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 (

Hong Kong Polytechnic University demonstrated that nitric acid at 35% 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 (

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 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

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 (

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 233C until the day of testing. Cubes of 707070 mm in size were used for the determination of hardened density, water absorption and compressive strength. 2525285 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 200100 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.

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 (2525285 mm) of the block specimens were measured. After the initial reading, the specimens were conveyed to a drying chamber with a temperature of 23 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 80C distilled water for 24 h. The samples were then transferred and immersed in 1N sodium hydroxide (NaOH) solution at 80C 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-1m-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

)( 23

masurfacearehmeSamplingtiMWQ

removalNO NONO

=

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/m3 and C-PB (with recycled coarse aggregate) was between 2319 kg/m3 and 2585 kg/m3. 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/m3)

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 - 45.0

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%

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

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

(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

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.

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

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 (

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. References ASTM C 1260, 2007. Standard test method for potential alkali reactivity of aggregates (mortar-bar method). ASTM International, USA. ASTM C349, 2008. Standard test method for compressive strength of hydraulic-cement mortars. ASTM International, USA. ASTM C642, 2006. Standard test method for density, absorption, and voids in hardened concrete. ASTM International, USA. BS 6073-Part 1, 1981. Precast concrete masonry units, specification for precast concrete masonry units. British Standards Institution, UK. Castro, S.D., Brito, J.D., 2013. Evaluation of the durability of concrete made with crushed glass aggregates. Journal of Cleaner Production 41, 7-14. Chen, J., Poon, C.S., 2009. Photocatalytic activity of titanium dioxide modified concrete material- Influence of utilizing recycled glass cullets as aggregates. Journal of Environmental Management 90, 3436-3442. Chidiac, S.E., Mihaljevic, S.N., 2011. Performance of dry cast concrete blocks containing waste glass powder or polyethylene aggregates. Cement and Concrete Composites 33, 855-863. ETWB TCW No. 24, 2004. Specifications facilitating the use of concrete paving units made of recycled aggregates. Works Bureau Technical Circular, Hong Kong Special Administrative Region, China. Gou, M.Z., Ling, T.C., Poon, C.S., 2012. TiO2-based self-compacting glass mortar: Comparison of photocatalytic nitrogen oxide removal and bacteria inactivation. Building and Environment 53, 1-6. Gou, M.Z., Ling, T.C., Poon, C.S., 2013. Nano-TiO2-based architectural mortar for NO removal and bacteria inactivation: Influence of coating and weathering conditions. Cement and Concrete Composites 36, 101-108. JIS R 1701- 1, 2004. Fine ceramics (advanced ceramics, advanced technical ceramics)- test method for air purification performance of photocatalytic materials- Part I: Removal of nitric oxide. Japanese Industrial Standard, Japan. Lam, C.S., Poon, C.S., Chan, D., 2007. Enhancing the performance of pre-cast concrete blocks by incorporating waste glass- ASR consideration. Cement and Concrete Composites 29: 616- 625. Lee, C.H., Chang, C.T., Fan, K.S., Chang, T.C., 2004. An overview if recycling and treatment of scrap computer. Journal of Hazardous Materials 114 (1-3), 93-100.

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