Use of recycled CRT funnel glass as fine aggregate in dry-mixed concrete paving blocks.

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First author: [email protected]

Original citation: Ling, T.-C., Poon, C.-S. (2014) Use of recycled CRT funnel glass as fine aggregate in dry-mixed concrete

paving blocks. Journal of Cleaner Production; 68: 209-215.

http://www.sciencedirect.com/science/article/pii/S0959652614000092

Use of recycled CRT funnel glass as fine aggregate in dry-mixed concrete

paving blocks

Tung-Chai Ling1,2

, and Chi-Sun Poon1,

* 1Department of Civil and Environmental Engineering,

The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong. 2School of Civil Engineering,

University of Birmingham, Edgbaston, Birmingham, United Kingdom

Abstract

This paper investigates the feasibility of using recycled glass derived from discarded cathode ray

tube (CRT) glass as an alternate fine aggregate for the production of dry-mixed concrete paving

blocks. The recycled CRT funnel glass used had been acid treated and regarded as a non

hazardous material based on the regulatory thresholds of the Toxicity Characteristic Leaching

Procedure (TCLP). Two series of concrete paving blocks were prepared, one with and one

without the use of coarse aggregate. Additionally, TiO2, a photo-catalyst was added to the surface

layer of the blocks during the fabrication process to effect photo-catalytic reaction for the

removal of an air pollutant, nitrogen oxide (NO). For each series, the CRT glass was used to

replace the fine aggregate by volume at different ratios. Their physical, mechanical and

durability properties, lead leachability, and photocatalytic air purifier performance were studied.

The results show that the use of up to 100% CRT funnel glass as fine aggregate in concrete

paving blocks not only have satisfactory levels in compressive strength (>45MPa) and ASR

expansion (<0.1%), but improved the resistance to water absorption, drying shrinkage and

photocatalytic performance for reducing air pollutants. However, the TCLP results reveal that the

casting method of producing dry-mixed concrete blocks had a significant influence on lead

leaching, and the replacement ratio of the CRT glass should be limited to about 25%.

Keywords: CRT funnel glass, concrete paving blocks, properties, leaching of lead, dry-mix

casting method

1. Introduction

With the recent widespread use of liquid crystal displays (LCDs) television and monitors,

environmental concern related to disposal of discarded cathode ray tube (CRT) glass has

becoming progressively significant (Poon, 2008). CRT funnel glass contains 20-25 % of PbO is

regulated as hazardous waste, and its Pb content if leached to the environment can pose a threat

to human health (Lee et al., 2004). Therefore, finding alternative methods to manage the huge

amount of CRT glass waste is important. Recently, a three-step recycling method for CRT glass

waste which comprises crushing, acid washing and water rinsing to remove the lead has been

developed and implemented in Hong Kong (Ling and Poon, 2011a). Experimental studies at the

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Hong Kong Polytechnic University demonstrated that nitric acid at 3–5% concentration levels

can be used to remove most of the lead from the crushed funnel glass surface and rendered it as a

non-hazardous waste based on the toxicity characteristics leaching procedure (TCLP) testing.

The details of the recycling process and results have been reported in the literature (Ling and

Poon, 2012a).

Use of the treated CRT funnel glass as a substitute of natural aggregate in concrete blocks

fabrication could be a possible recycling option. As for non-toxic/non-hazardous glass waste

derived from beverage glass bottles, Turgut (2008) studied the properties of masonry blocks

produced with waste limestone sawdust and glass powder. It was found that glass powder had a

positive effect to the concrete structure and led to better compressive strength. Chidiac et al.

(2011) also stated that waste glass powder can be incorporated to produce a better quality dry-

cast concrete blocks at industrial settings. Poon and Lam (2008) found that smooth surface of

coarser glass cullets (<5mm) could weaken the bonding between the glass particle and cement

paste, and resulted in the loss of concrete strength. But the study focused in the possible use of a

hazardous glass material (crushed CRT funnel glass) for use in concrete blocks after an acid

treatment step.

A major concern for using recycled glass in concrete is the potential risk of expansion due to

alkali-silica reaction (ASR) (Park and Lee, 2004). This is because the rich-silica crystal structure

of glass is very reactive in an alkaline environment. Lam et al. (2007) found that without the use

of ASR suppressant it was not possible to produce a durable concrete paving block containing

100% recycled beverage glass. Works have proved that ASR can be suppressed by incorporating

pozzolan such as fly ash, granulated blast furnace slag etc. since the amount of alkali hydroxide

is limited by the pozzolanic reaction (Shi et al., 2007). Lee at al. (2011) found that concrete

blocks produced by a dry-mix casting method contained a relatively higher porosity were able to

accommodate more amount of ASR gel. The ASR expansion of dry-mixed concrete block

containing 100% glass aggregate was found to be 44% lower when compared to the concrete

produced by using a traditional wet-mix method.

The purpose of the paper is to investigate the feasibility of using recycled CRT funnel glass for

the production of concrete paving blocks, as it can positively promote green and environmental

sustainability. The recycled CRT funnel glass used had been treated and rendered as non-

hazardous material by an acid washing process. Two series of dry-mixed paving blocks were

prepared, one with and one without the inclusion of coarse aggregate. In both series, the effects

of CRT funnel glass content (0%, 50% and 100% was used to replace equal volume of recycled

fine aggregate) on the properties of concrete paving blocks were investigated. Additionally,

methods that can reduce the potential leachability of lead from the CRT glass were also assessed.

2. Experimental procedures

2.1. Materials

ASTM type I ordinary Portland cement (OPC) and fly ash (PFA) complying with ASTM class F

were used as cementitious materials for the production of the concrete paving blocks. The

chemical compositions and physical properties are shown in Table 1.

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Table 1: Chemical compositions and physical properties of cement and fly ash

Chemical compositions (%) OPC PFA * Class F

Fly ash

**CRT funnel

glass

Calcium oxide (CaO) 63.15 <3 - 3.74

Silicon dioxide (SiO2) 19.61 56.79 - 50.50

Aluminium oxide (Al2O3) 7.33 28.21 - 2.40

Ferric oxide (Fe2O3) 3.32 5.31 - -

(SiO2 +Al2O3+ Fe2O3) 30.26 92.32 >70 -

Magnesium oxide (MgO) 2.54 5.21 - 1.17

Sodium oxide (Na2O) 0.13 0.45 - 3.00

Potassium oxide (K2O) 0.39 1.34 - 9.27

Sulfur trioxide (SO3) 2.13 0.68 <5.0 -

Lead oxide (PbO) - - - 24.50

Barium oxide (BaO) - - - 2.90

Strontium oxide (SrO) - - - 2.10

Zirconium Oxide (ZrO2) - - - 0.41

Loss on ignition 2.97 3.9 <6.0 -

Physical properties

Specific gravity 3.16 2.31 - -

Blaine fineness (cm2/g) 3519 3960 - -

* The chemical composition of the Class F Fly ash is according to the ASTM C618.

**The chemical composition of CRT funnel glass powder was measured using X-Ray

Fluorescence (XRF) method.

Coarse and fine aggregates used in this study were recycled concrete aggregate obtained from a

construction and demolition (C&D) waste recycling facility in Hong Kong. The particle size

range of the recycled coarse aggregate (RCA) and the recycled fine aggregate (RFA) were 5-10

mm and 0-5 mm (see Table 2). The recycled CRT funnel glass was obtained from a CRT waste

recycling center in Hong Kong, in which the CRT funnel glass was crushed, acid treated and

water washed. The treated CRT funnel glass with leachable Pb value of 2.2 mg/L was rendered

as a non hazardous material based on the regulatory thresholds of the Toxicity Characteristic

Leaching Procedure (TCLP). The particle size distribution and other physical properties of these

aggregates are shown in Table 2. The particle size distribution expressed as a percent passing (%)

is the percentage of total aggregate by weight passing through a given sieve size.

A commercially available nano-TiO2 powder (P25, Degussa) was used as the photocatalyst. The

particle size of the TiO2 was 20-50 nm, with a specific BET surface area of 50 ±15 m2 g

-1.

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Table 2: Particle size distributions and physical properties of RCA, RFA and CRT funnel glass

Sieve size and physical

properties

Percentage passing (%)

RCA (5-10mm) RFA (< 5mm) CRT (<5 mm)

10 mm 100 100 100

5 mm 19.6 99.6 99.6

2.36 mm 0.86 72.0 77.7

1.18 mm 0.80 44.8 48.6

0.6 mm 0.76 26.3 24.5

0.3 mm 0.73 15.7 8.3

0.15 mm 0.63 7.2 1.5

Fineness modulus 5.77 3.40 3.34

Relative density 2.47 2.53 3.00

Water absorption (%) 4.37 5.80 0

2.2. Mix proportion of paving blocks

To achieve required propertied of standard concrete paving blocks, the control concrete mixes

were designed with a fixed OPC content of 340 kg/m3, aggregate-to-cement ratio of 4:1 and

water-to-cement ratio (w/c) of 0.3. Two series of paving blocks were prepared in this study, i)

paving blocks without RCA and ii) paving blocks with RCA. For the paving blocks with RCA,

25% of RFA was replaced by RCA weight. For each concrete series, the crushed and processed

CRT funnel glass was used to replace RFA by volume at 0%, 50% and 100%. Table 3 shows the

materials composition (in kg/m3) of the 6 different concrete mixes prepared in this investigation.

Table 3: Mix proportions of paving block with and without recycled coarse aggregate (kg/m3)

Mix

notation

Material compositions

OPC PFA RCA RFA CRT Water w/c

PB0 340 113 0 1811 0 136 0.3

PB50 340 113 0 906 1074 136 0.3

PB100 340 113 0 0 2148 136 0.3

C-PB0 340 113 453 1358 0 136 0.3

C-PB50 340 113 453 679 805 136 0.3

C-PB100 340 113 453 0 1611 136 0.3 Note: the notation of paving blocks without coarse aggregate is expressed as PB;

the notation of paving blocks with coarse aggregate is expressed as C-PB.

Separately, to study the effect using the crushed CRT funnel glass on the photocatalytic air

purifying performance, an additional of 5% nano-TiO2 by cementitious materials weight was

added to the mixture. Only the most upper layer of the concrete blocks (5mm thick concrete

surface layer) was incorporated with the TiO2 because in actual application the air pollutant (e.g.

NO) only comes in contact with the surface of the blocks. Due to the thickness limitation, the

maximum particle sizes of RFA and CRT glass used to cast the concrete surface layers was

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below 2.36 mm. The mix proportions of the photocatalytic surface layer are shown in Table 4.

Table 4: Mix proportions of concrete surface layers (kg/m3)

Mix notation Material compositions

OPC PFA RFA CRT Water TiO2

PSL0 340 113 1811 0 136 22.7

PSL50 340 113 906 1074 136 22.7

PSL100 340 113 0 2148 136 22.7

Note: the notation of photocatalytic surface layer is expressed as PSL.

2.3. Paving block specimen preparation

All paving block specimens were fabricated in the laboratory using a dry-mix method (Xiao et al.,

2011). Cementitious materials and aggregates were first mixed in a pan mixer for approximately

3 min. Water was added slowly into the bowl of the mixer and the mixture was further mixed for

another 3 min. The fresh concrete (with zero slump value) was placed into steel moulds in three

layers of about equal thickness. After each of the first two layers was laid, compaction was

applied manually by hammering a wooden plank on the surface layer to provide an evenly

distributed compaction. The last layer was prepared by slightly overfilling the top of the mould

(approximately 5 mm) and the overfilled materials were subjected to a static compaction twice

by using a compression machine. A compression force of 25 N/mm2 was applied for the first

compaction. After removing the excessive materials with a trowel, a second compaction force of

30 N/mm2 was applied and the specimen was left in a laboratory environment for the first 24 h.

After one day, the specimens were demoulded and cured in water at an average temperature of

23±3°C until the day of testing.

Cubes of 70×70×70 mm in size were used for the determination of hardened density, water

absorption and compressive strength. 25×25×285 mm prisms were used for measuring the

dimension change of dry shrinkage and the expansion due to ASR. 5 mm think concrete layers

with a surface area size of 200×100 mm were prepared for the photocatalytic reaction test.

2.4. Test methods

2.4.1. Hardened density

The hardened densities of the block specimens were determined by using a water displacement

method according to ASTM C 642 (2006) for hardened concrete. The reported results were the

average values of three specimens.

2.4.2. Water absorption

The cold water absorption values of the block specimens were determined in accordance with

ASTM C 642 (2006). The reported results were the average values of three specimens.

2.4.3. Compressive strength

According to ASTM C 349 (2008), the compressive strength was determined by using a

universal testing machine with maximum a load capacity of 3000 kN. The loading was applied to

the nominal area of the block specimen. Three samples for each mix were tested at 7, 28 and 56

days.

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2.4.4. Dry shrinkage

The dry shrinkage of the concrete block specimens was determined according to BS 6073 (1981).

After 28 days of room temperature curing, the initial lengths (25×25×285 mm) of the block

specimens were measured. After the initial reading, the specimens were conveyed to a drying

chamber with a temperature of 23°C and with a relatively humidity of 55% until further

measurement at 14th

days.

2.4.5. Expansion due to the alkali-silica reaction (ASR)

An accelerated ASR test was carried out in accordance with ASTM C 1260 (2007). A zero

reading was taken after storing the prism samples in 80°C distilled water for 24 h. The samples

were then transferred and immersed in 1N sodium hydroxide (NaOH) solution at 80°C until

testing time. The measurements of ASR expansion were taken at 14th

days according to the

ASTM C1260. Each value represents the average of three specimens.

2.4.6. Toxic characteristic leaching procedure (TCLP)

The leachability of lead, as the main concern of the feasible use of CRT glass in concrete blocks,

was determined by using TCLP method according to the U.S. Environmental Protection Agency

method 1311 (2011). The samples were taken from the broken pieces after completing the

compressive strength testing at 28th

day and were crushed to pass through a 10 mm sieve. 20 g of

crushed sample was put into 400 ml of the TCLP leachant (prepared by diluting 5.7 mL of glacial

acetic acid in 2 L of distilled water) for tumbling 18 h in a rotary mixer. The lead concentration

in the leachate was then determined using an inductively coupled plasma-optical emission

spectral photometer (ICP). Each value represents the average of six samples.

2.4.7. Photocatalytic removal of nitrogen oxide (NO)

The influence of CRT glass content on the degradation of NO by the photo-catalytic reaction was

investigated. The experimental set up used has been described previously (Guo et al., 2013) and

it was carried out in a reactor with the flow of the testing gas (1000 ppb NO) was adjusted by

two flow controllers to a rate of 3 L/min. The UV lamp (intensity 10 W/m2) was positioned 100

mm above the concrete surface layer. Prior to all photocatalytic conversion processes, the testing

gas stream was introduced to the reactor with the absence of UV radiation for at least half an

hour to obtain a desired RH as well as gas-solid adsorption-desorption equilibrium. Then the UV

lamp was turned on for the photocatalytic process to begin. For each sample, the NO removal

test lasted for 1 h and the concentration changes of NO at the outlet were monitored continuously

by a Chemilluminescence NO analyzer (42C, Thermo Environmental Instruments Inc.). The

calculation of the amount of NO removal follows the instruction in the JIS R 1701-1 (2004). The

specific NO removal in unit of mgh-1

m-2

is calculated by using the following formula:

Where

QNO: The amount of nitrogen monoxide removed by the test sample (mol)

MWNO: The molecular weight of NO

3. Results and discussion

The hardened density, water absorption, compressive strength, dry shrinkage, expansion due to

)()(

10)(

2

3

masurfacearehmeSamplingti

MWQremovalNO NONO

×

××=

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ASR and lead leaching test results of all paving blocks with and without RCA are shown in Table

5. Each of the test results are discussed in details in the following sections.

Table 5: Test results of block specimens

*SD – Standard deviation

3.1. Hardened density

Fig.1 shows the hardened density of the paving block specimens (PB and C-PB series) with

different CRT glass replacement contents. The hardened density of PB (without recycled coarse

aggregate) was between 2294 kg/m3 to 2595 kg/m

3 and C-PB (with recycled coarse aggregate)

was between 2319 kg/m3 and 2585 kg/m

3. For both series of paving blocks, the density increased

with an increase of CRT glass content. This phenomenon is probably related to the high specific

gravity of CRT funnel glass due to the presence of lead compounds within the glass (Ling et al.,

2013).

At 0% and 50% CRT glass content, C-PB sample showed higher density than that of PB sample.

This could be due to the fact that C-PB series with coarse aggregate could provide a better

packing density (continuous grading pattern) than that of PB series which only contained a single

sized fine aggregate fraction (< 5 mm). However, when the CRT glass content was increased to

100%, it appears that the density effect (high specific gravity of CRT glass) had outweighed the

packing density effect rendering a higher density in the PB100 mix.

3.2. Water absorption

Fig. 2 shows the water absorption of the paving block specimens with different CRT glass

contents. The control mixes without CRT glass showed the highest water absorption values, 8.0

% for PB0 and 6.7 % for C-PB0. It is clear that the water absorption of paving blocks was

reduced by increasing the CRT glass content due to impermeable properties of the glass (Ling

and Poon, 2011b).

Mix

notation Density (kg/m

3)

Water absorption (%)

28-day compressive strength (MPa)

14-day dry shrinkage (%)

14-day ASR expansion (%)

Lead

content (mg/L)

PB0 2294 7.8 64.9 0.060 0.001 0.21 *SD 6.85 0.14 0.46 0.002 0.0006 0.029

PB50 2460 5.6 45.4 0.057 0.005 7.69 SD 6.95 0.35 2.94 0.002 0.0021 0.063

PB100 2595 3.4 45.0 0.032 0.056 10.47 SD 10.60 0.29 4.60 0.002 0.0025 0.053

C-PB0 2319 6.7 68.6 0.056 0.015 0.17 SD 5.93 0.26 0.97 0.002 0.0009 0.029

C-PB50 2465 4.4 68.3 0.037 0.025 3.40 SD 16.59 0.32 1.08 0.003 0.0020 0.032

C-PB100 2585 3.0 55.6 0.025 0.042 9.77 SD 11.08 0.09 0.10 0.001 0.0021 0.018

Standard

limit - <6.0 >45.0 <0.060 <0.100 <5.00

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Fig. 1: Density of PB and C-PB with different CRT glass contents

Fig. 2: Water absorption of PB and C-PB with different CRT glass contents

For a same CRT glass content, the water absorption of the paving blocks with coarse aggregate

(C-PB series) was lower than that of the paving blocks without coarse aggregate (PB series).

This may be attributed to better packing density and the lower water absorption capacity of RCA.

As prescribed by ETWB of Hong Kong (2004) for Grade A paving block for pedestrian areas,

Standard Limit of 6%

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only the control mixes (PB0 and C-PB0) exceeded the requirement of 6.0%.

3.3. Compressive strength

The results of the 7-day, 28-day and 56-day compressive strength of PB and C-PB mixes with

different CRT glass contents are shown in Figs. 3 and 4. It can be observed that for a given glass

content, the compressive strength of C-PB was slightly higher than that of PB mix. This result is

in line with the previous density and water absorption results. The higher density and lower

absorption specimens gave rise to higher compressive strength results.

In Table 5, the 28-day compressive strength of the blocks in the PB series was ranged from 45.0

to 64.9 MPa, while the C-PB series was varied from 55.6 to 68.6 MPa. It should be emphasised

that in general, the percentage of strength gain at 7 days was more than 80% of their

corresponding 28-day compressive strength, which reveals that including CRT funnel glass did

not delay the hardening process. The figures also show that the percentage of strength increase at

56 days for the concrete blocks with CRT glass are slightly higher than the control blocks. The

strength improvement at this stage could be partly attributed to the pozzolanic reaction of the

very fine glass particles in CRT glass, and this was also observed previously (Lee et al., 2013).

Fig. 3: Compressive strength development of PB series with different CRT glass contents

Standard Limit of 45 MPa

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Fig. 4: Compressive strength development of C-PB series with different CRT glass contents

Fig. 5: Microstructure of (a) C-PB0 and (b) C-PB100

(a)

(b)

Standard Limit of 45 MPa

CRT glass Cement paste

Weak ITZ

Loss of bonding due

to smooth surface of

glass particle

Micro-crack

Micro-crack

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(28 day crushed samples; magnified ×30 and ×1,000)

Observing the influence of glass content, the compressive strength became lower with increasing

CRT glass content, probably due to the weak bonding between the glass aggregate and the

cement paste (Ling et al., 2011). Similar results were also reported elsewhere (Castro and de

Brito, 2013). This is supported by SEM observations, as denser structures are present on the

surface of C-PB0 (see Fig 5(a)); whereas a smooth surface of CRT glass can be clearly seen in

Fig. 5(b), which led to a weaker interface and reduced the mechanical strength of the concrete

mixes. However, all the 28-day compressive strength results fulfilled the minimum strength (45

MPa) requirement, as prescribed by ETWB of Hong Kong (2004) for Grade A and B paving

block for pedestrian and trafficked areas.

3.4. Dry shrinkage

The dry shrinkage values (measured at 14 days) of the samples are shown in Fig. 6. The results

show the drying shrinkage values decreased with increasing percentage of CRT glass in the

mixes. In fact, for the mixes prepared with 100% CRT glass replacing RFA, the drying shrinkage

values were reduced by more than 46% and 56% as compared to their respective control mixes

of PB0 and C-PB0. This occurs because the CRT glass decreased the total water content and

thereby reduced the shrinkage.

Fig. 6: 14-day dry shrinkage of paving blocks with and without RCA

3.5. Expansion due to the alkali-silica reaction (ASR)

The expansion due to ASR measured at 14 days is shown in Fig. 7, where the expansion

increased with increasing CRT glass content. It should be noted that the ASR expansion of C-

PB100 was slightly lower than that of PB100, probably due to the lower amount of CRT glass in

the mix (because in the C-PB series, 25% of the total aggregate was RCA). All the expansion

values were still within the prescribed limit of 0.1% based on ASTM C1260 (2007). This is

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because the alkalis in the concrete was partly consumed by the pozzolanic reaction of fly ash

which led to lowering in ASR reactivity of the glass aggregate (Ling and Poon, 2012b). The

results also indicate that 25% replacement of cement with fly ash is usually sufficient for

controlling the deleterious expansion in concrete glass blocks. Similar results were also reported

in Lee et al. (2011) who stated that the dry-mixed concrete blocks having larger pores can

accommodate the formation of certain amount of ASR gel resulting in lower expansion.

Fig. 7: 14-day ASR expansions of paving blocks with and without RCA

3.6. Lead leaching

The leaching of lead from the PB and C-PB samples containing CRT glass was assessed by the

toxicity characteristic leaching procedure (TCLP). Fig. 8 shows the lead leaching was

significantly increased with the incorporation of CRT glass in the concrete mixes, and the

leaching value of PB series were slightly higher than that of C-PB series. As it can be seen only

control mixes and C-PB50 mix satisfied the TCLP limit of 5 mg/L despite previous TCLP test

conducted on the original processed CRT glass showed much lower leachable concentrations

(see Table 2). This perhaps due to the fragmentation of the CRT glass (breakage of glass) by

vigorous manual compaction applied during casting, and thus increased leachability of lead from

the broken glass.

In order to further assess effect of different level of CRT glass on the leaching of lead from the

paving blocks, an additional mix was prepared using 25% CRT glass in PB and C-PB. As

expected, the leaching of lead of the block samples were reduced to 1.42 mg/L and 0.86 mg/L

respectively, as shown in Table 6. This is consistent with the previous studies (Ling and Poon,

2012c) indicated that the alkaline environment of cement hydration was able to stabilize and

prevent the lead leaching to some extent.

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Fig. 8: TCLP results of PB and C-PB with different CRT glass contents

Table 6: TCLP result of PB and C-PB with 25% of CRT glass

CRT content Pb concentration (mg/L)

PB C-PB

25% 1.42 0.86

Standard deviation 0.034 0.055

TCLP limit <5.0

Previous studies (Ling and Poon, 2013) showed it was feasible to use 100% processed CRT

funnel glass as fine aggregates in wet-mixed cement mortar or concrete and the leaching of lead

satisfied the TCLP standard. In the wet-mixed cement mortar of concrete, manual compaction by

hammering with a wooden plank was not necessary. Therefore, it was expected that a higher

percent CRT glass could be used in the concrete mixes of the dry-mixed concrete products, if the

manual compaction could be eliminated. As such, further trials were conducted to cast concrete

blocks with 100% CRT glass replacing RFA by using only the compression machine as the

mechanical means of compaction. A comparison of the TCLP results for PB100 and C-PB100

produced by both (manual+mechanical) and (mechanical only) casting methods are shown in

Table 7. It is clearly shown that lead leaching could be reduced by about 60% from both the PB

and C-PB blocks by eliminating the manual compaction process.

But in actual factory production of paving blocks using mechanical compaction, a high

frequency of vibration and compaction forces are used. Therefore, it is suggest that a pilot plant-

scale production should be carried out before the CRT concrete paving blocks with satisfactory

leaching characteristics can be safely produced.

TCLP limit of 5 mg/L

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Table 7: TCLP results of the PB100 and C-PB100 produced by two casting methods

3.7. Photocatalytic degradation of nitrogen oxide (NO)

The 5mm thick concrete surface layers prepared with TiO2- intermixed cement mortar was used

for assessing pollutant removal performance. Fig. 9 illustrates the NO removal results of

photocatalytic surface layers containing different CRT glass contents. The NO removal rates for

surface layers with 50% and 100% CRT glass were enhanced by 5% and 7%, respectively as

compared to the control mix (without glass). This is consistent with the results of previous

studies (Chen and Poon, 2009; Guo et al., 2012) which showed the incorporation of crushed

glass cullet in the surface layer improved the photo-catalytic activity. But in comparison, the

enhancement effect of using transparent recycled beverage glass was higher than that of CRT

glass. This is probably due to the lead content and the darker colour CRT glass decreased the

penetration of UV light to the photo-catalysis.

Fig. 9: NO removal of concrete surface layers with different percentages of CRT glass

replacements.

4. Conclusion

In this study, the feasibility of using CRT funnel glass as a fine aggregate for the production of

concrete paving blocks has been demonstrated. The results show that the use of 100% CRT

funnel glass as the fine aggregate in concrete paving blocks not only had satisfactory levels in

compressive strength (>45MPa) and ASR expansion (<0.1%), but also improved resistance to

Mix notation Pb concentration (mg/L)

(Manual + Mechanical) (Mechanical only)

PB100 10.47 3.81


C-PB100 9.77 3.79


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water absorption, drying shrinkage. The photocatalytic performance for reducing air pollutants

was also improved. To limit the possible leaching of lead, it is recommended to prepare the

concrete blocks with < 25% of CRT glass. But the experimental results also demonstrated that a

higher percentage of CRT glass (up to 100%) can be incorporated in the blocks if alternate block

forming and compaction methods are used to reduce the fragmentation of the incorporated CRT

glass. To facilitate large scale applications, further pilot-plant studies are needed to ascertain the

environmental performance of the CRT glass concrete blocks produced at an industrial setting

(using a vibration and compaction casting method).

Acknowledgment

The authors would like to thank the Environment and Conservation Fund and the Woo Wheelock

Greed Fund, and The Hong Kong Polytechnic University for funding support.

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Use of recycled CRT funnel glass as fine aggregate in dry-mixed concrete paving blocks.

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