Artificial aggregate made from waste stone sludge and waste silt

Journal of Environmental Management 91 (2010) 2289e2294 Contents lists available at ScienceDirect Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman Artificial aggregate made from waste stone sludge and waste silt Fang-Chih Chang a, *, Ming-Yu Lee b, Shang-Lien Lo c, Jyh-Dong Lin d a The Instrument Center, National Cheng Kung University, No.1, University Road, Tainan City 70101, Taiwan, ROC Yien-Gu Co., LTD., Taipei 235, Taiwan, ROC c Research Center for Environmental Pollution Prevention and Control Technology, Graduate Institute of Environmental Engineering, National Taiwan University, 71 Chou-Shan Road, Taipei 106, Taiwan, ROC d Department of Civil Engineering, National Central University, No. 300, Jhongda Rd., Jhongli City, Taoyuan County 32001, Taiwan, ROC b a r t i c l e i n f o a b s t r a c t Article history: Received 2 November 2009 Received in revised form 18 May 2010 Accepted 11 June 2010 In this research, waste stone sludge obtained from slab stone processing and waste silt from aggregate washing plants were recycled to manufacture artificial aggregate. Fine-powdered stone sludge was mixed with waste silt of larger particle size; vibratory compaction was applied for good water permeability, resulting in a smaller amount of solidifying agent being used. For the densified packing used in this study, the mix proportion of waste stone sludge to waste silt was 35:50, which produced artificial aggregate of more compact structure with water absorption rate below 0.1%. In addition, applying vibratory compaction of 33.3 Hz to the artificial aggregate and curing for 28 days doubled the compressive strength to above 29.4 MPa. Hence, recycling of waste stone sludge and waste silt for the production of artificial aggregate not only offers a feasible substitute for sand and stone, but also an ecological alternative to waste management of sludge and silt. Ó 2010 Elsevier Ltd. All rights reserved. Keywords: Slab stone processing sludge Waste silt Artificial aggregate Vibratory compaction Resource recycling 1. Introduction Stone slabs with natural lines and lustrous polished finish as well as properties of hardness and fire resistance, make them ideal construction materials. They have been widely used for floor and wall paving in modern architecture. However, in the processing of construction slabs, which involves cutting, grinding, and polishing, about 25% of the raw stone will be turned into waste stone sludge. Containing fine particles of small diameter, the waste stone sludge has extremely good water permeability of less than 10 7 cm/s and low dehydration rate. Without treatment or recycling, the waste stone sludge will cause environmental pollution (Huang, 1998). Recycling attempts sintering sewage sludge ash into bricks and tiles have been made (Wiebusch and Seyfried, 1997). However, the many pores inside the bricks and tiles result in low compressive strength. Lightweight artificial aggregate has also been produced from sintered sludge (Cheeseman and Virdi, 2005; Tay and Show, 1997; Wainwright and Cresswell, 2001). The variations in chemical composition of the waste sludge cause quality control problems, not to mention the large amount of energy consumed in drying and sintering. These drawbacks undermine the possibility of commercial recycling of sludge for the production of lightweight aggregate. * Corresponding author. Tel.: þ886 2 2362 5373; fax: þ886 2 2392 8830. E-mail address: d90541003@ntu.edu.tw (F.-C. Chang). 0301-4797/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2010.06.011 Previous studies have reused sewage sludge ash (SSA) as pozzolanic material in cement mixing (Paya et al., 1999; Monzo et al., 1999, 2003) and as absorbent for copper removal from wastewater (Pan et al., 2003). Moreover, a solid absorbent for dry flue desulfurization systems has been developed from waste stone sludge (Lin, 1998), and activated carbon has also been made from sewage sludge (Chen et al., 2002). Sludge cakes made from dried sludge for production of cement and concrete (Kikuchi, 2001; Onaka, 2000; Valls et al., 2004) have poor workability and involve high processing cost, making recycling uneconomical. Despite the abovementioned resource recovery efforts, the amount of sludge produced from various sources is too large to be reused completely, thus leading to waste management problems. Rapid city development with new construction being planned continuously has increased the demand for aggregate for cement and concrete production. Natural aggregates are obtained by mining sand and stone, which are then crushed, washed and sorted. Waste silt is produced at typical aggregate washing plants at between 5 and 80 ton per hour depending on plant size and materials being processed. In general, 10e15% of the raw sand and stone will become waste silt. It is estimated that 16.5 million tons of waste silt is generated annually in Taiwan. It is an inert waste that is typically deposed of in landfills. When dried, the waste silt turns into dust that pollutes the air, another environmental hazard. Given the large quantities of waste silt produced coupled with its transport and diminishing landfill space, waste silt disposal is becoming 2290 F.-C. Chang et al. / Journal of Environmental Management 91 (2010) 2289e2294 2. Materials and methods 2.1. Materials 2.1.1. Stone sludge Sludge from slab stone processing was obtained from a marble processing factory in Hualien. The sludge was composed mainly of SiO2 and CaO, which account for 70% of the total weight, and had a water content of around 32%. Table 1 shows the chemical composition of the stone sludge oven dried at 105  C, and Fig. 1 displays the particle size distribution (weight percentage) in the sludge cake. As can be seen, over 75% of the particles had diameter below 0.044 mm and could pass through an ASTM sieve #325. The XRD analyses of waste stone sludge are shown in electronic annex Fig. A1. 2.1.2. Waste silt Washing and sorting of natural aggregates produce slurry. Slurry from waste silt was obtained from a sand and gravel processing plant in Hualien. The precipitated slurry was dried on the sludge bed (temperature: 40  C, flow speed: 0.1 m/s, and relative humidity: 50%) for 7 days. The waste silt thus obtained comprised mainly of SiO2 and CaO, which accounted for 73% of the total weight as shown in Fig. 1, and contained less than 5% moisture. Particle size distribution of the waste silt at 150 mm and 300 mm diameter accounted for 50% and 25%, respectively, hence not much of the waste silt could go through an ASTM sieve #200. The XRD analyses of waste silt are shown in electronic annex Fig. A1. 100 90 P er cent p ass in g b y we igh t ( %) a significant issue. Waste silt from sediments comprises abundant SiO2 and shows good water permeability. Previous studies showed that silt can be beneficially reused using geopolymerisation for the production of aggregates (Lampris et al., 2009). Silt geopolymers cured at room temperature had average 7-day compressive strengths of 18.7 MPa. Efforts have been made to reuse waste silt for soil improvement as well as for producing lightweight artificial aggregate or concrete. However, the varying chemical composition of waste silt not only causes problems during production, but also undermines the quality of the final products (Lee, 2001). In this study, waste stone sludge obtained from slab stone processing and waste silt from aggregate washing plants were recycled to manufacture artificial aggregate. They share similar chemical properties and have complementary physical characteristics. Fine-powdered stone sludge was mixed with waste silt of larger particle size. As low water absorption still depends on densified packing, vibratory compaction was applied, filling the voids between silt aggregates with waste stone sludge. The structure thus obtained was more densified, thus reducing the amount of solidifying agent used, lowering the required water-to-cement ratio, and above all, enhancing the compressive strength achieved. Waste stone sludge Waste silt 80 70 60 50 40 30 20 10 0 0.01 0.1 Grain size (mm) 1 Fig. 1. Size distribution of dried sludge cake and waste silt. 2.2. Sample preparation Waste stone sludge from slab stone processing plants was mixed with waste silt and solidifying agent (Lee et al., 2008). Solidifying agents were added in the vibratory mold and the waste was compacted into recycled materials. The study analyzed its compressive strength and absorption, as well as its resistance to abrasion by use of the Los Angeles index (an index to distinguish the resistance of materials to abrasion and grading levels of hardness) under vibration. Taking the operative and economic feasibility of waste recycling into consideration, the proportion of solidifying agents was controlled at 15% (weight ratio). The ratio of derived stone sludge ranged within 5e55%, increasing by 10% intervals, while ratios of waste silt ranged within 80e30%, decreasing by 10% intervals, with a total of 6 ratios. The materials were thoroughly mixed in a planetary gear driven mixer for 15 min, keeping the water content at 12%. Specific gravity of artificial aggregate was within the range of 2.65e2.73 for the different waste stone sludge and waste silt ratios. The test samples were prepared as follows: Firstly, a 5  5  16 cm compaction mold was filled with the thoroughlymixed materials and sealed. The materials were then compacted with the vibratory compactor at three vibration frequencies, 16.7, 33.3, and 50.0 Hz. The compaction speed was set at 0.15 MPa per second. After compacting each sample at 9.8, 19.6, 29.4, 39.2, and 49.0 MPa, they were retrieved and immediately enclosed in plastic wraps to prevent dehydration. The samples were dried in an oven at constant temperature of 55  C for 24 h and subsequently put under saturated vapor for 28 days. Then we examined their compressive strength with a uniaxial compression tester, absorption and resistance to abrasion with the Los Angeles index, and compaction characteristics with a Metallurgical Microscope. 3. Results and discussion 3.1. Effect of stone sludge to waste silt ratio 2.1.3. Solidifying agent The solidifying agent purchased from Stein Corporation, Japan contains 92% pozzolanic cement. As shown in Table 1, its chemical composition is very similar to that of Type I Portland cement. Table 1 Chemical composition of dried sludge cake, waste silt, and solidifying agent. Content (%) SiO2 CaO Fe2O3 Al2O3 MgO Others Dried sludge cake Waste silt Solidifying agent 54.7a 60.5 15.2 15.4 13.5 64.8 6.5 3.1 2.3 4.3 8.2 14.4 3.7 1.5 2.5 15.4 13.3 0.8 a The triplicated and averaged results were presented. To examine experimental results at different levels of proportions, the content ratio of stone sludge was increased from 5% to 55% while that of waste silt was reduced from 80% to 30%, both at 10% intervals. The uniaxial compaction result shows that the ratio of sludge to waste silt of 35%:50% under the same compaction pressure as that of other ratios obtained higher compressive strength (Electronic annex Fig. A2). That is because stone sludge filled the gaps in the waste silt. When the stone sludge is insufficient, the gaps in the waste silt packing cannot be filled entirely, which lowers the compressive strength. On the other hand, when the amount of sludge is excessive (over 45%), the compacted packing of waste silt bursts. The stone sludge, and not the waste silt 2291 F.-C. Chang et al. / Journal of Environmental Management 91 (2010) 2289e2294 40 25 Sludge: Waste Silt 45 : 40 Compaction pressure 9.8 MPa 20 30 15 10 Frequency (Hz) none 16.7 33.3 50.0 5 0 Compressive strength (Mpa) Compressive strength (MPa) 35 25 20 Compaction pressure 15 9.8 19.6 29.4 39.2 49.0 10 5 5:80 15:70 25:60 35:50 45:40 55:30 0 none Sludge: Waste Silt Fig. 2. Tendency of ratios and compressive strength under different vibration frequency. artificial aggregates, receives the direct force with compacted packing, which lowers the level of compressive strength. The coarse powder particles of waste silt have a smaller surface area than the fine stone sludge particles and demand less solidifying agent than the latter. Under an identical added ratio of solidifying agents, more waste silt helps to increase the compressive strength. In addition to the added contents of solidifying agents, the proper ratio is the most significant factor influencing compressive strength in recycling. The compressive strengths at different ratios are shown in Fig. 2. It seems the compressive strengths, regardless of groups, tended to increase with a rising sludge content; the compressive strength was the highest at ratio 35:50, while it descended by a small amount with other ratios. After vibratory compaction, the artificial aggregate packing at each ratio distinctly improved and reached a more compacted structure with higher compressive strength. 16.7 33.3 Vibration frequency (Hz) 50.0 Fig. 4. Comparison of vibration frequency and compressive strength under different compaction pressures. with a total of 20% after 28 days’ curing. It can be inferred that vibratory compaction has a strong influence on compressive strength; however, under compaction pressure over 29.4 MPa, each 9.8 MPa increase resulted in compressive strength being increased less than 10% after curing for 28 days. The experimental results under condition of no frequency show that compaction pressure plays a significant role in influencing the compressive strength as the ultimate compressive strength grew by a large degree with an increase of compaction pressure (Fig. 4). In contrast, the compressive strength tended to slightly decrease at a compaction pressure over 29.4 MPa under conditions of frequency. Consequently, the proper vibration frequency contributes to a lower the compaction pressure needed for the proper strength. 3.3. Effect of frequency 3.2. Effect of compaction pressure The samples at each ratio in the experiment were compacted at different pressures of 9.8, 19.6, 29.4, 39.2, and 49.0 MPa. The larger the compaction pressure, the more the compacted packing obtained. After the 28 days interval, with pressure at 49.0 MPa, the compressive strength of each group (except for the condition of no vibration) exceeded 28.0 MPa; in each group at 19.6e39.2 MPa, except for the condition of no frequency, more than 19.6 MPa of compressive strength was obtained. However, at insufficient compaction pressure of 9.8 MPa, the compressive strength hardly surpassed 19.6 MPa in each group. From Fig. 3 we can see that at the same ratio under different compaction pressures, the increase of compaction pressure distinctly enhanced the compressive strength. Moreover, up to 29.4 MPa, with each increase in compaction pressure by 9.8 MPa, the compressive strength increased about 10%, 3.3.1. Results of vibration frequency and compressive strength under different compaction pressures The compressive strength of each sample at any pressure without vibration did not exceed 19.6 MPa, while the largest compressive strength obtained under vibration frequency of 16.7 Hz reached up to 33.8 MPa, and that under 33.3 Hz, up to 37.7 MPa. Under compaction pressure of 19.6 MPa and frequency of 16.7 Hz, the minimum compressive strength of samples also surpassed 19.6 MPa. However, the compressive strength under vibration frequency of 33.3 Hz doubled compared to the condition of no vibration, which reveals that proper vibration frequency helps to improve packing structure. Analyzing the conditions of frequencies 33.3 and 50.0 Hz, the compressive strength of samples slightly 30 Compaction pressure 19.6 MPa 35 25 Vibration frequency 16.7 (Hz) 25 20 Sludge : Waste Silt 5:80 15:70 25:60 35:50 45:40 55:30 15 10 5 0 9.8 19.6 29.4 39.2 49.0 Compaction pressure (MPa) Compressive strength (MPa) Compressive strength (MPa) 30 20 15 Sludge : Waste Silt 5:80 15:70 25:60 35:50 45:40 55:30 10 5 0 none 16.7 33.3 50 Vibration frequency (Hz) Fig. 3. Comparison of compaction pressure and compressive strength at different ratios. Fig. 5. Comparison of vibration frequency and compressive strength at different ratios. 2292 F.-C. Chang et al. / Journal of Environmental Management 91 (2010) 2289e2294 increased, but the average did not differ much. This indicates that increasing the frequency once the compacted structure has been achieved has no significant benefits. The relations of vibration frequency and compressive strength are shown in Fig. 4. 3.3.2. Results of frequency and compressive strength at different ratios Under the conditions of stone sludge content at 5% and no vibration, the minimum compressive strength only reached Fig. 6. Water absorption at different ratios under different compaction pressure. F.-C. Chang et al. / Journal of Environmental Management 91 (2010) 2289e2294 7.5 MPa. With an increase in frequency, the compressive strength apparently increased. Under the frequency of 33.3 Hz, the compressive strength of the sample, 16.9 MPa, is almost 2.3 times higher than that at no vibration. At the most compacted theoretical ratio and under the conditions of sludge content at 35% and compaction pressure at 9.8 MPa, the compressive strength increased from 10.1 MPa under no vibration to 19.7 MPa under 33.3 Hz. On the other hand, under the compaction pressure of 29.4 MPa and frequency of 33.3 Hz, the compressive strengths at sludge contents 5%, 15%, 25%, 35%, 45%, and 55% were respectively 28.0, 30.1, 31.3, 31.2, and 29.8 MPa. The influence of different ratios differed slightly, only accounting for 6%, indicating that various ratios corresponded closely with terminal compressive strengths. The comparisons of vibration frequency and compressive strength at different ratios are shown in Fig. 5. 2293 compaction 50.0 Hz at 49.0 MPa, while the ratio with the smallest compressive strength, 5%:80%, had absorption of only 0.17% under the lower compaction pressure (Fig. 6). After vibratory compaction tests all the samples had lower absorption than cement concrete. After hydration reaction and solidification, the concrete in this study produced Calcium Silicate Hydrates with great porosity. After the vibratory compaction, the compacted structure had an effect of prohibiting water. At the same ratio, the larger the compaction pressure, the lower the absorption tends to be, as shown in Fig. 6. The optimal ratio, 35%:50%, with higher porosity has the highest absorption, 0.17%, under the larger vibration frequency of 50.0 Hz and compaction pressure of 49.0 MPa. The ratio lower than the compacted ratio of 35%:50% has absorption of 3.5% under vibration frequency of 16.7 Hz and compaction pressure of 9.8 MPa. In view of the above it can be concluded that vibratory compaction is a desired mechanical pulling force to gain minimum absorption. 3.4. Water absorption test Experiment in this study showed that there was a large variation of absorption under the condition of no vibration, the highest being up to 27.33%, the lowest 4.71%, and the remainder ranging within 10e20%. The absorption distinctly dropped under the frequency of 16.7 Hz, mostly to within 3e6%, while it was less than 1% and mostly under 0.5% under the frequencies of 33.3 Hz and 50.0 Hz. Compressive strength grew and absorption decreased in direct proportion to increase in compaction pressure and frequency. The stone sludge/waste silt ratio with the largest compressive strength, 35%:50%, had the lowest absorption, only 0.04%, under vibratory 3.5. Tests of resistance to abrasion using the Los Angeles index The Los Angeles index is used to distinguish resistance to abrasion in order to grade materials as to levels of hardness. In this research, resistance to abrasion of each group as measured using the Los Angeles index increased with an increase in compressive strength, and all met the requirement for coarse artificial aggregates of concrete, namely abrasion less than 38% (Electronic annex Fig. A3). Fig. 7. Microanalysis. (a) No vibration/compaction pressure 9.8 MPa, (b) frequency 16.7 Hz/compaction pressure 19.6 MPa, (c) frequency 33.3 Hz/compaction pressure 29.4 MPa. 2294 F.-C. Chang et al. / Journal of Environmental Management 91 (2010) 2289e2294 3.6. Microanalysis Appendix Under a metallurgical microscope, the artificial aggregate packing without vibration and with low pressure appeared slack with the artificial aggregates separated from each other and with air in-between (Fig. 7(a)). The artificial aggregates became densely packed as they were compacted under the frequency of 16.7 Hz and the compaction pressure of 19.6 MPa (Fig. 7(b)). In general, compaction pressure is the chief determinant of compressive strength, while vibration frequency influences compact density, which in turn affects compressive strength (Lee et al., 2008). Increasing the vibration frequency and compaction pressure up to 33.3 Hz and 29.4 MPa, respectively, resulted in excellent compaction, as shown in Fig. 7(c). That is to say, the theory of vibratory compaction producing better compaction is proved and this leads to an increase of compressive strength and a decrease of absorption, thus attaining the goal of efficient complementary waste recycling. Supplementary material associated with this article can be found in the online version at doi:10.1016/j.jenvman.2010.06.011. 4. Conclusions a. This research uses the method of vibratory compaction to recycle waste into artificial aggregate. By exerting densified packing in compacted structure, it proves that fine waste silt powder over #30 with a particle size of 0.3e0.15 mm owns the compressive strength of more than 29.4 MPa due to its compacted packing, which makes it a more valuable recycling product. b. The results show that by combining 35% of stone sludge with 50% of waste silt and 15% of dried solidifying agents, the recycling of waste stone sludge and waste silt into construction materials can create brand-new products and offer an ecological alternative to waste treatment. c. This research shows that the ratio of compacted packing is one of the influential factors that impact compressive strength and that compaction pressure is the main factor that contributes to the compacted structure, which differs from the theory pertaining to cement concrete. d. The research shows that with a larger compressive strength, the recycled material with fine compacted structure has lower absorption (only 0.08%), which is quite different from the effect of general cement concrete hydration. References Cheeseman, C.R., Virdi, G.S., 2005. Properties and microstructure of lightweight aggregate produced from sintered sewage sludge ash. Resour. Conserv. Recycl. 45, 18e30. Chen, X., Jeyaseelan, S., Graham, N., 2002. Physical and chemical properties study of the activated carbon made from sewage sludge. Waste Manage. 22, 755e760. Huang, C.C., 1998. Feasibility Research of Cement Industry on Stone Sludge Recycling. MD thesis, National Dong Hwa University. Kikuchi, R., 2001. Recycling of municipal solid waste for cement production: pilotscale test for transforming incineration ash of solid waste into cement clinker. Resour. Conserv. Recycl. 31, 137e147. Lampris, C., Lupo, R., Cheeseman, C.R., 2009. Geopolymerisation of silt generated from construction and demolition waste washing plants. Waste Manage. 29, 368e373. Lee, M.Y., 2001. Solidification of Waste From the Remnant of Constructional Materials. MD thesis, National Central University. Lee, M.Y., Ko, C.H., Chang, F.C., Lo, S.L., Lin, J.D., 2008. Artificial stone slab production using waste glass, stone fragments and vacuum vibratory compaction. Cement Concr. Compost. 30, 583e587. Lin, C.H., 1998. Develop a Solid Absorbent for Dry Flue Desulfurization System From Waste Sludge Discharge From Stone Industries. MD thesis, National Dong Hwa University. Monzo, J., Paya, J., Borrachero, M.V., Peris-Mora, E., 1999. Mechanical behavior of mortars containing sewage sludge ash (SSA) and portland cements with different tricalcium aluminate content. Cement Concr. Res. 29, 87e94. Monzo, J., Paya, J., Borrachero, M.V., Girbes, I., 2003. Reuse of sewage sludge ashes (SSA) in cement mixtures: the effect of SSA on the workability of cement mortars. Waste Manage. 23, 373e381. Onaka, T., 2000. Sewage can make Portland cement: a new technology for ultimate reuse of sewage sludge. Water Sci. Technol. 41, 93e98. Pan, S.C., Lin, C.C., Tseng, D.H., 2003. Reusing sewage sludge ash as adsorbent for copper removal from wastewater. Resour. Conserv. Recycl. 39, 79e90. Paya, J., Borrachero, M.V., Monzo, J., Bonilla, M., 1999. Properties of portland cement mortars incorporating high amounts of oil-fuel ashes. Waste Manage. 19, 1e7. Tay, J.H., Show, K.Y., 1997. Resource recovery of sludge as a building and construction material e a future trend in sludge management. Water Sci. Technol. 36, 259e266. Valls, S., Yague, A., Vazquez, E., Mariscal, C., 2004. Physical and mechanical properties of concrete with added dry sludge from a sewage treatment plant. Cement Concr. Res. 34, 2203e2208. Wainwright, P.J., Cresswell, D.J.F., 2001. Synthetic aggregate from combustion ashes using an innovative rotary kiln. Waste Manage. 21, 241e246. Wiebusch, B., Seyfried, C.F., 1997. Utilization of sewage age sludge ashes in the brick and tile industry. Water Sci. Technol. 36, 251e258.