Preparation and characterization of whisker-reinforced ceramics from coal fly ash

Author’s Accepted Manuscript Preparation and characterization of whiskerreinforced ceramics from coal fly ash Yang Luo, Shuhua Ma, Zhenqing Zhao, Zehua Wang, Shili Zheng, Xiaohui Wang www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(16)31742-4 http://dx.doi.org/10.1016/j.ceramint.2016.09.211 CERI13871

To appear in: Ceramics International Received date: 12 September 2016 Revised date: 28 September 2016 Accepted date: 30 September 2016 Cite this article as: Yang Luo, Shuhua Ma, Zhenqing Zhao, Zehua Wang, Shili Zheng and Xiaohui Wang, Preparation and characterization of whisker-reinforced ceramics from coal fly ash, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.09.211 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Preparation and characterization of whisker-reinforced ceramics from coal fly ash Yang Luoa,b, Shuhua Maa, Zhenqing Zhaoa, Zehua Wanga,b, Shili Zhenga*, Xiaohui Wanga a

National Engineering Laboratory for Hydrometallurgy Cleaner Production Technology,

Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China b

University of Chinese Academy of Science, Beijing 100049, PR China

*

Corresponding author. Fax: +86 10 82544858. [email protected]

Abstract A new type of ceramic is developed based on tobermorite whiskers, in which aluminum replaces some of the original silicon atoms. These Al-tobermorite whiskers are synthesized from coal fly ash through a dynamic hydrothermal method. Their fine fiber morphology and Al-substitution produce bulk ceramics with excellent mechanical properties. When sintered at 900 °C, a temperature lower than the one used for

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conventional ceramics, these whisker-reinforced ceramics have a flexural strength of 52.27 MPa, an open porosity of 9.51%, and a bulk density of 2.15 g/cm3. These properties are attributed to the Al/Si ratio in the fly ash within an ideal range to improve the crystal form of the tobermorite as well as the wollastonite produced by sintering, without leaving residual Al that may affect the mechanical properties of the ceramics.

Abbreviations: CFA, coal fly ash; TCFA, tobermorite synthesized from CFA; CRE, ceramic; TOB, tobermorite.

Keywords: Coal fly ash, Dynamic hydrothermal method, Tobermorite fiber, Low temperature sintering, Reinforced ceramic

1. Introduction

Economic development in China has caused energy demands to soar, with coal being the major fuel for electricity generation in the country. The total production of coal fly ash (CFA) is estimated to exceed 4.5 × 1011 kg per year [1]. Finding a use for this CFA has become a significant problem [2,3], especially because the increasing number of coal power plants entails the production of even greater volumes of CFA in the future [4]. The CFA not only occupies valuable farmland, but also can create severe pollution issues in the surrounding environment [5,6]. While the efficient disposal of 2

CFA is especially challenging in China, it is also a global issue. The practical use of CFA started in the 1930s in the USA [3]; since then, many new applications have been attempted. These include CFA use in construction; as a low-cost adsorbent for removing organic compounds, flue gases, and metals; as a lightweight aggregate, mine backfill, or road sub-base; and in the synthesis of zeolites. Despite these efforts, only a small percentage of CFA (15%) is currently utilized in China [7].

Tobermorite [Ca5Si6O17·5H2O] is a crystalline calcium silicate hydrate synthesized by hydrothermal reactions in the CaO-SiO2-H2O system. Tobermorite synthesis has been studied globally, but generally only when using raw materials from natural resources [8]. The use of CFA as a precursor in tobermorite synthesis has not been reported.

Many studies have shown that fibrous tobermorite-type materials, and particularly tobermorite whiskers, have great potential for structural use because they show excellent mechanical properties and durability. However, the mechanical strength of directly formed tobermorite-type materials is generally insufficient for most applications. This study therefore combines sintering with a hydrothermal method for converting CFA into fibrous Al-tobermorite “whiskers,” to be used as reinforcing materials in architectural ceramics. The use of CFA as a building material has two-fold environmental benefits of reducing the need to mine materials and diminishing the 3

accumulation of waste CFA. The low cost of CFA could also permit significant economic gains.

2. Experimental procedure

2.1. Materials

The CFA used as the primary raw material in this experiment was sourced from a thermoelectric power plant in Inner Mongolia, China. The chemical composition is listed in Table 1, which shows that the Al2O3 and SiO2 contents of 21.47 and 55.57%, respectively, are very similar to those of conventional raw materials for ceramics [9,10]. Loss on ignition (LOI), a test where the amount of mass lost after a sample is exposed to strong heat indicates the prevalence of volatile compounds in the raw sample, was ~2.83%. The CFA was prepared by planetary ball milling for 2 min at a speed of 200 rpm.

Other reagents, aluminum hydroxide and calcium oxide, were of reagent grade and obtained from Xilong Chemical Co., Ltd. All were used as received without further purification.

2.2. Sample preparation

2.2.1. Preparation of aluminum-substituted tobermorite whiskers

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A hydrothermal reaction was performed in a 1000-mL stainless steel autoclave lined with pure nickel and equipped with a mechanical agitator and a control system used to maintain the desired temperature within an error of ±0.5 °C. The milled CFA was treated in the presence of calcium hydroxide to synthesize Al-tobermorite whiskers via a mild hydro-chemical reaction, after which the mixture was quickly cooled to 90 °C using a cold water flow. The reaction conditions used were those found in our earlier optimization study [11], namely: mass ratio of solution to solid reactant (L/S) of 30, temperature of 220 °C, Ca/(Al+Si) molar ratio of 1, and residence time of 2 h. The resulting slurry was filtered to obtain a solid composite, which was then washed twice with deionized water at 80 °C. Finally, the solid composite reinforced with the whiskers was dried at 105 °C for 8 h. A small amount of the composite was used for subsequent analyses of its physical properties, while the rest was used to prepare ceramics for testing.

Sodium silicate, calcium oxide, and aluminum hydroxide were mixed together in the same autoclave described above to synthesize a series of purer Al-tobermorite whiskers as comparison samples. To investigate the effects of Al2O3 present in the fly ash, tobermorite was also prepared from chemical reagents with varying molar ratios of alumina to silica (Al/Si) of 0 (TOB0), 0.1 (TOB0.1), 0.15 (TOB0.15), 0.2 (TOB0.2), and 0.25 (TOB0.25) by adjusting the amount of aluminum hydroxide added. All other

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conditions were kept constant.

2.2.2. Preparation of whisker-reinforced ceramics The prepared Al-tobermorites were mixed with deionized water to achieve a mass ratio of 10:2, and then pressed into 100 × 100 × 4 mm cuboids under 20 MPa of pressure using a uniaxial tablet press (Model WE-300B). The resulting green compacts were dried in an oven at 105 °C for 12 h, and then sintered at a temperature between 850 and 1000 °C for 2 h in a laboratory-type electrical furnace (Model HTF1400). Furnace cooling was applied to all samples. Additional ceramic samples were produced using the same procedure from the comparison tobermorite synthesized from reagents.

2.3. Characterization techniques The precursor CFA was analyzed by X-ray diffraction (XRD, X'Pert Pro MPD, PANalytical Company) at 40 kV and 30 mA using Cu Kα radiation. After milling, a laser particle size analyzer was used to determine the particle size distribution of the CFA powder.

Inductively coupled plasma-optical emission spectrometry (ICP-OES, Optimal 5300DV, PerkinElmer Instruments) was performed at 1300 W with a carrier gas flow of 0.08 L/min and a peristaltic pump flow of 1.5 mL/min to analyze the chemical composition of the CFA samples. The dried solid composite powders were characterized 6

by XRD (X'Pert Pro MPD, PANalytical Company) at 40 kV and 30 mA within the 2θ range of 5–90° using Cu Kα radiation to identify the crystalline phases present. The surface morphology was observed by scanning electron microscopy (SEM, Sirion 200, FEI), while the concentrations of individual elements were detected by energy dispersive X-ray (EDX) spectroscopy. The morphology and transmission electron diffraction patterns of the fibers were characterized by field emission transmission electron microscopy (FETEM, JEM-2100, JEOL).

The flexural strength of the ceramic samples was measured by an electronic universal testing machine (WDW-20E, Jinan Shidai Shijin Testing Machine Group Co., Ltd.). The bulk density of the samples was calculated from their weights and dimensional measurements, and the open porosity was evaluated using the Archimedes displacement method after ultrasonic processing for 10 min in deionized water. The particle size distribution of the powders was measured using a laser particle size analyzer (Beckman Coulter, LS 13320). Nitrogen adsorption-desorption isotherms were recorded with a nitrogen adsorption apparatus (ASAP 2000) and the specific surface areas were calculated by the Brunauer-Emmett-Teller (BET) method. The heat treatment procedure was confirmed by thermogravimetric differential scanning calorimetry (TG– DSC, NETZSCH STA 449C) under flowing air at a heating rate of 10 °C/min.

Fourier transform infrared (FTIR) spectra were recorded on a PerkinElmer 7

Spectrum 100FT-IR spectrometer in the wavenumber range of 4000–400 cm-1. For this, 2 mg of each material studied was mixed with KBr powder in a ratio of 1:100 and pressed into a pellet by 8.9 × 104 N of force, using a manually operated hydraulic press (Specac). All FTIR measurements were acquired with a 2 cm-1 spectral resolution using a global light source and liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector.

3. Results and discussion

3.1. Coal fly ash characterization and reaction principle

The X-ray diffraction (XRD) patterns in Fig. 1(a) show that the main phases of the raw CFA are quartz and mullite, along with some amorphous material. The laser particle size analyzer used to determine the particle size distribution, reveals the milled CFA to have a d(0.5) size of 75.32 μm, as shown in Fig. 1(b).

A schematic showing the fabrication of whisker-reinforced ceramics is presented in Fig. 2. Under hydrothermal conditions, the highly reactive silicon dioxide in fly ash reacts with calcium oxide when in a aqueous solution, as per the following reaction:

5CaO+6SiO2+5H2O=5CaO·6SiO2·5H2O Meanwhile, since silicon and aluminum atoms share very similar structures and chemical properties, Al3+ can partly replace Si4+ during the synthesis of tobermorite, 8

resulting the formation of an Al-tobermorite phase (e.g., Ca5Si5Al(OH)O17·5H2O) [12]. Though the structure of crystalline minerals in CFA can be destroyed in a hydrothermal environment, causing them to gradually participate in the reaction [13,14], the Al-tobermorite synthesized in the present study has an excellent whisker structure. It also converts into wollastonite with a very fine whisker structure after sintering, which gives the final ceramic products excellent mechanical properties.

3.2. Characterization of aluminum-substituted tobermorite whiskers The chemical composition of the tobermorite synthesized from CFA (hereafter referred to as TCFA) is listed in Table 2, showing that the aluminum content is relatively high. This is because of the significant Al2O3 levels present in the original fly ash, which affects the tobermorite [15] as discussed later in Section 3.5. The XRD pattern in Fig. 3(f) confirms that the hydrothermal product is tobermorite, while the SEM images in Fig. 3(a) and Fig. 3(b) show that it consists of many tiny dendritic crystals combined into a porous structure. The synthetic tobermorite fibers are aggregated into balls of uniform size; at high magnification, the fibers are observed to be fine and homogeneous. The FETEM images clearly confirm that the single whiskers have fibrous shapes, with an electron diffraction pattern shown in Fig. 3(e) typical of a single crystal. The width of the fiber is less than 200 nm. The draw ratio of most fibers is greater than 25 and the specific surface area, as measured by the BET method, is 117.8 m2/g. 9

3.3. Characterization of whisker-reinforced ceramics

In order to identify the most appropriate sintering plan, the Al-tobermorite powders were subjected to TG–DSC analysis (Fig. 4) with an applied heating rate of 10 °C/min. The resulting DSC curve is divided into three stages:

1) At temperatures from 50 to 350 °C, the TG curve slopes to form a broad endothermic valley. The loss of free water is responsible for the 7.12% mass loss and the absorption of heat. In this temperature range, free water is released through gaps between the dendritic crystals of tobermorite.

2) Between temperatures of 350 and 650 °C, a notable mass loss of 7.84% is associated with an endothermic hump in the DSC curve at ~444 °C. This can be explained by the destruction of the phase structure, with the subsequent loss of water molecules originally located between layers.

3) Between 650 and 1000 °C, the exothermic peak at ~819 °C is characteristic of the appearance of an amorphous, disordered, and metastable state of tobermorite, known as dehydroxylation tobermorite. The Si-O-H chemical bond is destroyed at this temperature and the O and H combine to form water molecules, which then leave the tobermorite structure. The weak exothermic peak at ~881 °C may represent a phase transformation from tobermorite to

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wollastonite. The dehydroxylation temperature and wollastonite formation temperature are both higher than those reported elsewhere [6,8,16], thus indicating that TCFA is more stable than naturally formed tobermorite during heating.

The phase transformation of ceramics heat-treated at different temperatures is shown in Fig. 5. The reflection peaks indicate that the main phase is wollastonite, though a small amount of gehlenite is also present. The crystal structure of wollastonite first forms at 800 °C, but at this temperature the degree of crystallinity is low. The intensity of the wollastonite reflection peaks increases with increasing the temperature from 800 to 850 °C, with well-crystallized wollastonite forming at 900 °C. This agrees well with the TG–DSC analysis, as it corresponds to the exothermic peak observed at ~895.4 °C in stage three. With an increase in temperature from 900 to 1000 °C, the reflection peaks of wollastonite decline and the XRD line broadens, which is consistent with the emergence and increasing prevalence of an amorphous phase. This indicates that melting and lattice disturbance of the ceramic occurs within this temperature range. The reflection peaks of gehlenite, however, change very little during the heating process. The gehlenite phase may therefore be comprised of impurities in the tobermorite whiskers [17], and because of its low concentration is unlikely to affect the mechanical properties of the ceramics.

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The SEM images in Fig. 6 show typical fracture surfaces in which the fiber structures of the final ceramic products are clearly observed. This structure mimics that of the as-synthesized Al-tobermorite whiskers, indicating that it is not damaged by the heat treatment process, despite the resultant phase transformation. In addition, the chemical composition of the fibers is consistent with that of wollastonite, as determined by EDX analysis. From 800 to 900 °C, the outline of the fibers remains clearly visible and their surface becomes smoother, which may be attributed to the increase in crystallinity with increasing temperature in this range. When the temperature exceeds 900 °C, an amorphous phase fuses with the surface of the fibers and their contours become blurred, with gradually more of the glassy phase appearing. The wollastonite whiskers tend to transform from crystalline to amorphous in state when the temperature exceeds 900 °C, which causes the fiber-based microstructure in the ceramic bodies to disappear.

The flexural strength, open porosity, and bulk density of the ceramics were averaged from three sets of data, and their variations as functions of sintering temperature are summarized in Fig. 7. When the temperature is increased from 800 to 900 °C, the flexural strength and bulk density are increased, with a small decrease in open porosity. This means that increasing the temperature from 800 to 900 °C can accelerate the sintering process and promote the densification of the ceramics. However,

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when heat treatment is performed at a temperature greater than 900 °C, the strength and density decrease while the open porosity increases. Based on the previous SEM images and XRD analysis in this temperature range, the onset of melting clearly damages the fiber-like microstructure of wollastonite whiskers and reduces the degree of crystallinity. This over-sintering not only degrades the mechanical properties of the bulk, but also produces microscale pores within the ceramic and irregular macroscale bulges on its surface. Among the tested sintering conditions used in this study, the treatment at 900 °C produced the densest and most solid ceramics, with flexural strength of 52.47 MPa, open porosity of 9.51%, and a bulk density of 2.15 g/cm3. This confirms that the fiber microstructure of wollastonite whiskers in the ceramic body is responsible for the improved mechanical properties of the ceramic.

From the analyses above, it can be concluded that this new type of ceramic has a low sintering temperature but better mechanical properties relative to traditional ceramics. This can be attributed to the high reaction activity of the raw materials, which is the result of both the high CaO content of the raw materials (>30%) and very small diameter (~100 nm) of the tobermorite fibers [18]. The high reaction activity permits the optimum sintering temperature of 900 °C, approximately 200 °C lower than that of traditional ceramics [17,19–21]. Significantly, the reinforcing effect of the whiskers works not only on the ceramic products, but also on the green bodies. The flexural

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strengths of the green bodies in this study reach 15 to 20 MPa, which are very high for the ceramics industry. This strength is attributed to the water contained within the crystalline structure of tobermorite [22]. With the fibrous morphology of the tobermorite whiskers, the flexural strength of the whiskered structure does not decrease during dehydration.

3.4. Effect of Al-tobermorite morphology on whisker-reinforced ceramics

The reaction temperature is very important in hydrothermal synthesis, as it greatly affects the morphology of the Al-tobermorite produced. Values ranging between 140 and 350 °C have been presented in the literature [13,23], but in the experiments described in the previous sections, the temperature was maintained at 220 °C. The next logical step in exploration is therefore to examine the influence of temperature on the morphology of the Al-tobermorite whiskers produced while maintaining constant values for all other variables, and to determine how the morphology affects the properties of the whisker-reinforced ceramics.

A comparison of the XRD patterns obtained from tobermorite specimens synthesized at different temperatures is shown in Fig. 8. The diffraction peaks become noticeably sharper with increases in the synthesis temperature, suggesting increases in crystallinity. As increasing the temperature can improve the rates of mass transfer and chemical reaction in the hydrothermal process, it follows that the crystals would grow 14

more completely at higher temperatures.

The SEM images of the tobermorite whiskers synthesized at different temperatures in Fig. 9 show distinct differences in crystal morphology. The tobermorite synthesized at 150 °C has a poor crystal structure and flake-like form, as illustrated in Fig. 9(a). When the temperature is increased to 180 °C, the flake-like material is transformed into plate-like strips, as shown in Fig. 9(b). A further increase in temperature to 200 °C causes the tobermorite to grow as needle-like fibers (Fig. 9(c)) with a length-to-diameter ratio of ~15. In Fig. 9(d) and at 250 °C, the width of these fibers is ~100 nm and the length-to-diameter ratio approaches 20.

These different morphologies of tobermorite—flake-like, plate-like, needle-like, and fibrous—were used to prepare ceramics sintered at 900 °C for 2 h, labeled as CRE150, CRE180, CRE200, and CRE220, respectively. A comparison of the XRD patterns of these ceramics (shown in Fig. S1 in the supplementary material) confirms that the principal crystalline phase of the ceramic products is wollastonite, with a small amount of gehlenite and some amorphous phase. Increasing the synthesis temperature is found to sharpen the diffraction peaks of wollastonite. From the SEM images of the typical fracture surfaces of the ceramics, CRE220 (Fig. 10(d)) has a distinct fiber microstructure, which is less obvious in CRE200 (Fig. 10(c)). Similarly, CRE150 and CRE180 (Fig. 10(a) and Fig. 10(b), respectively) retain their original flake- and 15

plate-shaped microstructures.

The variations in flexural strength, open porosity, and bulk density of the ceramics are presented in Table 3. These test results agree with the upward trend seen with increasing reaction temperature in the XRD and SEM data, and show that CRE220 has the highest flexural strength and bulk density, with the lowest open porosity. From this, we conclude that ceramics offering outstanding performance can be successfully produced using Al-tobermorite reinforcing whiskers with excellent fiber morphology.

3.5. Effects of aluminum substitution on whisker-reinforced ceramics

The XRD patterns in Fig. 11 reveal that, with an increase in the Al/Si ratio from 0 to 0.2, the degree of crystallinity is increased, as shown by the sharpened diffraction peaks of tobermorite. A further increase in the Al/Si ratio from 0.2 to 0.25 causes no change in the diffraction peaks, possibly because aluminum substitution has become saturated in the tobermorite crystals. Comparing the enlarged view of the diffraction peaks in the small-angle range, it is evident that greater aluminum substitution causes the diffraction peaks at 2θ ≈ 7.8° and 28.9° to shift to lower angles. Thus, in the presence of aluminum hydroxide, the substitution of Si4+ with Al3+ facilitates tobermorite formation, but induces lattice distortion by increasing the basal spacing between silicate layers along the c-axis [24]. The XRD pattern of TCFA is also very similar to that of TOB0.2 and TOB0.25, indicating that the aluminum substitution rate is 16

very high in TCFA.

Fig. 12 shows the morphological change in the tobermorite with increasing Al/Si ratio. Note that, although the tobermorite whiskers are generally slender and fibrous, increasing the Al/Si ratio from 0 to 0.25 causes them to enlarge noticeably. The surfaces of the whiskers also become smoother and the whiskers are distributed more evenly. The TCFA whisker size is similar to that of TOB0.2 and TOB0.25.

In order to further understand the impact of aluminum substitution on the morphology of tobermorite, FTIR spectra were obtained from different samples, as shown in Fig. 13. The stretching, bending, and deformation vibration modes of the SiO4 tetrahedra are evident in all spectra at 400–600 and 900–1200 cm-1 [8,25], and a band attributable to the scissoring bending mode of water appears at ~1635 cm-1 [25,26]. A broad hydroxyl stretching band is seen at ~3445 cm-1. As the Al/Si is increased from 0 to 0.25, absorption bands gradually appear at ~1177 and ~912 cm-1. The former of these is caused by the substitution of Si in Q3 sites [6,27] (where it is connected with another three SiO4 tetrahedra), while the latter is due to the substitution of Si in Q2 sites [6,27] (where it is connected with two SiO4 tetrahedra). Thus, an increase in the Al/Si ratio causes incremental substitution of Al for Si in Q3 and Q2 sites, which induces growth in the crystal structure of tobermorite [28].

XRD analysis of the final ceramics revealed that changing the Al/Si ratio from 0 to 17

0.25 increases the crystallinity of wollastonite, which agrees with the XRD results of the Al-tobermorite samples. Subsequent SEM imaging found that the ceramics retained the fiber-like microstructures of the Al-tobermorite specimens. These XRD and SEM results are respectively shown in Fig. S2 and Fig. S3 in the supplementary material. The flexural strength, open porosity, and bulk density of the ceramics were averaged from three sets of data, with their variation presented in Fig. 14 as functions of the Al/Si ratio. An increase in the Al/Si ratio from 0 to 0.2 accompanies a significant improvement in the flexural strength and bulk density of the ceramics, along with a great reduction in the open porosity. However, when the Al/Si ratio reaches 0.25, the high substitution of aluminum negatively affects the mechanical properties. This can be explained by the upper limit of aluminum substitution in tobermorite, which has been reported to reach 20 mol.% (i.e., Al/Si = 0.2), with any residual aluminum hydroxide in the tobermorite damaging the performance of the ceramics. Thus, an increase in the Al/Si ratio can benefit the mechanical properties of whisker-reinforced ceramics, but only within a suitable range. When the XRD, SEM, and FTIR analysis results are considered, it can be inferred that the aluminum substitution level in TCFA is between 0.2 and 0.3 mol.%.

The effects of aluminum substitution on the mechanical properties of whisker-reinforced ceramics can be attributed to the promotion of tobermorite whisker growth by aluminum. These well-developed tobermorite whiskers are subsequently

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converted into well-developed wollastonite whiskers upon sintering and provide good strength support for the final ceramic bodies. Notably, the flexural strength of the whisker-reinforced ceramics greatly exceeds the requirement of 15 MPa prescribed in GB/T 4100-2015 (the Chinese national standard for ceramic tiles).

4. Conclusions

In this study, whisker-reinforced ceramics with flexural strengths in the range of 44.96 to 52.47 MPa were successfully produced using tobermorite whiskers synthesized from CFA using a dynamic hydrothermal method. The fine fiber morphology of these whiskers and the effect of aluminum substitution were shown to provide this new type of ceramic with excellent mechanical properties. The high reaction activity of tobermorite also permitted a necessary sintering temperature ~200 °C lower than that of traditional ceramics. The optimum sintering temperature of 900 °C produced the reinforced ceramic with the greatest flexural strength (52.47 MPa), highest bulk density (2.15 g/cm3), and lowest open porosity (9.51%). The morphology of the tobermorite had a significant impact on the mechanical properties of this new type of ceramic, with optimum performance achieved using Al-tobermorite whiskers with fiber-like morphologies. Aluminum substitution promoted the crystal growth of tobermorite, which improved the wollastonite whiskers produced by sintering and provided greater support for the final ceramic bodies. Thus, increases in the Al/Si ratio could benefit the 19

mechanical properties of similar whisker-reinforced ceramics, as long as the ratio does not exceed the upper limit of Al substitution in tobermorite.

The prepared ceramics could be used as new engineering and building materials applied in rigor conditions or for specific purposes, as they showed strengths exceeding national standards for such components. This study provides a new method for utilization of CFA, which is of great guiding significance for high value added utilization of CFA in ceramic fields.

Although the proposed whisker-reinforced ceramics possess excellent mechanical properties, other properties (e.g., thermal shock resistance and corrosion resistance) are not investigated in this study. To expand the application fields of the whisker-reinforced ceramics, a more comprehensive characterization of the ceramics is suggested for further studies.

Acknowledgements The authors gratefully acknowledge the financial support received from the Major State Basic Research Development Program of China (973 Program) under Grant No. 2013CB632601. We would like to thank Editage [www.editage.cn] for English language editing.

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[21] R. Ji, Z. Zhang, C. Yan, M. Zhu, Z. Li, Preparation of novel ceramic tiles with high Al2O3 content derived from coal fly ash, Constr. Build. Mater. 114 (2016) 888-895. [22] J. Kikuma, M. Tsunashima, T. Ishikawa, S. Matsuno, A. Ogawa, K. Matsui, M. Sato, Effects of quartz particle size and water-to-solid ratio on hydrothermal synthesis of tobermorite studied by in-situ time-resolved X-ray diffraction, J. Solid. State. Chem. 184(2011) 2066-2074.

[23] F. Luo, C. Wei, B. Xue, S. Wang, Y. Jiang, Dynamic hydrothermal synthesis of Al-substituted 11 Å tobermorite from solid waste fly ash residue-extracted Al2O3, Res. Chem. Intermediat. 39 (2013) 693-705.

[24] O. Turkmen, A. Kucuk, S. Akpinar, Effect of wollastonite addition on sintering of hard porcelain, Ceram. Int. 41 (2015) 5505-5512.

[25] N. Y. Mostafa, A. A. Shaltout, H. Omar, S. A. Abo-El-Enein, Hydrothermal synthesis and characterization of aluminium and sulfate substituted 1.1 nm tobermorites, J. Alloy. Compd. 46 (2009) 332-337.

[26] J. Bai, Y. Li, J. Xiang, L. Ren, M. Mao, M. Zeng, X. Zhao, Preparation of the monolith of hierarchical macro-/mesoporous calcium silicate ultrathin nanosheets with low thermal conductivity by means of ambient-pressure drying, Chem-Asian. J. 10 (2015) 1394-1401. 24

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[28] E. Bonaccorsi, S. Merlino, A. R. Kampf, The crystal structure of tobermorite 14 Å (plombierite), a C-S-H phase, J. Am. Ceram. Soc. 88 (2005) 505-512.

Table 1 Chemical composition of CFA

Composition

Al2O3

SiO2

Fe2O3

TiO2

CaO

MgO

Na2O

K2O

LOI

Content (wt.%)

21.47

55.57

6.8

0.01

5.12

2.97

3.42

1.22

2.83

25

Table 2 Chemical composition of two kinds of aluminum-substituted tobermorite

Composition

Al2O3

SiO2

Fe2O3

TiO2

CaO

MgO

Na2O

K2O

Content (wt.%)

10.45

35.39

2.46

1.21

30.58

1.32

1.22

0.54

Table 3 Flexural strength, open porosity and bulk density of ceramics

Flexural strength (MPa)

Open porosity (%)

Bulk density (g/cm3)

Sample Max

Min

Mean

Max

Min

Mean

Max

Min

Mean

CRE 100 32.05

28.34

30.27

25.12

24.63

24.89

1.83

1.75

1.78

CRE 150 39.23

36.81

37.68

15.83

14.89

15.34

2.08

2.01

2.05

CER 200 45.32

44.35

44.98

7.21

6.74

6.98

2.22

2.10

2.18

CER 220 52.71

51.99

52.47

4.53

4.28

4.44

2.35

2.24

2.29

26

12

A 10

Differencial Volume/%

Intensity(a.u.)

A---Quartz B---Mullite

A B

B BB

A

AB A

A

50

60

A

A

8

6

4

2

0 10

20

30

40

70

80

90

1

Diffraction Angle,2/degree(CuK)

(a)

10

100

1000

Particle Diameter/μm

(b) Fig. 1. Characterization of CFA: (a) XRD pattern and (b) size distribution.

27

Coal fly ash

Calcium oxide

Hydrothermal reaction

Aluminum substituted tobermorite whiskers

Molding

Low temperature sintering

Whisker reinforced ceramics A

Fig. 2. Schematic for the fabrication of whisker-reinforced ceramics.

(a)

(b)

28

(c)

(d)

T

Intensity(a.u.)

T

T T

T T

10

20

30

40

50

60

70

80

90

Diffraction Angle,2/degree(CuK)

(e)

(f)

Fig. 3. Characterization of TCFA: (a, b) SEM images, (c, d) FETEM images, (e) electron diffraction pattern, (f) XRD pattern.

29

100 16

819℃

98

14 -7.12%

96

12 10 8

92

-17.44%

6

881℃

90

4

-7.84%

88

2

86

0

84

-2

DTA /mWmg

TG/wt %

-1

94

-4

82

444℃ 0

100

200

300

400

500

600

700

800

-6 1000

900

Temperature / C

Fig. 4. TG–DSC spectrum of TCFA.

WG G

WW

WW

G

W---Wollastonite

WW

G---Gehlenite

WG

1000

W W

W G

950

WW

W G

900

WW

WG

850

WW

W G

800

℃

W G

W WWW GG

℃

Intensity(a.u.)

W

G

W WWW GG

℃

W G

WWWW G G

℃

W G

5

WW WW G G

℃

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

Diffraction Angle,2/degree(CuK)

Fig. 5. XRD patterns of ceramics heat-treated at different temperatures.

30

(a)

(b)

(c)

(d)

(e) 31

Fig. 6. SEM images of typical fracture surfaces of ceramics heat-treated at: (a) 800, (b) 850, (c) 900, (d) 950, and (e) 1000 °C.

Flexural strength Open porosity Bulk density

54

11

2.30

10

2.25

8 48 7

46

42

5

2.15 2.10 2.05

-3

44

6

2.20

Bulk density/gcm

9 50

Open porosity/%

Flexural strength/MPa

52

2.00

40

4 800

850

900

Temperature/

950

1.95 1.90

1000

℃

Fig. 7. Flexural strength, open porosity, and bulk density of ceramics as functions of sintering temperature.

32

T

T T

Intensity(a.u.)

TT

T

220℃

T

200℃

T

180℃

T

150℃

T

T T

TT T

T T T

5

T---Tobermorite

TT T TT

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

Diffraction Angle,2/degree(CuK)

Fig. 8. Comparison of XRD patterns of tobermorite whiskers synthesized at different temperatures.

(a)

(b)

33

(c)

(d)

Fig. 9. SEM images of tobermorite whiskers synthesized at (a) 150, (b) 180, (c) 200, and (d) 250 °C.

34

(a)

(b)

(c)

(d)

Fig. 10. SEM images of ceramics made with different morphologies of tobermorite: (a) CRE100, (b) CRE150, (c) CRE200, (d) CRE220.

35

T

T---Tobermorite

T T T

Intensity(a.u.)

T

T

5

TCFA

TT

T

Al/Si=0.25

T

Al/Si=0.2

TT

T

T T

T

T

Al/Si=0.15

T

T TT

T

Al/Si=0.1

T

T TT

T

T

T

T

T

T

T

T

T T

Al/Si=0

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

Diffraction Angle,2/degree(CuK)

(a)

TCFA

TCFA

Al/Si=0.25

Intensity(a.u.)

Intensity(a.u.)

Al/Si=0.25

Al/Si=0.2

Al/Si=0.15

6.00 6.25 6.50 6.75 7.00 7.25

Al/Si=0.2

Al/Si=0.15

Al/Si=0.1

Al/Si=0.1

Al/Si=0

Al/Si=0

7.50 7.75 8.00 8.25 8.50 8.75 9.00

28.4

Diffraction Angle,2/degree(CuK)

28.6

28.8

29.0

29.2

29.4

Diffraction Angle,2/degree(CuK)

(b)

(c)

Fig. 11. (a) Comparison of XRD patterns of tobermorite specimens with different Al/Si ratios, (b, c) detailed view with enlarged scale.

36

(a)

(c)

(b)

(d)

(e)

(f) 37

Fig. 12. SEM images of tobermorite specimens with Al/Si ratios of (a) 0, (b) 0.1, (c) 0.15, (d) 0.2, (e) 0.25, and (f) TCFA.

TMFA

Al/Si=0.25

T(%)

Al/Si=0.2

Al/Si=0.15

Al/Si=0.1

Al/Si=0

4000

3500

3000

2500

2000

1500

1000

-1

Wavenumbers(cm )

Fig. 13. IR spectra of tobermorite with different Al/Si ratios.

38

500

50 45

30

2.6

28

2.5

26

2.4

24 Flexural strength Open porosity Bulk density

30

20 18

25

16

20

14

2.2 2.1 2.0 1.9

12

1.8

10

1.7

10

8

5

6

-3

15

2.3

Bulk density/gcm

22

35

Open porosity/%

Flexural strength/MPa

40

1.6 1.5

4 0 -0.05

1.4 0.00

0.05

0.10

0.15

0.20

0.25

0.30

Al/Si

Fig. 14. Flexural strength, open porosity, and bulk density of ceramics as functions of Al/Si ratio.

39