CEMENTITIOUS MATERIALS FOR SELF-CLEANING AND DE- POLLUTING FACADE SURFACES

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1 CEMENTITIOUS MATERIALS FOR SELF-CLEANING AND DE- POLLUTING FACADE SURFACES F. Vallée 1), B. Ruot 1), L. Bonafous 2), L. Guillot 2), N. Pimpinelli 3), L. Cassar 3), A. Strini 4) E. Mapelli 4), L. Schiavi 4), C. Gobin 5), H. André 5), N. Moussiopoulos 6), A. Papadopoulos 6), J. Bartzis 7), T. Maggos 7), R. McIntyre 8), C. Lehaut-Burnouf 9), A. Henrichsen 10), P. Laugesen 10), R. Amadelli 11), D. Kotzias 12), P. Pichat 13) 1) CSTB, Marne-la-Vallée, France; 2) CTG Italcementi Group, Guerville, France; 3) CTG Italcementi Group, Bergamo, Italy; 4) ITC-CNR, Milano, Italy; 5) GTM Construction, Nanterre, France; 6) Aristotle University Thessaloniki, Greece; 7) NCSR Demokritos, Athens, Greece; 8) Millennium Chemicals, Grimsby, UK; 9) Millennium Chemicals, Rueil Malmaison, France; 10) Dansk Beton Teknik, Hellerup, Denmark; 11) CNR Ferrara, Ferrara, Italy; 12) JRC, Ispra, Italy; 13) Ecole Centrale de Lyon, France. Abstract The aim of the European Project PICADA, as Photocatalytic Innovative Coverings Applications for De-pollution Assessment, is actually to develop a range of novel materials and to evaluate large scale effect (typically in a canyon street). The self-cleaning and de-polluting abilities arise from the photocatalytic properties of titanium dioxide which can be introduced in the building matrix. This paper presents, in particular, results obtained on the cementitious formulations containing nanoparticle composites of titanium dioxide. Investigations were undertaken in the laboratory to evaluate the self-cleaning and de-polluting properties of these products on organic molecules as well as volatile organic compounds (VOCs) and nitrogen oxides (NO x ). 1. Introduction The improvement of air quality and the preservation of the urban aesthetic are major needs for citizen urban areas. The main purpose of the research carried out in the frame of the European Project PICADA (Photocatalytic Innovative Coverings Applications for De-pollution Assessment), is : To make a substantial contribution to solving the phenomena of facades soiling and staining, exacerbated by traffic and industrial pollution; To make available, affordable, really sustainable products whose large applications may, combined with already existing techniques, improve the quality of urban built environment. The basic principle consists of introducing a nanoparticle of titanium dioxide mainly in anatase form into both cement-based materials and organic-based coatings. The photocatalytic properties of the anatase form of TiO 2 originates from its ability to initiate, when exposed to

2 UV-light, oxidation-reduction (redox) cyclic reactions that lead to the decomposition of the organic molecules adsorbed on its surface [1,2]. As indicated, the behaviour of TiO 2 is that of a photocatalyst and therefore it is not consumed during the process. This gives rise to its chemical durability. On this basis, investigations were undertaken on materials containing TiO 2 to study their behaviour towards different types of pollutants. To date there is significant evidence relating to the fact that TiO 2 will exhibit its photocatalytic properties on the surface of these materials when it is incorporated in the cementitious matrix [3, 4, 5]. This paper presents some results obtained with the cement-based materials. Self-cleaning properties were evaluated through the monitoring via colorimetric measurements of the disappearance of a dye tainted into the surface whereas de-polluting properties were monitored through the abatement of volatile organic compounds (VOCs) and nitrogen oxides (NO x ). The preliminary results show good performances and efficiency for the considered materials. This paper presents some results obtained with the cement-based materials. 2. Materials Cementitious products were developed based on a 10 mm-thick rendering (mixture of cement, lime and sand). The material contains nano-sized titanium oxide anatase phase with an average crystallite size of 20 nm. The performances of these products were evaluated by comparison with a reference: equivalent product without TiO 2 addition or CEN mortar [6]. The 10 mm-thick rendering (water / dry mix = 0.133) was directly cast in 10 cm-diameter Petri dishes so that only the upper surface is exposed to the surrounding air. 3. Self-cleaning tests This test consisted in monitoring the rate of photocatalytic decomposition of an organic dye, rhodamine B [7], by colorimetric measurements on polluted samples exposed to UV irradiation during a 30 h period. Samples were polluted by spraying the dye on the sample surface. Rhodamine B was selected in particular because it has an anthracene moiety and as such is related to a PAH (polycyclic aromatic hydrocarbon). PAHs are among the soiling agents found in urban environments [8]. After pollution, the samples are conserved for 16 h in air at 70 % relative humidity. Samples were then placed under a UV-light (300 W Ultra-Vitalux lamp from Osram) which provides an efficient wavelength band as contained in natural sunlight [9]. The distance between the lamp and samples was 1 m (figure 1), which corresponds to an intensity of 3700 Lux. This intensity has been chosen to limit the direct photolysis of rhodamine B and simultaneously allows photocatalytic decomposition to continue.

3 1.05 m sample Fig. 1: UV lamp lighting. Colorimetric measurements were carried out using a tristimulus colorimeter MINOLTA CR 231. The results are expressed in the CIE LAB system [10, 11, 12] with L*, a* and b* colorimetric parameters (figure 2). Colour differences are established in terms of the following variations: + L* = lighter + a* = redder + b* = yellower - L* = darker - a* = greener - b* = bluer Fig. 2: L*a*b* colorimetric system. Photocatalytic efficiency is assessed according to the sample ability to recover its initial coloration more or less rapidly. A convenient criteria to express and compare activity levels obtained for different materials is the dye destruction velocity, calculated between 0 and 3 h according to the following formula (identically for all parameters L*, a*, b*):

4 With: = t = = 3h a *(t) Va *.dt t 0 t a * (t) = A e B t + C a * unp t + D + A, B, C and D are parameters which are determined by fitting experimental curves. a* unp corresponds to sample colour before pollution with rhodamine. The velocity is calculated between 0 and 3 h so as to say in a first order reaction stage. Figure 3 shows the activity of the samples due to the photo-destruction of the rhodamine dye for a 10 mm-thick rendering. In particular it was evaluated the colour change rate. The kinetic curve of the photo-catalytic effect due to the anatase addition shows an exponential form, which is typical of a first order reaction. The reproducibility was proven (Fig 4) Fig. 3: Evolution of colour change (chrominant a*) vs. time for the cement-based materials. Fig. 4: Reproducibility of the test on the TiO 2 -added materials.

5 4. VOCs tests Typically, the evaluation of catalytic properties of materials is obtained in closed or in flowthrough system, were the catalyst is allowed to react with a substrate. The measurement of the substrate disappearance and/or the product appearance rate allows the determination of the catalytic efficiency (and other parameters) of the catalyst under study. The evaluation of the potential outdoor VOCs de-pollution properties of the photocatalytic material needs the design of a specific test system. Particularly, it is very important to measure the photocatalytic activity in conditions closer to the real application. This means to irradiate the photocatalytic surface with an irradiance in the active band (in this case UV-A) comparable to the medium solar irradiance in the typical application site. Moreover, the pollutants composition and concentration should match those found in the expected target environment (in this case, polluted urban sites) [13, 14]. Due to these reasons a specific dynamic photocatalytic chamber was designed for the VOCs de-pollution tests. In such system the internal atmosphere of the chamber is constantly mixed and substituted at fixed rate by a stream of fresh synthetic air. As VOCs pollutant standard a mixture of benzene, toluene, ethylbenzene and o-xylene (BTEX) was used in concentrations from 50 to 500 ng L -1. The supply synthetic air is obtained by adding the BTEX standard to an ultra pure humidified oxygen/nitrogen mixture in order to obtain the final desired pollutant concentration. A mixing fan assures the uniform BTEX and humidity concentration (figure 5). Fig. 5: VOCs photocatalytic testing chamber (top view). The irradiation box (figure 6) includes the catalytic chamber and a 300 W Ultra-Vitalux lamp from Osram. The resulting UV-A irradiance, measured inside the catalytic chamber, was about 12.4 W m 2.

6 Fig. 6: VOCs irradiation system (side view). For each test, a 24 h equilibration with the lamp switched off allows verification of the attainment of a steady-state equilibrium (or a quasi steady-state equilibrium) and the measurement of the BTEX concentration in the chamber without UV irradiation. This step is followed by a 24 h equilibration with the lamp turned on and a new BTEX concentration measurement. This allows the verification of the attainment of a steady-state or quasi steadystate equilibrium under irradiation conditions. The air samples are taken from the exhaust air outlet. The analytical method is based on adsorption on a solid substrate, followed by thermal desorption and GC/FID analysis. The test procedure lies within the determination of the steady-state VOCs concentration without irradiation (dark conditions) as a reference, and then the steady-state concentration under irradiated conditions. From these figures, and with the air change flow and the sample area, it was possible to calculate the photocatalytic destruction rate (in µg m -2 h -1 ) and the photocatalytic activity (as destruction rate/vocs concentration, in m h -1 ) of the employed system in the given concentration, temperature and irradiation conditions. Previous tests on a CEN mortar sample containing the same amount of TiO 2 within the 10 mmthick rendering were carried out with three nominal concentrations of BTEX: 50, 300 and 500 ng L -1. The results showed that the BTEX destruction rate versus BTEX concentration was linear (figure 8).

7 VOC Reduction rate (µg m -2 h -1 ) BENZENE TOLUENE ETILBENZENE ORTO-XILENE Concentration µg m -3 Fig. 7: shows the photocatalytic activity of the TiO 2 -containing materials. 2,5 catalytic activity (m/h) 2 1,5 1 0,5 benzene ethylbenzene toluene o-xylene 0 CEN mortar, no TiO2 10 mm-thick rendering 1 mm-thick mineral paint Fig. 8: Photocatalytic activity of the cement-based materials at 300 ng/l BTEX nominal concentration. 5. NO x tests This test involved measurement of the NO 2 removal rate by samples under UV irradiation in different chambers.

8 In a first experiment the sample was placed in a 1.5 L testing chamber followed by purging rapidly with 10 L of air containing 400 ppb NO 2. In order to separate the two main contributions, adsorption and oxidation (destruction), the amount of nitrite ions NO 2 - and nitrate ions NO 3 - remaining on the samples was measured at the end of the experiment. The amount of NO 2 remaining in the chamber was measured with the chemiluminescence s method. The measurements are carried out after different periods, with or without light. The accuracy of the measurement is 2.5 ppb for a concentration of 400 ppb. The irradiation was undertaken utilising a 300 W Ultra-Vitalux lamp placed at a distance of 25 cm from the sample surface. A Pyrex optical window separates the sample chamber from the lamp, allowing a normal light transmission. The first measurement was obtained 3 min after the end of the NO 2 introduction in the chamber. The later measurement showed a pseudo equilibrium which was achieved 20 minutes later during experiments without light (dark). At the end of the experiment, the sample was rinsed and the amounts of NO 2 - and NO 3 - ions were measured. The mass balance shows that approximately 90% of the species are extracted from the samples. It was shown that photocatalysis can induce the oxidation of adsorbed nitrite ions into nitrate ions, as it has been reported for the aqueous medium [15]. The important consequence of this result is that NO 2 adsorbed in the dark (mainly on account of the presence of the cement matrix) is subsequently transformed on the surface or within the material top layers during illumination, when the TiO 2 is active. In other words, NO 2 adsorbed during the night can undergo photocatalytic oxidation during the following day. The efficiency rate is obtained by multiplying the amount of NO 2 consumed by the ratio of NO 3 - over NO 2 - as expressed in Table 1. NO 2 consumed NO 3 - /NO 2 - destruction efficiency CEN mortar, no TiO 2 (ref.) TiO 2 modified 10 mmthick mortar Table 1: Destruction efficiency with NO 2 initial concentration of 400ppb for cementitious materials under UV light. In a second experiment the efficiency rate was determined for the cement-based materials, using a concentration of 200 ppb of NO (Fig. 9). Tests were conducted at the Research Centre of EU in Ispra (VA) in a30-m 3 Indoortron room (Fig. 10).

9 NO percentage Room loss or cement based material without TiO Cement based material with TiO Time time (min) Figure 9: Photocatalytic activity of the cement-based materials using a concentration of 200 ppb. When light is on the NO curve rapidly decreases. The other curves showed in Figure 9 concern the NO decrease due to the room s loss in the dark and in the light. Fig. 10: A scheme of the Indoortroon room.

10 6. Conclusion Experimental tests carried out on a laboratory scale clearly demonstrate the fact that the nanoparticle anatase titanium dioxide exhibits its photocatalytic properties when mixed with cementitious materials. Self-cleaning measurements show fast dye destruction kinetics. The cementitous materials show a good activity towards NO x and BTEX and a significant activity towards that are realistic urban pollutants considered at common levels. In particular the activity towards NO x were demonstrated also by increasing the experimental room Works of PICADA are in progress : an experimentation site with a specific geometry such as canyon streets has been build in Guerville (Paris) and it will be possible to evaluate the behaviour of the photocatalytic materials in a real open environment. References [1] Pichat, P., In: Tarr, M.A., Chemical Degradation Methods for Wastes and Pollutants: Environmental and Industrial Applications. Marcel Dekker, Inc., New York, Basel, pp , [2] Cassar, L., Photocatalysis of cementitious materials: clean buildings and clean air. MRS Bulletin, pp , May [3] EP Patent Nr , Italcementi SpA, [4] Cassar, L., Pepe, C., Tognon, G., Guerrini, G.L., and Amadelli, R. Proceedings of the 11th International Congress on the Chemistry of Cement (ICCC), Durban, South Africa, [5] US Patent Nr. 6,117,229, Italcementi SpA, [6] EN Standard: Methods of testing cement determination of strength, [7] Ying, M., Yao, J.-N., Chemosphere, 1999, 38, [8] Gustafson, K.E., Dickhut, R.M., Environ. Sci. Technol., 1997, 31, [9] Koller, L.R., Ultra-violet radiation, Wiley, New York, [10] ISO 10526/CIE S005 Standard: CIE standard illuminants for colorimetry, 1999 [11] ISO 10527/CIE S002 Standard: Colorimetric observers, 1991 [12] CIE S 004 Standard: Colours of light signals, 2001 [13] Shah, J.J., Singh, H.B., Environ. Sci. Technol. 1988, 32, [14] Jones, A.P. Atmos. Environ., 1999, 63, [15] Milis, A., Peral, J., Domenech, X., Navio, J.A., J. Mol. Catal., 1994, 87,