO₂ on SiO₂, coprecipitation, impregnation, chemical vapour deposition). The most interesting point about silica-supported TiO₂ is that silica itself is a common ingredient of cement-based materials, which makes the integration of these composite nanopowders easier (Bellardita et al., 2010).

13.3.1 Photocatalytic degradation of pollutants

TiO₂ photocatalytic activity can be exploited to solve manifold purification issues arising not only from industrial uses or production of harmful substances, but also from heating and transportation. It exhibits good efficiency in the degradation of organic pollutants as well as inorganic compounds, from nitrogen oxides to complex metal salts, in gas and liquid phase.

Several works on photocatalysis focus on the removal of organic pollutants from wastewaters, specially referring to dyes, which are toxic to microorganisms and aquatic life: as reported by Konstantinou and Albanis (2004), up to 20% of dyes used in manufacturing processes are dispersed in waste-waters, therefore efficient processes that avoid their release into natural water sources are vital to preserve the ecosystem. Several reviews can be found on TiO₂ effectiveness in dye degradation: we therefore suggest the reader refers to specific literature for further information (e.g., Akpan and Hameed, 2009; Han et al., 2009).

However, in recent years the aspect of removing gas phase organic compounds has also attracted attention. Extensive studies have been carried out on the removal of outdoor and indoor pollutants, and more specifically volatile organic compounds (VOCs), by TiO₂ nanoparticles under UV illumination. These particles are often integrated in air purification devices, which are commercially available from several companies. VOCs are often responsible for malodorous air in buildings. They derive from several sources, such as cooking, cleaning products, furniture, etc.; in outdoor environments, they are usually part of combustion gases, industrial exhausts, cigarette smoke (Fujishima et al., 2007). A long list of experimental studies is available in the scientific literature on this topic as well.

Photocatalysts are not only used for breaking down large volumes of soilage, they are also capable of destroying it as it accumulates, e.g., to prevent cigarette smoke residue stains, or unpleasant odours due to the presence of VOCs, of the order of 10 ppb by volume. At these concentrations, TiO₂ should be able to decompose such compounds even with scarce UV light (even as low as 1 μW/cm² according to some authors). Bright UV lamps are also used in city and highway tunnels to reduce pollution released by traffic, degrading both VOCs and nitrogen oxides, NO_x, as described by Demeestere et al. (2008), Toma et al. (2004), and many others.

13.3.2 Self-cleaning

Besides photocatalytic applications of TiO₂, another fascinating phenomenon arises from UV irradiation, that is, the alteration of TiO₂ wettability and formation of a highly hydrophilic surface state: this behaviour is defined as photoinduced superhydrophilicity (Wang et al., 1997), and involves the reduction of Ti⁴⁺ to Ti³⁺ by electrons and simultaneous hole trapping at lattice sites. This reduces the bond strength between reduced titanium and the closest oxygen, which is then removed when another water molecule arrives in contact with the surface and adsorbs on it. This creates a highly hydroxylated surface layer, which is responsible for hydrogen bonds with water and consequent increased hydrophilicity of the surface, as depicted in Fig. 13.4.

In this way, water can reach a contact angle close to zero on the surface of irradiated TiO₂ (Drelich et al., 2011). Moreover, this surface is not solely hydrophilic: on the contrary, it presents an amphiphilic nature, with hydrophobic and hydrophilic domains of nanometre size alternating across the surface. This allows both oils and water to spread easily on the photoactivated TiO₂ surface (Fujishima and Zhang, 2006).

Regardless of the in-depth study of the chemical and photochemical mechanisms involved, this effect is of extreme practical importance, as it defines one of the most renowned abilities of titanium dioxide and the reason why it has found so many applications in building materials: the self-cleaning effect.

The formation of an amphiphilic domain network is accompanied by photocatalytic activity, as both have a common origin: UV irradiation. This double photoinduced phenomenon results in the self-cleaning effect: surface contaminants are first photomineralized, at least in part, and subsequently washed away by water, which spreads below them in tight contact with the TiO₂ surface. Moreover, drop formation on superhydrophilic surfaces is avoided, which in turn precludes stain formation due to slow water evaporation from the surface (Fig. 13.5). Another effect, anti-fogging, also arises from the same mechanism: no water droplets form on surfaces with a contact angle lower than 20°, as in the case of irradiated TiO₂ (Fujishima and Zhang, 2006).

To be precise, this property of self-cleaning should be referred to as ‘easy cleaning’: in fact, dirt and particles can adhere on the surface of titanium dioxide, even when irradiated; it is then extremely easy to remove them, as only UV light (available in natural sunlight) and water (that can be provided by rain) are required to remove stains and dirt.

Self-cleaning has become popular in a number of fields, even in clothing: some companies are producing self-cleaning cotton fabric, for an easier cleaning and deodorizing of clothes. Yet, their chief success is in the built environment, with the production of self-cleaning glass, tiles, paints, mortars and many other materials and components. In building facades, soiling is due to the adhesion of particulate matter on the surface of the materials they are made of, which are often porous (and therefore more prone to adsorbing such compounds). Particulate usually attaches to the surface through organic bonds, such as fatty acid chains and carboxylic groups. The twofold role of TiO₂ is then the photocatalytic degradation of such groups and the onset of superhydrophilicity, which drives rainwater in direct contact with the surface, removing particles which are at that point loosely adherent, thus reducing the need for maintenance.

13.3.3 Antibacterial and anti-vegetative properties

Another possible way of exploiting the photocatalytic properties of TiO₂ is its ability to mediate the destruction of bacteria, viruses and other biological materials. This function is often referred to as photosterilization, and it is of great interest for applications connected to air depuration – possibly of private housing, but mainly of medical-related environments, such as operating theatres, common rooms and patient rooms – due to the chance to induce the death of bacteria, viruses, as well as allergens and fungi.

The mechanism is similar to that of photocatalytic degradation: active species are once again surface hydroxyl radical species produced by photogenerated holes, and superoxide ions produced by photogenerated electrons, which damage or destroy cell walls of biological materials (Mills and Lee, 2002; Gerrity et al., 2008). A more detailed explanation is given by Hamal et al. (2009), who ascribe the photosterilization effect to the oxidation of complex proteins and to the inhibition of enzymatic functions of bacterial cells, which lead to ultimate cell death. In this respect, the employment of Ag-doped titanium dioxide is gaining much attention so as to achieve an easy decontamination and disinfection of common rooms and offices, as well as of medical equipment and operating theatres (Page et al., 2007; Wysocka-Krol et al., 2011).

Antibacterial activity was also described as a means to control biological growth on concrete surfaces and avoid unsightly stains. As described by Kurth et al. (2007), biofilm growth on concrete and mortar causes the triggering of undesirable chemical and aesthetical changes. The introduction of TiO₂ in the material does not explicate a strictly bactericidal activity in this frame, but more properly an anti-vegetative effect. Polo et al. (2011) investigated the possibility of using titanium dioxide as a control technology to counteract biofilm microorganism growth, and reported the photokilling effect of immobilized TiO₂ nanopowders on planktonic cells; conversely, no cell inactivation was observed on young biofilms, suggesting that possible applications of TiO₂ should focus on preventative measures rather than on the disaggregation of already existing biofilms. Linkous et al. (2000), and Linkous and Robertson (2006) also reported the inhibition of algae adhesion on cement substrates and on roofing membranes modified with TiO₂ (and WO₃).

13.4.1 Interaction with hydraulic and non-hydraulic binders

The introduction of TiO₂ nanoparticles in building materials surely brings advantages, mainly from an economic point of view (i.e., the decrease in 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 environment can benefit 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 TiO₂ 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 constituents 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 TiO₂ usually finds its collocation, acting as a further aggregate, or nano-filler. Therefore, all reaction products of cement hydration that remain unbounded in the material porosities can adsorb on the TiO₂ 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 mechanism 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 influence of the alkaline hosting material on TiO₂ photoactivated properties; it is then immediate to wonder whether TiO₂ itself may lead to changes in the material characteristics. Concerning the fresh state, the key modification induced by TiO₂ 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 TiO₂. Attention has been focused on the observed increase in the compressive resistance of the material: this was mainly ascribed to the already cited filling effect, which was addressed by several works and summarized in the review by Sanchez and Sobolev (2010). Yet, controversies arise when a possible active behaviour of TiO₂ is considered. Lackhoff et al. (2003) and Li et al. (2007) hypothesize a pozzolanic activity of TiO₂; while the former justify this as an indirect consequence of an accelerated cement hydration observed with NMR relaxometry, in the latter case no experimental validation 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 a consequence of observed reduction in setting time and final porosity of TiO₂-containing concrete, in spite of Chen and Poon’s observations (2009a) rebutting any pozzolanic nature of TiO₂. Their experimental tests showed no TiO₂ 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 field are related to hydraulic binder-based materials, as they represent the larger volume of products, especially concerning the realization of new structures; a few works are also available on non-hydraulic binders, focusing on the conservation of historical and contemporary structures. In this specific context, Karatasios et al. (2010) investigated the ageing of lime mortars, and specifically their carbonation. They attributed to the presence of TiO₂ an effect of CO₂ release, which accelerated carbonation reactions. In fact, TiO₂ presence would have caused the photocatalytic degradation of organic substances adsorbed on the material surface, releasing CO₂ as a reaction product. The process was considered beneficial 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 TiO₂ and its hosting environment is just as important as studying its beneficial 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% (Ceukelaire and Nieuwenburg, 1993; Castellote et al., 2009). These precipitates tend to obstruct active sites, thus decreasing the photocatalytic efficiency of TiO₂ mainly for a shielding effect (Chen and Poon, 2009b). This effect builds up with the accumulation of contaminants on surfaces exposed to the environment, as noted in a report of the Hong Kong Environmental Protection Department (Yu, 2003) (Fig. 13.8).

Although some data are already available, the influence of carbonation on the photocatalytic and self-cleaning efficiency of TiO₂ embedded in construction materials has not been examined thoroughly yet. This can be a vital aspect to define 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 importantly if the whole structure is built with photocatalytic concrete, since its degradation cannot be easily overcome by substitution or re-application. A guideline dedicated to solving design issues with photocatalytic materials would therefore be a desirable target of further research.

• photocatalytic activity: measurement of the extent of gas phase degradation of inorganic pollutants (NO_x) or volatile organic compounds (VOCs) at the material surface

• self-cleaning: introduction of the material in a closed chamber with soiling atmosphere, or impregnation with an organic dye, and monitoring of colour loss (colour recovery); analysis of chromatic changes during extended exposure of materials to the external environment

• superhydrophilicity: measurement of contact angle of water on the material surface

• antibacterial and anti-vegetative effect: inhibition of biofilm formation, algae adhesion and proliferation, and sterilization effects.

13.5.1 Air depollution

Investigations on photocatalytic activity of cement pastes, mortars and concrete containing TiO₂ nanopowders have been performed in most cases by flowthrough methods. Nitrogen monoxide, or directly a NO_x (NO + NO₂) mixture, are used as polluting source with typical concentration of approximately 1 ppmv (1 part per million in volume), and NO and NO₂ concentrations are compared in the inlet and outlet gas flow; a chemiluminescent NO_x analyser is used as measuring unit. Tests involve a first phase of gas flow in the absence of any irradiation, to reach an equilibrium of air composition in the reactor chamber and of the NO absorption in the cementitious material itself, which clearly must not be considered in the calculation of NO degradation. Afterwards, a UV light source (or solar spectrum lamp) is switched on and the outlet gas composition sampled at predefined time intervals: NO concentration usually drops immediately by some percent, and finally goes back to its initial value when the lamp is switched off and the test interrupted.

In NO_x degradation tests, it is important to keep in mind that the first chemical reaction taking place at the TiO₂ surface is the reduction of NO to NO₂. Therefore, after a rapid decrease of NO_x concentration, a slight increase in its value can be observed in the steady state of reaction, since the drop of NO concentration is accompanied by the formation of NO₂. Finally, NO₂ is reduced as well, by the following reactions:

[13.1]

[13.2]

Degradation efficiencies of laboratory specimens range from approximately 50–60% in flow conditions to the total degradation of pollutants in batch conditions, depending on gas concentration, flux and irradiation time. Humidity was observed in many cases to slightly reduce photocatalytic activity when above a certain threshold, as the adsorption of water molecules on the surface of TiO₂ nanoparticles is competitive with respect to pollutant adsorption and consequent degradation (Hüsken et al., 2009).

The result of the reaction chain of NO_x degradation has drawn much attention and some concern, since the final reaction step involves the dissolution of nitrate ions in rain to form nitric acid, and the consequent acidification of rainwater that reaches the sewers. Nonetheless, the concentration of NO_x in air is in the order of magnitude of tens of ppb (parts per billion), and consequently the possible concentration of HNO₃ that could reach sewers is extremely limited and is not expected to produce any detrimental effect.

Similar tests are performed with VOCs as polluting source, among which propanol, butanol, toluene, formaldehyde and acetone are the most diffuse model reactants. Also in this case, removal efficiencies are almost 100% in batch conditions and close to 60% in flow conditions. Experimental tests performed in flow 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 first studies on the self-cleaning attribute of photoactive materials 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 TiO₂. 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 concentration 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 spectrophotometer (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, defined by the Commission Internationale de I’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 (Δcol), or to analyse only the changes in hue (Δhue), or to identify changes in the saturation of a single hue (Δsat). 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 representative parameter of dye concentration. In the case of rhodamine B, which exhibits a strong magenta colour, a decreased intensity of red component 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 sufficient to break the molecule chromophore groups to induce the colour loss. This is why these tests often refer to dye ‘decolonization’ 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 environment can be created artificially or it is possible to expose some specimens directly to urban atmosphere. In the latter case, since experimental conditions (humidity, temperature, irradiation) cannot be chosen and regulated, it is fundamental to have a monitoring station close to the exposure site, from where atmospheric data can be obtained. It is then possible to correlate them with the colour measurements, the most relevant parameters being the amount of radiation reaching the surface during the day and the time of wetness, which is closely correlated to the number of rainy days. As for the interpretation of data, a lower colour change in time is expected from a self-cleaning material. Moreover, if the soiled surface of a photoactive material returns towards its original colour right after a rainy event, and the same trend is not observed in a similar ‘blank’ material (without TiO₂), then this behaviour will be due to the onset of self-cleaning (Diamanti et al., 2008).

13.5.3 A case study of life cycle assessment

A life cycle assessment (LCA) of TiO₂-containing coatings for concrete pavements was performed by Hassan in 2010. The author assumed that an improvement in air quality cannot be used as the only criterion for the complete evaluation of a material that should be considered sustainable, as critical environmental factors may be omitted. Therefore, a life-cycle inventory (LCI) was performed to quantify the energy, abiotic raw material inputs and emission from cradle to grave. A Building for Environmental and Economic Sustainability (BEES) impact assessment model (Lippiat, 2007) was applied to analyse the inventory.

The reduction of the environmental impact of the material was found to act on four main categories: acidification, eutrophication, air pollutants and smog formation. Parallel increases in global warming, fossil fuel depletion, water intake, ozone depletion and impacts on human health were also assessed, caused mainly by the production phase and fossil energy consumption. Yet, the total environmental performance of the product led to the conclusion that the titanium dioxide coatings tested have an overall beneficial effect on the environment.

13.7.1 Overview of current patents

Nowadays, hundreds of patents exist on photocatalytic materials for air purification, self-cleaning surfaces, antibacterial surfaces, and many other sub-categories. Just as an example, in 2009 approximately 60 patent applications were submitted to the WIPO (World Intellectual Property Organization) based on a keyword search of the concept ‘photocatalysis’, among which 41% come from Japan: the major players are TOTO Ltd (Japan) and Carrier Corp. (USA), followed by Saint Gobain Glass (France) and Italcementi (Italy). Air purification is by far the most popular, and includes air purification devices and photocatalytic filters, which are beyond the scope of this chapter. A short list, surely not comprehensive, of some patents related to photocatalytic cementitious materials is reported in Table 13.1.

13.7.2 Standards for materials testing

Japan was undoubtedly the first country to appreciate the potential of photocatalytic materials, to invest in their application nationwide and to formulate appropriate standards to evaluate their efficiency. All standards that have been, or are being, developed are therefore based on Japanese experience, and often refer to the corresponding JIS (Japanese Industrial Standard), which include the test methods of photocatalytic materials for air purification performance (JIS R 1701), antibacterial activity (JIS R 1702), self-cleaning performance (JIS R 1703), water-purification performance (JIS R 1704) and antifungal activity (JIS R 1705).

In fact, every single outcome of the photoactivation of titanium dioxide requires a different standard, and also the type of supporting material strongly influences the test methods and the expected results: porous cementitious materials will necessarily undergo a different procedure with respect to compact ceramics, and further differences will characterize the testing of coatings. This short survey will only focus on standards referring to cement-based, and therefore intrinsically porous, photocatalytic materials.

The formulation of such standards in Europe was entrusted to an official working group of CEN (European Committee for Standardization), who was asked to define technical specifications and guidelines: the activity is not closed yet, and the group is still working on it. In the meanwhile, some documents were produced by the ISO (International Standards Organization) working group on photocatalytic materials, starting in 2007, focusing on photocatalytic fine ceramics (Table 13.2). Other standards are under evaluation and will be published probably in the next few years. These documents will surely become a reference to assess product performances for both public and private building contractors.

Akpan, U.G., Hameed, B.H. Parameters affecting the photocatalytic degradation of dyes using TiO₂-based photocatalysts: a review. J. Hazard. Mater. 2009; 170:520–529.

Ballari, M.M., Hunger, M., Hüsken, G., Brouwers, H. NO_x photocatalytic degradation employing concrete pavement containing titanium dioxide. Appl. Catal. B. 2010; 95:245–254.

Beeldens, A., Environmental friendly concrete pavement blocks: air purification in the centre of Antwerp. Proceedings of 10th International Symposium on Concrete Roads. 2006. [Brussels].

Bellardita, M., Addamo, M., Di Paola, A., Marcì, G., Palmisano, L., Cassar, L., Borsa, M. Photocatalytic activity of TiO₂/SiO₂ systems. J. Hazard. Mater. 2010; 174:707–713.

Carp, O., Huisman, C.L., Reller, A. Photoinduced reactivity of titanium dioxide. Progr. Solid State Chem. 2004; 32:33–177.

Cassar, L. Photocatalysis of cementitious materials: clean buildings and clear air. MRS Bull. 2004; 29:328–331.

Castellote, M., Fernandez, L., Andrade, C., Alonso, C. Chemical changes and phase analysis of OPC pastes carbonated at different CO₂ concentrations. Mater. Struct. 2009; 42:515–525.

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