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Development of porous ceramsite from construction and demolition waste ab
a
a
Chuan Wang , Jian-Zhi Wu & Fu-Shen Zhang a
Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China b
Graduate University of Chinese Academy of Sciences, Beijing, China Published online: 06 Feb 2013.
To cite this article: Chuan Wang, Jian-Zhi Wu & Fu-Shen Zhang (2013) Development of porous ceramsite from construction and demolition waste, Environmental Technology, 34:15, 2241-2249, DOI: 10.1080/09593330.2013.765918 To link to this article: http://dx.doi.org/10.1080/09593330.2013.765918
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Environmental Technology, 2013 Vol. 34, No. 15, 2241–2249, http://dx.doi.org/10.1080/09593330.2013.765918
Development of porous ceramsite from construction and demolition waste Chuan Wanga,b , Jian-Zhi Wua and Fu-Shen Zhanga∗ a Research
Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China; b Graduate University of Chinese Academy of Sciences, Beijing, China
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(Received 25 May 2012; final version received 28 December 2012 ) The disposal of construction and demolition (C&D) waste has become a serious problem in China due to the rapid increase of Chinese construction industry in recent years. In the present study, typical C&D waste was employed for ceramsite fabrication so as to find a new way for its effective recycling. A novel process was developed for manufacturing high-quality porous ceramsite according to the special chemical composition and properties of C&D waste. Most importantly, a unique bloating agent was developed for the porous structure formation since it was difficult to obtain a suitable porous structure using traditional bloating agents. The effects of processing parameters such as sintering temperature, heating rate and soaking time were investigated, and the bloating mechanism for ceramsite was discussed. The C&D waste ceramsite (CDWC), with highintensity, low density and homogeneous mechanical properties, was much more suitable for application in the construction field. This study provides a practical process for efficient recycling of the rapidly increasing quantities of C&D waste. Keywords: construction and demolition (C&D) waste; ceramsite; lightweight aggregate; recycle; bloating mechanism
1. Introduction Due to the rapid development of the economy, China builds over an area in excess of 2 billion m2 each year, being the largest in the world, and many old buildings are simultaneously being demolished. Therefore, the amount of construction and demolition (C&D) waste generated is very large, forming 30% to 40% of gross municipal refuse [1]. However, the reuse and recycle ratio of C&D waste in China is much lower than in most of the developed countries, such as Japan, whose recycle ratio is estimated to be more than 90% [2]. C&D waste has caused various environmental problems in recent years [3]. Firstly, this type of waste usually occupies large areas of land, especially in big cities such as Beijing, where there are usually not enough landfill sites for the total C&D waste generated. Secondly, C&D waste contains much dust, which can cause atmosphere pollution if directly exposed to the external environment. Thirdly, C&D waste changes the components of natural soil, and causes degradation of much cultivated land, which leads to a huge economic loss. The promotion of environmental management has exerted pressure demanding the adoption of proper methods to deal with the problems caused by C&D waste. The best way to relieve the environmental impacts caused by C&D waste is to reuse or recycle the waste. Research by concrete engineers has clearly suggested the possibility of appropriately treating and reusing such waste as aggregate in new concrete, especially in applications such as embankments ∗ Corresponding
author. Email:[email protected]
© 2013 Taylor & Francis
or sub-base layer, etc. [4,5]. However, the portion of fine granules in C&D waste could hardly be utilized as aggregate directly. It is necessary to develop new, efficient methods as supplements for reusing or recycling C&D waste. Ceramsite is a special type of construction material with low density and high strength, usually applied as a lightweight aggregate in concrete. Generally, this type of material is prepared by high-temperature sintering of natural materials such as clay, shale, perlite, vermiculite and polystyrene [6]. In the last few decades, due to the lack of natural resources, many kinds of solid wastes have been expected to be developed as the raw materials for ceramsite, such as sewage sludge [7–15], coal fly ash [16,17], mining residues [18], incinerator fly ash or bottom ash [19], reservoir sediments [20,21], iron ore tailings [22], etc. In particular, Mueller et al. [23] reported the fabrication of lightweight aggregate from masonry rubble, which suggested the possibility for producing ceramsite from C&D waste. C&D waste has a similar chemical composition to clay-like materials. Therefore, it is a potential material for producing ceramsite. If possible, it is ideal to recycle C&D waste in the construction industry, for wide applicability and high recycling efficiency. However, as far as we know, so far there has been a less successful case of producing ceramsite from C&D waste. Riley researched the sintering behaviour of clay and proposed a mechanism for the bloating effect as follows: (1) formation of the vitreous phase to trap gas; (2) emission
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of gas to form a porous structure [6]. In general, material composition and sintering conditions influence the formation of the vitreous phase, and bloating agents influence the emission of gas. Therefore, it is important to investigate the influences of variables such as material composition, sintering conditions and bloating agents. In this study, C&D waste was tested as the major raw material for the fabrication of ceramsite, and straw ash was utilized as bloating agent. The chemical composition of C&D waste was analysed in comparison with clay, which was successfully used to produce lightweight aggregate. Sintering parameters, such as sintering temperature, heating rate and soaking time were investigated to obtain the optimized product. The properties of ceramsite obtained, such as cylinder compressive strength, water adsorption, density, leaching characteristics of heavy metals, etc. were tested. Furthermore, C&D waste ceramsite (CDWC) was utilized in concrete masonry units (CMUs), and the engineering properties of CMUs made from CDWC were examined.
2.
Materials and methods
2.1. Materials The C&D waste employed in this study was obtained from a baking-free brick plant in Xianyang, Shaanxi province, China. In the local region there was much C&D waste to be disposed of because of its serious environmental impacts on the fluvial system. The C&D waste was mainly derived from the demolition of brick-structure constructions, and was crushed into fragments onsite using crushing machines. The as-obtained C&D waste for the present study was made up of brick scraps, concrete scraps, cement, soil, etc. Straw ash, which was utilized as bloating agent, was collected from a waste-to-energy plant located in Hegang, Heilongjiang province, China. The straw ash was treated with a thermal process before use as bloating agent. Cement used for CMUs was type I Portland cement with a minimum compressive strength of 32.5 MPa at 28 d. The fine aggregate used was natural sand. The aggregate selected from C&D waste (CDWA) was obtained from C&D waste by crushing and sorting. CDWC was obtained from C&D waste by pulverizing, pelletizing and sintering, etc.
2.2. Ceramsite fabrication Following the characteristics of the C&D waste, a fabrication process was established by modifying the traditional lightweight aggregate fabrication process. As shown in Figure 1, the modified fabrication process mainly included pre-treatment, pulverizing, mixing, pelletizing, sintering, etc. The pre-treatment consisted of two steps, i.e. crushing the C&D waste into small scraps of 1 to 3 cm, then heating at 550◦ C for 30 min in a muffle roaster. The aims of preheating were to removal some of the combined water and
Figure 1.
Flow chart for ceramsite fabrication from C&D waste.
to improve the efficiency of the following pulverizing process. The raw materials (C&D waste and straw ash) were pulverized by ball grinder and sieved to 100 mesh, and then the C&D waste was mixed with a bloating agent. The pelletizing process was carried out by a granulating disc with a disc diameter of 0.8 m. Water by weight percentage of 15% was added into the mixture powder during the pelletizing process. Sintering was performed in a muffle roaster. To determine the best sintering temperature, different sintering temperatures from 600◦ C to 1000◦ C were tested and thermal analysis done. To investigate the influence of process parameters on sintered products the heating time and soaking time were set as the variable factors. The heating rates were 6, 8, 10, 12, 14 and 16◦ C/min, and the soaking times were 0, 5, 10, 20, 30 and 40 min. 2.3. Characterization and analysis method Particle density, bulk density, water absorption and cylinder compressive strength were measured according to Chinese standard: Lightweight aggregates and its test methods (GB/T 17431-2010). X-ray fluorescence (XRF) analysis was utilized to determine the chemical composition of raw materials (C&D waste and straw ash) with a scanning rate of 8◦ Cdeg/min using XRF-1800 (Schimadu, Japan). Thermal analysis of C&D waste was investigated by thermogravimetry (Seteram Labsys-16, France). In a typical measurement, 30 mg of sample was heated in an Al2 O3 crucible at a constant heating rate of 10◦ C/min in air. X-ray diffraction (XRD) analysis for C&D waste, sintered product and straw ash were performed using a Philips X-ray diffraction meter (Philips PW 1700, Holland). Operating conditions were 45 kV and 250 mA using Cu-K radiation. The samples were scanned from 10◦ to 80◦ . Morphology of the sintered product was observed using a scanning electron micrograph (SEM) (Hitachi S-3000N, Japan). The pore size of ceramsite was obtained by analysing SEM pictures of ceramsite cross section using Nano Measurer (copyright 2008 Jie Xu, China). In order to evaluate the
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Figure 2.
Table 1.
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Composition diagram of bloating clay. From Riley [6].
Proportion of CMUs.
I (CDWA) II (CDWC)
Water (kg/m3 )
Cement (kg/m3 )
Fine aggregate (kg/m3 )
Coarse aggregate (kg/m3 )
230 230
350 350
576 576
1344 798
heavy metal leaching properties of CDWC, a toxicitycharacteristic leaching procedure (TCLP) test was performed in accordance of EPA test method 1311. The TCLP results were obtained from an inductively-coupled plasma optical emission spectrometer (ICP-OES) (PerkinElmer Optima 2000, USA). 2.4. Fabrication and test of CMUs The CMUs were made from water, cement, fine aggregate, CDWC and CDWA, with a mixing ratio as shown in Table 1. The concrete mix proportion was calculated by volume. Fine aggregate and CDWC (or CDWA) were prepared in a saturated surface dry condition before use. In mixing, the cement, fine aggregate and coarse aggregate were blended first, followed by added water. Mixing continued until a uniform concrete was obtained. After that, the mixture was cast into a concrete specimen mould with dimensions of 100 × 100 × 100 mm. After curing for 28 d, unit weight and compressive strength of the CMUs were tested. Each value is the average of measurements of three units.
3. Results and discussion 3.1. Characterization of materials It has been found that the chemical composition of raw materials imposes an influence on the bloating effect for ceramsite fabrication. Furthermore, the bloating effect influences ceramsite properties such as density and compressive strength. Previously, Riley presented a composition diagram to describe the relation of the clay chemical composite and the bloating effect, as shown in Figure 2 [6]. As shown in Table 2, the chemical composition of the C&D waste mainly comprises silicon oxide (SiO2 ), followed by calcium oxide (CaO), aluminum oxide (Al2 O3 ), iron oxide (Fe2 O3 ), potassium oxide (K2 O), magnesium oxide (MgO) and sodium oxide (Na2 O). Compared with clay, the C&D waste contains a higher content of CaO and a lower content of Al2 O3 . However, the composition point is near to the composition areas that represent a good bloating effect. It indicates that C&D waste, due to its chemical position, is a potential material for bloating ceramsite fabrication. XRD results show that the main phases in the C&D waste are quartz, calcite, microcline and albite. As for the bloating agent, the residual carbon content of the straw ash is 15.6%. The non-carbonaceous composition of straw ash mainly includes silicon oxide (SiO2 ), potassium oxide (K2 O), iron oxide (Fe2 O3 ), calcium oxide (CaO), magnesium oxide (MgO) aluminum oxide (Al2 O3 ) and sodium oxide (Na2 O). Sintering effects at different sintering temperature were investigated and the experimental results are shown in Figure 3. It was indicated that C&D waste could not
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Chemical composition of C&D waste and bloating agent (wt%).
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C&D waste Bloating agent
SiO2
CaO
Al2 O3
Fe2 O3
K2 O
MgO
Na2 O
Other
57.4 87.8
14.9 5.4
12.3 2.3
5.5 1.1
3.1 0.7
2.9 0.4
1.8 0.1
2.1 2.2
Figure 3.
Sintering effects at different temperatures: (a) 600◦ C; (b) 800◦ C; (c) 900◦ C (d) 1000◦ C.
Figure 4.
Thermal analysis curves for C&D waste.
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peak can be observed. The raw material is a mixture and the constituents dos not possess an identical melting point, thus the endothermic peak representing the formation of the vitreous phase occupies a wide range of temperatures. For the fabrication of CDWC, the proper amount of the vitreous phase is essential for sintering and bloating. According to the sintering experiment results and thermal analysis, the sintering temperature was selected to be 950◦ C. 3.2. Optimization of the sintering parameters Process parameters largely influence the properties of sintered products. The effects of heating rate and soaking time were investigated to obtain a sintered product with the proper porous structure.
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be sintered below 900◦ C and was excessively sintered at 1000◦ C. To determine sintering temperature precisely, thermal analysis was conducted from room temperature to 1000◦ C. It can be seen from Figure 4 that exothermic reactions occur at around 324◦ C and 424◦ C, while obvious mass loss was observed. The two reactions, which persist in a relatively wide temperature range, can be referred to as the emission of water of hydration and organic matter from the raw material. At around 790◦ C an endothermic reaction occurs with an obvious mass loss. According to the reaction temperature, the main reaction occurring is the decomposition of calcium carbonate. The mass loss ratio (from the TG curve), which is caused by the release of CO2 , is approximately 5%. As the temperature exceeds 930◦ C, the vitreous phase begin to form, hence an endothermic
Figure 5.
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Effects of sintering conditions on CDWC properties: (a) heating rate; (b) soaking time.
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As shown in Figure 5(a), average pore size gradually increased from 0.08 mm to 1.45 mm as the heating rate increased from 8◦ C/min to 16◦ C/min. An experiment was also performed with a heating rate of 6◦ C/min, but a nonbloating lightweight aggregate was obtained due to the absence of the vitreous phase, thus the data is not presented. When the heating rate was >8◦ C/min, the amount of vitreous phase increased with the increase of the heating rate. More vitreous phase caused a decline of the viscosity of the mixture; hence average pore size increased due to the aggregation of pores. The particle density first declined with an increase of the heating rate due to the enhanced bloating effect. The minimum particle density was obtained at a heating rate of 12◦ C/min. It should be noted that the particle density of the sintered product began to increase when the heating rate exceeded 12◦ C/min. This result could be attributed to the excessively low viscosity, which caused a shrinkage of volume of the sintered product.
Figure 6.
Figure 5(b) shows the effect of soaking time on particle density and average pore size. With an increase of soaking time, porous structure was enhanced by the greater amount of gas generated in the sintering process, which resulted in a decline of the particle density. Meanwhile, the average pore size gradually increased due to the aggregation of small pores. According to the experimental results, an optimization for the sintering parameters is as follows: (1) the sintering temperature is 950◦ C; (2) the heating rate is 8–12◦ C/min; (3) the soaking time is 10 to 20 min.
3.3. Bloating mechanism According to our preliminary study, it was found that it was quite difficult to get porous ceramsite. Thus a special bloating agent, which was patented, was developed in the present study. Being different from the fusion of crystal, the
X-ray diffraction analysis: (a) C&D waste and sintered product; (b) straw ash and straw ash after sintering.
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Environmental Technology formation of the vitreous phase in C&D waste is a gradual process in a relatively wide range of temperatures. Therefore, the final sintering temperature and heating rate both affect the amount of vitreous phase formed. By increasing the sintering temperature or heating rate, a lower viscosity can be obtained due to the increase of the amount of vitreous phase. Accordingly, the viscosity of the mixture at a high temperature could be controlled by varying sintering parameters. However, it must be pointed out that an excessively low viscosity is harmful for the fabrication of ceramsite, because a too-low viscosity results in an inefficient bloating and the fusion of small pores into bigger ones. In summary, the control of sintering temperature, heating rate and soaking time is essential to get a sintered product with proper properties. Figure 6(a) indicates that the phase component has an obvious variation during the sintering process. The decrease of the intensities of the quartz peaks can be attributed to the reaction between quartz and metal oxides. Calcite peaks almost disappear because of the decomposition of calcite above 850◦ C. During the sintering process, microcline and albite are transformed to anorthite due to the presence of calcium oxide. The increase of the diffraction peak width indicates that much vitreous phase has been formed. As shown in Figure 6(b), the main composition of straw ash was opal phase, whose chemical composition was mainly SiO2 . Opal phase cannot be sustained at high temperature, and it will undergo a phase transformation during sintering. An XRD analysis was conducted for pure straw ash after sintering, and the result showed that the mainly product phases were quartz and low quartz, as shown
Figure 7.
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in Figure 6(b). The bloating effect of straw ash could be attributed to SiO2 -compound and the residual carbon. For confirming the affirmation, some experiments were conducted. For example, pure carbon powder and pure SiO2 powder were tested as bloating agents, but the results show that neither of them had a bloating effect when used alone. In summary, the bloating effect of straw ash could be attributed to the combined function of SiO2 -compound and residual carbon. 3.4. Physical properties of the sintered product According to the above experiment results, CDWC with proper properties was obtained in an optimized condition as follows: sintering temperature of 950◦ C, heating rate of 12◦ C and soaking time of 10 min. As shown in Figure 7(a), the CDWC obtained is in a good shape with a ceramic surface. As shown in Figure 7(b), the big pores inside the CDWC are uniform with a size range of 0.2–0.5 mm. In Figure 7(c), some small pores located on the wall of large pores can be observed. As shown in Table 3, CDWC has a much lower bulk density (0.54 g/cm3 ) than CDWA (1.64 g/cm3 ). In addition, the water absorption of CDWC is 0.7%, much lower than that of CDWA. The low water absorption of CDWC is mainly due to the formation of a CDWC ceramic surface, which is compacted and waterproof so as to protect the porous structure of CDWC from water. The cylinder compressive strength of CDWC is 3.78 MPa, meeting the Chinese standard for lightweight aggregate. The properties of CDWC, such as low bulk density, low water absorption and enough cylinder
Pictures of CDWC: (a) appearance of CDWC; (b,c) porous internal structure of CDWC.
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Table 3.
CDWA CDWC
Table 5. Compressive strength of CMUs derived from CDWC and CDWA.
Physical properties of C&D waste and CDWC. Particle density (g/cm)
Bulk density (g/cm3 )
Water absorption
Cylinder compressive strength (MPa)
1.86 0.77
1.64 0.54
11.9% 0.7%
– 3.78
CMU
Compressive strength (MPa)
Specific gravity
Minimum
Maximum
Average
1.96 1.44
12.73 18.97
16.49 19.96
14.65 19.22
I (CDWA) II (CDWC)
compressive strength, indicate that CDWC is an appropriate lightweight aggregate for use in the construction industry.
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3.5.
Toxicity-characteristic leaching procedure (TCLP) tests
One of the environmental risks caused by C&D waste is heavy metals contamination. For the purpose of relieving the environmental impact of C&D waste, stabilization of heavy metals is also a target during the preparation of CDWC. Table 4 shows the toxicity-characteristic leaching procedure (TCLP) results for CDWC and the C&D waste in different particle dimensions. The main heavy metals contained in the C&D waste used in this study are Cr, Mn, Ba, Ni, Cu, Zn, and Pb is also investigated due to serious contamination. The C&D waste of 80–100 mesh had a much higher heavy metal leaching concentration, especially the leaching concentration for Cr (6.96 mg/L), which exceeded the maximum concentration required by Chinese standard [24]. It indicated that heavy metals were not stable in C&D waste, so the heavy metal leaching concentration increased largely with the increase of surface area. As for CDWC, the heavy metal leaching concentration is much lower than that of the C&D waste. It can be inferred that the vitreous phase formed is the main factor stabilizing heavy metals. Heavy metals were trapped within the glass phase, which was compacted and acid-resistant. Furthermore, during the sintering process, heavy metals could also be stabilized by the formation of metal oxide compounds, such as PbAl2 O4 [25]. In summary, TCLP tests showed that heavy metals contained in C&D waste could be well stabilized by sintering during the preparation of CDWC.
3.6. Application of CDWC in concrete For the purpose of testing the practicability of CDWC in concrete, CMUs were fabricated with CDWA and CDWC, and a comparison was made by measuring specific gravity and compressive strength. As shown in Table 5 the CMUs made with CDWC have an average specific gravity of 1.44, with 26% less weight than the CMUs made with CDWA, which could be attributed to: (1) the lower bulk density of CDWC which decreases the weight of the CMU directly; (2) the lower water absorption of CDWC, which decreases the amount of water contained in CMUs. The average compressive strength of CMUs made with CDWA is much lower than that of CMUs made with CDWC. By observing the fracture plane of test samples, it was found that most of the CDWA granules on the fracture plane were crushed during the compressive testing, whereas most of the CDWC granules remained in a good condition. Therefore, the fracture mechanisms of the two kinds of CMU samples were quite different. For the CMUs made with CDWA, cracks were first formed inside some CDWA granules. With the increase of pressure, the cracks extended from CDWA granules to the cement stone structure. Finally, the sample collapsed due to the destruction of the cement stone structure. CDWC granules had a uniform and high compressive strength, thus CDWC granules could hardly be destroyed during the compressive tests. In addition, the smooth shape of CDWC granules could relieve stress concentration so as to hamper the formation and the extension of cracks. In summary, results indicate that CDWC, which
Table 4. Results of toxicity-characteristic leaching procedure (TCLP) testing for C&D waste and CDWC. Leaching concentration (mg/L)
Element Cr Mn Ba Ni Cu Zn Pb
C&D waste
C&D waste powder (200 mesh)
CDWC
CDWC powder (200 mesh)
Regulatory level (Chinese standard)
0.47 4.61 1.88 0.94 1.07 ND 0.92
6.96 11.08 2.01 10.41 4.74 0.95 3.45
0.02 0.22 0.07 0.04 ND 0.39 0.34
0.04 0.24 0.11 0.02 ND 0.39 0.52
5 – 100 5 – 100 100
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4. Conclusions Based on the results obtained in the present research, the following conclusions could be drawn: (1) High-quality ceramsite could be fabricated using C&D waste as raw material by the special process developed in this study. During the sintering process, the sintering temperature, heating rate and soaking time were the major factors controlling the properties of the product. The optimized sintering temperature, heating rate and soaking time were 950◦ C, 8–12◦ C/min and 10–20 min, respectively. (2) CDWC was an excellent product with homogeneous internal porous structure and ceramic surface. Compared with CDWA, CDWC had high strength, lower density, lower water absorption and more homogeneous mechanical properties. TCLP results showed that heavy metals were well stabilized in CDWC by the sintering process in comparison with C&D waste. Acknowledgements This work was made possible by financial support from the National Water Pollution Control and Management Programs (2009ZX07212-002, 2012ZX07202-005) and the Environmental Public Welfare Project (201009026).
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[9] Y.Q. Zhao, Q.Y. Yue, R.B. Li, M. Yue, S.X. Han, B.Y. Gao, Q. Li, and H. Yu, Research on sludge-fly ash ceramic particles (SFCP) for synthetic and municipal wastewater treatment in biological aerated filter (BAF), Bioresource Technol. 100 (2009), pp. 4955–4962. [10] Q.Y. Yue, S.X. Han, M. Yue, B.Y. Gao, Q. Li, H. Yu, Y.Q. Zhao, and Y.F. Qi, The performance of biological anaerobic filters packed with sludge-fly ash ceramic particles (SFCP) and commercial ceramic particles (CCP) during the restart period: Effect of the C/N ratios and filter media, Bioresource Technol. 100 (2009), pp. 5016–5020. [11] S.Q. Wu, Q.Y. Yue, Y.F. Qi, B.Y. Gao, S.X. Han, and M. Yue, Preparation of ultra-lightweight sludge ceramics (ULSC) and application for pharmaceutical advanced wastewater treatment in a biological aerobic filter (BAF), Bioresource Technol. 102 (2011), pp. 2296–2300. [12] X. Wang, Y. Jin, Z. Wang, R.B. Mahar, and Y. Nie, A research on sintering characteristics and mechanisms of dried sewage sludge, J. Hazard. Mater. 160 (2008), pp. 489–494. [13] Y.F. Qi, Q.Y. Yue, S.X. Han, M. Yue, B.Y. Gao, H. Yu, and T. Shao, Preparation and mechanism of ultra-lightweight ceramics produced from sewage sludge, J. Hazard. Mater. 176 (2010), pp. 76–84. [14] K.Y. Show, D.J. Lee, J.H. Tay, S.Y. Hong, and C.Y. Chien, Lightweight aggregates from industrial sludge-marine clay mixes, J. Environ. Eng.-ASCE 131 (2005), pp. 1106–1113. [15] J.H. Tay, S.Y. Hong, and K.Y. Show, Reuse of industrial sludge as pelletized aggregate for concrete, J. Environ. Eng.ASCE 126 (2000), pp. 279–287. [16] Y.L. Zhao, J.W. Ye, X.B. Lu, M.G. Liu, Y. Lin, W.T. Gong, and G.L. Ning, Preparation of sintered foam materials by alkali-activated coal fly ash, J. Hazard. Mater. 174 (2010), pp. 108–112. [17] M.R. Little, V. Adell, A.R. Boccaccini, and C.R. Cheeseman, Production of novel ceramic materials from coal fly ash and metal finishing wastes, Resour. Conserv. Recy. 52 (2008), pp. 1329–1335. [18] S.C. Huang, F.C. Chang, S.L. Lo, M.Y. Lee, C.F. Wang, and J.D. Lin, Production of lightweight aggregates from mining residues, heavy metal sludge, and incinerator fly ash, J. Hazard. Mater. 144 (2007), pp. 52–58. [19] H.J. Chen, S.Y. Wang, and C.W. Tang, Reuse of incineration fly ashes and reaction ashes for manufacturing lightweight aggregate, Constr. Build. Mater. 24 (2010), pp. 46–55. [20] Y.L. Wei, J.C. Yang, Y.Y. Lin, S.Y. Chuang, and H.P. Wang, Recycling of harbor sediment as lightweight aggregate, Mar. Pollut. Bull. 57 (2008), pp. 867–872. [21] C.W. Tang, H.J. Chen, S.Y. Wang, and J. Spaulding, Production of synthetic lightweight aggregate using reservoir sediments for concrete and masonry, Cement Concrete Comp. 33 (2011), pp. 292–300. [22] Y. Liu, F. Du, L. Yuan, H. Zeng, and S. Kong, Production of lightweight ceramisite from iron ore tailings and its performance investigation in a biological aerated filter (BAF) reactor, J. Hazard. Mater. 178 (2010), pp. 999–1006. [23] A. Mueller, S.N. Sokolova, and V.I. Vereshagin, Characteristics of lightweight aggregates from primary and recycled raw materials, Constr. Build. Mater. 22 (2008), pp. 703–712. [24] G.B.5085.3, Identification standards for hazardous wastes identification for extraction toxicity [S], China Environmental Science Press, China, 2007. [25] X.W. Lu and K.M. Shih, Phase transformation and its role in stabilizing simulated lead-laden sludge in aluminum-rich ceramics, Water Res. 45 (2011), pp. 5123–5129.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Development of porous ceramsite from construction and demolition waste ab
a
a
Chuan Wang , Jian-Zhi Wu & Fu-Shen Zhang a
Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China b
Graduate University of Chinese Academy of Sciences, Beijing, China Published online: 06 Feb 2013.
To cite this article: Chuan Wang, Jian-Zhi Wu & Fu-Shen Zhang (2013) Development of porous ceramsite from construction and demolition waste, Environmental Technology, 34:15, 2241-2249, DOI: 10.1080/09593330.2013.765918 To link to this article: http://dx.doi.org/10.1080/09593330.2013.765918
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Environmental Technology, 2013 Vol. 34, No. 15, 2241–2249, http://dx.doi.org/10.1080/09593330.2013.765918
Development of porous ceramsite from construction and demolition waste Chuan Wanga,b , Jian-Zhi Wua and Fu-Shen Zhanga∗ a Research
Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China; b Graduate University of Chinese Academy of Sciences, Beijing, China
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(Received 25 May 2012; final version received 28 December 2012 ) The disposal of construction and demolition (C&D) waste has become a serious problem in China due to the rapid increase of Chinese construction industry in recent years. In the present study, typical C&D waste was employed for ceramsite fabrication so as to find a new way for its effective recycling. A novel process was developed for manufacturing high-quality porous ceramsite according to the special chemical composition and properties of C&D waste. Most importantly, a unique bloating agent was developed for the porous structure formation since it was difficult to obtain a suitable porous structure using traditional bloating agents. The effects of processing parameters such as sintering temperature, heating rate and soaking time were investigated, and the bloating mechanism for ceramsite was discussed. The C&D waste ceramsite (CDWC), with highintensity, low density and homogeneous mechanical properties, was much more suitable for application in the construction field. This study provides a practical process for efficient recycling of the rapidly increasing quantities of C&D waste. Keywords: construction and demolition (C&D) waste; ceramsite; lightweight aggregate; recycle; bloating mechanism
1. Introduction Due to the rapid development of the economy, China builds over an area in excess of 2 billion m2 each year, being the largest in the world, and many old buildings are simultaneously being demolished. Therefore, the amount of construction and demolition (C&D) waste generated is very large, forming 30% to 40% of gross municipal refuse [1]. However, the reuse and recycle ratio of C&D waste in China is much lower than in most of the developed countries, such as Japan, whose recycle ratio is estimated to be more than 90% [2]. C&D waste has caused various environmental problems in recent years [3]. Firstly, this type of waste usually occupies large areas of land, especially in big cities such as Beijing, where there are usually not enough landfill sites for the total C&D waste generated. Secondly, C&D waste contains much dust, which can cause atmosphere pollution if directly exposed to the external environment. Thirdly, C&D waste changes the components of natural soil, and causes degradation of much cultivated land, which leads to a huge economic loss. The promotion of environmental management has exerted pressure demanding the adoption of proper methods to deal with the problems caused by C&D waste. The best way to relieve the environmental impacts caused by C&D waste is to reuse or recycle the waste. Research by concrete engineers has clearly suggested the possibility of appropriately treating and reusing such waste as aggregate in new concrete, especially in applications such as embankments ∗ Corresponding
author. Email:[email protected]
© 2013 Taylor & Francis
or sub-base layer, etc. [4,5]. However, the portion of fine granules in C&D waste could hardly be utilized as aggregate directly. It is necessary to develop new, efficient methods as supplements for reusing or recycling C&D waste. Ceramsite is a special type of construction material with low density and high strength, usually applied as a lightweight aggregate in concrete. Generally, this type of material is prepared by high-temperature sintering of natural materials such as clay, shale, perlite, vermiculite and polystyrene [6]. In the last few decades, due to the lack of natural resources, many kinds of solid wastes have been expected to be developed as the raw materials for ceramsite, such as sewage sludge [7–15], coal fly ash [16,17], mining residues [18], incinerator fly ash or bottom ash [19], reservoir sediments [20,21], iron ore tailings [22], etc. In particular, Mueller et al. [23] reported the fabrication of lightweight aggregate from masonry rubble, which suggested the possibility for producing ceramsite from C&D waste. C&D waste has a similar chemical composition to clay-like materials. Therefore, it is a potential material for producing ceramsite. If possible, it is ideal to recycle C&D waste in the construction industry, for wide applicability and high recycling efficiency. However, as far as we know, so far there has been a less successful case of producing ceramsite from C&D waste. Riley researched the sintering behaviour of clay and proposed a mechanism for the bloating effect as follows: (1) formation of the vitreous phase to trap gas; (2) emission
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of gas to form a porous structure [6]. In general, material composition and sintering conditions influence the formation of the vitreous phase, and bloating agents influence the emission of gas. Therefore, it is important to investigate the influences of variables such as material composition, sintering conditions and bloating agents. In this study, C&D waste was tested as the major raw material for the fabrication of ceramsite, and straw ash was utilized as bloating agent. The chemical composition of C&D waste was analysed in comparison with clay, which was successfully used to produce lightweight aggregate. Sintering parameters, such as sintering temperature, heating rate and soaking time were investigated to obtain the optimized product. The properties of ceramsite obtained, such as cylinder compressive strength, water adsorption, density, leaching characteristics of heavy metals, etc. were tested. Furthermore, C&D waste ceramsite (CDWC) was utilized in concrete masonry units (CMUs), and the engineering properties of CMUs made from CDWC were examined.
2.
Materials and methods
2.1. Materials The C&D waste employed in this study was obtained from a baking-free brick plant in Xianyang, Shaanxi province, China. In the local region there was much C&D waste to be disposed of because of its serious environmental impacts on the fluvial system. The C&D waste was mainly derived from the demolition of brick-structure constructions, and was crushed into fragments onsite using crushing machines. The as-obtained C&D waste for the present study was made up of brick scraps, concrete scraps, cement, soil, etc. Straw ash, which was utilized as bloating agent, was collected from a waste-to-energy plant located in Hegang, Heilongjiang province, China. The straw ash was treated with a thermal process before use as bloating agent. Cement used for CMUs was type I Portland cement with a minimum compressive strength of 32.5 MPa at 28 d. The fine aggregate used was natural sand. The aggregate selected from C&D waste (CDWA) was obtained from C&D waste by crushing and sorting. CDWC was obtained from C&D waste by pulverizing, pelletizing and sintering, etc.
2.2. Ceramsite fabrication Following the characteristics of the C&D waste, a fabrication process was established by modifying the traditional lightweight aggregate fabrication process. As shown in Figure 1, the modified fabrication process mainly included pre-treatment, pulverizing, mixing, pelletizing, sintering, etc. The pre-treatment consisted of two steps, i.e. crushing the C&D waste into small scraps of 1 to 3 cm, then heating at 550◦ C for 30 min in a muffle roaster. The aims of preheating were to removal some of the combined water and
Figure 1.
Flow chart for ceramsite fabrication from C&D waste.
to improve the efficiency of the following pulverizing process. The raw materials (C&D waste and straw ash) were pulverized by ball grinder and sieved to 100 mesh, and then the C&D waste was mixed with a bloating agent. The pelletizing process was carried out by a granulating disc with a disc diameter of 0.8 m. Water by weight percentage of 15% was added into the mixture powder during the pelletizing process. Sintering was performed in a muffle roaster. To determine the best sintering temperature, different sintering temperatures from 600◦ C to 1000◦ C were tested and thermal analysis done. To investigate the influence of process parameters on sintered products the heating time and soaking time were set as the variable factors. The heating rates were 6, 8, 10, 12, 14 and 16◦ C/min, and the soaking times were 0, 5, 10, 20, 30 and 40 min. 2.3. Characterization and analysis method Particle density, bulk density, water absorption and cylinder compressive strength were measured according to Chinese standard: Lightweight aggregates and its test methods (GB/T 17431-2010). X-ray fluorescence (XRF) analysis was utilized to determine the chemical composition of raw materials (C&D waste and straw ash) with a scanning rate of 8◦ Cdeg/min using XRF-1800 (Schimadu, Japan). Thermal analysis of C&D waste was investigated by thermogravimetry (Seteram Labsys-16, France). In a typical measurement, 30 mg of sample was heated in an Al2 O3 crucible at a constant heating rate of 10◦ C/min in air. X-ray diffraction (XRD) analysis for C&D waste, sintered product and straw ash were performed using a Philips X-ray diffraction meter (Philips PW 1700, Holland). Operating conditions were 45 kV and 250 mA using Cu-K radiation. The samples were scanned from 10◦ to 80◦ . Morphology of the sintered product was observed using a scanning electron micrograph (SEM) (Hitachi S-3000N, Japan). The pore size of ceramsite was obtained by analysing SEM pictures of ceramsite cross section using Nano Measurer (copyright 2008 Jie Xu, China). In order to evaluate the
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Figure 2.
Table 1.
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Composition diagram of bloating clay. From Riley [6].
Proportion of CMUs.
I (CDWA) II (CDWC)
Water (kg/m3 )
Cement (kg/m3 )
Fine aggregate (kg/m3 )
Coarse aggregate (kg/m3 )
230 230
350 350
576 576
1344 798
heavy metal leaching properties of CDWC, a toxicitycharacteristic leaching procedure (TCLP) test was performed in accordance of EPA test method 1311. The TCLP results were obtained from an inductively-coupled plasma optical emission spectrometer (ICP-OES) (PerkinElmer Optima 2000, USA). 2.4. Fabrication and test of CMUs The CMUs were made from water, cement, fine aggregate, CDWC and CDWA, with a mixing ratio as shown in Table 1. The concrete mix proportion was calculated by volume. Fine aggregate and CDWC (or CDWA) were prepared in a saturated surface dry condition before use. In mixing, the cement, fine aggregate and coarse aggregate were blended first, followed by added water. Mixing continued until a uniform concrete was obtained. After that, the mixture was cast into a concrete specimen mould with dimensions of 100 × 100 × 100 mm. After curing for 28 d, unit weight and compressive strength of the CMUs were tested. Each value is the average of measurements of three units.
3. Results and discussion 3.1. Characterization of materials It has been found that the chemical composition of raw materials imposes an influence on the bloating effect for ceramsite fabrication. Furthermore, the bloating effect influences ceramsite properties such as density and compressive strength. Previously, Riley presented a composition diagram to describe the relation of the clay chemical composite and the bloating effect, as shown in Figure 2 [6]. As shown in Table 2, the chemical composition of the C&D waste mainly comprises silicon oxide (SiO2 ), followed by calcium oxide (CaO), aluminum oxide (Al2 O3 ), iron oxide (Fe2 O3 ), potassium oxide (K2 O), magnesium oxide (MgO) and sodium oxide (Na2 O). Compared with clay, the C&D waste contains a higher content of CaO and a lower content of Al2 O3 . However, the composition point is near to the composition areas that represent a good bloating effect. It indicates that C&D waste, due to its chemical position, is a potential material for bloating ceramsite fabrication. XRD results show that the main phases in the C&D waste are quartz, calcite, microcline and albite. As for the bloating agent, the residual carbon content of the straw ash is 15.6%. The non-carbonaceous composition of straw ash mainly includes silicon oxide (SiO2 ), potassium oxide (K2 O), iron oxide (Fe2 O3 ), calcium oxide (CaO), magnesium oxide (MgO) aluminum oxide (Al2 O3 ) and sodium oxide (Na2 O). Sintering effects at different sintering temperature were investigated and the experimental results are shown in Figure 3. It was indicated that C&D waste could not
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Chemical composition of C&D waste and bloating agent (wt%).
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C&D waste Bloating agent
SiO2
CaO
Al2 O3
Fe2 O3
K2 O
MgO
Na2 O
Other
57.4 87.8
14.9 5.4
12.3 2.3
5.5 1.1
3.1 0.7
2.9 0.4
1.8 0.1
2.1 2.2
Figure 3.
Sintering effects at different temperatures: (a) 600◦ C; (b) 800◦ C; (c) 900◦ C (d) 1000◦ C.
Figure 4.
Thermal analysis curves for C&D waste.
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peak can be observed. The raw material is a mixture and the constituents dos not possess an identical melting point, thus the endothermic peak representing the formation of the vitreous phase occupies a wide range of temperatures. For the fabrication of CDWC, the proper amount of the vitreous phase is essential for sintering and bloating. According to the sintering experiment results and thermal analysis, the sintering temperature was selected to be 950◦ C. 3.2. Optimization of the sintering parameters Process parameters largely influence the properties of sintered products. The effects of heating rate and soaking time were investigated to obtain a sintered product with the proper porous structure.
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be sintered below 900◦ C and was excessively sintered at 1000◦ C. To determine sintering temperature precisely, thermal analysis was conducted from room temperature to 1000◦ C. It can be seen from Figure 4 that exothermic reactions occur at around 324◦ C and 424◦ C, while obvious mass loss was observed. The two reactions, which persist in a relatively wide temperature range, can be referred to as the emission of water of hydration and organic matter from the raw material. At around 790◦ C an endothermic reaction occurs with an obvious mass loss. According to the reaction temperature, the main reaction occurring is the decomposition of calcium carbonate. The mass loss ratio (from the TG curve), which is caused by the release of CO2 , is approximately 5%. As the temperature exceeds 930◦ C, the vitreous phase begin to form, hence an endothermic
Figure 5.
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Effects of sintering conditions on CDWC properties: (a) heating rate; (b) soaking time.
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As shown in Figure 5(a), average pore size gradually increased from 0.08 mm to 1.45 mm as the heating rate increased from 8◦ C/min to 16◦ C/min. An experiment was also performed with a heating rate of 6◦ C/min, but a nonbloating lightweight aggregate was obtained due to the absence of the vitreous phase, thus the data is not presented. When the heating rate was >8◦ C/min, the amount of vitreous phase increased with the increase of the heating rate. More vitreous phase caused a decline of the viscosity of the mixture; hence average pore size increased due to the aggregation of pores. The particle density first declined with an increase of the heating rate due to the enhanced bloating effect. The minimum particle density was obtained at a heating rate of 12◦ C/min. It should be noted that the particle density of the sintered product began to increase when the heating rate exceeded 12◦ C/min. This result could be attributed to the excessively low viscosity, which caused a shrinkage of volume of the sintered product.
Figure 6.
Figure 5(b) shows the effect of soaking time on particle density and average pore size. With an increase of soaking time, porous structure was enhanced by the greater amount of gas generated in the sintering process, which resulted in a decline of the particle density. Meanwhile, the average pore size gradually increased due to the aggregation of small pores. According to the experimental results, an optimization for the sintering parameters is as follows: (1) the sintering temperature is 950◦ C; (2) the heating rate is 8–12◦ C/min; (3) the soaking time is 10 to 20 min.
3.3. Bloating mechanism According to our preliminary study, it was found that it was quite difficult to get porous ceramsite. Thus a special bloating agent, which was patented, was developed in the present study. Being different from the fusion of crystal, the
X-ray diffraction analysis: (a) C&D waste and sintered product; (b) straw ash and straw ash after sintering.
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Environmental Technology formation of the vitreous phase in C&D waste is a gradual process in a relatively wide range of temperatures. Therefore, the final sintering temperature and heating rate both affect the amount of vitreous phase formed. By increasing the sintering temperature or heating rate, a lower viscosity can be obtained due to the increase of the amount of vitreous phase. Accordingly, the viscosity of the mixture at a high temperature could be controlled by varying sintering parameters. However, it must be pointed out that an excessively low viscosity is harmful for the fabrication of ceramsite, because a too-low viscosity results in an inefficient bloating and the fusion of small pores into bigger ones. In summary, the control of sintering temperature, heating rate and soaking time is essential to get a sintered product with proper properties. Figure 6(a) indicates that the phase component has an obvious variation during the sintering process. The decrease of the intensities of the quartz peaks can be attributed to the reaction between quartz and metal oxides. Calcite peaks almost disappear because of the decomposition of calcite above 850◦ C. During the sintering process, microcline and albite are transformed to anorthite due to the presence of calcium oxide. The increase of the diffraction peak width indicates that much vitreous phase has been formed. As shown in Figure 6(b), the main composition of straw ash was opal phase, whose chemical composition was mainly SiO2 . Opal phase cannot be sustained at high temperature, and it will undergo a phase transformation during sintering. An XRD analysis was conducted for pure straw ash after sintering, and the result showed that the mainly product phases were quartz and low quartz, as shown
Figure 7.
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in Figure 6(b). The bloating effect of straw ash could be attributed to SiO2 -compound and the residual carbon. For confirming the affirmation, some experiments were conducted. For example, pure carbon powder and pure SiO2 powder were tested as bloating agents, but the results show that neither of them had a bloating effect when used alone. In summary, the bloating effect of straw ash could be attributed to the combined function of SiO2 -compound and residual carbon. 3.4. Physical properties of the sintered product According to the above experiment results, CDWC with proper properties was obtained in an optimized condition as follows: sintering temperature of 950◦ C, heating rate of 12◦ C and soaking time of 10 min. As shown in Figure 7(a), the CDWC obtained is in a good shape with a ceramic surface. As shown in Figure 7(b), the big pores inside the CDWC are uniform with a size range of 0.2–0.5 mm. In Figure 7(c), some small pores located on the wall of large pores can be observed. As shown in Table 3, CDWC has a much lower bulk density (0.54 g/cm3 ) than CDWA (1.64 g/cm3 ). In addition, the water absorption of CDWC is 0.7%, much lower than that of CDWA. The low water absorption of CDWC is mainly due to the formation of a CDWC ceramic surface, which is compacted and waterproof so as to protect the porous structure of CDWC from water. The cylinder compressive strength of CDWC is 3.78 MPa, meeting the Chinese standard for lightweight aggregate. The properties of CDWC, such as low bulk density, low water absorption and enough cylinder
Pictures of CDWC: (a) appearance of CDWC; (b,c) porous internal structure of CDWC.
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Table 3.
CDWA CDWC
Table 5. Compressive strength of CMUs derived from CDWC and CDWA.
Physical properties of C&D waste and CDWC. Particle density (g/cm)
Bulk density (g/cm3 )
Water absorption
Cylinder compressive strength (MPa)
1.86 0.77
1.64 0.54
11.9% 0.7%
– 3.78
CMU
Compressive strength (MPa)
Specific gravity
Minimum
Maximum
Average
1.96 1.44
12.73 18.97
16.49 19.96
14.65 19.22
I (CDWA) II (CDWC)
compressive strength, indicate that CDWC is an appropriate lightweight aggregate for use in the construction industry.
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3.5.
Toxicity-characteristic leaching procedure (TCLP) tests
One of the environmental risks caused by C&D waste is heavy metals contamination. For the purpose of relieving the environmental impact of C&D waste, stabilization of heavy metals is also a target during the preparation of CDWC. Table 4 shows the toxicity-characteristic leaching procedure (TCLP) results for CDWC and the C&D waste in different particle dimensions. The main heavy metals contained in the C&D waste used in this study are Cr, Mn, Ba, Ni, Cu, Zn, and Pb is also investigated due to serious contamination. The C&D waste of 80–100 mesh had a much higher heavy metal leaching concentration, especially the leaching concentration for Cr (6.96 mg/L), which exceeded the maximum concentration required by Chinese standard [24]. It indicated that heavy metals were not stable in C&D waste, so the heavy metal leaching concentration increased largely with the increase of surface area. As for CDWC, the heavy metal leaching concentration is much lower than that of the C&D waste. It can be inferred that the vitreous phase formed is the main factor stabilizing heavy metals. Heavy metals were trapped within the glass phase, which was compacted and acid-resistant. Furthermore, during the sintering process, heavy metals could also be stabilized by the formation of metal oxide compounds, such as PbAl2 O4 [25]. In summary, TCLP tests showed that heavy metals contained in C&D waste could be well stabilized by sintering during the preparation of CDWC.
3.6. Application of CDWC in concrete For the purpose of testing the practicability of CDWC in concrete, CMUs were fabricated with CDWA and CDWC, and a comparison was made by measuring specific gravity and compressive strength. As shown in Table 5 the CMUs made with CDWC have an average specific gravity of 1.44, with 26% less weight than the CMUs made with CDWA, which could be attributed to: (1) the lower bulk density of CDWC which decreases the weight of the CMU directly; (2) the lower water absorption of CDWC, which decreases the amount of water contained in CMUs. The average compressive strength of CMUs made with CDWA is much lower than that of CMUs made with CDWC. By observing the fracture plane of test samples, it was found that most of the CDWA granules on the fracture plane were crushed during the compressive testing, whereas most of the CDWC granules remained in a good condition. Therefore, the fracture mechanisms of the two kinds of CMU samples were quite different. For the CMUs made with CDWA, cracks were first formed inside some CDWA granules. With the increase of pressure, the cracks extended from CDWA granules to the cement stone structure. Finally, the sample collapsed due to the destruction of the cement stone structure. CDWC granules had a uniform and high compressive strength, thus CDWC granules could hardly be destroyed during the compressive tests. In addition, the smooth shape of CDWC granules could relieve stress concentration so as to hamper the formation and the extension of cracks. In summary, results indicate that CDWC, which
Table 4. Results of toxicity-characteristic leaching procedure (TCLP) testing for C&D waste and CDWC. Leaching concentration (mg/L)
Element Cr Mn Ba Ni Cu Zn Pb
C&D waste
C&D waste powder (200 mesh)
CDWC
CDWC powder (200 mesh)
Regulatory level (Chinese standard)
0.47 4.61 1.88 0.94 1.07 ND 0.92
6.96 11.08 2.01 10.41 4.74 0.95 3.45
0.02 0.22 0.07 0.04 ND 0.39 0.34
0.04 0.24 0.11 0.02 ND 0.39 0.52
5 – 100 5 – 100 100
Environmental Technology has lower density, lower water absorption and higher compressive strength, is superior to CDWA for application in aggregates.
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4. Conclusions Based on the results obtained in the present research, the following conclusions could be drawn: (1) High-quality ceramsite could be fabricated using C&D waste as raw material by the special process developed in this study. During the sintering process, the sintering temperature, heating rate and soaking time were the major factors controlling the properties of the product. The optimized sintering temperature, heating rate and soaking time were 950◦ C, 8–12◦ C/min and 10–20 min, respectively. (2) CDWC was an excellent product with homogeneous internal porous structure and ceramic surface. Compared with CDWA, CDWC had high strength, lower density, lower water absorption and more homogeneous mechanical properties. TCLP results showed that heavy metals were well stabilized in CDWC by the sintering process in comparison with C&D waste. Acknowledgements This work was made possible by financial support from the National Water Pollution Control and Management Programs (2009ZX07212-002, 2012ZX07202-005) and the Environmental Public Welfare Project (201009026).
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