Journal of Materials Processing Technology 168 (2005) 56–61

An advanced technique for recycling fly ash and waste glass

Soon-Do Yoona, Yeon-Hum Yunb,∗a Department of Chemical Technology Engineering, Chonnam National University, 300 Yongbong-dong, Buk-gu,

Kwangju 500-757, South Koreab Department of Mineral and Energy Resources Engineering, Chonnam National University, 300 Yongbong-dong,

Buk-gu, Kwangju 500-757, South Korea

Received 30 April 2003; received in revised form 7 June 2004; accepted 27 October 2004

Abstract

FA glass–ceramic was prepared by using fly ash from the thermal power plant and waste glass. A mixture of two different powders, about60 g of fly ash and waste glass, was mechanically ground in a disk type ball mill for 4 h (700 rpm). After milling, the mixture was pressedinto a cylindrical form having a diameter of 10 mm and length of 30 mm without using any binder. The formed specimens was placed in abox-type SiC furnace, and the temperature was increased to 5◦C min−1 and sintered at different temperatures (800, 900, and 1000◦C for 1 h,r

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espectively).Crystallinity, morphological properties, and chemical compositions were observed by X-ray diffraction (XRD), field emission-s

lectron microscopy (FE-SEM), and energy dispersive X-ray spectrometer (EDX). Various properties, such as density, compressiending strength and chemical durability were also examined.2004 Elsevier B.V. All rights reserved.

eywords:FA glass–ceramic; Fly ash; Waste glass

. Introduction

Fly ash is produced as a by-product of coal combustionn power stations all over the world. Currently, only a smallercentage of this waste is utilized, mainly in the cement

ndustry [1]. Available in finer powder form and in greateruantities than slag[2], fly ash is much more economical andractical than steel slags in glass–ceramic production.

The ash residue from the incineration of municipal solidastes contains a large amount of hazardous materials such asioxins and heavy metals. It therefore needs further treatment

or it to become reasonably safe for the environment[3]. Inecent years, new ceramic and glass–ceramic materials madey recycling fly ash have been getting much attention fromesearch and development[4–7] because attempts to resolvenvironmental and waste recycling problems are needed for

he healthy life of future generations.

∗ Corresponding author. Fax: +82 62 530 1729.E-mail address:yhhumm@empal.com (Y.-H. Yun).

Fly ash containing a large amount of CaO, SiO2, andAl2O3 can be a good raw material for the CaO–Al2O3–SiO2system of glass–ceramic production[1]. Mixing of the rawmaterials, fly ash and glass cullet, and heat-treatmenacquire glass–ceramic production of the various newtalline phases and properties. The crystalline phases anal properties of the glass–ceramic production can betrolled by the raw material’s composition and by its hetreatment[8].

In this work, fly ash glass–ceramic (FA glass–ceramwas prepared by using fly ash from the thermal poplant and waste glass cullet. The aim of this study ifind the optimum annealing temperature for crystallizingglass–ceramic. Although recent papers have reported ouse of fly ashes and waste glass cullet for the produof glass–ceramic[9,10], there are many problems sucheconomical loss of twice thermal step. The purpose ofwork is to obtain the FA glass–ceramic by means ofchanical grinding applying the disk type ball mill and otime thermal step. For recycling, it is useful to obtain the

924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2004.10.012

S.-D. Yoon, Y.-H. Yun / Journal of Materials Processing Technology 168 (2005) 56–61 57

glass–ceramic which have many advantages in mechanicaland thermal properties[11].

Field emission–scanning electron microscopy (FE-SEM)was performed to investigate the changes in crystallinity andmorphological properties by varying the heat-treatment tem-peratures, and the chemical composition was analyzed byenergy dispersive X-ray spectrometer (EDX). Mechanicalproperties were investigated on FA glass–ceramic synthe-sized from fly ash and waste glass cullet.

2. Experimental procedures

FA glass–ceramic was prepared from fly ash and wasteglass cullet as raw materials. Fly ash from thermal powerplant (Yeocheon, Chonnam) in South Korea and waste glasscullet mixed with various kinds of waste glass (bottle, auto-mobile windowshield, plate, etc.) were used. The chemicalcompositions of the raw materials, fly ash and waste glass cul-let, are shown inTable 1. The chemical composition of thefly ash consists of oxides (in wt.%); 45.10% SiO2, 22.55%CaO, 5.32% MgO, 22.86% Al2O3, 4.17% Fe2O3.

Fly ash of the minute powder (−150 mesh) from the ther-mal power plant was used at the experiment. Waste glasscullet was washed and dried by dry oven at 60◦C for 24 h.W ma-t schG y)fm

d att sus:o w-d hani-c f-t apeh outu in ab asedt 900a omt

g oft rontt ding

TC

O sh

SNCMAKF

Fig. 1. Process of FA glass–ceramic preparation.

by heat-treatment and expensive cost of several times thermalstep.

After the heat-treatment, the FA glass–ceramic specimenswere cleaned with ethyl alcohol in an ultrasonic cleaner anddried at 70◦C for 10 h.Fig. 1 shows scheme of the experi-mental procedure in this study.

Crystalline phases in the specimens were measured byX-ray diffraction (XRD, Rigaku Co., D-Max-1200, Japan)with Cu K� radiation generated at 40 kV and 30 mA, in the5◦ < 2θ < 50◦ range at a scan speed of 2◦ 2θ/min. Morphologyand surface composition of the specimens were evaluated us-ing field emission–scanning electron microscopy (FE-SEM,S-4700, Hitachi Co., Japan) equipped with an energy dis-persive X-ray spectrometer (EDX) that has a robinson typebackscattered electron detector. Various properties such asdensity, bending strength, and chemical durability were alsoexamined. Density was measured using an Electronic Den-simeter (ED-120T, MFD BY A&D Co., Ltd., Japan). Thecompressive strength was investigated by universal tester (In-stron 4302, Instron Co., England), and the bending strengthwas determined from the three-point bending strength testin a universal tester (Instron N8872, Instron Co., England).The Vickers hardness was estimated by using a Vickers’hardness tester (Shimadzu Co., HMV-2 series, Japan). Thechemical durability was observed by the measurement ofweight change. To investigate the chemical durability, the FAg cidics ,s 0f

3

omt p ofa obileg Mm heri-c icalpX is

aste glass powder was obtained by grinding the rawerial, waste glass cullet, in a disk type ball mill (RetmbH & Co.KG, D-42781 HAAN, TYPE:RS1, German

or 20 min (700 rpm). The waste glass powder of about−150esh was used to produce FA glass–ceramic.Waste glass powder and fly ash powder were fixe

he weight ratio of four parts waste glass powder verne part fly ash powder. The mixture of two different poers about 60 g of waste glass and fly ash was mecally ground in disk type ball mill for 4 h (700 rpm). Aer milling, the mixture was pressed into a cylindrical shaving a diameter of 10 mm and length of 30 mm withsing any binder. The formed specimens was placedox-type SiC furnace and, the temperature was incre

o 5◦C min−1, sintered at different temperatures (800,nd 1000◦C for 1 h, respectively) and then cooled to ro

emperature.It is important to note that the mechanical processin

he disk type ball milling solves many problems that confhe recycling of fly ash and glass such as chemical bon

able 1hemical composition (wt.%) of the raw materials used in this study

xide Waste glass Fly a

iO2 75.17 45.10a2O 11.08 –aO 6.87 22.55gO 2.22 5.32l2O3 2.85 22.86

2O 1.81 –e2O3 – 4.17

lass–ceramic specimens were immersed into 15 mL aolution (1 N H2SO4) at 60◦C for 48 h. After immersingpecimens were washed by distilled water and dried at 8◦Cor 12 h.

. Results and discussion

FA glass–ceramic was obtained by mixing fly ash frhe thermal power plant and waste glass cullet made ull types of waste glass (bottle glass, plate glass, automlass) using starting materials.Fig. 2(a) presents a FE-SEorphological analysis of the fly ash sample, showing sp

ally and irregularly round-shaped powder particles, a typarticle morphology for fly ash powders[12]. Fig. 2(a) givesRD pattern of fly ash powder (−150 mesh) used in th

58 S.-D. Yoon, Y.-H. Yun / Journal of Materials Processing Technology 168 (2005) 56–61

Fig. 2. FE-SEM image (a) and XRD pattern (b) for the fly ash used in this study.

study. As shown inFig. 2(b), mineral phases of the fly ashpowder are identified as�-quartz (SiO2), mullite (Al6Si2O13)and enstatite [(Mg,Fe)SiO3]. The peaks are detected in theXRD pattern of the fly ash powder.

The XRD patterns of the FA glass–ceramic heated to800, 900 and 1000◦C for 1 h, respectively, are shownin Fig. 3(a)–(c). The crystalline phase present in theFA glass–ceramic corresponds to diopside [Ca(Mg,Al)(Si,Al)2O6], Augite [Ca(Mg,Fe)Si2O6] and wollastonite

F ted at8

[CaSiO3]. Peak intensities corresponding to the diop-side + wollastonite crystal, wollastonite crystal were gradu-ally increased, and the augite + diopside crystal, augite crystalwere decreased as the heat-treatment temperature increasedfrom 800 and 900 to 1000◦C. As clearly shown inFig. 3,with the increase of heat-treated temperature from (a) 800◦Cto (c) 1000◦C for 1 h, the peaks at 2θ = 29.9◦ and 36◦ identi-fied as the diopside + wollastonite crystals are increased, andthe peak intensities at around 2θ = 27◦ and 30◦ correspond-ing to augite + diopside are decreased. With the increase ofheat-treatment temperature to 1000◦C, the small intensitiesof the peaks at around 2θ = 23◦, 25◦ and 28◦ correspond-ing to wollastonite were increased, and the newly formedpeak at around 2θ = 25◦ corresponding to wollastonite wasidentified. However, peaks at 2θ = 29.8◦, which correspondto augite, were decreased with the increase of heat-treatmenttemperature at 1000◦C. These decreases, diopside + augiteand augite crystals, are caused by the formation of wollas-tonite crystals and the increase of heat-treatment temperatureto 1000◦C that brought about the growth of the wollastonitecrystals.

Figs. 4–6show the surface morphologies and microstruc-tures of the FA glass–ceramic heat-treated at 800, 900 and1050◦C.Fig. 4(a) presents the result of morphological analy-sis of the specimen heat-treated at 800◦C by using FE-SEM.Fig. 4(a) shows many round-shape grains of the diameter(ms s and

ig. 3. XRD patterns for the FA glass–ceramic specimens heat-trea00, 900, and 1000◦C.

average: 25�m) and acicular type grains about 5�m in theatrix. In the FA glass–ceramic specimen (Fig. 4), Fig. 4(b)

hows the grain’s surface condition of the homogeneou

S.-D. Yoon, Y.-H. Yun / Journal of Materials Processing Technology 168 (2005) 56–61 59

Fig. 4. FE-SEM images for the FA glass–ceramic heat-treated at 800◦C.

small size. As can be seen inFig. 4, crystallization degreeis investigated with a tough and complicated surface mi-crostructure. As seen inFig. 5, with the increase of heat-treatment temperature to 900◦C, specimen’s surface condi-tions consist of the round-shape and acicular grains. Fromthe micrograph (Fig. 5(a)), it is observed that two different

microstructures are present, one round-shape and the otheracicular (characteristic of the wollastonite), and the aciculargrains are different in size from 5�m up to 12�m. Fig. 5(b)shows grain’s surface to be irregular and unsystematic, it isa medium step of the crystallization process in which theround-shape changed into the acicular grains.Fig. 6shows the

FA gla

Fig. 5. FE-SEM images for the ss–ceramic heat-treated at 900◦C.

60 S.-D. Yoon, Y.-H. Yun / Journal of Materials Processing Technology 168 (2005) 56–61

Fig. 6. FE-SEM images for the FA glass–ceramic heat-treated at 1000◦C.

surface morphology of the FA glass–ceramic at 1000◦C.Morphological analysis of the specimen heat-treated at1000◦C by using FE-SEM shows that well-crystallized aci-cular type crystals are generally aggregated in the matrix(Fig. 6(a)). It is a typical SEM micrograph of the wollas-tonite type glass–ceramic of the heat-treatment temperatureat 1000◦C.Fig. 6(b) shows the microstructure of the aciculargrain, and the surface condition is homogeneous and smoothcompared withFig. 5(b). As clearly shown inFigs. 4–6, pro-gressive increases of heat-treated temperature cause changesin the grain shape and the surface condition of specimens.Therefore, the important factor in the crystal formation is aheat-treatment temperature.

As shown inTable 2, compressive strength and bendingstrength of the specimens at 800, 900, and 1000◦C wereinvestigated. All the tests were done with five times/eachspecimens. The compressive strength increases from 219.7to 273.3 MPa as the heat-treatment temperature increasesfrom 800 to 1000◦C. Bending strength is also examinedfrom 124.8 to 150.3 MPa. It was quite evident fromFig. 6(a)that the increase of the compressive and bending strength at1000◦C is due to the increasing acicular type crystals, wol-lastonite crystals, in the FA glass–ceramic. Generally, the

Fig. 7. Vickers hardness values of the FA glass–ceramic specimens heat-treated at 800, 900 and 1000◦C.

acicular type crystals included in the glass–ceramic showedgood mechanical strength.

Fig. 7shows representation of the Vickers hardness mea-surements of the FA glass–ceramic specimens heat-treated atvarious temperatures. It is clear fromFig. 7that the hardnessvalue increases with the heat-treatment temperature from 800

Table 2Properties of FA glass–ceramic heat-treated at 800, 900 and 1000◦C

P 900◦C 1000◦C

D 2.647± 0.002 2.699± 0.003C 246.8± 19.4 273.3± 18.7B 146.9± 20.3 150.3± 15.6W 0.151± 0.045 0.139± 0.051

roperties 800◦C

ensity (g cm−3) 2.551± 0.013ompression strength (MPa) 219.7± 24.1ending strength (MPa) 124.8± 13.2eight change (%) (1 N H2SO4) 0.146± 0.081

S.-D. Yoon, Y.-H. Yun / Journal of Materials Processing Technology 168 (2005) 56–61 61

to 1000◦C. Therefore, the acicular type crystal growth atthe highest temperature accounts for the increase in hard-ness value. As shown inFig. 7, the FA glass–ceramic speci-men heat-treated at 1000◦C has the maximum hardness valueof 5799± 49 MPa. In this work, the development of well-crystallized acicular type crystals in the FA glass–ceramic canimprove the compressive and bending strength at 1000◦C, soit is concluded that specimens through all the heat-treatmenttemperature-ranges is perfectly sufficient for enough me-chanical strength of practical usage.

Table 2shows chemical durability (weight change %) ofthe specimens heat-treated at 800, 900, and 1000◦C. Theweight change values were evaluated according to the fol-lowing equation:

chemical durability (weight change %)= m1 − m2

m1× 100

wherem1 andm2 are the weights of the samples before andafter immersing in acidic solution[13]. The weight changesof the specimens are not affected by the increase of theheat-treatment temperatures, and it is hard to suggest anyrelation of the weight change according to heat-treatmenttemperature. Moreover, the mechanical properties and thechemical durability should be researched to use them practi-cally. The future work will be focused on the environmentaland recycling problems, such as the advanced technique ofg s (flya astew

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theg ngth( ,1

124.8± 13.2 MPa, 900◦C: 146.9± 20.3 MPa, 1000◦C:150.3± 15.6 MPa)] proved to be sufficient mechanicalstrength. Therefore, the utility of the FA glass–ceramic issuitable for industrial buildings, external and internal wallfacings and pavement materials.

Acknowledgments

This work was supported by the Post-doctoral Fellow-ship Program of Korea Science & Engineering Foundation(KOSEF).

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. Conclusion

FA glass–ceramics was synthesized from fly ash ohermal power plant and waste glass cullet that mixeaste glasses as starting materials. From FE-SEM re

he round-shape grains in glass matrix decreased andeared, while well-advanced acicular type crystals wereificantly crystallized by increasing the heat-treatment terature from 800 to 1000◦C.

The most suitable temperature to generate glass–ceontaining acicular type crystalline phase with good meccal property was 1000◦C. Suitable glass–ceramic prodas acquired by controlling heat-treatment temperatureroper ratios of the used wastes.

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