NO removal efficiency of photocatalytic paving blocks prepared with recycled materials
Construction and Building Construction and Building Materials 21 (2007) 1746–1753 MATERIALS www.elsevier.com/locate/conbuildmat NO removal eﬃciency of photocatalytic paving blocks prepared with recycled materials C.S. Poon *, E. Cheung Department of Civil and Structural Engineering, The Hong Kong, Polytechnic University, Hung Hom, Hong Kong Received 30 August 2005; received in revised form 18 April 2006; accepted 30 May 2006 Available online 27 September 2006Abstract This paper presents the results of a study on the eﬀectiveness of incorporating air cleaning agents such as titanium dioxide (TiO2) intothe technique of producing concrete paving blocks, using local waste materials to remove nitrous oxide (NO). Factors which would aﬀectthe performance of the blocks were studied including the porosity of blocks, the type of waste materials used within the mix design, thetypes and percentage of TiO2 added within the mix design. The results show that the photodegradation of NO is related to the porosity of the blocks. When the porosity of the block wasincreased so was the NO removal ability. Hence the choice, size and content of aggregate material used in the mix design are important.In addition, crushed recycled glass cullet was used to place part of the aggregates in the blocks and was found to beneﬁt the NO removalability due to its light transmitting characteristic. Three types of TiO2 were tested in this study and their inﬂuence on NO removal wasquantiﬁed. Based on the experimental results, an optimum mix design was selected which incorporates recycled glass, sand, cement andTiO2.Ó 2006 Elsevier Ltd. All rights reserved.Keywords: Photocatalyst; Titanium dioxide; Nitrogen oxides; Recycled aggregates1. Introduction lion people . The numerous tall buildings, particularly in the urban area, hinder and prevent the dispersion of The construction industry is the major solid waste gen- air pollutants generated by a high concentration of vehicleserator in Hong Kong . The extensive building and infra- at the street level. It is apparent that there is a need tostructure development projects as well as redevelopment of remove pollutants, such as nitrogen oxides (NOX) and sul-old districts have led to an increase in construction and phur dioxide (SO2) from the atmosphere. Not only do thesedemolition waste (C&DW) generation in the last decade. gases pose a threat to health, they are also causing degra-This has caused the disposal of the wastes to become a dation to many inner city buildings. Despite attempts tosevere social and environmental problem in the territory. lower these emissions by using cleaner vehicles, it appearsUp to present this problem has been dealt with by dispos- that a way of removing such pollutants once in the atmo-ing the waste at landﬁlls and public ﬁlling areas locally. sphere needs to be sought.There is an increasing interest to explore new ways to Photocatalysis, such as titanium dioxide, have alreadyrecycle aggregates derived from C&D waste . been tested in Japan for concrete paving materials that Additionally, Hong Kong also faces a growing concern can facilitate a photocatalytic reaction converting the moreof air pollution due to having to provide habitats and toxic forms of air pollutants to less toxic forms (e.g. NOXtransportation for a high population density of seven mil- to HNO3) [4–7]. Under the illumination of ultraviolet light, photocatalysis shows diverse functions, such as the decom- * Corresponding author. Tel.: +852 2766 6024; fax: +852 2334 6389. position of air and water contaminants and deodorization, E-mail address: email@example.com (C.S. Poon). as well as self cleaning, antifogging, and antibacterial0950-0618/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.conbuildmat.2006.05.018
C.S. Poon, E. Cheung / Construction and Building Materials 21 (2007) 1746–1753 1747 two other types of TiO2 were sourced from an industrial source in Mainland China due to their low prices compared to P25. The two types of TiO2 included one in the form of anatase crystal structure and the other in the rutile crystal structure. The two types of TiO2 are referenced as anatase and rutile respectively in this paper. The properties of the TiO2 powders used are shown in Figs. 2 and 3, and Table 1. Fig. 2 shows that the particle size decreased in the order Fig. 1. The design of the photocatalytic paving block. of P-25, followed by the rutile form and then the anatase form. Fig. 3 show that P25 possessed TiO2 in the forms of anatase and rutile. Whereas the anatase form of TiO2actions [8–11]. Practical applications of photocatalysts contained only the anatase form and the rutile form ofhave rapidly expanded in recent years. Photocatalytic TiO2 contained only the rutile form.materials for outdoor puriﬁcation are in urgent demand Furnace bottom ash (FBA) used is a by-product of coal-because energy and labour saving advantages have been ﬁred electricity generation. FBA is the coarser material thatrealized when applied to building or road construction falls to the bottom of the furnace during the burning ofmaterials in large cities where urban air pollution is very coal. Chemically, it is very similar to pulverized ﬂy ashserious . but due to its coarse grain size, it is not commonly used Based on the current environmental problems the main in cement and concrete applications [14,15]. In Hongobjective of this study is to analyze the eﬀectiveness of Kong, the produced FBA is currently dumped at an ashincorporating air cleaning agents such as TiO2 into the lagoon as waste. FBA used in this study was obtained fromtechnique of producing concrete paving blocks using local the Castle Peak Power Station and sieved in the laboratory.waste materials. The paving block consists of a concrete Only the portion that passed through a 2.36 mm sieve wasbase layer made from cement and recycled aggregates, used for making the surface layer.and a thin surface layer made of cement, various aggregatematerials and a small amount of titanium dioxide. Thedesign of this photocatalytic block is shown in Fig. 1. In 16this paper, the focus is on optimizing the surface layer P25 Rutile Anatase 14design of the block. To achieve this, surface layers wereproduced using diﬀerent material combinations, and tested 12 Distribution (%)for their NO removal abilities. The followings were studied: 10 8 Compare natural aggregates with recycled aggregates and decide which beneﬁts the pollutant removal ability 6 of the paving blocks. 4 Study the factors aﬀecting the NO photodegradation of 2 the blocks. These include porosity, cement content, dif- 0 ferent particle size of aggregates and curing time of the .84 .69 4 .68 05 15 23 35 53 81 23 86 83 52 91 3 80 .0 4. blocks. 0. 0. 0. 0. 1. 1. 2. 9. 0. 0. 6. 22 34 15 52 Particle diameter (micrometre) Compare diﬀerent sources of TiO2 and their eﬀects towards pollutant removal ability. Fig. 2. Particle size distribution of TiO2 powders. Anatase Rutile P252. Experimental details A R R R A A A A R A2.1. Materials R R Intensity (a.u.) The cementitious material used in this study was anOrdinary Portland Cement (OPC) commercially availablein Hong Kong, complying with BS 12  and ASTM TypeI . Three sources of titanium dioxide (TiO2) were used. Theﬁrst was P-25 sourced from Degussa, which was used dueto its high purity and accurate speciﬁcations. It is com- 10 20 30 40 50 60 70 80 90 100 2 theta (degree)monly used in the industry and research community, hencewould be useful for comparison with works of others. The Fig. 3. XRD spectrum of TiO2 powders (A: anatase, R: rutile).
1748 C.S. Poon, E. Cheung / Construction and Building Materials 21 (2007) 1746–1753Table 1 Table 3Properties of TiO2 Mixes prepared with RA, FBA and sandProperties P25 Anatase Rutile Mix notation Relative proportions (by weight)Moisture (%) 1.5 0.04 0.46 Cement RA Sand FBA P25 TiO2 WaterWater solubility (%) – 0.05 0.03 R1:2 1 2 – – 0.06 0.28Ignition loss (%) 2.0 0.01 0.17 R1:2.5 1 2.5 – – 0.07 0.30pH 3.0–4.0 7.5 6.7 R1:3 1 3 – – 0.08 0.32Oil adsorption (g/g) – 22/100 23/100 R*1:2 1 2 – – 0.06 0.28Color eliminating capacity (per min) – 100 100 R*1:2.5 1 2.5 – – 0.07 0.30Sieve residue, Mocker 45 (%) 0.05 0.05 0.05 R*1:3 1 3 – – 0.08 0.32TiO2 (%) 99.5 98.5 91 S1:2 1 – 2 – 0.06 0.24 S1:2.5 1 – 2.5 – 0.07 0.26 S1:3 1 – 3 – 0.08 0.28 The recycled aggregate (RA) used in this study was a S*1:2 1 – 2 – 0.06 0.24crushed CD waste sourced from a temporary recycling S*1:2.5 1 – 2.5 – 0.07 0.26facility in Hong Kong. In the plant the CD waste under- S*1:3 1 – 3 – 0.08 0.28 RF1:2 1 1.5 – 0.50 0.06 0.32went a process of mechanized sorting, crushing and sieving RF1:2.5 1 1.88 – 0.63 0.07 0.34to produce both ﬁne and coarse aggregates according to RF1:3 1 2.25 – 0.75 0.08 0.36the particle size requirements of BS 812 . Only the smal- RF*1:2 1 1.5 – 0.50 0.06 0.32ler ﬁne aggregate proportion was used for making the sur- RF*1:2.5 1 1.88 – 0.63 0.07 0.34face layer of the blocks in this study. The maximum size of RF*1:3 1 2.25 – 0.75 0.08 0.36 SF1:2 1 – 1.5 0.50 0.06 0.28the recycled ﬁne aggregate used was 2.36 mm. The proper- SF1:2.5 1 – 1.88 0.63 0.07 0.30ties of the RA are shown in Table 2. SF1:3 1 – 2.25 0.75 0.08 0.32 The recycled glass (RG) used in this study was mainly SF*1:2 1 – 1.5 0.50 0.06 0.28post-consumer beverage bottles sourced locally. The glass SF*1:2.5 1 – 1.88 0.63 0.07 0.30bottles were washed and crushed by mechanical equipment. SF*1:3 1 – 2.25 0.75 0.08 0.32The RG was sieved in the laboratory to pass though the2.36 mm sieve. The properties of the RG are shown inTable 2. Table 4 The sand used was ﬁne natural river sand commercially Mixes prepared with recycled glassavailable in Hong Kong. The properties of sand are shown Mix notation Relative proportions (by weight)in Table 2. Cement RG Sand P25 TiO2 Water2.2. Mix proportions GS100 1 3 – 0.08 0.28 GS75 1 2.25 0.75 0.08 0.28 GS50 1 1.5 1.5 0.08 0.282.2.1. Mixes prepared with RA, FBA and sand GS25 1 0.75 2.25 0.08 0.28 This study focused on utilizing recycled materials, so a GS0 1 – 3 0.08 0.28series of mixes (as shown in Table 3) were prepared to ﬁndout the eﬀects of diﬀerent materials and proportions on NOremoval eﬃciency. Mixes with diﬀerent cement to aggre-gate ratios, ranging from 1:2, 1:2.5 and 1:3 were prepared. 2.2.3. Mixes prepared with varying amount and types ofMost of the mixes were prepared using aggregate sizes from TiO20 to 2.36 mm. But selected mixes (those identiﬁed by ‘*’) The eﬀects of varying the amount and types TiO2 werewere prepared with aggregate sizes only between 300 and studied by preparing specimens with the TiO2 (P-25) con-2.36 mm with the À300 lm portion removed. tent ranging from 0% to 10% at 2% intervals (Table 5) and diﬀerent sources of TiO2 (Table 6).2.2.2. Mixes prepared with RG The light transmitting characteristic of glass wasthought to beneﬁt NO photodegradation when used in Table 5the mix design of the surface layers. Hence mix proportions Mixes prepared with varying TiO2 contentprepared with recycled glass were designed (Table 4). Mix notation (%) Relative proportions (by weight) Cement Glass Sand P25 TiO2 WaterTable 2 0 1 1.5 1.5 0 0.26Properties of RA, RG and sand 2 1 1.5 1.5 0.08 0.28 4 1 1.5 1.5 0.16 0.36 RA (ﬁne) RG Sand 6 1 1.5 1.5 0.24 0.40Density (kg/m3) 2093 2531 2651 8 1 1.5 1.5 0.32 0.48Water absorption (%) 10.28 0 0.87 10 1 1.5 1.5 0.40 0.64
C.S. Poon, E. Cheung / Construction and Building Materials 21 (2007) 1746–1753 1749Table 6Mixes prepared with diﬀerent sources of TiO2Mix notation Relative proportions (by weight) Cement Glass Sand P-25 TiO2 Anatase TiO2 Rutile TiO2 WaterP-25 1 1.5 1.5 0.08 – – 0.26Anatase 1 1.5 1.5 – 0.08 – 0.26Rutile 1 1.5 1.5 – – 0.08 0.262.3. Sample preparation the specimen, which is expressed as the approximate indica- tion of the porosity of the specimens. Surface layers for the diﬀerent mix designs were pre-pared to test for their ability to remove NO. The surface 2.5. Test of photodegradation of NOlayers were fabricated in steel moulds with internal dimen-sions of 200 · 100 · 5 mm. The wet mixed materials 2.5.1. Equipmentweighed between 220 and 280 g for each surface layer The testing equipment used was a self-designed ﬂowdepending on the diﬀerent materials. The steel moulds were reactor adapted from an existing design , but with alter-over ﬁlled by hand compaction, and then further com- ations. The reactor provides a physical boundary to enablepressed using a compression machine at a rate of a photocatalytic material, in our case a photocatalytic sur-600 KN/min twice, ﬁrst to 500 KN and secondly to face layer of a paving block, to be examined for its pollu-600 KN. After 1 day, the surface layers were removed from tant removal capability. The dimensions of the chambertheir moulds and cured in a chamber with a controlled reactor were 700 mm in length, 400 mm in width andhumidity of 75% and temperature of 25 °C until testing. 130 mm in height. The reactor consists of a sampling inletThe surface layers were tested for NO photodegradation and outlet. Two 10 W UV-A ﬂuorescent lamps (blackat 28, 56 and 90 days. lights), which emit primary UV light wavelengths at 365 nm were used to provide photoirradiation to activate2.4. Determination of physical properties the photocatalyst. The intensity was measured using a UV meter (Spectroline DRC-100X) to be 10 W mÀ2 at2.4.1. Porosity the centre of the reactor which was also where the test sam- The method requires the specimens to be crushed into ples were placed. The light source was positioned outsideapproximately 10 mm diameter sizes and then oven dried the reactor and the distance from the reactor was adjustedat 105 °C for 24 h. Hundred grams of the prepared speci- till the required intensity was achieved. The design enablesmens were soaked in acetone in a sealed container for the reactor to be used as a continuous ﬂow reactor or a24 h. The specimens were then removed from the acetone batch ﬂow reactor. The reactor needed to be constructedand the specimen surfaces were dried by an acetone wetted with materials of low adsorption ability and resistance totissue paper. The weights of the specimens were then ultraviolet irradiation, hence stainless steel was used. Rub-weighed and the diﬀerence between the original weights ber was used as sealant for the reactor. The lid of the reac-indicated the amount of acetone that can be absorbed by tor was designed using transparent glass rimmed with Light source 2 x 10 W UV-A lamps (Intensity = 10 Wm-2 at centre of sample surface) Standard gas (NO) with flow controller Reactor NO analyzer Computer Humidity = 10 % Zero air with flow controller Fig. 4. Schematic diagram of the experimental setup.
1750 C.S. Poon, E. Cheung / Construction and Building Materials 21 (2007) 1746–1753stainless steel. A temperature sensor, humidity sensor, fan due to the porous nature of RA compared to that of sand.(for circulation), and adjustable rack to support the speci- The results also indicate that the NO removal slightlymens were placed inside the reactor. Fig. 4 further illus- increased when FBA was included in the mix design. Thistrates the equipment setup described. is believed to be due to the higher porosity of FBA particles which was exempliﬁed by its relatively low speciﬁc density2.5.2. Testing procedure compared to those of sand and RA. The reactor was connected to a supply of standard gas(NO) and a zero air generator (Thermo Environmental 3.1.2. Eﬀect of cement/aggregate ratio on NOInc. Model 111). The standard gas was obtained from a photodegradationcompressed gas cylinder with nitrogen as the balanced With diﬀerent cement to aggregate ratios, the resultsgas (NIST certiﬁed). A humidity of 10% was achieved for (Fig. 6) show that the NO removal increased when thethe reactor by passing the reactant stream and the zero cement content decreased. An increase in NO removal ofair stream directly through the reactor. It is possible to approximately 30% was experienced for specimens whenachieve a higher humidity by passing the zero air stream the cement to aggregate ratio dropped from 1:2 to 1:3.through a water bath, but in the procedure the humidity The increase in NO removal due to the change in cementwas kept at 10% as at this humidity level observation pat- content was a result of the ﬁne particle size of cementterns can be more easily interpreted. Also for comparison grains and hydrated cement particles could easily ﬁll uppurposes this is believed to be reasonable. The gas streams the voids within the specimens, the surface area availablewere then adjusted by the ﬂow controllers to achieve an ini- for pollutants was reduced. Although reducing the cementtial NO concentration of 1000 ppb and a ﬂowrate of content was favourable towards NO removal, the adopted6 L minÀ1, these testing conditions in a similar set-up have cement content of the surface layers should also consideralso been used by Yu  and are believed to be the most the necessary mechanical strength required for pavingideal from trial. After the inlet and the outlet NO concen- applications. Preliminary studies showed that a minimumtrations reached equilibrium (1/2 h), the UV lamps were cement to aggregate ratio of 1:3 should be used in the sur-turned on to begin the removal process (1 h). The NO con- face layer .centration was continuously measured by the NO analyzer(Thermo Environmental Instruments Inc. Model 42c). To 3.1.3. Eﬀect of curing age on NO photodegradationcomplete the experimental procedure the lamps were then All specimens prepared were tested at the curing ages ofturned oﬀ and the supply gas changed to zero air only 28, 56 and 90 days to investigate the inﬂuence of curing age(1/2 h). on the NO removal. The results displayed in Fig. 7 only show some of the selected mixes, all with a cement to aggre-3. Results and discussion gate ratio of 1:3. The results show that NO removal decreased with increasing age from 28 to 90 days by3.1. Factors aﬀecting the NO photodegradation approximately 8%. This drop was a result of the reduction in the number of active sites due to the closing up of pores3.1.1. Eﬀect of aggregate material on NO photodegradation as a result of the continuous hydration and carbonization The results of NO photodegradation as shown in Fig. 5 of the hydrated cement particles. Lackhoﬀ et al.  inves-indicate that the RA mixes achieved a much higher NO tigated the possibility of using photocatalyst modiﬁedremoval compared to the sand mixes. This is probably cement samples for the degradation of pollutants and they 3 3 S* SF* R* RF* 2.5 2.8 NO removal (mg hr m ) NO removal (mghr -1m-2) -2 2.6 -1 2 2.4 1.5 2.2 2 1 1.8 0.5 1.6 0 1.4 S* 1:3 R* 1:3 SF* 1:3 RF* 1:3 1:2 1:2.5 1:3 Mixes Cement to aggregate ratioFig. 5. Comparison of diﬀerent materials to remove NO at 90 days Fig. 6. A comparison of NO removal for mixes prepared with diﬀerenttesting. cement contents at 90 days testing (Table 3).
C.S. Poon, E. Cheung / Construction and Building Materials 21 (2007) 1746–1753 1751 3.4 2.8 3.2 NO removal (mghr m ) 2.7 -2 NO removal (mghr m ) -2 3 -1 -1 2.6 2.8 2.5 2.6 2.4 2.4 2.2 2.3 2 28 56 90 2.2 Days 7 8 9 10 11 12 13 R1:3 R*1:3 S1:3 S*1:3 RF1:3 RF*1:3 SF1:3 SF*1:3 Acetone absorbed per 100g of dry specimenFig. 7. NO removal of specimens tested at diﬀerent curing ages (Table 2). Fig. 9. Comparison of NO removal and porosity for selected mixes.also demonstrated that photocatalytic activity decreasedduring the ageing of the hardened cement pastes. 3.2. Eﬀect of incorporating recycled crushed glass cullet3.1.4. Eﬀect of particle size of aggregates on NO The results as shown in Fig. 10 indicate that when recy-photodegradation cled glass was used as aggregates the NO removal ability The specimens prepared with diﬀerent aggregate sizes was enhanced. This is believed to be related to the highwas believed to aﬀect their ability to remove NO as altering light transmitting characteristic of the recycled glass parti-the particle size distribution of aggregates would eﬀectively cles. Light could be carried to a greater depth activating thechange the porosity of the specimens. The specimens were TiO2 on the surface as well as within the surface layer. Thisdivided into to two groups, one was prepared with all was also supported by Murata et al.  when they utilizedaggregate sizes below 2.36 mm included and the other with glass beads in the design of photocatalytical paving blocks.aggregate sizes only between 300 lm and 2.36 mm The results in Fig. 11 also show quite clearly that NOincluded. Fig. 8 shows the specimens tested at 90 days with removal increased with increasing amounts of recycleda cement to aggregate ratio of 1:3. The results indicate that glass. As alkali aggregate reaction has been recognized asthe specimens prepared with aggregate sizes between a limiting factor in using crushed glass in concrete mixes,300 lm and 2.36 mm (the more porous specimens) a replacement level of 50% of sand by recycled glass wasachieved approximately 4% higher NO removal. chosen as the optimal mix based on our separate study.3.1.5. Eﬀect of porosity on NO photodegradation 3.3. Eﬀect of diﬀerent contents and sources of TiO2 The factors aﬀecting the NO removal discussed abovewere all associated with porosity. Fig. 9 shows the mea- The results as shown in Fig. 12 indicate that there was asured porosity compared with NO removal of some of signiﬁcant increase in NO removal with an increase in TiO2the selected mixes. An obvious trend can be observed content. At 90 days, NO removal increased fromshowing NO removal increased with increase in porosity. 2.56 mg hÀ1 mÀ2 for specimens with 2% TiO2 to 3.5 SF 3 NO removal (mghr-1m-2 ) 2.5 RF 2 1.5 S 1 R 0.5 0 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 GS100 (Glass) R1:3 (RA) S1:3 (Sand) RF1:3 (RA+FBA) SF1:3 (S+FBA) -1 -2 NO removal (mghr m ) Mix notation 2.36mm 300-2.36um Fig. 10. Eﬀect of recycled glass in the mix towards NO photodegradationFig. 8. NO removal of specimens with diﬀerent aggregate sizes (Table 3). when compared to other materials at 90 days curing age (Table 3 and 4).
1752 C.S. Poon, E. Cheung / Construction and Building Materials 21 (2007) 1746–1753 3.5 3.5 3.3 3 NO removal (mghr-1m-2 ) NO removal (mghr m ) 3.1 -2 2.5 -1 2.9 2.7 2 2.5 1.5 2.3 2.1 1 1.9 0.5 1.7 0 1.5 28 56 90 0 25 50 75 100 Days Days P25 Rutile AnataseFig. 11. NO removal of mixes with diﬀerent recycled glass contents at Fig. 13. NO removal of specimens containing diﬀerent sources of TiO290 days curing age. (Table 6). 4.5 . In addition, Deng et al.  showed that the activities 4 28 56 90 Days of pure anatase TiO2 and pure rutile TiO2 catalysts with almost the same surface area for photocatalytic oxidation NO removal (mg hr m ) -2 3.5 of hexane were similar. -1 3 The TiO2 powder Degussa P-25 consists of both the ana- 2.5 tase and rutile forms at a ratio of 70:30. P-25 has often been 2 considered as one of the best and most frequently used 1.5 TiO2 photocatalysts. Ohno et al.  explained the excel- 1 lent ability of P-25 was due to a synergy eﬀect as a result of the anatase and rutile particles in contact. It was sug- 0.5 gested that anatase and rutile TiO2 particles exist sepa- 0 0% 2% 4% 6% 8% 10% rately by forming agglomerates in P-25. Therefore TiO2 (% of whole weight) electron transfer was feasible via the agglomerates which lead to the high activity of P-25.Fig. 12. The eﬀect of NO removal at diﬀerent TiO2 contents (Table 6). There are a wide range of factors which could possibly aﬀect the activity of TiO2 used in this study. These included4.01 mg hÀ1 mÀ2 for specimens with 10% TiO2. The corre- the crystalline forms, surface area, particle size, porosity,sponding increase was 57%. Although NO removal surface acidity and density of surface adsorbed water andincreases as the TiO2 content increases up to 10% by weight hydroxyl groups . It seems that the larger P-25 TiO2of the whole mix, the eﬀectiveness of the use of higher TiO2 particles were more beneﬁcial to the photocatalytic activitycontent on the NO removal needs to be veriﬁed by future in the current study. Small TiO2 particles have often beenstudies. believed to beneﬁt photoactivity due to the increased sur- Three types of commercially available TiO2 powders face area. The apparent contrary results might be attrib-were compared. Specimens of the same mix design were uted to the presence of other small sized materials suchproduced in which the only variable was the source of as the cement in the production of the surface layers whichTiO2 (Table 6). The results as shown in Fig. 13 indicate covered the TiO2 particles. Further research is required tothat, at all test ages, P-25 showed the best removal ability, conclude this ﬁnding.followed by the rutile form of TiO2 and the anatase form ofTiO2. Indeed, the NO removal for P-25 and the rutile form 4. Conclusionof TiO2 was very close. At 90 days, the NO removal of P-25was higher by only 8% compared to the removal of the This paper reports on the ﬁndings on assessing the fac-rutile form. On the other hand the NO removal for the ana- tors which would aﬀect the ability of the prepared surfacetase form of TiO2 was 53% lower compared to that of P-25. layer of the paving blocks to remove NO by photocatalytic The rutile form of TiO2 performed better photocatalyt- activities. The results indicate that porosity of the surfaceically when used in the mix design compared to the anatase layer is important which eﬀectively increased the area avail-form despite the anatase form is generally believed to be able to reacting with the pollutants. The porosity of themore photoactive . But it has also been reported in a surface layer was aﬀected by the type of materials withprevious study that the rutile form can perform more which they were prepared. Materials with a lower densityactively or as well as the anatase form in certain situation. led to a higher porosity of the blocks. The particle size
C.S. Poon, E. Cheung / Construction and Building Materials 21 (2007) 1746–1753 1753distributions of the materials used also aﬀected the porosity  Fujishima A, Rao TN, Tryk DA. Titanium dioxide photocatalysis. Jof the paving blocks. Materials with less ﬁne particles Photochem Photobiol C: Photochem Rev 2000;1:1–21.  Fujushima A, Hashimoto K, Watanabe T. TiO2 photocatalysisincreased the porosity of the surface layers. The change fundamentals and applications. Japan: BKC; 1999.in the cement to aggregate ratio of the mixes had an obvi-  Murata Y, Tawara H, Obata H, Takeuchi K. Air purifying pavement:ous relationship to the NO removal ability. Mixes prepared development of photocatalytic concrete blocks. J Adv Oxidat Technolwith a lower cement to aggregate ratio were more eﬀective 1999;4(2):227–30.at removing NO. Specimens tested at diﬀerent curing ages  BS 12. Speciﬁcation for Portland cement. British Standards Institu- tion, 1996.were found to show diﬀerent abilities to remove NO. The  ASTM C150 REV A. Standard speciﬁcation for Portland cement.results show that photocatalytic activity decreased with Annual book of ASTM standards, 2004.age but the decrease stabilized at the age of 90 days.  Dawson AR. Furnace bottom ash: its engineering properties and its The eﬀect of incorporating recycled crushed glass cullet use as a sub-base material. In: Proceedings of the institution of civilin the mix design of the surface layer was also investigated. engineers, 1991. p. 993–1009.  Kayabali K, Bulus G. The usability of bottom ash as an engineeringThe improved performance of the surface layers when material when amended with diﬀerent matrices. Eng Geologycrushed recycled glass was used was believed to be due to 2000;56:293–303.the high light transmitting characteristic of the glass cullet.  BS 812-103.1. Testing aggregates—methods for determination of Furthermore, diﬀerent types of photocatalysts were particle size distribution. British Standards Institution, 1985.studied. P-25 TiO2 was compared to an anatase and a rutile  National Institute of Advanced Industrial Science and Technology (NIAIST). Technical report (draft proposal, provisional transla-forms sourced from an industrial source. P-25 showed the tion)—testing method for air puriﬁcation performance of photocat-best photocatalytic ability. But the use of the rutile form alytic material. Japan. 2001.instead of P-25 was particularly appealing in terms of cost.  Yu JCM. Deactivation and regeneration of environmentally exposed titanium dioxide based products—testing report prepared for theAcknowledgement environmental protection department of HKSAR E183413. Hong Kong. 2003.  Poon CS, Cheung E. Air pollutant removing paving blocks produced The authors wish to thank the Hong Kong Polytechnic with recycled waste materials. In: Proceedings of the RILEMUniversity for funding support. international symposium on environment conscious materials and systems for sustainable development, Japan, 2004.  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