Green Paving nich....kyknya seru kl diaplikasi di perkotaan

1. 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 2006
Abstract
This paper presents the results of a study on the effectiveness of incorporating air cleaning agents such as titanium dioxide (TiO2) into
the technique of producing concrete paving blocks, using local waste materials to remove nitrous oxide (NO). Factors which would affect
the performance of the blocks were studied including the porosity of blocks, the type of waste materials used within the mix design, the
types 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 was
increased 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 removal
ability due to its light transmitting characteristic. Three types of TiO2 were tested in this study and their influence on NO removal was
quantified. Based on the experimental results, an optimum mix design was selected which incorporates recycled glass, sand, cement and
TiO2.
Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: Photocatalyst; Titanium dioxide; Nitrogen oxides; Recycled aggregates
1. Introduction lion people [3]. 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 vehicles
erator in Hong Kong [1]. The extensive building and infra- at the street level. It is apparent that there is a need to
structure 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 these
demolition 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 to
severe social and environmental problem in the territory. lower these emissions by using cleaner vehicles, it appears
Up 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 already
recycle aggregates derived from C&D waste [2]. been tested in Japan for concrete paving materials that
Additionally, Hong Kong also faces a growing concern can facilitate a photocatalytic reaction converting the more
of air pollution due to having to provide habitats and toxic forms of air pollutants to less toxic forms (e.g. NOX
transportation 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: cecspoon@polyu.edu.hk (C.S. Poon). as well as self cleaning, antifogging, and antibacterial
0950-0618/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.conbuildmat.2006.05.018

2. 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 TiO2
actions [8–11]. Practical applications of photocatalysts contained only the anatase form and the rutile form of
have 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 that
realized when applied to building or road construction falls to the bottom of the furnace during the burning of
materials in large cities where urban air pollution is very coal. Chemically, it is very similar to pulverized fly ash
serious [11]. 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 Hong
objective of this study is to analyze the effectiveness of Kong, the produced FBA is currently dumped at an ash
incorporating air cleaning agents such as TiO2 into the lagoon as waste. FBA used in this study was obtained from
technique 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 was
base layer made from cement and recycled aggregates, used for making the surface layer.
and a thin surface layer made of cement, various aggregate
materials and a small amount of titanium dioxide. The
design of this photocatalytic block is shown in Fig. 1. In 16
this paper, the focus is on optimizing the surface layer P25 Rutile Anatase
14
design of the block. To achieve this, surface layers were
produced 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 P25
2. Experimental details A
R R
R A A A A R A
2.1. Materials R R
Intensity (a.u.)
The cementitious material used in this study was an
Ordinary Portland Cement (OPC) commercially available
in Hong Kong, complying with BS 12 [12] and ASTM Type
I [13].
Three sources of titanium dioxide (TiO2) were used. The
first was P-25 sourced from Degussa, which was used due
to 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, hence
would be useful for comparison with works of others. The Fig. 3. XRD spectrum of TiO2 powders (A: anatase, R: rutile).

3. 1748 C.S. Poon, E. Cheung / Construction and Building Materials 21 (2007) 1746–1753
Table 1 Table 3
Properties of TiO2 Mixes prepared with RA, FBA and sand
Properties P25 Anatase Rutile Mix notation Relative proportions (by weight)
Moisture (%) 1.5 0.04 0.46 Cement RA Sand FBA P25 TiO2 Water
Water solubility (%) – 0.05 0.03
R1:2 1 2 – – 0.06 0.28
Ignition loss (%) 2.0 0.01 0.17 R1:2.5 1 2.5 – – 0.07 0.30
pH 3.0–4.0 7.5 6.7
R1:3 1 3 – – 0.08 0.32
Oil adsorption (g/g) – 22/100 23/100
R*1:2 1 2 – – 0.06 0.28
Color eliminating capacity (per min) – 100 100
R*1:2.5 1 2.5 – – 0.07 0.30
Sieve residue, Mocker 45 (%) 0.05 0.05 0.05
R*1:3 1 3 – – 0.08 0.32
TiO2 (%) 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 0.24
crushed CD waste sourced from a temporary recycling S*1:2.5 1 – 2.5 – 0.07 0.26
facility 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 0.32
went a process of mechanized sorting, crushing and sieving RF1:2.5 1 1.88 – 0.63 0.07 0.34
to produce both fine and coarse aggregates according to RF1:3 1 2.25 – 0.75 0.08 0.36
the particle size requirements of BS 812 [16]. Only the smal- RF*1:2 1 1.5 – 0.50 0.06 0.32
ler fine aggregate proportion was used for making the sur- RF*1:2.5 1 1.88 – 0.63 0.07 0.34
face 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 0.28
the recycled fine aggregate used was 2.36 mm. The proper- SF1:2.5 1 – 1.88 0.63 0.07 0.30
ties 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 0.28
post-consumer beverage bottles sourced locally. The glass SF*1:2.5 1 – 1.88 0.63 0.07 0.30
bottles were washed and crushed by mechanical equipment. SF*1:3 1 – 2.25 0.75 0.08 0.32
The RG was sieved in the laboratory to pass though the
2.36 mm sieve. The properties of the RG are shown in
Table 2.
Table 4
The sand used was fine natural river sand commercially Mixes prepared with recycled glass
available in Hong Kong. The properties of sand are shown
Mix notation Relative proportions (by weight)
in Table 2.
Cement RG Sand P25 TiO2 Water
2.2. 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 0.28
2.2.1. 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 0.28
series of mixes (as shown in Table 3) were prepared to find
out the effects of different materials and proportions on NO
removal 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 of
Most of the mixes were prepared using aggregate sizes from TiO2
0 to 2.36 mm. But selected mixes (those identified by ‘*’) The effects of varying the amount and types TiO2 were
were 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 was
thought to benefit NO photodegradation when used in
Table 5
the mix design of the surface layers. Hence mix proportions Mixes prepared with varying TiO2 content
prepared with recycled glass were designed (Table 4).
Mix notation (%) Relative proportions (by weight)
Cement Glass Sand P25 TiO2 Water
Table 2 0 1 1.5 1.5 0 0.26
Properties 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 0.40
Density (kg/m3) 2093 2531 2651 8 1 1.5 1.5 0.32 0.48
Water absorption (%) 10.28 0 0.87 10 1 1.5 1.5 0.40 0.64

4. C.S. Poon, E. Cheung / Construction and Building Materials 21 (2007) 1746–1753 1749
Table 6
Mixes prepared with different sources of TiO2
Mix notation Relative proportions (by weight)
Cement Glass Sand P-25 TiO2 Anatase TiO2 Rutile TiO2 Water
P-25 1 1.5 1.5 0.08 – – 0.26
Anatase 1 1.5 1.5 – 0.08 – 0.26
Rutile 1 1.5 1.5 – – 0.08 0.26
2.3. 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 NO
layers were fabricated in steel moulds with internal dimen-
sions of 200 · 100 · 5 mm. The wet mixed materials 2.5.1. Equipment
weighed between 220 and 280 g for each surface layer The testing equipment used was a self-designed flow
depending on the different materials. The steel moulds were reactor adapted from an existing design [17], but with alter-
over filled by hand compaction, and then further com- ations. The reactor provides a physical boundary to enable
pressed 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 chamber
their moulds and cured in a chamber with a controlled reactor were 700 mm in length, 400 mm in width and
humidity of 75% and temperature of 25 °C until testing. 130 mm in height. The reactor consists of a sampling inlet
The surface layers were tested for NO photodegradation and outlet. Two 10 W UV-A fluorescent lamps (black
at 28, 56 and 90 days. lights), which emit primary UV light wavelengths at
365 nm were used to provide photoirradiation to activate
2.4. Determination of physical properties the photocatalyst. The intensity was measured using a
UV meter (Spectroline DRC-100X) to be 10 W mÀ2 at
2.4.1. 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 outside
approximately 10 mm diameter sizes and then oven dried the reactor and the distance from the reactor was adjusted
at 105 °C for 24 h. Hundred grams of the prepared speci- till the required intensity was achieved. The design enables
mens were soaked in acetone in a sealed container for the reactor to be used as a continuous flow reactor or a
24 h. The specimens were then removed from the acetone batch flow reactor. The reactor needed to be constructed
and the specimen surfaces were dried by an acetone wetted with materials of low adsorption ability and resistance to
tissue 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.

5. 1750 C.S. Poon, E. Cheung / Construction and Building Materials 21 (2007) 1746–1753
stainless 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 slightly
mens were placed inside the reactor. Fig. 4 further illus- increased when FBA was included in the mix design. This
trates the equipment setup described. is believed to be due to the higher porosity of FBA particles
which was exemplified by its relatively low specific density
2.5.2. 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 NO
Inc. Model 111). The standard gas was obtained from a photodegradation
compressed gas cylinder with nitrogen as the balanced With different cement to aggregate ratios, the results
gas (NIST certified). A humidity of 10% was achieved for (Fig. 6) show that the NO removal increased when the
the reactor by passing the reactant stream and the zero cement content decreased. An increase in NO removal of
air stream directly through the reactor. It is possible to approximately 30% was experienced for specimens when
achieve 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 cement
was kept at 10% as at this humidity level observation pat- content was a result of the fine particle size of cement
terns can be more easily interpreted. Also for comparison grains and hydrated cement particles could easily fill up
purposes this is believed to be reasonable. The gas streams the voids within the specimens, the surface area available
were then adjusted by the flow controllers to achieve an ini- for pollutants was reduced. Although reducing the cement
tial NO concentration of 1000 ppb and a flowrate of content was favourable towards NO removal, the adopted
6 L minÀ1, these testing conditions in a similar set-up have cement content of the surface layers should also consider
also been used by Yu [18] and are believed to be the most the necessary mechanical strength required for paving
ideal from trial. After the inlet and the outlet NO concen- applications. Preliminary studies showed that a minimum
trations 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 [19].
centration was continuously measured by the NO analyzer
(Thermo Environmental Instruments Inc. Model 42c). To 3.1.3. Effect of curing age on NO photodegradation
complete the experimental procedure the lamps were then All specimens prepared were tested at the curing ages of
turned 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 by
3.1. 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 pores
3.1.1. 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. [20] inves-
indicate that the RA mixes achieved a much higher NO tigated the possibility of using photocatalyst modified
removal 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 ratio
Fig. 5. Comparison of different materials to remove NO at 90 days Fig. 6. A comparison of NO removal for mixes prepared with different
testing. cement contents at 90 days testing (Table 3).

6. 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 specimen
Fig. 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 decreased
during the ageing of the hardened cement pastes. 3.2. Effect of incorporating recycled crushed glass cullet
3.1.4. 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 high
was 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 the
change the porosity of the specimens. The specimens were TiO2 on the surface as well as within the surface layer. This
divided into to two groups, one was prepared with all was also supported by Murata et al. [6] when they utilized
aggregate 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 NO
included. Fig. 8 shows the specimens tested at 90 days with removal increased with increasing amounts of recycled
a cement to aggregate ratio of 1:3. The results indicate that glass. As alkali aggregate reaction has been recognized as
the 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 was
achieved 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 above
were all associated with porosity. Fig. 9 shows the mea- The results as shown in Fig. 12 indicate that there was a
sured porosity compared with NO removal of some of significant increase in NO removal with an increase in TiO2
the selected mixes. An obvious trend can be observed content. At 90 days, NO removal increased from
showing 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 photodegradation
Fig. 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).

7. 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 Anatase
Fig. 11. NO removal of mixes with different recycled glass contents at
Fig. 13. NO removal of specimens containing different sources of TiO2
90 days curing age.
(Table 6).
4.5 [22]. In addition, Deng et al. [23] 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. [24] 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 included
4.01 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 and
increases as the TiO2 content increases up to 10% by weight hydroxyl groups [25]. It seems that the larger P-25 TiO2
of the whole mix, the effectiveness of the use of higher TiO2 particles were more beneficial to the photocatalytic activity
content on the NO removal needs to be verified by future in the current study. Small TiO2 particles have often been
studies. 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 such
produced in which the only variable was the source of as the cement in the production of the surface layers which
TiO2 (Table 6). The results as shown in Fig. 13 indicate covered the TiO2 particles. Further research is required to
that, 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 of
TiO2. Indeed, the NO removal for P-25 and the rutile form 4. Conclusion
of TiO2 was very close. At 90 days, the NO removal of P-25
was 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 surface
tase 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 surface
ically 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 the
more photoactive [21]. But it has also been reported in a surface layer was affected by the type of materials with
previous study that the rutile form can perform more which they were prepared. Materials with a lower density
actively or as well as the anatase form in certain situation. led to a higher porosity of the blocks. The particle size

8. C.S. Poon, E. Cheung / Construction and Building Materials 21 (2007) 1746–1753 1753
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