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Photocatalytic activities of titanium dioxide incorporated architectural mortars: Effects of weathering and activation light
Accepted Manuscript Photocatalytic activities of titanium dioxide incorporated architectural mortars: Effects of weathering and activation light Ming-Zhi Guo, Anibal Maury-Ramirez, Chi Sun Poon PII:
S0360-1323(15)30108-6
DOI:
10.1016/j.buildenv.2015.08.027
Reference:
BAE 4238
To appear in:
Building and Environment
Received Date: 2 July 2015 Revised Date:
20 August 2015
Accepted Date: 30 August 2015
Please cite this article as: Guo M-Z, Maury-Ramirez A, Poon CS, Photocatalytic activities of titanium dioxide incorporated architectural mortars: Effects of weathering and activation light, Building and Environment (2015), doi: 10.1016/j.buildenv.2015.08.027. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Photocatalytic activities of titanium dioxide incorporated architectural mortars:
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Effects of weathering and activation light
Ming-Zhi Guoa, Anibal Maury-Ramireza,b, Chi Sun Poona* a
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Department of Civil and Environmental Engineering (CEE), The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong. E-mails: [email protected], [email protected]* (corresponding author).
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b
Department of Civil and Industrial Engineering, Pontificia Universidad Javeriana Cali, Calle 18 # 118-250, Av. Cañas Gordas, Cali, Colombia. E-mail: [email protected] Abstract
Self-cleaning and air-purifying properties obtained through photocatalytic reactions on building
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materials containing TiO2 are functions aimed to reduce cleaning activities on buildings and alleviate air pollution. However, due to the significant effect of weathering on the removal efficiencies at outdoors and low UV-A intensity at indoors, the application of these materials has
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been limited. In this study, two methods of introduction of TiO2 particles on building materials
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were carried out and their photocatalytic performances, in particular the resistance to weathering, were compared. The results showed that a novel transparent photocatalytic coating applied on the architectural mortar performed better than the TiO2 intermixed samples in
terms of air
purifying (indicated by NOx removal) and self-cleaning (indicated by Rhodamine b removal) properties under both UV-A and visible light (VL) irradiation conditions. Moreover, no obvious deterioration was observed on the self-cleaning and air purification ability after the application of both a simulated facade weathering process, which represents a usage period of about 20 years
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under Hong Kong weather conditions, and an accelerated carbonation process. Findings from this study demonstrate that the TiO2-containing paint coated self-compacting mortar shows great promise as an attractive strategy to simultaneously combat air pollution problems and reduce
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maintenance costs in a sustainable way.
Key words: Self-compacting architectural mortar; photocatalytic NOx removal; Rhodamine b
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degradation; self-cleaning; air-purifying
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1. Introduction
In order to develop more environmentally-friendly cementitious materials, TiO2 photocatalysis has been applied on building materials as a strategy to endow the end products with self-cleaning, antimicrobial and air-purifying properties [1-7]. When exposed to an activation light (e.g. UV-A), TiO2 is able to generate simultaneously oxidative (e.g. •OH) and reductive
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(e.g. O2•-) species, which can subsequently degrade different organic and inorganic compounds responsible for causing air pollution and materials fouling [8]. Thanks to its inert nature, TiO2
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has been conveniently and widely incorporated in building materials to improve aesthetic appearance and hygiene of urban infrastructures and combat the urban air pollution problem [9].
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However, the significant effect of weathering on the removal efficiencies at outdoors and a commonly low UV-A intensity at indoors adversely impede a broader application of these materials.
For example, after a laboratory simulated and accelerated weathering process was applied on the TiO2 coated natural stones (travertine), a significant decrease in the photocatalytic activity (indicated by its RhB removal reduction) was observed [10]. Under real weathering conditions, the photocatalytic performance of concrete paving blocks containing TiO2 was investigated in
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five different street locations (sidewalks) in Hong Kong. The results indicated that after 4 months of exposure, the NOx removal efficiency dropped significantly [11]. Moreover, washing (with water alone or with detergent) or abrading the surface of the photocatalytic paving blocks were
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not effective to fully recover the NOx removal activity when they were initially installed [12]. On the contrary, one study observed that aging process and weathering conditions exerted negligible influences on the TiO2 coatings of the fired clay brick façades [13]. After undergoing aging, the
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self-cleaning ability of TiO2 remained almost unchanged. Moreover, it has been reported that wearing of the concrete pavement specimens with 3% TiO2 led to a slight improvement in the
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NO removal ability [14]. It seems that apart from the TiO2 adding method, the properties of the substrates also play a vital role in determining the weathering resistant ability of the end products [15]. Therefore, for different TiO2 incorporated cementitious materials, their distinct weathering resistance deserves special attention, especially for the TiO2 coated products.
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On the other hand, as fluorescent lighting is the most frequently used light type in indoors and it contains almost no UV-A radiation, until now, most of the TiO2 incorporated photocatalytic materials have been limited to outdoor applications. Thus, broadening TiO2 activation light
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wavelength towards the visible region is a current scientific challenge. For this purpose,
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introduction of impurities/defect states in the TiO2 band gap such as doping with transition metals and non-metallic anionic species, and forming reduced TiOx, have been intensively investigated [16, 17]. Although the photocatalytic performances of such synthesized novel photocatalysts were encouraging, their current application in cementitious materials was rather scarce due to high cost, unstable chemical nature, and complexity of production [18]. Apart from the strategy of modifying TiO2 photocatalysts, other alternative photocatalysts, which are effective under visible light illumination, have been also employed for indoor air purification
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[19, 20]. Such suitable candidates included BiOBr, (BiO)2CO3, α-Bi2O3, Zn2SnO4 and ZnAl2O4. However, due to either their prohibitive cost or the compatibility with the cementitious materials, their large-scale applications in the building and construction materials remain rather limited.
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Therefore, the successful application of building materials with self-cleaning, antimicrobial and air-purifying functions will largely pivot on the long lasting photocatalytic materials at outdoors as well as more flexibility in relation to the activation light to generate the TiO2 photocatalytic
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activity.
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In this study, two methods of introduction of TiO2 particles on building materials are carried out, and their respective photocatalytic performance in terms of RhB abatement and NOx removal is subsequently examined. Special attention was paid to the effects of weathering (façade weathering and carbonation) and light activation on the corresponding photocatalytic behaviour
2. Materials and Methods
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of the testing samples.
2.1 Architectural Mortar Samples
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A self-compacting-based approach was used to prepare the architectural mortar samples due to the high quality surfaces that this technology offers [21]. To achieve this, a superplasticizer
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(ADVA 109, Grace Construction Products) was added to the mix until the mini-slump flow test reached 25 cm as indicated by the EFNARC test for self-compacting mortars [22]. For enhancing the photocatalytic performance of the architectural mortars, river sand traditionally used as the fine aggregate was completely replaced by recycled glass cullet. Recycled glass (RG) has the potential of facilitating light access to the catalyst particles. Increasing light irradiation on the TiO2 catalyst may increase the generation of reactive oxygen
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species (e.g. •OH and O2•-), which are responsible for the oxidation of different pollutants [23, 24].
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A mix proportion (by mass) of 0.8:0.2:2.0:0.4 (white cement: metakaolinite: recycled glass: water) was used for preparing all the samples based on our previous feasibility studies on the use of recycled glass in architectural cement mortars [25, 26]. In this mix design, white ordinary
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Portland cement (Onoda, Taiheiyo Cement Corp.) and metakaolinite (2nd grade, Diamond) were used as the cementitious phases. The crushed recycled glass was obtained through a local glass
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waste recycling company (Laputa Eco-Construction Material Co. Ltd) and it was derived from crushing post-consumer beverage glass bottles. The recycled glass contained a range of particle sizes varying from 5 mm to 0.15 mm (Table 1).
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Table 1. Particle size distribution of the recycled glass used for the architectural mortars % passing Sieve size recycled (mm) glass 10 100 5 99 2.36 89 1.18 56 0.6 29 0.3 10 0.15 7 0 0 As previously reported, the open porosity (%) and roughness (Ra), determined respectively by the ASTM C-642-06 method (permeable void content) and profilometry (Veeko Dektak 8), of the architectural mortar used were 17 ± 2% and 6.7 ± 0.5 µm, respectively [25]. All mortar samples were prepared and cast as thin layers (20 cm × 10 cm × 0.5 cm thick) which were allowed to cure for 28 days (after demolding at 1 day) at room conditions (25℃ and 50% relative humidity). The fluidity and compressive strength of the samples are tested and given in Table 2.
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Table 2. Fluidity and compressive strength of the reference and 5% TiO2-intermixed SCAM Superplasticizer Compressive strength Average Dosage (kg/m3)
Dosage increase (%)
Reference (without TiO2)
245
10.34
-
5% TiO2-intermixed SCAM
238
15.63
51.2
MPa
Percent increase (%)
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mini-slump value (mm)
58.0
-
64.9
11.9
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Mix notation
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To study photocatalytic architectural mortars with self-cleaning and air-purifying properties, two different TiO2 incorporation approaches (i.e. TiO2 intermixing and TiO2 coating) were used in this research. A TiO2 coating (PC-S7, Cristal Active) was obtained through an aqueous dispersion (sol) of ultrafine single anatase particles (surface area = 300 m2·g-1). The TiO2 content
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in the paint is about 10% (by weight of the paint) according to the information provided by the supplier. The TiO2 (P25, Degussa-Hüls AG) for the intermixed samples was a nanopowder (surface area = 50 m2·g-1) composed of 75% anatase and 25% rutile phases [27, 28]. For the
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coating approach, the samples (PC-S7 coating) were prepared by brushing the anatase aqueous dispersion in 3 layers (dried at room temperature and humidity for approx. 15 min between
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applying successive layers) onto the prepared architectural mortars. On one hand, the adopted coating procedures are able to render homogeneously distributed films on the substrate surface (easily observed by naked eyes). On the other hand, since the photocatalytic activity is heavily dependent on the outer layer (which is directly exposed to light irradiation), applying more TiO2 coatings does not necessarily lead to a proportional increase in photocatalytic efficiency (proven by pre-conditioning tests) [13].
For the intermixed samples, the TiO2 (5% by weight of binder)
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particles were intermixed with the other materials (white cement, metakaolinite, recycled glass, water, superplasticizer) by an electric mixer (CE-207XG, Fargo – Century Equipment Ltd.) at a significantly high mixing rate (190 rpm) which had been proven to prevent particles
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agglomeration. The mixing process included first mixing the dry constitutes (white cement, metakaolinite, recycled glass and TiO2) and then adding the appropriate amounts of respective water and surperplasticizer contents. The fluidity and compressive strength of the 5% TiO2-P25
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intermixed samples are also reported in Table 2.
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2.2 Photocatalytic removal of RhB Rhodamine
b
([9-(2-carboxyphenyl)-6-diethylamino-3-xanthenylidene]-diethylammonium
(RhB) chloride)
was
selected as an organic dye model to simulate air particulate pollutants because its molecule structure is similar to some airborne particulate compounds such as polycyclic aromatic
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hydrocarbons (PAHs) [29-31]. Using a pipette, a RhB solution (0.1 mL e.a.) with a concentration of 5 × 10-4 g·mL-1 was applied evenly on 3 standardized positions (5 cm2 e.a.) on the mortar specimen surfaces (PC-S7 coating, 5% TiO2-P25, and reference) and allowed to oven-dry (60°C)
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overnight. The RhB contaminated sample was then exposed to UV-A or VL irradiation under
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laboratory conditions (T = 25°C and RH = 50%). Light irradiation was provided by UV-A lamps (2 × F8T5 BLB, HRK) and visible lamps (2 × YZ08 T5, NVC) as required by the test. During the UV-A and VL irradiation, the UV-A intensity was maintained between 310-350 µW·cm-2 and 50-60 µW·cm-2, respectively, as indicated by a UV light meter (LT Lutron, Digital Instruments) calibrated at 340 nm (The distance selected for irradiance measurement was about 100 mm, which was the same distance between the light sources and the testing samples).
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The colour changes of the dye before and after the UV light irradiation were measured by a portable sphere spectrophotometer (SP60, X-Rite). The readings were expressed with L*, a* and
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b* colorimetric coordinated in the CIE LAB system (Figure 1).
Figure 1. CIE Lab colour system
As can be seen in Figure 1, (L*) plots the lightness of luminance from white to black, (a*)
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represents values between red and green, and (b*) does the same with values between blue and yellow. Analysis of the RhB degradation was then based on the comparison of the colour parameter a* before (a*(0h)) and after 4h (a*(4h)) and 26h (a*(26h)) of UV light irradiation. These
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RhB removal efficiencies (R4, R26) can be calculated as indicated in Equations 2 and 3. Finally,
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the material is considered as photocatalytic if R4 > 20% and R26 > 50% [32]. ∗
% =
% =
∗
∗
0ℎ −
0ℎ −
∗
4ℎ
×
∗
26ℎ
×
∗
0ℎ 0ℎ
× 100% Eq. 1 × 100%
Eq. 2
2.3 Photocatalytic removal of NOx A continuous flow reactor, modified according to the specifications of JIS R1701-1, was used in this study. The reactor, with a dimension of 300 mm in length, 150 mm in width and 130 mm in
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height, was completely sealed with no detectable leakage. A single pass plug flow pattern was adopted for the photocatalytic gaseous pollutant degradation experiments. Figure 2 shows the schematic diagram of the experimental set-up. A zero air generator (Model 111, Thermo
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Environmental Inc.) was used to supply clean air flow constantly. The certified gas 50 ppm nitric oxide (NO, BOC Gases) was mixed with the zero air stream to the desired concentrations by adjusting mass flow controllers. The zero air was passed through a water bubbler to achieve the
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targeted relative humidity (RH), and the RH level was monitored by a humidity sensor at the centre of the reactor. The light irradiation conditions were the same as those described in section
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2.2. The NOx concentration was continuously measured by a chemiluminescence NOx analyser (Model 42c, Thermo Environmental Instruments Inc.).
All the experiments were carried out at ambient temperature (25±3°C). The exhaust vent was open to the outdoor environment, so that the experiments were run almost at ambient pressure.
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The total inlet air flow rate was 3 L·min-1 and the relative humidity was controlled at 30%. Prior to all photocatalytic conversion processes, the testing gas stream was allowed to pass through the reactor in the absence of the light radiation for at least half an hour to obtain the desired RH as
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well as gas-solid adsorption-desorption equilibrium. The concentration of inlet NO was 1000±50
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ppb and when the concentration of the gaseous pollutant reached equilibrium in the reactor, the lamps were turned on and the photocatalytic degradation was allowed to go on for 30 min. The details of the calculation of the amount of NOx removal have been described previously [22], expressed as a subtraction of the NO2 generated from the NO removed. The calculation of the amount of NOx removal is shown below: =
.
" #$ %&' () − %&' ( *+ − $ %&' ( − %&' () *+,/ -×.
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Eq. 3
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where QNOx (µmol·m-2·h-1) is the amount of nitric oxides removed by the test sample, [NO]0 and [NO2]0 (ppm) are the inlet concentration of nitrogen monoxide and nitrogen dioxide, respectively, [NO] and [NO2] (ppm) are the outlet concentration of nitrogen monoxide and
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nitrogen dioxide, respectively, t (min) is the time of removal operation, f (L·min-1) is the flow rate converted into that at the standard state (0oC, 1.013 kPa), A (m2) is the surface area of cement paste samples, T (0.5 h for all experiments) is the duration of the photocatalytic process,
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and 22.4 represents that the volume of 1 mole ideal gas at the standard state is 22.4 L (ideal gas
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law).
Figure 2. Schematic diagram of gaseous NOx removal experimental set-up
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2.4 Weathering process
In order to determine the effect of weathering on the self-cleaning and air-purifying properties, photocatalytic removal of RhB and NOx was monitored before and after subjecting the architectural mortar samples to two independent weathering processes, a lab-simulated façade weathering process and carbonation. 2.4.1 Simulated façade weathering
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The lab-simulated façade weathering process (Figure 3) mimics the weathering process caused by rain water and sunlight on a building material applied vertically at outdoors. More details about the operation of this test method can be found in reference [33]. Briefly, the test set-up
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consisted of three inclined (45º) polyvinyl chloride (PVC) compartments, which house the testing samples. The wet condition was imposed on each compartment by pumping 1.5 L of tap water during 12 h by means of an aquarium pump (New-Jet 400, Aquarium Systems-Newa). On
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the other hand, the dry condition was applied by simply switching off the pumps for the following 12 h. The day and night conditions were coordinated respectively with the dry and rain
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conditions and each lasted also 12 h. For the day condition, light irradiation was supplied by three UV-A (3 × F8T5 BLB, HRK) and two visible lamps (T514W, Aqua Gem). The UV-A intensity striking on the surface of samples was 350 ± 10 µW·cm-2, measured by UV light meter (LT Lutron, Digital Instruments) calibrated at 340 nm. The duration of the coordinated ‘rain/dry’
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and ‘day/night’ cycles was 2 weeks. Considering the simulated ‘rain’ conditions (290 mm·h-1) selected for operating the test set-up on the one hand, and the Hong Kong average rainfall (2315 mm·year-1) on the other hand, the weathering process represents approximately 20 years of
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weathering. Although other factors, such as UV exposure, do exert influences on the deterioration of the samples, their contributions were found less significant than the dry-wet
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cycles. For example, only UV radiation (up to 1800 h) caused no observable physical degradation of TiO2 nanocoating, while wet/dry cycles resulted in a certain degradation of the nanocoating [13]. Thus, in this study, a rough estimation of weathering years in real conditions in Hong Kong was only based on the volume of rainfall.
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Figure 3. Weathering test set-up and operating conditions (UV-A intensity = 350 ± 10 µW·cm-2; Rain water intensity = 290 mm·h-1). 2.4.2 Carbonation
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To evaluate the resistance of the samples to carbonation, the international standard method ISO/DIS 1920-12 was followed. The accelerated carbonation was carried out in a carbonation chamber (Yue Fat Engineering and Oven Works) with 4 % concentration (20℃ and 60%
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relative humidity). The carbonation reaction was allowed to go on for during 70 days before the
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samples were taken for subsequent photocatalytic tests. 3. Results and Discussion
3.1 Photocatalytic removal of RhB under UV-A irradiation 3.1.1 Effect of façade weathering RhB removal efficiencies on architectural mortar samples under UV-A irradiation before and after the application of a façade weathering process are shown in Figure 4. Before subject to the weathering condition, the PC-S7 coated samples achieved a satisfactory RhB removal
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performance, with both R4 (51.4%) and R26 (63.6%) larger than the photocatalytic boundaries (R4 > 20% and R26 > 50%) [32]. In contrast, the 5% TiO2-P25 intermixed samples experienced significantly lower RhB removal efficiencies (R4 = 4.7% and R26 = 12.5%) under the same
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testing conditions, indicating that the intermixing method adversely impaired the photocatalytic performance. This observed reduction in RhB removal is mainly due to the encapsulation of the intermixed TiO2 particles by the accumulation of surrounding hydration products. Therefore,
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compared to the PC-S7 coated samples, the 5% TiO2-P25 intermixed samples are less exposed to the photons and the dye (RhB). Previous evaluation of the photocatalytic RhB removal under
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UV-A irradiation (1.5-2.5 mW·cm-2) on white cement pastes intermixed with TiO2 (0, 5, 10, 15% on a weight basis) found that only samples containing 15% TiO2 were nearly satisfactory (R4 = 33.9% and R26 = 45.8%) to be considered as photocatalytic material [34]. Taken together, it seems that although the solid mass layer can form a stable support to protect the TiO2 particles
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against abrasion or erosion, they may also unfavourably weaken the photocatalytic activity. This explanation was supported by a previous study which proposed that when nano-particles were embedded in cement-based materials, they could become potential nucleation sites for cement
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hydration products. As the hydration reaction proceeded, the hydration products (such as calcium silicate hydrate and calcium hydroxide) gradually bonded the individual TiO2 together, forming a
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dense coating on the TiO2 surface [35]. As for the PC-S7 coated samples, the ultra-fine particle size of TiO2 in the paint directly translates into a much larger surface area, meaning an extended exposure to light irradiation for the active TiO2 particles. In addition, the content of TiO2 in the paint (10% by weight of paint) is much higher than that added by the intermixing method (5% by weight of binder). This, coupled with the fact that the coating method gives the coated TiO2
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particles much easier access to light irradiation, helps explain the above observed relatively higher RhB removal efficiencies possessed by the PC-S7 coated mortar.
80
5%TiO2-P25 PC-S7 coating Reference
70 60 Condition 2: R26>50%
50 40 30
Condition 1: R4>20%
20 10 0 R262
before weathering
R43
4
R26
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R4 1
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Rhodamine B removal (%)
90
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100
after weathering ~ 20 years
Figure 4. Photocatalytic
removal under UV-A irradiation of RhB impregnated on mortar
samples (PC-S7-coated and 5% TiO2-intermixed) before and after application of an accelerated
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facade weathering process (approx. 20 years exposure under Hong Kong weather conditions). It is well known that rain and wind will cause a significant detachment of the coatings on the
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surface of substrate materials [36]. Thus, it is necessary to evaluate the weathering resistant ability of the developed photocatalytic products to justify their viable potential for real
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application. After exposure to the façade weathering process, the PC-S7 coated samples still displayed higher RhB removal efficiencies (R4 = 50.2% and R26 = 52.6%), which satisfied the boundaries (R4 > 20% and R26 > 50%) to be considered as photocatalytic material. On the other hand, the 5% TiO2-P25 intermixed samples experienced a significant improvement in the RhB removal efficiencies (R4 = 27.2% and R26 = 43.5%) after the application of the weathering process, although still not enough to satisfy the photocatalytic boundaries. It is highly likely that the weathering process helped to expose more TiO2 particles to the photons and RhB, resulting in
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a higher photocatalytic RhB degradation efficiency. Thus, existing texturing surface treatments might be a good option for enhancing the photocatalytic activity of such materials. Similar results have been reported by a previous study in which an increase of about 30% and 70% on
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the RhB removal efficiencies R26 and R4, respectively, was observed on the 5% TiO2 intermixed mortar samples, which were previously abraded with sand paper (P150 – ISO/FEPA Grit designation) [37]. It seems that TiO2 particles do not tend to appear on the casting surface due to
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a lower density compared to the other cementitious materials used to prepare the architectural
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mortars. 3.1.2 Effect of carbonation
In the natural environments, carbonation of cementitious materials represents another concern about the long term durability of the self-cleaning ability of the architectural mortar samples. It is
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a slow process occurring between calcium hydroxide in the cement past and carbon dioxide from the air [38]. RhB removal efficiencies of different architectural mortar samples under UV-A irradiation both before and after the application of the accelerated carbonation weathering
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process are shown in Figure 5. For the 5% TiO2-P25 intermixed samples, a slight reduction in RhB degradation was observed after undergoing the accelerated carbonation process. For
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example, the RhB removal efficiency of the 5% TiO2-P25 intermixed sample just after 4 h of UV-A light irradiation was reduced by about 45% due to the effect of carbonation. Apparently, the formation of tiny calcium carbonate (CaCO3) crystals from the carbonation reaction adversely affects the contact between the RhB molecules and the surface TiO2 particles. Consequently, the photocatalytic reaction is slowed down. Similar phenomena have been observed in previous studies. Reductions up to 60% in atrazine degradation for 10% TiO2 intermixed samples have been reported after two months of carbonation [39]. As the carbonation
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can only occur between the calcium hydroxide (CH) in the substrate and CO2 in the air, no obvious difference in RhB degradation was found in the PC-S7 coated samples before and after
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the carbonation process.
Figure 5. Photocatalytic removal under UV-A irradiation of RhB impregnated on mortar samples
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(PC-S7-coated and 5% TiO2-intermixed) before and after application of an accelerated carbonation process
3.2 Photocatalytic removal of RhB under visible light irradiation
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3.2.1 Effect of façade weathering
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RhB photocatalytic removal efficiencies of architectural mortar samples under visible light irradiation both before and after application of the façade weathering process can be seen in Figure 6. For all the samples, generally, similar trends to those obtained under UV-A light irradiation were observed under VL irradiation, with a slight reduction in RhB removal efficiencies. The PC-S7 coated samples also displayed higher RhB removal efficiencies with R4 (42.0%) and R26 (61.7%) larger than the photocatalytic boundaries (R4 > 20% and R26 > 50%) before undergoing the weathering conditions. After the application of the façade weathering
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process, RhB removal efficiencies (R4 = 40% and R26 = 51%) for the PC-S7 coated samples still satisfied the photocatalytic boundaries for considering this as photocatalytic material. It seems that a combination of the 405-410 nm wavelength fraction near the UV-A region (Figure
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S1) from the VL lamps (YZO8T5, NVC) and a high surface area (300 m2·g-1) of the photocatalyst particles in the PC-S7 paints are sufficient enough to ensure a high photocatalytic activity under VL irradiation. For instance, investigation on the degradation of RhB on
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TiO2-P25, TiO2 nano-rod and TiO2 nano-stripe (all have a high surface area) under VL irradiation has shown that the morphology and crystal structure of the catalyst have an important
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effect on the degradation activity [40]. It can be seen that the PC-S7 coated architectural mortars have promising potential for indoor application, in which environment the self-cleaning ability is certainly a benefit to people who spend a considerable time for indoor activities. Contrary to the appealing photocatalytic activity possessed by the PC-S7 coated samples, the 5%
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TiO2-intermixed mortars failed to deliver a sufficient enough photocatalytic RhB removal ability after subject to the weathering conditions.
100
70 60 50
5%TiO2-P25 intermixed PC-S7 coating Reference
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80
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Rhodamine B removal (%)
90
Condition 2: R26>50%
40 30 20
Condition 1: R4>20%
10 0
R4 1 R26 2 before weathering
R43 R426 after weathering
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Figure 6. Photocatalytic removal under visible light irradiation of RhB impregnated on mortar samples (PC-S7-coated and 5% TiO2-intermixed) before and after application of an accelerated
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facade weathering process (approx. 20 years exposure under Hong Kong weather conditions). 3.2.2 Effect of carbonation
Under visible light irradiation, there is a similar story about the effect of the accelerated
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carbonation process on RhB degradation performances for both the TiO2 intermixed and PC-S7 coated samples (Figure 7). The carbonation process imposed much more pronounced decreases
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in RhB degradation on the 5% TiO2 intermixed sample in comparison with the PC-S7 coated sample. It should also be noted that even after exposure to the accelerated carbonation weathering, the PC-S7 coated sample still delivered a satisfactory RhB removal performance (R4 > 20% and R26 > 50%). Again, the efficacy and long term service-life of such products have been
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well demonstrated in both outdoor and indoor applications.
Figure 7. Photocatalytic removal under visible light irradiation of RhB impregnated on mortar samples (PC-S7-coated and 5% TiO2-intermixed) before and after application of an accelerated carbonation process
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3.3 Photocatalytic NOx removal under UV-A irradiation Figure 8 shows photocatalytic NOx removal performances of PC-S7-coated and 5% TiO2-P25 intermixed architectural mortar samples under UV-A irradiation before and after the application
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of the façade weathering process. Compared with the 5% TiO2-P25 intermixed samples, the PC-S7-coated mortars displayed a significantly higher NOx removal efficiency (259.3 µmol·m-2·h-1) without exposure to the facade weathering. After subject to the facade weathering,
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no obvious reduction in photocatalytic NOx removal activity was observed for the PC-S7-coated samples (only a 2% reduction). In stark contrast, the 5% TiO2-P25 intermixed samples suffered a
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significant decrease in photocatalytic NOx removal efficiency, dropping from 181.0 to 21.5 µmol·m-2·h-1. This unexpected observation was in contradiction with the results from photocatalytic RhB degradation, which experienced a slight increase in the efficiencies for the 5% TiO2-P25 intermixed samples. The exact reason for this seeming contradiction was not clear. It is
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possible that the repeated raining process relocated some impurities on the surface of the mortars to the pores, blocking the light from reaching the underlying TiO2 particles. Therefore, although the weathering process can expose more TiO2 particles on the surface to intimately contacte RhB
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(resulting in higher photocatalytic RhB degradation efficiencies), the blocked pores by the accumulation of impurities adversely reduced the NOx removal ability by allowing less light and
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NO gas to come into contact with the TiO2 particles.
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Figure 8. Photocatalytic removal under UV-A irradiation of gaseous NOx on mortar samples
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(PC-S7-coated and 5% TiO2-intermixed) before and after application of an accelerated facade weathering process (approx. 20 years exposure under Hong Kong weather conditions). 3.4 Photocatalytic NOx removal under visible light irradiation
The photocatalytic NOx removal performances of architectural mortar samples under VL
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irradiation both before and after the application of the façade weathering process are provided in Figure 9. Compared with UV-A irradiation, VL irradiation exerted a negligible influence on photocatalytic NOx removal ability (251.9 µmol·m-2·h-1) for the PC-S7-coated samples before
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exposure to the weathering conditions, demonstrating high potential for indoor application. However, the 5% TiO2-P25 intermixed samples experienced a significant reduction in
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photocatalytic NOx removal (93.5 µmol·m-2·h-1). This can be attributed to the fact that P25 is mainly effective within the UV light spectrum. After the application of the facade weathering, the PC-S7-coated samples exhibited no apparent NOx removal change under VL irradiation, while the 5% TiO2 intermixed samples suffered a considerable reduction in the NOx removal activity (a 92% reduction). This trend is similar to that observed under UV-A irradiation, and the explanation offered above can be applied here.
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Figure 9. Photocatalytic removal under visible light of gaseous NOx on mortar samples
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(PC-S7-coated and 5% TiO2-intermixed) before and after application of an accelerated facade weathering process (approx. 20 years exposure under Hong Kong weather conditions). 4. Conclusion
This paper compared the photocatalytic performance of two TiO2 incorporation methods on
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cementitious materials under both different weathering and incident light irradiation conditions. The PC-S7-coated samples delivered a similar performance in photocatalytic RhB and NOx removal under both visible light and UV-A light irradiation. Compared with the 5%
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TiO2-intermixed mortars, the PC-S7-coated samples also garnered a robust weathering-resistant
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ability, reflected by an almost unchanged ability to photocatalytic degrade both RhB and NOx after exposure to a facade weathering process (representing a usage period of about 20 years under Hong Kong weather conditions), as well as an accelerated carbonation process. The results suggest that self-cleaning and air-purifying properties coupled with a robust weathering-resistant ability could make the TiO2 coated architectural mortars an attracting product for both indoor and outdoor applications.
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Acknowledgements The authors wish to thank the Environment and Conservation Fund, the Woo Wheelock Green
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Fund and the Hong Kong Polytechnic University for funding supports.
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[16] Asahi R, Morikawa T. Nitrogen complex species and its chemical nature in TiO2 for visible-light ensitized photocatalysis, Chem Phys 2007; 339: 57-63.
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[17] Blöß SP, Elfenthal L. Doped titanium dioxide as photocatalyst for UV and visible light. In Baglioni, P. and Cassar, L. (Eds) Proc. Int. RILEM Symposium on Photocatalysis, Environment and Construction Materials. Florence, Italy. 8-9 October 2007: 31-38.
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[26] Ling TC, Poon CS, Kou SC. Feasibility of using recycled glass in architectural cement
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Mater 2013; 44: 309–316.
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[30] Folli A, Pade C, Bæk Hansen T, De Marco T, Macphee DE. TiO2 photocatalysis in
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[31] Ruot B, Plassais A, Olive F, Guillot L, Bonafous L. TiO2-containing cement pastes and mortars: Measurements of the photocatalytic efficiency using a rhodamine B-based colourimetric test. Sol Energ 2009; 83-10: 1794–1801. [32] Cassar L, Beeldens A, Pimpinelli N, Guerrini G. Photocatalysis of cementitious materials. Proceedings International RILEM Symposium on Photocatalysis, Environment and Construction Materials - TDP 2007. RILEM Publications SARL. Florence, Italy. 8-9 October 2007, 131-143.
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[33] Maury-Ramirez A, Demeestere K, De Belie N. Photocatalytic activity of titanium dioxide nanoparticle coatings applied on autoclaved aerated concrete: Effect of weathering on coating
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physical characteristics and gaseous toluene removal. J Hazard Mater 2012; 211–212: 218–225. [34] Maury-Ramirez A, De Belie N, Demeestere K. Self-cleaning and/or air-purifying cementitious materials: evaluation method. Proceedings 11th Firw PhD Symposium, Ghent
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[36] Diamanti MV, Ormellese M, Pedeferri MP. Characterization of photocatalytic and superhydrophilic properties of mortars containing titanium dioxide. Cem Concr Res 2008; 38:
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[37] Diamanti MV, Del Curto B, Ormellese M, Pedeferri MP. Photocatalytic and self-cleaning
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activity of colored mortars containing TiO2. Constr Build Mater 2013; 46: 167-174. [38] Neville AM. Properties of Concrete, 5th Edition, Prentice Hall, 2011.
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[39] Lackhoff M, Prieto X, Nestle N, Dehn F, Niessner R. Photocatalytic activity of semiconductor modified cement-influence of semiconductor type and cement ageing. Appl Catal B 2003; 43: 205.
[40] Li JY, Ma WH, Lei PX, Zhao JC. Detection of intermediates in the TiO2-assisted photodegradation of Rhodamine B under visible light irradiation. J Environ Sci 2007; 19: 892-896.
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Supporting Information to
architectural mortars:
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Photocatalytic activities of titanium dioxide incorporated
Effects of weathering and activation light
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Ming-Zhi Guoa, Anibal Maury-Ramireza,b, Chi Sun Poona*
a
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Department of Civil and Environmental Engineering (CEE), The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong. E-mails: [email protected], [email protected]* (corresponding author).
b
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Department of Civil and Industrial Engineering, Pontificia Universidad Javeriana Cali, Calle 18 # 118-250, Av. Cañas Gordas, Cali, Colombia. E-mail: [email protected] 300
250
Silicone
Acrylic
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NOx Removal/µmol m-2 h-1
PC-S7
200
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150 100 50
0
UVA
Sunlight LED 395 Different Light Sources
LED 405
Figure S1. Photocatalytic NOx removal of three different paints coated mortars under different lights irradiation
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ACCEPTED MANUSCRIPT Highlights Two TiO2 adding methods conferred self-cleaning and air-purifying ability on mortars. TiO2 paint coated mortars performed better both in RhB degradation and NOx removal. A robust weathering-resistant capacity was displayed by paint-coated mortars.
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Paint-coated products show great promise for both indoor and outdoor applications.
S0360-1323(15)30108-6
DOI:
10.1016/j.buildenv.2015.08.027
Reference:
BAE 4238
To appear in:
Building and Environment
Received Date: 2 July 2015 Revised Date:
20 August 2015
Accepted Date: 30 August 2015
Please cite this article as: Guo M-Z, Maury-Ramirez A, Poon CS, Photocatalytic activities of titanium dioxide incorporated architectural mortars: Effects of weathering and activation light, Building and Environment (2015), doi: 10.1016/j.buildenv.2015.08.027. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Photocatalytic activities of titanium dioxide incorporated architectural mortars:
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Effects of weathering and activation light
Ming-Zhi Guoa, Anibal Maury-Ramireza,b, Chi Sun Poona* a
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Department of Civil and Environmental Engineering (CEE), The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong. E-mails: [email protected], [email protected]* (corresponding author).
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b
Department of Civil and Industrial Engineering, Pontificia Universidad Javeriana Cali, Calle 18 # 118-250, Av. Cañas Gordas, Cali, Colombia. E-mail: [email protected] Abstract
Self-cleaning and air-purifying properties obtained through photocatalytic reactions on building
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materials containing TiO2 are functions aimed to reduce cleaning activities on buildings and alleviate air pollution. However, due to the significant effect of weathering on the removal efficiencies at outdoors and low UV-A intensity at indoors, the application of these materials has
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been limited. In this study, two methods of introduction of TiO2 particles on building materials
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were carried out and their photocatalytic performances, in particular the resistance to weathering, were compared. The results showed that a novel transparent photocatalytic coating applied on the architectural mortar performed better than the TiO2 intermixed samples in
terms of air
purifying (indicated by NOx removal) and self-cleaning (indicated by Rhodamine b removal) properties under both UV-A and visible light (VL) irradiation conditions. Moreover, no obvious deterioration was observed on the self-cleaning and air purification ability after the application of both a simulated facade weathering process, which represents a usage period of about 20 years
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under Hong Kong weather conditions, and an accelerated carbonation process. Findings from this study demonstrate that the TiO2-containing paint coated self-compacting mortar shows great promise as an attractive strategy to simultaneously combat air pollution problems and reduce
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maintenance costs in a sustainable way.
Key words: Self-compacting architectural mortar; photocatalytic NOx removal; Rhodamine b
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degradation; self-cleaning; air-purifying
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1. Introduction
In order to develop more environmentally-friendly cementitious materials, TiO2 photocatalysis has been applied on building materials as a strategy to endow the end products with self-cleaning, antimicrobial and air-purifying properties [1-7]. When exposed to an activation light (e.g. UV-A), TiO2 is able to generate simultaneously oxidative (e.g. •OH) and reductive
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(e.g. O2•-) species, which can subsequently degrade different organic and inorganic compounds responsible for causing air pollution and materials fouling [8]. Thanks to its inert nature, TiO2
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has been conveniently and widely incorporated in building materials to improve aesthetic appearance and hygiene of urban infrastructures and combat the urban air pollution problem [9].
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However, the significant effect of weathering on the removal efficiencies at outdoors and a commonly low UV-A intensity at indoors adversely impede a broader application of these materials.
For example, after a laboratory simulated and accelerated weathering process was applied on the TiO2 coated natural stones (travertine), a significant decrease in the photocatalytic activity (indicated by its RhB removal reduction) was observed [10]. Under real weathering conditions, the photocatalytic performance of concrete paving blocks containing TiO2 was investigated in
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five different street locations (sidewalks) in Hong Kong. The results indicated that after 4 months of exposure, the NOx removal efficiency dropped significantly [11]. Moreover, washing (with water alone or with detergent) or abrading the surface of the photocatalytic paving blocks were
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not effective to fully recover the NOx removal activity when they were initially installed [12]. On the contrary, one study observed that aging process and weathering conditions exerted negligible influences on the TiO2 coatings of the fired clay brick façades [13]. After undergoing aging, the
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self-cleaning ability of TiO2 remained almost unchanged. Moreover, it has been reported that wearing of the concrete pavement specimens with 3% TiO2 led to a slight improvement in the
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NO removal ability [14]. It seems that apart from the TiO2 adding method, the properties of the substrates also play a vital role in determining the weathering resistant ability of the end products [15]. Therefore, for different TiO2 incorporated cementitious materials, their distinct weathering resistance deserves special attention, especially for the TiO2 coated products.
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On the other hand, as fluorescent lighting is the most frequently used light type in indoors and it contains almost no UV-A radiation, until now, most of the TiO2 incorporated photocatalytic materials have been limited to outdoor applications. Thus, broadening TiO2 activation light
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wavelength towards the visible region is a current scientific challenge. For this purpose,
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introduction of impurities/defect states in the TiO2 band gap such as doping with transition metals and non-metallic anionic species, and forming reduced TiOx, have been intensively investigated [16, 17]. Although the photocatalytic performances of such synthesized novel photocatalysts were encouraging, their current application in cementitious materials was rather scarce due to high cost, unstable chemical nature, and complexity of production [18]. Apart from the strategy of modifying TiO2 photocatalysts, other alternative photocatalysts, which are effective under visible light illumination, have been also employed for indoor air purification
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[19, 20]. Such suitable candidates included BiOBr, (BiO)2CO3, α-Bi2O3, Zn2SnO4 and ZnAl2O4. However, due to either their prohibitive cost or the compatibility with the cementitious materials, their large-scale applications in the building and construction materials remain rather limited.
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Therefore, the successful application of building materials with self-cleaning, antimicrobial and air-purifying functions will largely pivot on the long lasting photocatalytic materials at outdoors as well as more flexibility in relation to the activation light to generate the TiO2 photocatalytic
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activity.
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In this study, two methods of introduction of TiO2 particles on building materials are carried out, and their respective photocatalytic performance in terms of RhB abatement and NOx removal is subsequently examined. Special attention was paid to the effects of weathering (façade weathering and carbonation) and light activation on the corresponding photocatalytic behaviour
2. Materials and Methods
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of the testing samples.
2.1 Architectural Mortar Samples
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A self-compacting-based approach was used to prepare the architectural mortar samples due to the high quality surfaces that this technology offers [21]. To achieve this, a superplasticizer
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(ADVA 109, Grace Construction Products) was added to the mix until the mini-slump flow test reached 25 cm as indicated by the EFNARC test for self-compacting mortars [22]. For enhancing the photocatalytic performance of the architectural mortars, river sand traditionally used as the fine aggregate was completely replaced by recycled glass cullet. Recycled glass (RG) has the potential of facilitating light access to the catalyst particles. Increasing light irradiation on the TiO2 catalyst may increase the generation of reactive oxygen
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species (e.g. •OH and O2•-), which are responsible for the oxidation of different pollutants [23, 24].
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A mix proportion (by mass) of 0.8:0.2:2.0:0.4 (white cement: metakaolinite: recycled glass: water) was used for preparing all the samples based on our previous feasibility studies on the use of recycled glass in architectural cement mortars [25, 26]. In this mix design, white ordinary
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Portland cement (Onoda, Taiheiyo Cement Corp.) and metakaolinite (2nd grade, Diamond) were used as the cementitious phases. The crushed recycled glass was obtained through a local glass
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waste recycling company (Laputa Eco-Construction Material Co. Ltd) and it was derived from crushing post-consumer beverage glass bottles. The recycled glass contained a range of particle sizes varying from 5 mm to 0.15 mm (Table 1).
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Table 1. Particle size distribution of the recycled glass used for the architectural mortars % passing Sieve size recycled (mm) glass 10 100 5 99 2.36 89 1.18 56 0.6 29 0.3 10 0.15 7 0 0 As previously reported, the open porosity (%) and roughness (Ra), determined respectively by the ASTM C-642-06 method (permeable void content) and profilometry (Veeko Dektak 8), of the architectural mortar used were 17 ± 2% and 6.7 ± 0.5 µm, respectively [25]. All mortar samples were prepared and cast as thin layers (20 cm × 10 cm × 0.5 cm thick) which were allowed to cure for 28 days (after demolding at 1 day) at room conditions (25℃ and 50% relative humidity). The fluidity and compressive strength of the samples are tested and given in Table 2.
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Table 2. Fluidity and compressive strength of the reference and 5% TiO2-intermixed SCAM Superplasticizer Compressive strength Average Dosage (kg/m3)
Dosage increase (%)
Reference (without TiO2)
245
10.34
-
5% TiO2-intermixed SCAM
238
15.63
51.2
MPa
Percent increase (%)
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mini-slump value (mm)
58.0
-
64.9
11.9
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Mix notation
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To study photocatalytic architectural mortars with self-cleaning and air-purifying properties, two different TiO2 incorporation approaches (i.e. TiO2 intermixing and TiO2 coating) were used in this research. A TiO2 coating (PC-S7, Cristal Active) was obtained through an aqueous dispersion (sol) of ultrafine single anatase particles (surface area = 300 m2·g-1). The TiO2 content
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in the paint is about 10% (by weight of the paint) according to the information provided by the supplier. The TiO2 (P25, Degussa-Hüls AG) for the intermixed samples was a nanopowder (surface area = 50 m2·g-1) composed of 75% anatase and 25% rutile phases [27, 28]. For the
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coating approach, the samples (PC-S7 coating) were prepared by brushing the anatase aqueous dispersion in 3 layers (dried at room temperature and humidity for approx. 15 min between
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applying successive layers) onto the prepared architectural mortars. On one hand, the adopted coating procedures are able to render homogeneously distributed films on the substrate surface (easily observed by naked eyes). On the other hand, since the photocatalytic activity is heavily dependent on the outer layer (which is directly exposed to light irradiation), applying more TiO2 coatings does not necessarily lead to a proportional increase in photocatalytic efficiency (proven by pre-conditioning tests) [13].
For the intermixed samples, the TiO2 (5% by weight of binder)
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particles were intermixed with the other materials (white cement, metakaolinite, recycled glass, water, superplasticizer) by an electric mixer (CE-207XG, Fargo – Century Equipment Ltd.) at a significantly high mixing rate (190 rpm) which had been proven to prevent particles
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agglomeration. The mixing process included first mixing the dry constitutes (white cement, metakaolinite, recycled glass and TiO2) and then adding the appropriate amounts of respective water and surperplasticizer contents. The fluidity and compressive strength of the 5% TiO2-P25
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intermixed samples are also reported in Table 2.
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2.2 Photocatalytic removal of RhB Rhodamine
b
([9-(2-carboxyphenyl)-6-diethylamino-3-xanthenylidene]-diethylammonium
(RhB) chloride)
was
selected as an organic dye model to simulate air particulate pollutants because its molecule structure is similar to some airborne particulate compounds such as polycyclic aromatic
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hydrocarbons (PAHs) [29-31]. Using a pipette, a RhB solution (0.1 mL e.a.) with a concentration of 5 × 10-4 g·mL-1 was applied evenly on 3 standardized positions (5 cm2 e.a.) on the mortar specimen surfaces (PC-S7 coating, 5% TiO2-P25, and reference) and allowed to oven-dry (60°C)
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overnight. The RhB contaminated sample was then exposed to UV-A or VL irradiation under
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laboratory conditions (T = 25°C and RH = 50%). Light irradiation was provided by UV-A lamps (2 × F8T5 BLB, HRK) and visible lamps (2 × YZ08 T5, NVC) as required by the test. During the UV-A and VL irradiation, the UV-A intensity was maintained between 310-350 µW·cm-2 and 50-60 µW·cm-2, respectively, as indicated by a UV light meter (LT Lutron, Digital Instruments) calibrated at 340 nm (The distance selected for irradiance measurement was about 100 mm, which was the same distance between the light sources and the testing samples).
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The colour changes of the dye before and after the UV light irradiation were measured by a portable sphere spectrophotometer (SP60, X-Rite). The readings were expressed with L*, a* and
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b* colorimetric coordinated in the CIE LAB system (Figure 1).
Figure 1. CIE Lab colour system
As can be seen in Figure 1, (L*) plots the lightness of luminance from white to black, (a*)
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represents values between red and green, and (b*) does the same with values between blue and yellow. Analysis of the RhB degradation was then based on the comparison of the colour parameter a* before (a*(0h)) and after 4h (a*(4h)) and 26h (a*(26h)) of UV light irradiation. These
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RhB removal efficiencies (R4, R26) can be calculated as indicated in Equations 2 and 3. Finally,
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the material is considered as photocatalytic if R4 > 20% and R26 > 50% [32]. ∗
% =
% =
∗
∗
0ℎ −
0ℎ −
∗
4ℎ
×
∗
26ℎ
×
∗
0ℎ 0ℎ
× 100% Eq. 1 × 100%
Eq. 2
2.3 Photocatalytic removal of NOx A continuous flow reactor, modified according to the specifications of JIS R1701-1, was used in this study. The reactor, with a dimension of 300 mm in length, 150 mm in width and 130 mm in
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height, was completely sealed with no detectable leakage. A single pass plug flow pattern was adopted for the photocatalytic gaseous pollutant degradation experiments. Figure 2 shows the schematic diagram of the experimental set-up. A zero air generator (Model 111, Thermo
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Environmental Inc.) was used to supply clean air flow constantly. The certified gas 50 ppm nitric oxide (NO, BOC Gases) was mixed with the zero air stream to the desired concentrations by adjusting mass flow controllers. The zero air was passed through a water bubbler to achieve the
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targeted relative humidity (RH), and the RH level was monitored by a humidity sensor at the centre of the reactor. The light irradiation conditions were the same as those described in section
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2.2. The NOx concentration was continuously measured by a chemiluminescence NOx analyser (Model 42c, Thermo Environmental Instruments Inc.).
All the experiments were carried out at ambient temperature (25±3°C). The exhaust vent was open to the outdoor environment, so that the experiments were run almost at ambient pressure.
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The total inlet air flow rate was 3 L·min-1 and the relative humidity was controlled at 30%. Prior to all photocatalytic conversion processes, the testing gas stream was allowed to pass through the reactor in the absence of the light radiation for at least half an hour to obtain the desired RH as
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well as gas-solid adsorption-desorption equilibrium. The concentration of inlet NO was 1000±50
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ppb and when the concentration of the gaseous pollutant reached equilibrium in the reactor, the lamps were turned on and the photocatalytic degradation was allowed to go on for 30 min. The details of the calculation of the amount of NOx removal have been described previously [22], expressed as a subtraction of the NO2 generated from the NO removed. The calculation of the amount of NOx removal is shown below: =
.
" #$ %&' () − %&' ( *+ − $ %&' ( − %&' () *+,/ -×.
9
Eq. 3
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where QNOx (µmol·m-2·h-1) is the amount of nitric oxides removed by the test sample, [NO]0 and [NO2]0 (ppm) are the inlet concentration of nitrogen monoxide and nitrogen dioxide, respectively, [NO] and [NO2] (ppm) are the outlet concentration of nitrogen monoxide and
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nitrogen dioxide, respectively, t (min) is the time of removal operation, f (L·min-1) is the flow rate converted into that at the standard state (0oC, 1.013 kPa), A (m2) is the surface area of cement paste samples, T (0.5 h for all experiments) is the duration of the photocatalytic process,
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and 22.4 represents that the volume of 1 mole ideal gas at the standard state is 22.4 L (ideal gas
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law).
Figure 2. Schematic diagram of gaseous NOx removal experimental set-up
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2.4 Weathering process
In order to determine the effect of weathering on the self-cleaning and air-purifying properties, photocatalytic removal of RhB and NOx was monitored before and after subjecting the architectural mortar samples to two independent weathering processes, a lab-simulated façade weathering process and carbonation. 2.4.1 Simulated façade weathering
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The lab-simulated façade weathering process (Figure 3) mimics the weathering process caused by rain water and sunlight on a building material applied vertically at outdoors. More details about the operation of this test method can be found in reference [33]. Briefly, the test set-up
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consisted of three inclined (45º) polyvinyl chloride (PVC) compartments, which house the testing samples. The wet condition was imposed on each compartment by pumping 1.5 L of tap water during 12 h by means of an aquarium pump (New-Jet 400, Aquarium Systems-Newa). On
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the other hand, the dry condition was applied by simply switching off the pumps for the following 12 h. The day and night conditions were coordinated respectively with the dry and rain
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conditions and each lasted also 12 h. For the day condition, light irradiation was supplied by three UV-A (3 × F8T5 BLB, HRK) and two visible lamps (T514W, Aqua Gem). The UV-A intensity striking on the surface of samples was 350 ± 10 µW·cm-2, measured by UV light meter (LT Lutron, Digital Instruments) calibrated at 340 nm. The duration of the coordinated ‘rain/dry’
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and ‘day/night’ cycles was 2 weeks. Considering the simulated ‘rain’ conditions (290 mm·h-1) selected for operating the test set-up on the one hand, and the Hong Kong average rainfall (2315 mm·year-1) on the other hand, the weathering process represents approximately 20 years of
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weathering. Although other factors, such as UV exposure, do exert influences on the deterioration of the samples, their contributions were found less significant than the dry-wet
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cycles. For example, only UV radiation (up to 1800 h) caused no observable physical degradation of TiO2 nanocoating, while wet/dry cycles resulted in a certain degradation of the nanocoating [13]. Thus, in this study, a rough estimation of weathering years in real conditions in Hong Kong was only based on the volume of rainfall.
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Figure 3. Weathering test set-up and operating conditions (UV-A intensity = 350 ± 10 µW·cm-2; Rain water intensity = 290 mm·h-1). 2.4.2 Carbonation
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To evaluate the resistance of the samples to carbonation, the international standard method ISO/DIS 1920-12 was followed. The accelerated carbonation was carried out in a carbonation chamber (Yue Fat Engineering and Oven Works) with 4 % concentration (20℃ and 60%
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relative humidity). The carbonation reaction was allowed to go on for during 70 days before the
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samples were taken for subsequent photocatalytic tests. 3. Results and Discussion
3.1 Photocatalytic removal of RhB under UV-A irradiation 3.1.1 Effect of façade weathering RhB removal efficiencies on architectural mortar samples under UV-A irradiation before and after the application of a façade weathering process are shown in Figure 4. Before subject to the weathering condition, the PC-S7 coated samples achieved a satisfactory RhB removal
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performance, with both R4 (51.4%) and R26 (63.6%) larger than the photocatalytic boundaries (R4 > 20% and R26 > 50%) [32]. In contrast, the 5% TiO2-P25 intermixed samples experienced significantly lower RhB removal efficiencies (R4 = 4.7% and R26 = 12.5%) under the same
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testing conditions, indicating that the intermixing method adversely impaired the photocatalytic performance. This observed reduction in RhB removal is mainly due to the encapsulation of the intermixed TiO2 particles by the accumulation of surrounding hydration products. Therefore,
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compared to the PC-S7 coated samples, the 5% TiO2-P25 intermixed samples are less exposed to the photons and the dye (RhB). Previous evaluation of the photocatalytic RhB removal under
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UV-A irradiation (1.5-2.5 mW·cm-2) on white cement pastes intermixed with TiO2 (0, 5, 10, 15% on a weight basis) found that only samples containing 15% TiO2 were nearly satisfactory (R4 = 33.9% and R26 = 45.8%) to be considered as photocatalytic material [34]. Taken together, it seems that although the solid mass layer can form a stable support to protect the TiO2 particles
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against abrasion or erosion, they may also unfavourably weaken the photocatalytic activity. This explanation was supported by a previous study which proposed that when nano-particles were embedded in cement-based materials, they could become potential nucleation sites for cement
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hydration products. As the hydration reaction proceeded, the hydration products (such as calcium silicate hydrate and calcium hydroxide) gradually bonded the individual TiO2 together, forming a
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dense coating on the TiO2 surface [35]. As for the PC-S7 coated samples, the ultra-fine particle size of TiO2 in the paint directly translates into a much larger surface area, meaning an extended exposure to light irradiation for the active TiO2 particles. In addition, the content of TiO2 in the paint (10% by weight of paint) is much higher than that added by the intermixing method (5% by weight of binder). This, coupled with the fact that the coating method gives the coated TiO2
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particles much easier access to light irradiation, helps explain the above observed relatively higher RhB removal efficiencies possessed by the PC-S7 coated mortar.
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5%TiO2-P25 PC-S7 coating Reference
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20 10 0 R262
before weathering
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Rhodamine B removal (%)
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after weathering ~ 20 years
Figure 4. Photocatalytic
removal under UV-A irradiation of RhB impregnated on mortar
samples (PC-S7-coated and 5% TiO2-intermixed) before and after application of an accelerated
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facade weathering process (approx. 20 years exposure under Hong Kong weather conditions). It is well known that rain and wind will cause a significant detachment of the coatings on the
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surface of substrate materials [36]. Thus, it is necessary to evaluate the weathering resistant ability of the developed photocatalytic products to justify their viable potential for real
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application. After exposure to the façade weathering process, the PC-S7 coated samples still displayed higher RhB removal efficiencies (R4 = 50.2% and R26 = 52.6%), which satisfied the boundaries (R4 > 20% and R26 > 50%) to be considered as photocatalytic material. On the other hand, the 5% TiO2-P25 intermixed samples experienced a significant improvement in the RhB removal efficiencies (R4 = 27.2% and R26 = 43.5%) after the application of the weathering process, although still not enough to satisfy the photocatalytic boundaries. It is highly likely that the weathering process helped to expose more TiO2 particles to the photons and RhB, resulting in
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a higher photocatalytic RhB degradation efficiency. Thus, existing texturing surface treatments might be a good option for enhancing the photocatalytic activity of such materials. Similar results have been reported by a previous study in which an increase of about 30% and 70% on
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the RhB removal efficiencies R26 and R4, respectively, was observed on the 5% TiO2 intermixed mortar samples, which were previously abraded with sand paper (P150 – ISO/FEPA Grit designation) [37]. It seems that TiO2 particles do not tend to appear on the casting surface due to
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a lower density compared to the other cementitious materials used to prepare the architectural
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mortars. 3.1.2 Effect of carbonation
In the natural environments, carbonation of cementitious materials represents another concern about the long term durability of the self-cleaning ability of the architectural mortar samples. It is
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a slow process occurring between calcium hydroxide in the cement past and carbon dioxide from the air [38]. RhB removal efficiencies of different architectural mortar samples under UV-A irradiation both before and after the application of the accelerated carbonation weathering
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process are shown in Figure 5. For the 5% TiO2-P25 intermixed samples, a slight reduction in RhB degradation was observed after undergoing the accelerated carbonation process. For
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example, the RhB removal efficiency of the 5% TiO2-P25 intermixed sample just after 4 h of UV-A light irradiation was reduced by about 45% due to the effect of carbonation. Apparently, the formation of tiny calcium carbonate (CaCO3) crystals from the carbonation reaction adversely affects the contact between the RhB molecules and the surface TiO2 particles. Consequently, the photocatalytic reaction is slowed down. Similar phenomena have been observed in previous studies. Reductions up to 60% in atrazine degradation for 10% TiO2 intermixed samples have been reported after two months of carbonation [39]. As the carbonation
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can only occur between the calcium hydroxide (CH) in the substrate and CO2 in the air, no obvious difference in RhB degradation was found in the PC-S7 coated samples before and after
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the carbonation process.
Figure 5. Photocatalytic removal under UV-A irradiation of RhB impregnated on mortar samples
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(PC-S7-coated and 5% TiO2-intermixed) before and after application of an accelerated carbonation process
3.2 Photocatalytic removal of RhB under visible light irradiation
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3.2.1 Effect of façade weathering
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RhB photocatalytic removal efficiencies of architectural mortar samples under visible light irradiation both before and after application of the façade weathering process can be seen in Figure 6. For all the samples, generally, similar trends to those obtained under UV-A light irradiation were observed under VL irradiation, with a slight reduction in RhB removal efficiencies. The PC-S7 coated samples also displayed higher RhB removal efficiencies with R4 (42.0%) and R26 (61.7%) larger than the photocatalytic boundaries (R4 > 20% and R26 > 50%) before undergoing the weathering conditions. After the application of the façade weathering
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process, RhB removal efficiencies (R4 = 40% and R26 = 51%) for the PC-S7 coated samples still satisfied the photocatalytic boundaries for considering this as photocatalytic material. It seems that a combination of the 405-410 nm wavelength fraction near the UV-A region (Figure
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S1) from the VL lamps (YZO8T5, NVC) and a high surface area (300 m2·g-1) of the photocatalyst particles in the PC-S7 paints are sufficient enough to ensure a high photocatalytic activity under VL irradiation. For instance, investigation on the degradation of RhB on
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TiO2-P25, TiO2 nano-rod and TiO2 nano-stripe (all have a high surface area) under VL irradiation has shown that the morphology and crystal structure of the catalyst have an important
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effect on the degradation activity [40]. It can be seen that the PC-S7 coated architectural mortars have promising potential for indoor application, in which environment the self-cleaning ability is certainly a benefit to people who spend a considerable time for indoor activities. Contrary to the appealing photocatalytic activity possessed by the PC-S7 coated samples, the 5%
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TiO2-intermixed mortars failed to deliver a sufficient enough photocatalytic RhB removal ability after subject to the weathering conditions.
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70 60 50
5%TiO2-P25 intermixed PC-S7 coating Reference
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Condition 2: R26>50%
40 30 20
Condition 1: R4>20%
10 0
R4 1 R26 2 before weathering
R43 R426 after weathering
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Figure 6. Photocatalytic removal under visible light irradiation of RhB impregnated on mortar samples (PC-S7-coated and 5% TiO2-intermixed) before and after application of an accelerated
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facade weathering process (approx. 20 years exposure under Hong Kong weather conditions). 3.2.2 Effect of carbonation
Under visible light irradiation, there is a similar story about the effect of the accelerated
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carbonation process on RhB degradation performances for both the TiO2 intermixed and PC-S7 coated samples (Figure 7). The carbonation process imposed much more pronounced decreases
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in RhB degradation on the 5% TiO2 intermixed sample in comparison with the PC-S7 coated sample. It should also be noted that even after exposure to the accelerated carbonation weathering, the PC-S7 coated sample still delivered a satisfactory RhB removal performance (R4 > 20% and R26 > 50%). Again, the efficacy and long term service-life of such products have been
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well demonstrated in both outdoor and indoor applications.
Figure 7. Photocatalytic removal under visible light irradiation of RhB impregnated on mortar samples (PC-S7-coated and 5% TiO2-intermixed) before and after application of an accelerated carbonation process
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3.3 Photocatalytic NOx removal under UV-A irradiation Figure 8 shows photocatalytic NOx removal performances of PC-S7-coated and 5% TiO2-P25 intermixed architectural mortar samples under UV-A irradiation before and after the application
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of the façade weathering process. Compared with the 5% TiO2-P25 intermixed samples, the PC-S7-coated mortars displayed a significantly higher NOx removal efficiency (259.3 µmol·m-2·h-1) without exposure to the facade weathering. After subject to the facade weathering,
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no obvious reduction in photocatalytic NOx removal activity was observed for the PC-S7-coated samples (only a 2% reduction). In stark contrast, the 5% TiO2-P25 intermixed samples suffered a
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significant decrease in photocatalytic NOx removal efficiency, dropping from 181.0 to 21.5 µmol·m-2·h-1. This unexpected observation was in contradiction with the results from photocatalytic RhB degradation, which experienced a slight increase in the efficiencies for the 5% TiO2-P25 intermixed samples. The exact reason for this seeming contradiction was not clear. It is
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possible that the repeated raining process relocated some impurities on the surface of the mortars to the pores, blocking the light from reaching the underlying TiO2 particles. Therefore, although the weathering process can expose more TiO2 particles on the surface to intimately contacte RhB
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(resulting in higher photocatalytic RhB degradation efficiencies), the blocked pores by the accumulation of impurities adversely reduced the NOx removal ability by allowing less light and
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NO gas to come into contact with the TiO2 particles.
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Figure 8. Photocatalytic removal under UV-A irradiation of gaseous NOx on mortar samples
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(PC-S7-coated and 5% TiO2-intermixed) before and after application of an accelerated facade weathering process (approx. 20 years exposure under Hong Kong weather conditions). 3.4 Photocatalytic NOx removal under visible light irradiation
The photocatalytic NOx removal performances of architectural mortar samples under VL
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irradiation both before and after the application of the façade weathering process are provided in Figure 9. Compared with UV-A irradiation, VL irradiation exerted a negligible influence on photocatalytic NOx removal ability (251.9 µmol·m-2·h-1) for the PC-S7-coated samples before
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exposure to the weathering conditions, demonstrating high potential for indoor application. However, the 5% TiO2-P25 intermixed samples experienced a significant reduction in
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photocatalytic NOx removal (93.5 µmol·m-2·h-1). This can be attributed to the fact that P25 is mainly effective within the UV light spectrum. After the application of the facade weathering, the PC-S7-coated samples exhibited no apparent NOx removal change under VL irradiation, while the 5% TiO2 intermixed samples suffered a considerable reduction in the NOx removal activity (a 92% reduction). This trend is similar to that observed under UV-A irradiation, and the explanation offered above can be applied here.
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Figure 9. Photocatalytic removal under visible light of gaseous NOx on mortar samples
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(PC-S7-coated and 5% TiO2-intermixed) before and after application of an accelerated facade weathering process (approx. 20 years exposure under Hong Kong weather conditions). 4. Conclusion
This paper compared the photocatalytic performance of two TiO2 incorporation methods on
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cementitious materials under both different weathering and incident light irradiation conditions. The PC-S7-coated samples delivered a similar performance in photocatalytic RhB and NOx removal under both visible light and UV-A light irradiation. Compared with the 5%
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TiO2-intermixed mortars, the PC-S7-coated samples also garnered a robust weathering-resistant
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ability, reflected by an almost unchanged ability to photocatalytic degrade both RhB and NOx after exposure to a facade weathering process (representing a usage period of about 20 years under Hong Kong weather conditions), as well as an accelerated carbonation process. The results suggest that self-cleaning and air-purifying properties coupled with a robust weathering-resistant ability could make the TiO2 coated architectural mortars an attracting product for both indoor and outdoor applications.
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Acknowledgements The authors wish to thank the Environment and Conservation Fund, the Woo Wheelock Green
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Fund and the Hong Kong Polytechnic University for funding supports.
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Supporting Information to
architectural mortars:
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Photocatalytic activities of titanium dioxide incorporated
Effects of weathering and activation light
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Ming-Zhi Guoa, Anibal Maury-Ramireza,b, Chi Sun Poona*
a
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Department of Civil and Environmental Engineering (CEE), The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong. E-mails: [email protected], [email protected]* (corresponding author).
b
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Department of Civil and Industrial Engineering, Pontificia Universidad Javeriana Cali, Calle 18 # 118-250, Av. Cañas Gordas, Cali, Colombia. E-mail: [email protected] 300
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Silicone
Acrylic
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NOx Removal/µmol m-2 h-1
PC-S7
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150 100 50
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UVA
Sunlight LED 395 Different Light Sources
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Figure S1. Photocatalytic NOx removal of three different paints coated mortars under different lights irradiation
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ACCEPTED MANUSCRIPT Highlights Two TiO2 adding methods conferred self-cleaning and air-purifying ability on mortars. TiO2 paint coated mortars performed better both in RhB degradation and NOx removal. A robust weathering-resistant capacity was displayed by paint-coated mortars.
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Paint-coated products show great promise for both indoor and outdoor applications.