Journal of Non-Crystalline Solids 337 (2004) 157–160

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Characterization of cordierite-based glass-ceramicsproduced from fly ash

Hua Shao *, Kaiming Liang, Feng Zhou, Guoliang Wang, Fei Peng

Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China

Received 18 December 2003; received in revised form 23 February 2004

Abstract

The cordierite-based glass-ceramics have been developed using fly ash as one of the starting raw materials. On the basis of DTA

analysis, nucleation experiments were carried out at 840 �C for 2 h and crystallization experiments were performed at 1020 �C for2 h. X-ray diffraction analysis revealed that the major phase of the synthesized glass-ceramics was cordierite along with a trace of

anorthite and iron cordierite. The properties of the synthesized cordierite-based glass-ceramics compared well with the values of

industrial cordierite. Results indicate an interesting potential for fly ash to produce useful materials.

� 2004 Elsevier B.V. All rights reserved.

PACS: 61.10.Nz; 61.43.Fs; 65.40.De

1. Introduction

Fly ash, a waste product of coal combustion in

thermal power plant, contains many hazardous sub-

stances such as heavy metals and toxic organic com-

pounds and thus is a major source for environment

pollution [1–3]. Currently in China a small percentage ofthis waste is mainly utilized for the manufacture of

concrete, cement and brick products, the remainder

being directly buried in fly ash ponds or landfills, which

is an unsatisfactory solution both from the ecological

and economical points of view. As a consequence, new

economical and reliable means have to be found out in

order to safeguard the environment and provide useful

way for its disposal. Because the fly ash contains largeamount of SiO2 and Al2O3, which are main glass net-

work formers, many research and development investi-

gations recently have been conducted in its utilization as

a starting material for glass and glass-ceramic produc-

tion [4–6].

Glass-ceramics are polycrystalline solids produced by

controlled crystallization of glasses. Currently there has

* Corresponding author. Tel.: +86-10 6277 3392.

E-mail address: sh00@mails.tsinghua.edu.cn (H. Shao).

0022-3093/$ - see front matter � 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.jnoncrysol.2004.04.003

been a considerable amount of interest in cordierite-

based glass-ceramics due to their good mechanical

properties, low dielectric constant and low thermal

expansion coefficient, therefore are widely used as kiln

furniture in whiteware industry as well as in micro-

electronic packaging industry [7–9].

The aim of the present work is to investigate thepossibility of using fly ash as one of the starting raw

materials to synthesize cordierite-based glass-ceramics.

For this purpose, microstructural characterization and

mechanical property investigations were carried out on

glass-ceramics devitrified from fly ash.

2. Experimental procedure

Fly ash used for the present experiments was sampled

from a power station in Beijing of China. The chemical

composition of as-received fly ash analyzed by X-ray

fluorescence spectroscopy (XRF) is shown in Table 1.

Parent glass was made from a powder mixture of

15MgO–10Al2O3–10SiO2–75fly ash corresponding to

the stoichiometric composition of cordierite phase(MgO 13.80%, Al2O3 34.80%, SiO2 51.4%), by adding

0.5% Sb2O3 and 1% NH4NO3 as fluxes.

10 20 30 40 50 60 70 80

*

#

+

Quartz (SiO 2) mullite (Al6Si2O13) anorthite (CaAl 2Si2O8) hematite (Fe 2O3) enstatite[(Mg,Fe)SiO3]

+

**

##

+

CPS

2 /(o)θ

Fig. 1. X-ray diffraction pattern of the as-received fly ash.

Fig. 2. SEM micrograph of the as-received fly ash.

Table 1

Chemical composition of as-received fly ash

SiO2 Al2O3 Fe2O3 CaO TiO2 K2O P2O5 SO3 MgO F

Component (wt%) 57.30 29.36 5.84 3.57 1.24 1.22 0.56 0.38 0.35 0.18

o

158 H. Shao et al. / Journal of Non-Crystalline Solids 337 (2004) 157–160

The glass was prepared by melting the starting pow-

ders in air using Pt-crucibles in an electrically heated

furnace at 1500 �C for 2 h. The melts were poured onto300 �C hot stainless steel plate. To remove thermal

residual stress, the cast glass was transferred to an

annealing furnace and held at 500 �C for 1 h, then

crushed and remelted at least three times to ensure

homogeneity.The resulting glass was crushed and sieved through a

200 mesh to produce glass powder suitable for differ-

ential thermal analysis (DTA) employing a Dupont

DTA with the temperature range of 20–1200 �C at theheating rate of 10 K/min. The glass powder with the

weight of 50 mg was contained in a platinum crucible

and the reference material was a-Al2O3 powders. Thedata were recorded by means of a chart recorder.The types of crystalline phases existing in a sample

after heat treatment were determined by X-ray powder

diffraction (D/max-RB) using CuKa radiation, workingvoltage 40 kV, working current 80 mA, scanning speed

of 4� min�1.After nucleation and crystallization, the samples were

polished and etched in 1% hydrofluoric acid for 30 s (20

�C), then washed, dried and coated with gold in an ionbeam coater, and analyzed by SEM (Hitachi S-450).

Several techniques were used to evaluate the proper-

ties of glasses and glass-ceramics. The bulk density and

porosity were measured by Archimedes’ method using

water as a medium. The thermal expansion coefficient

ðaÞ was measured by TMA with a heating rate of 10 K/min in air atmosphere. The bending strength of the

samples was measured by a four-point method withspasm of 20 and 40 mm at a loading rate of 100 lm/min.Hardness was measured by an indentation method using

the Vickers indenter.

600 800 1000

EXO

.EN

O.

Tp=1020

Tg=840

Temperature/ oC

C

oC

Fig. 3. Typical DTA trace of the as-cast glass sample.

3. Results

Fig. 1 shows the X-ray diffraction pattern of the as-received fly ash sample. As seen in Fig. 1, the raw fly ash

sample comprises some quantity of glassy phase and the

following mineral phases: quartz (SiO2), mullite (Al6Si2-

O13), enstatite ((Mg,Fe)SiO3), anorthite (CaAl2Si2O8)

and hematite (Fe2O3). Fig. 2 is the typical SEM micro-

graph of the fly ash sample, showing predominantly

spherically shaped powder particles, whereas the particle

size varies between 1 and 3 lm, the average particle sizeis about 1.5 lm.

10 20 30 40 50 60 70

(b)(a)

CPS

2 /(o)

+

++

+

cordierite (Mg2Al4Si5O18)anorthite (CaAl2Si2O8)iron-cordierite[(Mg,Fe )2 Al4Si5O18

]

θ

Fig. 4. X-ray diffraction patterns of cordierite-based glass-ceramics

sample at different heat treatment conditions: (a) 840 �C for 2 h; (b)840 �C for 2 h+ 1020 �C for 2 h.

Fig. 5. SEM micrograph of cordierite-based glass-ceramics sample

nucleated at 840 �C for 2 h and crystallized at 1020 �C for 2 h.

Table 2

Comparison of the properties of glass and synthesized cordierite-based glass-ceramics with those of industrial cordierite

Glass Cordierite-based glass-ceramics Industrial cordierite

Vickers micro-hardness (MPa) 4020± 87 6250± 116 –

Density (g/cm3) 2.34± 0.21 2.49± 0.14 2.5

Bending strength (MPa) 65± 5 90± 3 110

Thermal expansion coefficient (a) (79± 7)� 10�7/K (35± 3)� 10�7/K 25� 10�7/K

H. Shao et al. / Journal of Non-Crystalline Solids 337 (2004) 157–160 159

Typical DTA trace of the glass sample crystallized at

the heating rate of 10 K/min is shown in Fig. 3. The

glass transition temperature (Tg) of the curve was evi-dently about 840 �C. The DTA curve exhibited one

exothermic peak at 1020 �C indicating the formation ofcrystalline phase. The crystallization of the glass sub-

jected to heat-treatment at 840 �C for 2 h presentedtypical X-ray pattern characteristic of glass structure asshown in Fig. 4(a). The XRD result of the sample

nucleated at 840 �C for 2 h and crystallized at 1020 �Cfor 2 h was represented in Fig. 4(b). As we can see from

Fig. 4(b), a-quartz and mullite of the as-received fly ashhave completely disappeared, and the major phase

clearly identified was cordierite along with anorthite and

iron cordierite. The low intensity of iron cordierite phase

in Fig. 4(b) was due to high Fe2O3 content in thecomposition. Fig. 5 shows the SEM of sample nucleated

at 840 �C for 2 h and crystallized at 1020 �C for 2 h. Aswe can see in Fig. 5, the crystals are elongated grains.

The properties of the glass and cordierite-based glass-

ceramics are summarized in Table 2. Properties of

industrial cordierite are also included in Table 2 for

comparision [10]. The Vickers hardness of the glass was

4020± 87 MPa and increased up to 6250± 116 MPa incordierite-based glass-ceramics, which was similar to the

results of Augis and Bennett [11]. From the bulk density

measurements, it can be seen that dense cordierite has

been obtained. Bending strength also improved from

65± 5 MPa in glass to 90± 3 MPa in glass-ceramics. The

thermal expansion coefficient of the fly ash glass-

ceramics is slight higher than the value of industrial

cordierite which could be either due to substitution of

Ti2þ for Mg2þ (and Ti3þ for Al3þ) or presence ofamorphous phase.

4. Conclusion

Cordierite-based glass-ceramics have been synthe-sized from fly ash. The major phase formed after

nucleated at 840 �C for 2 h and crystallized at 1020 �Cfor 2 h was cordierite along with anorthite and iron

cordierite. The glass-ceramics showed good mechanical

properties with a hardness of 6250± 116 MPa, bending

strength of 90 ± 3 MPa and density of 2.49 0.14 g/cm3

which were comparable with properties of industrial

cordierite.

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