Ceramic Processing Research

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1 Jurnal f Ceramic Prcessing Research. Vl. 9, N. 2, pp. 135~139 (2008) J O U R N A L O F Ceramic Prcessing Research Chemical durability f glass-ceramics btained frm waste glass and fly ash Sn-D Yn a and Yen-Hum Yun b, * a Department f Applied Chemical Engineering, Chnnam Natinal University, 300 Yngbng-dng, Buk-gu, Gwangju , Suth Krea b Department f Gesystem Engineering, Chnnam Natinal University, 300 Yngbng-dng, Buk-gu, Gwangju , Suth Krea Applying the mechanical milling methd and annealing prcedure, we prepared glass-ceramics using fly ash frm a thermal pwer plant mixed with waste glass cullet t reslve the envirnmental waste recycling prblem and find ut the ptimum crystallizing mechanism. The chemical durability f the heat-treated specimens [850 C, 950 C and 1000 C] was investigated by measuring the weight changes, mrphlgical prperties, and chemical cmpsitin at the sample surfaces befre and after acid immersin by field emissin scanning electrn micrscpy (FE-SEM) and energy dispersive X-ray spectrmetry (EDS). Varius prperties, such as density, cmpressive strength, bending strength and Vickers hardness f specimens [befre and after acid immersin] were bserved. Regardless f chemical treatment, glass-ceramics heat-treated at 850 C, 950 C and 1000 C shwed sufficient mechanical prperties fr practical usage. Key wrds: Glass-ceramics, Chemical durability, Milling methd, Mechanical prperties. Intrductin Fly ash is a by-prduct f cal-, il-fired electric pwer statins and thermal pwer plants. An increasing amunt f fly ash frm pwer plants and urban slid waste incineratrs has caused an envirnmental prblem with technlgical and ecnmic damages all ver the wrld. Recently, by prperly selecting raw materials and ther wastes in the frms f bttm ash, fly ash, slag and mining residue, new prducts have been develped such as glass fibers and glass-ceramics fr ptential architectural and decrative applicatins which have chemical and mechanical prperties cmparable t, if nt better than, cmmercial nes [1-2]. The prductin f glass-ceramic materials has fund their applicatins in the field f abrasin-resistant materials, that is, industrial flr cverings, wall facings, abrasinresistant linings and high temperature insulatrs. Mrever, develping apprpriate methds f new glass-ceramic materials frm recycling fly ash has acquired particular imprtance [3-6]. Hwever, much mre effrt is needed in reslving envirnmental waste recycling prblems fr the life and health f future generatins. Many researches have been cnducted n the utilizatin f fly ash as a starting material fr glass-ceramic prductin [7-9]. Glass-ceramics have been prepared using fly ash frm a thermal pwer plant and waste glass [10]. It is *Crrespnding authr: Tel : Fax: yhhumm@empal.cm imprtant t nte that the waste materials such as fly ash, and glass are recycled, and that many recycling prblems such as chemical bnding by heat-treatment and ecnmic lss caused by several thermal steps are slved by a mechanical prcessing methd. In the present study, we investigated the chemical durability and mechanical prperties f glass-ceramics. Experimental prcedures The experimental prcedure and the starting materials used in this study have been reprted n previusly [10]. Briefly, fly ash and waste glass were used as starting materials t prepare glass-ceramics. Fly ash ( 200 mesh) frm a thermal pwer plant was used in its raw state in these experiments. Waste glass pwder was btained by grinding the raw material, waste glass cullet, in a disk type ball mill (Retsch GmbH & C.KG., D HAAN, TYPE : RS1, Germany) fr 20 minutes (700 rpm). The pwder size f the waste glass btained was abut 150 mesh t prduce the glass-ceramic. Table 1 shws Table 1. Chemical cmpsitin (wt.%) f the raw materials. Oxide Waste glass Fly ash SiO Na 2 O CaO MgO Al 2 O K 2 O Fe 2 O

2 136 Sn-D Yn and Yen-Hum Yun the cmpsitin f the raw materials, fly ash, and waste glass pwder used in this study. Waste glass pwder and fly ash pwder were mixed at a weight rati f three parts waste glass pwder t ne part fly ash pwderu Tw different pwder mixtures, abut 80 g f waste glass and fly ash, were mechanically grund in a disk type ball mill fr 6 h (700 rpm). After milling, the mixtures were pressed int cylindrical shapes having a diameter f 10 mm and length f 30 mm withut using any binder. The glass disks frmed were annealed at 850 C, 950 C, and 1000 C at a rate f 5 K minute 1 fr 1 h, in a tube-type furnace and allwed t cl inside the furnace. After the heat treatment, the glass-ceramic specimens were cleaned with ethyl alchl in an ultrasnic cleaner and dried at 70 C fr 10 h. T investigate the chemical durability, heat-treated samples were immersed in a 20 ml acidic slutin (1N H 2 SO 4 ) at 60 C fr 48 h. After immersin, the samples were successfully washed with distilled water and dried at 80 C fr 12 h in air. The chemical durability was analyzed by measurement f weight change and by bservatin f the surface mrphlgy by field-emissin scanning electrn micrscpy (FE-SEM, S-4700, Hitachi C., Japan). The variatin f the chemical cmpsitin at the sample surfaces befre and after acid immersin was als evaluated by energy-dispersive X-ray spectrmetry (EDS) with a Rbinsn-type backscattered electrn detectr. Furthermre, varius prperties such as density, cmpressive strength and bending strength were als investigated. Density was measured using an Electrnic Densimeter (ED-120T, MFD BY A&D CO., LTD, Japan). The cmpressive strength was examined by a universal tester (Instrn 4302, Instrn C., England) and the bending strength was determined frm a 3-pint bending strength test in a universal tester (Instrn N8872, Instrn C., England). The Vickers hardness was estimated using a Vickers hardness tester (Shimadzu C., HMV-2 series, Japan). Results and discussin Table 2 shws the density and weight change (%) f the specimens heat treated at 850 C~1000 C. T calculate the weight change, we defined the degree f the weight change (%) as fllws: Weight change (%) = (m 1 -m 2 )/m 1 Ý 100 (1) where m 1 and m 2 are the weights f the specimens befre and after immersing in the acidic slutin, respectively. The weight changes in the specimens are affected neither by immersing in the acidic slutin nr by increasing the heat-treatment temperature. Because it was difficult t examine the exact chemical durability f the specimens wing t their small weight changes befre and after immersing, we culd nt clearly explain the variatin f the chemical durability f the specimens by weight gain alneu Table 2. Weight change(%)f the glass-ceramics heat-treated at 850 C, 950 C and 1000 C befre acid immersin and after acid immersin Prperties 850 C 950 C 1000 C Density(gcm 3 ) Weight-change(%) (1 N H 2 SO 4 ) T btain sund infrmatin abut the chemical durability f the samples befre and after immersin, the surface mrphlgy and chemical cmpsitin were evaluated by FE-SEM and EDX. Fig. 1(a, b), 2(a, b) and 3(a, b) shw the surface mrphlgies and chemical cmpsitins fr the glass ceramics heat-treated at 850 C, 950 C and 1000 C befre acid immersin, respectively. In all the specimens, silicn, calcium, aluminum and alkali ins such as sdium, magnesium, and ptassium were detected n the surface grains fr the samples befre acid immersin. A mrphlgical analysis f the specimen heat-treated at 950 C and 1000 C [Fig. 2(a) and Fig. 3(a)] using FE-SEM shws that well-crystallized acicular type crystals are generally aggregated in the matrix. The acicular crystals are typical SEM results frm a Wllastnite [CaSiO 3 ] type glass-ceramic heat treated at 1000 C. As clearly shwn in Fig. 3(b), fr the sample heat-treated at 1000 C, relatively strng peak intensities crrespnding t calcium were identified at a whisker type crystal area, which prbably were caused by the frmatin f highly crystallized Wllastnite [CaSiO 3 ]. Mrever, the calcium cntent f the grain shwn in Fig. 1(b) at 850 C was smaller in grain area than at 1000 C [Fig. 3(b)]. This is additinal evidence f the grwth f acicular type Wllastnite crystals in the specimen heated at 1000 C, since the frmatin f Wllastnite needs mre calcium ins. Therefre, the imprtant factr in the surface crystallizatin mechanism f the acicular type crystals is the heat treatment temperature. Fig. 1(c, d), 2(c, d) and 3(c, d) shw the surface mrphlgies and chemical cmpsitins fr the glassceramics heat treated at 850 C, 950 C and 1000 C, respectively, after acid immersin. As shwn in Figs. 1(b) and 1(d), fr the sample heat-treated at 850 C, it is difficult t identify the variatin f calcium in cntent in the grains by EDX. At 950 C and 1000 C, as seen in Figs. 2(d) and 3(d), the calcium in cntent in the aciculartype grains decreased slightly after treatment in the acidic slutin [cmpare with Figs. 2(b) and 3(b)]. The calcium in cntent f the acicular-type grain f all specimens [850 C, 950 C and 1000 C] was nt affected significantly by the chemical treatment in the acidic slutin, althugh there was a small decrease f the specimen heat-treated at 950 C and 1000 C. Fig. 4 shws the variatins in the sdium cntents by weight rati (Na/Si) f the glass-ceramics surface [grain] at varius temperatures [850 C, 950 C and 1000 C]

3 Chemical durability f glass-ceramics btained frm waste glass and fly ash 137 FE-SEM image and chemical cmpsitin f the surface frm EDS fr the glass-ceramic heat-treated at 850C, befre acid immersin (a), (b) and after acid immersin (c), (d). Fig. 1. FE-SEM image and chemical cmpsitin f the surface frm EDS fr the glass-ceramic heat-treated at 950C, befre acid immersin (a), (b) and after acid immersin (c), (d). Fig. 2.

4 Sn-D Yn and Yen-Hum Yun 138 FE-SEM image and chemical cmpsitin f the surface frm EDS fr the glass-ceramic heat-treated at 1000C, befre acid immersin (a), (b) and after acid immersin (c), (d). Fig. 3. phase than in the residual glass phase. Cnsequently, fr glass-ceramic materials t have gd chemical resistance, it is necessary that the crystalline phase cntains high cncentratins f alkali metal xide [11]. Frm the result f the EDS analysis f the surface grains fr the specimens befre acid immersin and after acid immersin, we can cnclude that the specimens heat-treated at 950 C and 1000 C were favrable fr gd chemical durability, since their crystalline phase (grain) slightly changed the amunt f alkali ins (such as Na) as cmpared with the specimen heat-treated at 850 C. Table 3 shws the results f investigatin n cmpressive strength and bending strength f the specimens at varius temperatures [befre and after acid immersin]. All the tests were dne with 5 times/each specimens. The data values are the average frm tests n specimens. The cmpressive strengths are MPa(befre acid immersin) and MPa (after acid immersin) and the bending strength als imprved frm 72.8 t 94.9 MPa (befre acid immersin) and frm 55.3 t 72.6 MPa (after acid immersin) as the heat treatment was temperature increased frm 850 C t 1000 C. The mechanical strength f all the specimens was nt influenced by the increase f heat-treatment temperature nt by the chemical treatment in acidic slutin. Vickers hardness measurements f the specimens heat-treated at varius temperatures [befre and after acid immersin] were als investigated. As shwn in Variatin in the sdium cntents f the glass-ceramic surfaces at varius temperatures between befre (BAI) and after acid-immersin (AAI). Fig. 4. between befre and after acid-immersin. The data values are the average f sdium cntents by weight rati (Na/Si) n 5 specimens by EDS. In the specimen heat-treated at 850 C, sdium ins shwed a larger in variatin in grain areas between befre and after acidimmersin. Hwever, wing t chemical stability f wellcrystallized grains, the crystalline phase f the grain areas f the specimen heat-treated at 950 C and 1000 C exhibited a smaller sdium in variatin between befre and after acid immersin. Generally, the chemical stability f glass-ceramic materials is affected by the cmpsitin f the crystalline phase and als by the cmpsitin and amunt f the residual glass phase and its mrphlgy. Alkali ins are much mre stable in the crystalline

5 Chemical durability f glass-ceramics btained frm waste glass and fly ash 139 Table 3. Varius prperties f the glass-ceramic heat-treated at 850 C~1000 C, Befre Acid Immersin (BAI) and After Acid Immersin (AAI). Prperties 850 C 950 C 1000 C BAI AAI BAI AAI BAI AAI Cmpressin Strength(MPa) Bending Strength(MPa) Vickers Hardness(GPa) table 3, the glass-ceramic specimen heat-treated at 1000 C [befre and after acid immersin] has the maximum hardness value f 5.3 GPa (befre acid immersin) and 4.7 GPa (after acid immersin). Generally, glass-ceramics f the well- crystallized whisker grain type shwed gd mechanical prperties. In this study, the aciculartype crystals in the glass-ceramic specimen heat-treated at 1000 C imprved the cmpressive strength, bending strength and Vickers hardness, and these findings led us t cnclude that, thrughut the temperature range and chemical treatment in acidic slutin, ur specimens shwed sufficient mechanical prperties and chemical durability fr practical usage. The future wrk will fcus n the envirnmental and recycling prblems, such as an advanced technique f glass-ceramic frmatin which has prper ratis f the wastes used (sludge, waste-incineratr fly ash, cal fly ash and waste glass) and emplys a filter ceramic fr waste water absrptin. Cnclusins Glass-ceramics were synthesized frm fly ash f a thermal pwer plant and waste glass cullet as starting materials. The FE-SEM and EDX analyses revealed that glass-ceramics heat-treated at 950 C and 1000 C exhibited favurable imprvements in chemical durability, since their crystalline phases f grain areas maintained the amunt f alkali ins, such as sdium ins, and shwed a smaller calcium in variatin between befre and after acid immersin. The cmpressive strength and the bending strength f the glass-ceramics were prven t be gd, regardless f whether befre r after immersin in the acidic slutin. The cmpressive strengths were MPa (befre acid immersin) and MPa (after acid immersin) and the bending strengths were MPa (befre acid immersin) and MPa (after acid immersin) as the heat treatment temperature increased frm 850 C t 1000 C. Thrughut all the heat-treatment temperature range [befre and after acid immersin], the specimens shwed mechanical prperties strng enugh fr practical use. Reference 1. A. Karamanv, M. Pelin, M. Salv and I. Metekvits, J. Eur. Ceram. Sc. 23 (2003) E. Bernard, M. Varrass, F. Cadamur and S. Hreglich, J. Nn-Cryst. Slids, 352 (2006) B. Cumpstn, F. Shadman and S. Risbud, J. Mater. Sci. 27 (1992) R. Ciffi, P. Pernice, A. Arnne, M. Cataur and G. Quattrni, J. Eur. Ceram. Sc. 13 (1994) I. Queralt, X. Querl, A. Lpez-Sler and F. Plana, Fuel 76 (1997) J. Ma. Rinc, M. Rmer and A. R. Bccaccini, J. Mater. Sci. 34 (1999) M. Erl, S. Kucukbayrak, A. Ersy-Mericbyu and M. L. Ovecglu, J. Eur. Ceram. Sc. 21 (2001) R. Ciffi, P. Pernice, A. Arnne, A. Martta and G. Quattrni, J. Mater. Sci. 28 (1993) C. Lery, M. C. Ferr, R. C. C. Mnteir and M. H. V. Fernandes, J. Eur. Ceram. Sc. 21 (2001) C. Y. Park, S. D. Yn and Y. H. Yun, J. Ceram. Prc. Res. 8[6] (2007) Z. Strnad, Glass-Ceramic Materials, Elsevier, New Yrk, 1986, p. 182.