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Preparation of novel ceramic tiles with high Al2O3 content derived from coal fly ash
Construction and Building Materials 114 (2016) 888–895
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Preparation of novel ceramic tiles with high Al2O3 content derived from coal fly ash Ru Ji a, Zuotai Zhang b,c,⇑, Chen Yan c, Mengguang Zhu c, Zhongmin Li c a
School of Mechanical Engineering, University of Science and Technology Beijing, 100083 Beijing, PR China School of Environmental Science and Engineering, South University of Science and Technology of China, 518055 Shenzhen, PR China c Department of Energy and Resources Engineering and Beijing Key Laboratory for Solid Waste Utilization and Management, College of Engineering, Peking University, 100871 Beijing, PR China b
h i g h l i g h t s A novel alumina-rich ceramic system was proposed to fabricate the ceramic tile. Macro-properties and microscopic structures of the samples were investigated. New ceramic system exerts great influences on the performance of the sample. The study provides deep insights into the usage of alumina-rich industry solid waste.
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Article history: Received 23 January 2016 Received in revised form 29 March 2016 Accepted 4 April 2016
Keywords: Ceramic tile Coal fly ash Alumina-rich ceramic system
a b s t r a c t In order to take full advantage of coal fly ash (FA) with high alumina content, the present study proposed a novel method to fabricate the ceramic tile by using FA as the main raw material. The influences of FA content on the macro-properties and microscopic structures were systematically investigated. Results revealed that the rupture modulus of the sample containing 60 wt.% FA and 4 wt.% quartz can reach 51.28 MPa at 1200 °C. Furthermore, its corresponding water absorption capacity, apparent porosity, linear shrinkage are 0.47%, 1.1% and 13.51%, respectively, which all exceed the requirements for porcelain tiles. The excellent properties may be ascribed to the proposed alumina-rich ceramic system, which exerts great influences on the crystalline phase composition and the densification process of the sample. This study definitely paves the way for the efficient utilization of FA and provides deep insights into the usage of other alumina-rich industry solid waste. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Ceramic tile, one of the most frequent-used building materials, has been widely developed to meet the demand in construction considering that the annual newly built residences could reach 2 billion square meters in China. The fabrication of ceramic tile requires a large number of raw materials, for instance, 1 square meters of ceramic tile needs 20 kg of raw materials according to the previous study [1]. Hence, the demand for raw materials would be striking. Faced with the shortage of high-quality raw materials and the rising cost, it is therefore in urgent need to seek alternative raw materials with high availability. ⇑ Corresponding author at: Department of Energy and Resources Engineering and Beijing Key Laboratory for Solid Waste Utilization and Management, College of Engineering, Peking University, 100871 Beijing, PR China. E-mail address: [email protected] (Z. Zhang). http://dx.doi.org/10.1016/j.conbuildmat.2016.04.014 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
To realize this target, researchers have made great efforts to prepare ceramic tiles using industrial waste, such as steel slag, coal fly ash and coal gangue [2–11]. For example, Erol et al. [2] prepared ceramic materials from FA with no additives; Olgun et al. [3] developed ceramic tiles from FA and tincal ore waste; Lü et al. [6] studied the environment-oriented ceramic membrane supports with coal gangue and bauxite; while Wei et al. [7] recycled steel slag and harbor sediment into construction materials. These previous studies are significant, because they not only relieve the shortage situation of traditional materials, but also verify the feasibility of the method by using industrial waste as an alternative raw material of ceramic tile. Moreover, the utilization of FA is conducive to facilitate the enhancement of ceramic properties. In particular, He et al. [10] produced glass-ceramics sintered from FA and the solid parts contain more than 74 vol.% cordierite and mechanism property is higher than 30 MPa. Zhang et al. [11] proposed a useful choice to recycle FA from power plants to manufacture glass-
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R. Ji et al. / Construction and Building Materials 114 (2016) 888–895 Table 1 The chemical composition of the raw materials. Content (wt.%)
SiO2
Al2O3
K2O
Na2O
CaO
Fe2O3
MgO
TiO2
S
LOI
Fly ash Clay Feldspar Quartz
41.97 34.96 68.60 91.30
39.90 36.57 17.50 4.47
0.501 0.385 7.74 1.76
0.197 0.088 3.98 1.19
6.41 0.492 0.530 0.424
1.96 0.824 0.408 0.166
0.600 0.221 0.171 0.152
1.20 1.47 0.0358 0.0125
3.15 – – –
3.64 24.38 0.83 0.44
ceramic which was considered harmless by using the toxicity characteristic leaching procedure (TCLP) method. However, all of these studies focused on the ceramic preparation with certain solid wastes, but the corresponding ceramic system was rarely proposed, which limited their potential applications to other solid waste utilization. Before the starting of this study, the analysis and investigation of the characteristics of FA are required. FA is the by-product of coal combustion in thermal power plants [12,13] and the annual generation has reached 580 million tonnes in China by 2015 [14– 16]. Although approximately 69% of FA are comprehensively utilized [17], there are still a large amount of FA untreated and dumped in landfills or ash ponds, which not only occupies valuable land resources but also causes serious environment problems, such as water pollution, air pollution, disruption of ecological cycles and environmental hazards [18–20]. Therefore, it is necessary to develop useful methods to solve this problem. Meanwhile, it is worth to note that the composition of FA is complex and its physicochemical properties depend on the type of raw coal and the combustion conditions [15,21]. In China, there are many alumina-rich FA in Inner Mongolia and Shanxi province [14], in which Al2O3 content is as much high as 40%. However, the traditional ceramic tile belongs to SiO2-Al2O3-Na2O (K2O) ternary system with the Al2O3 content in the range of 15–25 wt.% and CaO content less than 3 wt.% in the raw materials mixture. For this reason, it is necessary to broaden the preparation area of the traditional ceramic. Thus, the present study is motivated to apply alumina-rich ceramic system (SiO2-Al2O3-CaO-K2O) to prepare high-quality ceramic tiles for the usage in special circumstances. In this paper, a novel alumina-rich ceramic system was proposed to fabricate the ceramic tile using FA as the raw materials in combination with the traditional raw materials. Both macroproperties (linear shrinkage rate, water absorption capacity, apparent porosity, rupture modulus) and microstructure analysis were systematically investigated. The proposed new ceramic system would not only realize the efficient utilization of alumina-rich FA, but also lay the foundation for the use of other alumina-rich waste.
2. Experimental procedure 2.1. Materials The FA is supplied by a thermal power plant, which is located in Shuozhou City, Shanxi province in China. Other raw materials including clay, feldspar and quartz, were derived from one region close to Shuozhou City. The chemical compositions of these materials were analyzed by X-ray fluorescence (XRF) and the results are shown in Table 1. For FA, the main compositions are SiO2, Al2O3, CaO and Fe2O3. Clay mainly consists of 34.96% SiO2 and 36.57% Al2O3. In feldspar, SiO2, Al2O3, K2O and Na2O are the major compositions. The main component of quartz is SiO2. The crystalline phases of the raw materials determined by X-Ray Diffraction (XRD) are shown in Fig. 1. XRD analysis results show that the FA is a heterogeneous material, and it is composed of glassy phases (about 58%) and parts of crystalline phases are quartz, mullite and a few of gypsum. This may be explained by the generation procedure of FA. The FA particles solidify rapidly while suspended in the flue gases. The major consequence of the rapid air cooling is that some minerals have no time to crystallize, i.e., part of FA exists with glassy phases. Nevertheless, some refractory phases in FA do not melt, and maintain the same crystal structure.
Fig. 1. XRD patterns of the different raw materials. (a) Fly ash, (b) Clay, (c) Feldspar, (d) Quartz.
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Table 2 Batch compositions of the samples (wt.%).
3. Results and discussion
(%)
1
2
3
4
5
2-1
2-2
2-3
2-4
Fly ash Clay Feldspar Quartz
50 20 30 –
60 20 20 –
70 20 10 –
80 20 – –
90 10 – –
60 20 16 4
60 20 12 8
60 20 8 12
60 20 4 16
In addition, XRD patterns show that boehmite and kaolinite are the main phases in clay. Feldspar mainly includes microcline, albite and quartz. The quartz has almost no impurities. 2.2. Sample preparation and characterization Raw materials were thoroughly mixed according to the batch composition as shown in Table 2. The FA and quartz contents in the batches varied from 50 to 90 wt.% and from 0 to 16 wt.%, respectively. Then the mixtures were wet ground in a ball mill for 15 h to obtain the homogeneous slurry. The slurry was sieved to pass through a 200-mesh screen and dried at 110 °C for 12 h. Subsequently, the mixtures were ground and sieved to pass through 100-mesh sieve. Then, the mixtures were granulated with the addition of a small amount of water (7 wt.%) which acted as a binder. Afterwards, samples of 61 mm 61 mm 7 mm were hydraulically compacted using uniaxial pressing at 20 MPa. Finally, the shaped samples were dried at 105 °C for 4 h, followed by calcination in a muffle furnace at different temperatures. The obtained samples were naturally cooled to room temperature in the furnace. The flow chart of the preparation process is shown in Fig. 2. The properties of the naturally cooled samples were characterized systematically. First, the microstructures and the phase characteristics were carried out by SEM and XRD at the room temperature, respectively. Second, the samples were tested regarding: linear shrinkage rate, water absorption capacity, apparent porosity and rupture modulus. On average, five specimens were used for each measurement. The linear shrinkage rate, LS (%), of fired samples has been determined by the following equation:
LS ¼
LS LC 100 LS
ð1Þ
where LS and LC are the diameters (mm) of the green and fired samples, respectively. The water absorption capacity, bulk density and apparent porosity were measured according to ASTM C373-88 [22], which involve drying the sample to constant mass (M), boiling in distilled water for 5 h and soaking for additional 24 h at ambient temperature. After impregnation, the mass (M 1 ) of the specimen while suspended in water and the saturated mass (M 2 ) are determined. Water absorption capacity, WA (%), expresses the relationship of the mass of water absorbed to the mass of the dry specimen (kg) as follows:
WA ¼
M2 M 100 M
M2 M 100 M2 M1
ð3Þ
The rupture modulus, R (MPa), measured according to ISO 10545-4 [23], is calculated by the following formula:
R¼
3Fl 2bh
2
3.1.1. The effect of the FA addition (or feldspar content) Fig. 3 shows the macro-properties versus FA addition at different sintering temperatures. According to the performance curves, the samples with different FA additions are classified into two groups. For samples 1–3, every macroscopic property shows an significant increasing trend, indicating the fast densification in the whole temperature range. This phenomenon could be attributed to the increasing alkali content originated from feldspar, which facilitates the generation of liquid phase and promotes the sintering reaction. Moreover, the beginning densification temperatures are 1100 °C for samples 1–2 and 1150 °C for sample 3, respectively, while this difference is also caused by the different alkali contents. In addition, the results show that the performances of samples at 1200 °C are in good agreement with standard requirements for ceramic wall tiles, such as fine earthenware tile, stoneware tile and fine stoneware tile [24]. However, for samples 4–5, the performance trends are basically constant, and don’t vary with the sintering temperature. In the investigated temperature range, all of the linear shrinkage rates are smaller than 3% and no sintering phenomenon is observed, which could be ascribed to low alkali content as no feldspar was added. Meanwhile, high FA content causes a high degree of polymerization (DOP) of the silicate structure in samples 4–5. These two factors result in the reduction of liquid in this sintering temperature range. Therefore, for samples 4–5, the values of the water absorption capacity and the rupture modulus are not satisfactory due to the standardized limit [24]. The above densification curve (linear shrinkage, water absorption capacity and apparent porosity versus sintering temperature) and rupture modulus versus sintering temperature were evaluated as parameters to find the optimum range of sintering temperature and FA addition content. It is therefore proposed that the optimum sintering temperature range is 1150–1200 °C with FA addition being 50–70 wt.%.
ð2Þ
The apparent porosity, AP (%), expresses the relationship of the volume of open pores to the exterior volume of the specimen:
AP ¼
3.1. Physical and mechanical properties
ð4Þ
where F is the breaking load, (N); l is the span between the support rods, (mm); b is the width of the test sample, (mm); h is the minimum thickness of the test specimen measured after the test along the broken edge, (mm).
3.1.2. The effect of quartz addition Fig. 4 shows the effect of quartz addition on the water absorption capacity and rupture modulus of the samples. Firstly, the water absorption capacity curve displays that with the increase of quartz addition, the water absorption capacity of the samples is increased. It can be explained as follows. For the samples 2-1 to 2-4, the decreasing feldspar content will cause the decrease of Na and K content, which will increase the softening temperature of the sample, leading to the decrease of the matrix densification. Hence, the sample 2 has the lowest water absorption capacity. However, for sample 2-1, although quartz addition leads to the slight enhancement of water absorption capacity, it still
Fig. 2. Schematic details of processing route for the specimens.
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Fig. 4. Rupture modulus and water absorption capacity values versus quartz addition at different sintering temperatures.
could meet the standard requirement of porcelain tile (maximum 0.5%) [24]. Second, it is also shown in Fig. 4 that, the proper quartz addition gives rise to the increase of the rupture modulus, but an excess of quartz addition decreases the rupture modulus of the samples. It is explained that, for the samples 2-1 to 2-3, the proper quartz addition may give rise to the increase of the mullite formation, which will increase the rupture modulus of the samples [25]. However, an excess of quartz addition will cause the volume change of the sample during the cooling process (Sample 2-4), which exerts a negative effect on the mechanical property [26]. For example, for sample 2-1, the highest rupture modulus reaches 51.28 MPa at 1200 °C, and its corresponding water absorption capacity is only 0.47%, which all exceed the standards with regard to highest quality ceramic tile (porcelain tile) [24]. 3.2. The microscopic structure and crystalline phases Fig. 5 shows the normalized ternary diagram of the samples 1– 5. Unlike the conventional K2O-SiO2-Al2O3 ternary ceramic system, the proposed novel system belongs to SiO2-Al2O3-CaO-K2O system. Firstly, this new system broadens the traditional ceramic crystal phase area (the green area in the Factsage phase diagrams) to a new phase area. Secondly, the contents of Al2O3 and CaO are much higher than that in traditional system [26–28]. Particularly, in sample 2 the contents of Al2O3 and CaO are high to about 40% and 5%, respectively. These two facts suggest that this new system will change the final crystal phase composition and densification process of the sample, which are discussed in the following sections.
Fig. 3. The values of macro-properties versus fly ash addition at different sintering temperatures.
3.2.1. The analysis of the effect of FA addition (or feldspar content) Effects of FA addition on samples 1–5 were investigated by SEM and XRD analysis combined with ternary diagrams, while the results are shown in Fig. 5. Fig. 5(a) displays that higher FA content leads to an increase of CaO (from sample 1-5 in the diagram), increasing the sintering process, in which CaO can act as a network modifier to reduce the viscosity of the amorphous phase. However, Fig. 5(b) shows that the decrease of feldspar content results in the reduction of K2O, which is a strong network modifier. In this way, for the samples from 1 to 5, lower K2O content will increase the viscosity of the amorphous phase and alleviate the sintering. According to the contents of K2O and CaO, the samples can be divided into two groups, which are consistent with the groups of the performance above.
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To verify the conjecture above, the microscopic structures of the samples 1–5 sintered at 1200 °C are shown in Fig. 6. The fracture surface of samples 4–5 is very loose with small solid particles, which is the indicative of no sintering, resulting in bad quality of samples. For sample 3, there is a relatively granular structure and partial sintering is suggested; while for samples 1–2, the surface is much smoother, indicating a higher degree of sintering. Therefore, it is manifested that the sintering phenomenon is conducive to the excellent performance. The crystalline phases of samples 1–5 are analyzed by XRD in Fig. 7. According to the XRD results, the samples can also be divided into two groups, which agree well with the above classification.
For samples 4–5, there is a characteristic peak of cristobalite due to the transformation from excess SiO2 [29], which will cause crystal transformation associated with large volume change (5%) in cooling process and decrease the strength property [30]. As shown in Fig. 6, all of these facts result in the decrease of the performance in view of no sintering in samples 4–5. However, for samples 1–3, the addition of R2O originated from feldspar will retard or even stop the formation of cristobalite [31]. Therefore, samples 1–3 exhibit better strength properties. It is interesting to note that, for samples 2–3, there is a new crystalline phase anorthite observed in XRD patterns. Anorthite is formed by the reaction of calcium sulfate with microcline and albite originated from feldspar, which can improve the strength property of
(a) System SiO2-Al2O3-CaO.
(b) System SiO2-Al2O3-K2O. Fig. 5. Ternary diagram of the samples 1–5 with different FA addition. (The points 1, 2, 3, 4, 5 correspond to the samples 1, 2, 3, 4, 5, respectively. The green area indicates the traditional ceramic crystal phase area.)
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Fig. 6. SEM micrographs of the samples with different FA content at the sintering temperature 1200 °C. (a) Sample 1, (b) Sample 2, (c) Sample 3, (d) Sample 4, (e) Sample 5.
Fig. 7. XRD patterns of the samples with different FA content at the sintering temperature of 1200 °C.
the sample [32]. However, for sample 1, Fig. 5(a) indicates that the content of CaO is very low owing to low FA addition, so no anorthite is formed. Therefore, it could clearly explain the phenomenon in which rupture modulus of sample 1 is smaller than that of sample 2, in spite of the observed densification trends. Especially for sample 2, there is no quartz, replaced by large amount of mullite, which is different from that in traditional porcelain stoneware tiles [27,28]. First, in the traditional tile, the existence of quartz will cause the change of volume during the cooling process, which exerts a negative effect on the mechanical property of the product such as cracking in the severe case [26]. Therefore, samples with no quartz addition will have better strength properties. Second, the large amount of mullite in our tiles is caused by the high Al2O3 content in the proposed ceramic system, in which Al2O3 content is varied from 33 to 40 wt.%. The formation of mullite will not only improve its strength property, but also lead to high antiknock characteristic [25]. 3.2.2. The analysis of the effect of sintering temperature The effect of the sintering temperature on the sample 2 was investigated by SEM and XRD analysis, respectively. Fig. 8 shows
fracture section morphology in the body of the sample 2 at different sintering temperatures. It can be observed that higher sintering temperature facilitates the improvement of sintering. At 1000 °C, the sample is still in an incompact state. With increasing temperature to 1100 °C, there is an appearance of the interconnected structure, suggesting the beginning of densification. This is in consistence with the results in performance analysis above in Fig. 3. As the sintering temperature reaches 1200 °C, the particles completely disappear and the sintering is completed, which can promote the improvement of its quality. Fig. 9 shows the XRD patterns of the sample 2 sintered at different temperatures. It can be seen that the generation of new crystalline phase starts at about 1100 °C, which is lower than the temperature of the complete densification (1200 °C). Hence, the sample in this study is an equilibrium material, which is conducive to enhance its performance. As shown in Fig. 9, the microcline and albite disappear at the temperature higher than 1100 °C, which are replaced by anorthite. Noteworthily, the characteristics peaks of quartz decrease or even disappear at 1200 °C accompanied by an increase of mullite. The change of the crystal phase will enhance the mechanical strength of the sample, which verifies the explanation in Fig. 7. 3.3. The characterization of the pilot scale sample In this part, the ceramic system (65 wt.% FA, 18 wt.% clay, 13 wt. % feldspar and 4% quartz) was introduced into the practical manufacturing process without any alteration to the existing production line. It can be found that there is scarcely any exfoliation or pinhole on the surface of the pilot sample. The surface and the overall texture were fully acceptable (Fig. 10). The values of water absorption capacity and rupture modulus were 0.29% and 58 MPa, respectively, which exhibited better performance than that of the standard porcelain tiles. The results indicate that it is possible to use FA as a potential secondary raw material for the production of ceramic tiles. What’s more, the proposed new method can bring great economic and environment benefits. In China, the annual newly built residences could reach about 2 billion square meters. We assumed that one tenth of the newly built residences are decorated with
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Fig. 8. SEM micrographs of the sample 2 at different sintering temperatures. (a) 1000 °C, (b) 1100 °C, (c) 1150 °C, (d) 1200 °C.
Fig. 9. XRD patterns of the sample 2 at different sintering temperatures.
ceramic floor tile. Because the production of 1 square meters of ceramic tile requires about 20 kg of raw materials [1], the corresponding consumption of the raw materials is 40 million tonnes per year. According to the market research, the average price of the traditional ceramic raw materials is 270 RMB/t. Hence, the annual raw material cost will be as high as 10.8 billion RMB. Assumed that two-tenth of ceramic tiles are manufactured by using FA addition, only if one-tenth of the traditional raw materials is replaced by FA, the annual cost will be reduced by 216 million RMB. In addition, the disposal of FA needs a huge filling cost. According to relevant policy in China (Levy standard for the disposition of solid waste and hazardous wastes), the cost of FA disposal is about 30 RMB/t. Therefore, the use of FA in production of ceramic tile will further save 24 million RMB per year. Moreover, both of the recycling of FA and the less consumption of the traditional ceramic raw material are beneficial to the protection of the environment.
Fig. 10. Pilot scale sample after sintering in case of 65 wt.% FA content.
R. Ji et al. / Construction and Building Materials 114 (2016) 888–895
4. Conclusions An alumina-rich ceramic system was proposed to fabricate the ceramic tile by using FA as the main raw material. The rupture modulus of the sample can reach 51.28 MPa, which exceeds 47% than the standard requirement. The excellent property is attributed to the new system, SiO2-Al2O3-CaO-K2O system with high Al2O3 and CaO contents. Furthermore, for the pilot scale samples, the performances are far better than the standard requirements for porcelain tiles. Moreover, the surface and the overall texture are fully acceptable. Therefore, this paper develops a new field for FA utilization, and provides a basis method for the application of other alumina-rich industry solid waste. Acknowledgements The paper was supported by the Fundamental Research Funds for the Central Universities (FRF-TP-15-085A1). Supports by the Key Projects in the National Science & Technology Pillar Program (2013BAC14B07) and Common Development Fund of Beijing and the National Natural Science Foundation of China (51522401, 51472007 and 51272005) are acknowledged. References [1] A. Zimmer, C.P. Bergmann, Fly ash of mineral coal as ceramic tiles raw material, Waste Manage. 27 (1) (2007) 59–68. [2] Zahide Bayer Ozturk, ElifEren Gultekin, Preparation of ceramic wall tiling derived from blast furnace slag, Ceram. Int. 41 (2015) 12020–12026. [3] Z. Jian, D. Wen, J. Li, et al., Utilization of coal fly ash in the glass–ceramic production, J. Hazard. Mater. 149 (2) (2007) 523–526. [4] J. Qin, C. Cui, X.Y. Cui, et al., Preparation and characterization of ceramsite from lime mud and coal fly ash, Constr. Build. Mater. 95 (2015) 10–17. [5] J. Ye, W. Zhang, D. Shi, Effect of elevated temperature on the properties of geopolymer synthesized from calcined ore-dressing tailing of bauxite and ground-granulated blast furnace slag, Constr. Build. Mater. 69 (11) (2014) 41– 48. [6] J. Cao, X. Dong, L. Li, et al., Recycling of waste fly ash for production of porous mullite ceramic membrane supports with increased porosity, J. Eur. Ceram. Soc. 34 (13) (2014) 3181–3194. [7] National Development and Reform Commission, The annual report of the comprehensive utilization of Chinese resources, Renewable Resour. Recycl. Economy 7 (10) (2014) (in Chinese). [8] Y. Chen, Y. Zhang, T. Chen, et al., Preparation and characterization of red porcelain tiles with hematite tailings, Constr. Build. Mater. 38 (2013) 1083– 1088. [9] A. Jonker, J.H. Potgieter, An evaluation of selected waste resources for utilization in ceramic materials applications, J. Eur. Ceram. Soc. 25 (13) (2005) 3145–3149.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Preparation of novel ceramic tiles with high Al2O3 content derived from coal fly ash Ru Ji a, Zuotai Zhang b,c,⇑, Chen Yan c, Mengguang Zhu c, Zhongmin Li c a
School of Mechanical Engineering, University of Science and Technology Beijing, 100083 Beijing, PR China School of Environmental Science and Engineering, South University of Science and Technology of China, 518055 Shenzhen, PR China c Department of Energy and Resources Engineering and Beijing Key Laboratory for Solid Waste Utilization and Management, College of Engineering, Peking University, 100871 Beijing, PR China b
h i g h l i g h t s A novel alumina-rich ceramic system was proposed to fabricate the ceramic tile. Macro-properties and microscopic structures of the samples were investigated. New ceramic system exerts great influences on the performance of the sample. The study provides deep insights into the usage of alumina-rich industry solid waste.
a r t i c l e
i n f o
Article history: Received 23 January 2016 Received in revised form 29 March 2016 Accepted 4 April 2016
Keywords: Ceramic tile Coal fly ash Alumina-rich ceramic system
a b s t r a c t In order to take full advantage of coal fly ash (FA) with high alumina content, the present study proposed a novel method to fabricate the ceramic tile by using FA as the main raw material. The influences of FA content on the macro-properties and microscopic structures were systematically investigated. Results revealed that the rupture modulus of the sample containing 60 wt.% FA and 4 wt.% quartz can reach 51.28 MPa at 1200 °C. Furthermore, its corresponding water absorption capacity, apparent porosity, linear shrinkage are 0.47%, 1.1% and 13.51%, respectively, which all exceed the requirements for porcelain tiles. The excellent properties may be ascribed to the proposed alumina-rich ceramic system, which exerts great influences on the crystalline phase composition and the densification process of the sample. This study definitely paves the way for the efficient utilization of FA and provides deep insights into the usage of other alumina-rich industry solid waste. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Ceramic tile, one of the most frequent-used building materials, has been widely developed to meet the demand in construction considering that the annual newly built residences could reach 2 billion square meters in China. The fabrication of ceramic tile requires a large number of raw materials, for instance, 1 square meters of ceramic tile needs 20 kg of raw materials according to the previous study [1]. Hence, the demand for raw materials would be striking. Faced with the shortage of high-quality raw materials and the rising cost, it is therefore in urgent need to seek alternative raw materials with high availability. ⇑ Corresponding author at: Department of Energy and Resources Engineering and Beijing Key Laboratory for Solid Waste Utilization and Management, College of Engineering, Peking University, 100871 Beijing, PR China. E-mail address: [email protected] (Z. Zhang). http://dx.doi.org/10.1016/j.conbuildmat.2016.04.014 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
To realize this target, researchers have made great efforts to prepare ceramic tiles using industrial waste, such as steel slag, coal fly ash and coal gangue [2–11]. For example, Erol et al. [2] prepared ceramic materials from FA with no additives; Olgun et al. [3] developed ceramic tiles from FA and tincal ore waste; Lü et al. [6] studied the environment-oriented ceramic membrane supports with coal gangue and bauxite; while Wei et al. [7] recycled steel slag and harbor sediment into construction materials. These previous studies are significant, because they not only relieve the shortage situation of traditional materials, but also verify the feasibility of the method by using industrial waste as an alternative raw material of ceramic tile. Moreover, the utilization of FA is conducive to facilitate the enhancement of ceramic properties. In particular, He et al. [10] produced glass-ceramics sintered from FA and the solid parts contain more than 74 vol.% cordierite and mechanism property is higher than 30 MPa. Zhang et al. [11] proposed a useful choice to recycle FA from power plants to manufacture glass-
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R. Ji et al. / Construction and Building Materials 114 (2016) 888–895 Table 1 The chemical composition of the raw materials. Content (wt.%)
SiO2
Al2O3
K2O
Na2O
CaO
Fe2O3
MgO
TiO2
S
LOI
Fly ash Clay Feldspar Quartz
41.97 34.96 68.60 91.30
39.90 36.57 17.50 4.47
0.501 0.385 7.74 1.76
0.197 0.088 3.98 1.19
6.41 0.492 0.530 0.424
1.96 0.824 0.408 0.166
0.600 0.221 0.171 0.152
1.20 1.47 0.0358 0.0125
3.15 – – –
3.64 24.38 0.83 0.44
ceramic which was considered harmless by using the toxicity characteristic leaching procedure (TCLP) method. However, all of these studies focused on the ceramic preparation with certain solid wastes, but the corresponding ceramic system was rarely proposed, which limited their potential applications to other solid waste utilization. Before the starting of this study, the analysis and investigation of the characteristics of FA are required. FA is the by-product of coal combustion in thermal power plants [12,13] and the annual generation has reached 580 million tonnes in China by 2015 [14– 16]. Although approximately 69% of FA are comprehensively utilized [17], there are still a large amount of FA untreated and dumped in landfills or ash ponds, which not only occupies valuable land resources but also causes serious environment problems, such as water pollution, air pollution, disruption of ecological cycles and environmental hazards [18–20]. Therefore, it is necessary to develop useful methods to solve this problem. Meanwhile, it is worth to note that the composition of FA is complex and its physicochemical properties depend on the type of raw coal and the combustion conditions [15,21]. In China, there are many alumina-rich FA in Inner Mongolia and Shanxi province [14], in which Al2O3 content is as much high as 40%. However, the traditional ceramic tile belongs to SiO2-Al2O3-Na2O (K2O) ternary system with the Al2O3 content in the range of 15–25 wt.% and CaO content less than 3 wt.% in the raw materials mixture. For this reason, it is necessary to broaden the preparation area of the traditional ceramic. Thus, the present study is motivated to apply alumina-rich ceramic system (SiO2-Al2O3-CaO-K2O) to prepare high-quality ceramic tiles for the usage in special circumstances. In this paper, a novel alumina-rich ceramic system was proposed to fabricate the ceramic tile using FA as the raw materials in combination with the traditional raw materials. Both macroproperties (linear shrinkage rate, water absorption capacity, apparent porosity, rupture modulus) and microstructure analysis were systematically investigated. The proposed new ceramic system would not only realize the efficient utilization of alumina-rich FA, but also lay the foundation for the use of other alumina-rich waste.
2. Experimental procedure 2.1. Materials The FA is supplied by a thermal power plant, which is located in Shuozhou City, Shanxi province in China. Other raw materials including clay, feldspar and quartz, were derived from one region close to Shuozhou City. The chemical compositions of these materials were analyzed by X-ray fluorescence (XRF) and the results are shown in Table 1. For FA, the main compositions are SiO2, Al2O3, CaO and Fe2O3. Clay mainly consists of 34.96% SiO2 and 36.57% Al2O3. In feldspar, SiO2, Al2O3, K2O and Na2O are the major compositions. The main component of quartz is SiO2. The crystalline phases of the raw materials determined by X-Ray Diffraction (XRD) are shown in Fig. 1. XRD analysis results show that the FA is a heterogeneous material, and it is composed of glassy phases (about 58%) and parts of crystalline phases are quartz, mullite and a few of gypsum. This may be explained by the generation procedure of FA. The FA particles solidify rapidly while suspended in the flue gases. The major consequence of the rapid air cooling is that some minerals have no time to crystallize, i.e., part of FA exists with glassy phases. Nevertheless, some refractory phases in FA do not melt, and maintain the same crystal structure.
Fig. 1. XRD patterns of the different raw materials. (a) Fly ash, (b) Clay, (c) Feldspar, (d) Quartz.
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Table 2 Batch compositions of the samples (wt.%).
3. Results and discussion
(%)
1
2
3
4
5
2-1
2-2
2-3
2-4
Fly ash Clay Feldspar Quartz
50 20 30 –
60 20 20 –
70 20 10 –
80 20 – –
90 10 – –
60 20 16 4
60 20 12 8
60 20 8 12
60 20 4 16
In addition, XRD patterns show that boehmite and kaolinite are the main phases in clay. Feldspar mainly includes microcline, albite and quartz. The quartz has almost no impurities. 2.2. Sample preparation and characterization Raw materials were thoroughly mixed according to the batch composition as shown in Table 2. The FA and quartz contents in the batches varied from 50 to 90 wt.% and from 0 to 16 wt.%, respectively. Then the mixtures were wet ground in a ball mill for 15 h to obtain the homogeneous slurry. The slurry was sieved to pass through a 200-mesh screen and dried at 110 °C for 12 h. Subsequently, the mixtures were ground and sieved to pass through 100-mesh sieve. Then, the mixtures were granulated with the addition of a small amount of water (7 wt.%) which acted as a binder. Afterwards, samples of 61 mm 61 mm 7 mm were hydraulically compacted using uniaxial pressing at 20 MPa. Finally, the shaped samples were dried at 105 °C for 4 h, followed by calcination in a muffle furnace at different temperatures. The obtained samples were naturally cooled to room temperature in the furnace. The flow chart of the preparation process is shown in Fig. 2. The properties of the naturally cooled samples were characterized systematically. First, the microstructures and the phase characteristics were carried out by SEM and XRD at the room temperature, respectively. Second, the samples were tested regarding: linear shrinkage rate, water absorption capacity, apparent porosity and rupture modulus. On average, five specimens were used for each measurement. The linear shrinkage rate, LS (%), of fired samples has been determined by the following equation:
LS ¼
LS LC 100 LS
ð1Þ
where LS and LC are the diameters (mm) of the green and fired samples, respectively. The water absorption capacity, bulk density and apparent porosity were measured according to ASTM C373-88 [22], which involve drying the sample to constant mass (M), boiling in distilled water for 5 h and soaking for additional 24 h at ambient temperature. After impregnation, the mass (M 1 ) of the specimen while suspended in water and the saturated mass (M 2 ) are determined. Water absorption capacity, WA (%), expresses the relationship of the mass of water absorbed to the mass of the dry specimen (kg) as follows:
WA ¼
M2 M 100 M
M2 M 100 M2 M1
ð3Þ
The rupture modulus, R (MPa), measured according to ISO 10545-4 [23], is calculated by the following formula:
R¼
3Fl 2bh
2
3.1.1. The effect of the FA addition (or feldspar content) Fig. 3 shows the macro-properties versus FA addition at different sintering temperatures. According to the performance curves, the samples with different FA additions are classified into two groups. For samples 1–3, every macroscopic property shows an significant increasing trend, indicating the fast densification in the whole temperature range. This phenomenon could be attributed to the increasing alkali content originated from feldspar, which facilitates the generation of liquid phase and promotes the sintering reaction. Moreover, the beginning densification temperatures are 1100 °C for samples 1–2 and 1150 °C for sample 3, respectively, while this difference is also caused by the different alkali contents. In addition, the results show that the performances of samples at 1200 °C are in good agreement with standard requirements for ceramic wall tiles, such as fine earthenware tile, stoneware tile and fine stoneware tile [24]. However, for samples 4–5, the performance trends are basically constant, and don’t vary with the sintering temperature. In the investigated temperature range, all of the linear shrinkage rates are smaller than 3% and no sintering phenomenon is observed, which could be ascribed to low alkali content as no feldspar was added. Meanwhile, high FA content causes a high degree of polymerization (DOP) of the silicate structure in samples 4–5. These two factors result in the reduction of liquid in this sintering temperature range. Therefore, for samples 4–5, the values of the water absorption capacity and the rupture modulus are not satisfactory due to the standardized limit [24]. The above densification curve (linear shrinkage, water absorption capacity and apparent porosity versus sintering temperature) and rupture modulus versus sintering temperature were evaluated as parameters to find the optimum range of sintering temperature and FA addition content. It is therefore proposed that the optimum sintering temperature range is 1150–1200 °C with FA addition being 50–70 wt.%.
ð2Þ
The apparent porosity, AP (%), expresses the relationship of the volume of open pores to the exterior volume of the specimen:
AP ¼
3.1. Physical and mechanical properties
ð4Þ
where F is the breaking load, (N); l is the span between the support rods, (mm); b is the width of the test sample, (mm); h is the minimum thickness of the test specimen measured after the test along the broken edge, (mm).
3.1.2. The effect of quartz addition Fig. 4 shows the effect of quartz addition on the water absorption capacity and rupture modulus of the samples. Firstly, the water absorption capacity curve displays that with the increase of quartz addition, the water absorption capacity of the samples is increased. It can be explained as follows. For the samples 2-1 to 2-4, the decreasing feldspar content will cause the decrease of Na and K content, which will increase the softening temperature of the sample, leading to the decrease of the matrix densification. Hence, the sample 2 has the lowest water absorption capacity. However, for sample 2-1, although quartz addition leads to the slight enhancement of water absorption capacity, it still
Fig. 2. Schematic details of processing route for the specimens.
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Fig. 4. Rupture modulus and water absorption capacity values versus quartz addition at different sintering temperatures.
could meet the standard requirement of porcelain tile (maximum 0.5%) [24]. Second, it is also shown in Fig. 4 that, the proper quartz addition gives rise to the increase of the rupture modulus, but an excess of quartz addition decreases the rupture modulus of the samples. It is explained that, for the samples 2-1 to 2-3, the proper quartz addition may give rise to the increase of the mullite formation, which will increase the rupture modulus of the samples [25]. However, an excess of quartz addition will cause the volume change of the sample during the cooling process (Sample 2-4), which exerts a negative effect on the mechanical property [26]. For example, for sample 2-1, the highest rupture modulus reaches 51.28 MPa at 1200 °C, and its corresponding water absorption capacity is only 0.47%, which all exceed the standards with regard to highest quality ceramic tile (porcelain tile) [24]. 3.2. The microscopic structure and crystalline phases Fig. 5 shows the normalized ternary diagram of the samples 1– 5. Unlike the conventional K2O-SiO2-Al2O3 ternary ceramic system, the proposed novel system belongs to SiO2-Al2O3-CaO-K2O system. Firstly, this new system broadens the traditional ceramic crystal phase area (the green area in the Factsage phase diagrams) to a new phase area. Secondly, the contents of Al2O3 and CaO are much higher than that in traditional system [26–28]. Particularly, in sample 2 the contents of Al2O3 and CaO are high to about 40% and 5%, respectively. These two facts suggest that this new system will change the final crystal phase composition and densification process of the sample, which are discussed in the following sections.
Fig. 3. The values of macro-properties versus fly ash addition at different sintering temperatures.
3.2.1. The analysis of the effect of FA addition (or feldspar content) Effects of FA addition on samples 1–5 were investigated by SEM and XRD analysis combined with ternary diagrams, while the results are shown in Fig. 5. Fig. 5(a) displays that higher FA content leads to an increase of CaO (from sample 1-5 in the diagram), increasing the sintering process, in which CaO can act as a network modifier to reduce the viscosity of the amorphous phase. However, Fig. 5(b) shows that the decrease of feldspar content results in the reduction of K2O, which is a strong network modifier. In this way, for the samples from 1 to 5, lower K2O content will increase the viscosity of the amorphous phase and alleviate the sintering. According to the contents of K2O and CaO, the samples can be divided into two groups, which are consistent with the groups of the performance above.
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To verify the conjecture above, the microscopic structures of the samples 1–5 sintered at 1200 °C are shown in Fig. 6. The fracture surface of samples 4–5 is very loose with small solid particles, which is the indicative of no sintering, resulting in bad quality of samples. For sample 3, there is a relatively granular structure and partial sintering is suggested; while for samples 1–2, the surface is much smoother, indicating a higher degree of sintering. Therefore, it is manifested that the sintering phenomenon is conducive to the excellent performance. The crystalline phases of samples 1–5 are analyzed by XRD in Fig. 7. According to the XRD results, the samples can also be divided into two groups, which agree well with the above classification.
For samples 4–5, there is a characteristic peak of cristobalite due to the transformation from excess SiO2 [29], which will cause crystal transformation associated with large volume change (5%) in cooling process and decrease the strength property [30]. As shown in Fig. 6, all of these facts result in the decrease of the performance in view of no sintering in samples 4–5. However, for samples 1–3, the addition of R2O originated from feldspar will retard or even stop the formation of cristobalite [31]. Therefore, samples 1–3 exhibit better strength properties. It is interesting to note that, for samples 2–3, there is a new crystalline phase anorthite observed in XRD patterns. Anorthite is formed by the reaction of calcium sulfate with microcline and albite originated from feldspar, which can improve the strength property of
(a) System SiO2-Al2O3-CaO.
(b) System SiO2-Al2O3-K2O. Fig. 5. Ternary diagram of the samples 1–5 with different FA addition. (The points 1, 2, 3, 4, 5 correspond to the samples 1, 2, 3, 4, 5, respectively. The green area indicates the traditional ceramic crystal phase area.)
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Fig. 6. SEM micrographs of the samples with different FA content at the sintering temperature 1200 °C. (a) Sample 1, (b) Sample 2, (c) Sample 3, (d) Sample 4, (e) Sample 5.
Fig. 7. XRD patterns of the samples with different FA content at the sintering temperature of 1200 °C.
the sample [32]. However, for sample 1, Fig. 5(a) indicates that the content of CaO is very low owing to low FA addition, so no anorthite is formed. Therefore, it could clearly explain the phenomenon in which rupture modulus of sample 1 is smaller than that of sample 2, in spite of the observed densification trends. Especially for sample 2, there is no quartz, replaced by large amount of mullite, which is different from that in traditional porcelain stoneware tiles [27,28]. First, in the traditional tile, the existence of quartz will cause the change of volume during the cooling process, which exerts a negative effect on the mechanical property of the product such as cracking in the severe case [26]. Therefore, samples with no quartz addition will have better strength properties. Second, the large amount of mullite in our tiles is caused by the high Al2O3 content in the proposed ceramic system, in which Al2O3 content is varied from 33 to 40 wt.%. The formation of mullite will not only improve its strength property, but also lead to high antiknock characteristic [25]. 3.2.2. The analysis of the effect of sintering temperature The effect of the sintering temperature on the sample 2 was investigated by SEM and XRD analysis, respectively. Fig. 8 shows
fracture section morphology in the body of the sample 2 at different sintering temperatures. It can be observed that higher sintering temperature facilitates the improvement of sintering. At 1000 °C, the sample is still in an incompact state. With increasing temperature to 1100 °C, there is an appearance of the interconnected structure, suggesting the beginning of densification. This is in consistence with the results in performance analysis above in Fig. 3. As the sintering temperature reaches 1200 °C, the particles completely disappear and the sintering is completed, which can promote the improvement of its quality. Fig. 9 shows the XRD patterns of the sample 2 sintered at different temperatures. It can be seen that the generation of new crystalline phase starts at about 1100 °C, which is lower than the temperature of the complete densification (1200 °C). Hence, the sample in this study is an equilibrium material, which is conducive to enhance its performance. As shown in Fig. 9, the microcline and albite disappear at the temperature higher than 1100 °C, which are replaced by anorthite. Noteworthily, the characteristics peaks of quartz decrease or even disappear at 1200 °C accompanied by an increase of mullite. The change of the crystal phase will enhance the mechanical strength of the sample, which verifies the explanation in Fig. 7. 3.3. The characterization of the pilot scale sample In this part, the ceramic system (65 wt.% FA, 18 wt.% clay, 13 wt. % feldspar and 4% quartz) was introduced into the practical manufacturing process without any alteration to the existing production line. It can be found that there is scarcely any exfoliation or pinhole on the surface of the pilot sample. The surface and the overall texture were fully acceptable (Fig. 10). The values of water absorption capacity and rupture modulus were 0.29% and 58 MPa, respectively, which exhibited better performance than that of the standard porcelain tiles. The results indicate that it is possible to use FA as a potential secondary raw material for the production of ceramic tiles. What’s more, the proposed new method can bring great economic and environment benefits. In China, the annual newly built residences could reach about 2 billion square meters. We assumed that one tenth of the newly built residences are decorated with
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Fig. 8. SEM micrographs of the sample 2 at different sintering temperatures. (a) 1000 °C, (b) 1100 °C, (c) 1150 °C, (d) 1200 °C.
Fig. 9. XRD patterns of the sample 2 at different sintering temperatures.
ceramic floor tile. Because the production of 1 square meters of ceramic tile requires about 20 kg of raw materials [1], the corresponding consumption of the raw materials is 40 million tonnes per year. According to the market research, the average price of the traditional ceramic raw materials is 270 RMB/t. Hence, the annual raw material cost will be as high as 10.8 billion RMB. Assumed that two-tenth of ceramic tiles are manufactured by using FA addition, only if one-tenth of the traditional raw materials is replaced by FA, the annual cost will be reduced by 216 million RMB. In addition, the disposal of FA needs a huge filling cost. According to relevant policy in China (Levy standard for the disposition of solid waste and hazardous wastes), the cost of FA disposal is about 30 RMB/t. Therefore, the use of FA in production of ceramic tile will further save 24 million RMB per year. Moreover, both of the recycling of FA and the less consumption of the traditional ceramic raw material are beneficial to the protection of the environment.
Fig. 10. Pilot scale sample after sintering in case of 65 wt.% FA content.
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4. Conclusions An alumina-rich ceramic system was proposed to fabricate the ceramic tile by using FA as the main raw material. The rupture modulus of the sample can reach 51.28 MPa, which exceeds 47% than the standard requirement. The excellent property is attributed to the new system, SiO2-Al2O3-CaO-K2O system with high Al2O3 and CaO contents. Furthermore, for the pilot scale samples, the performances are far better than the standard requirements for porcelain tiles. Moreover, the surface and the overall texture are fully acceptable. Therefore, this paper develops a new field for FA utilization, and provides a basis method for the application of other alumina-rich industry solid waste. Acknowledgements The paper was supported by the Fundamental Research Funds for the Central Universities (FRF-TP-15-085A1). Supports by the Key Projects in the National Science & Technology Pillar Program (2013BAC14B07) and Common Development Fund of Beijing and the National Natural Science Foundation of China (51522401, 51472007 and 51272005) are acknowledged. References [1] A. Zimmer, C.P. Bergmann, Fly ash of mineral coal as ceramic tiles raw material, Waste Manage. 27 (1) (2007) 59–68. [2] Zahide Bayer Ozturk, ElifEren Gultekin, Preparation of ceramic wall tiling derived from blast furnace slag, Ceram. Int. 41 (2015) 12020–12026. [3] Z. Jian, D. Wen, J. Li, et al., Utilization of coal fly ash in the glass–ceramic production, J. Hazard. Mater. 149 (2) (2007) 523–526. [4] J. Qin, C. Cui, X.Y. Cui, et al., Preparation and characterization of ceramsite from lime mud and coal fly ash, Constr. Build. Mater. 95 (2015) 10–17. [5] J. Ye, W. Zhang, D. Shi, Effect of elevated temperature on the properties of geopolymer synthesized from calcined ore-dressing tailing of bauxite and ground-granulated blast furnace slag, Constr. Build. Mater. 69 (11) (2014) 41– 48. [6] J. Cao, X. Dong, L. Li, et al., Recycling of waste fly ash for production of porous mullite ceramic membrane supports with increased porosity, J. Eur. Ceram. Soc. 34 (13) (2014) 3181–3194. [7] National Development and Reform Commission, The annual report of the comprehensive utilization of Chinese resources, Renewable Resour. Recycl. Economy 7 (10) (2014) (in Chinese). [8] Y. Chen, Y. Zhang, T. Chen, et al., Preparation and characterization of red porcelain tiles with hematite tailings, Constr. Build. Mater. 38 (2013) 1083– 1088. [9] A. Jonker, J.H. Potgieter, An evaluation of selected waste resources for utilization in ceramic materials applications, J. Eur. Ceram. Soc. 25 (13) (2005) 3145–3149.
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