The possibihty of producing wall-covering glass-ceramic materials from raw materials by crystallization of the glass is demonstrated. Glass-ceramics based on β-wollastonite and diopside with various ratios were designed from raw materials such as limestone and dolomite. The phases crystallized were examined by X-ray analysis. Thermal expansion and mechanical properties such as bending strength and microhardness for glasses and glass-ceramics were measured

Abdel-Hameed, Salwa A. M.

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Repositorium für Naturwissenschaften und Technik

Origina PaperDevelopment of wall-covering glass-ceramics from raw materialsSalwa A. M. Abdel-HameedGlass Research Dept., National Research Center, Cairo (Egypt)The possibihty of produeing wall-covering glass-ceramic materials from raw materials by crystallization of the glass is demonstrated.Glass-ceramics based on ^-wollastonite and diopside with various ratios were designed from raw materials such as limestone anddolomite. The phases crystallized were examined by X-ray analysis. Thermal expansion and mechanical properties such as bendingstrength and microhardness for glasses and glass-ceramics were measured.1. IntroductionThe present paper describes an attempt to show the possibihty of using raw materials such as limestone, dolo-mite and sand for the production of wall-covering glass-ceramic materials based on ^-wollastonite and diopsidewhich, in addition to their attractive appearance similarto that of granite, possess physico-mechanical and ther-mal properties surpassing those of natural wall-cover-ing materials.Glass-ceramics obtained from crystallization of glasscompositions located in the ternary S i 0 2 -CaO-MgOsystem may contain several crystalline phases such aswollastonite, forsterite, diopside, protoenstatite and akermenite. Owing to the peculiar durability and mechan-ical properties of the phases present in these glass-ce-ramics, they may be suitable for many applications [1],but initiation of internal bulk crystallization is relativelydifficult in these glasses owing to nucleation problems[2].There are two kinds of glass-ceramic wall-coveringmaterials that are produced by sintering and crystalliza-tion of the frits [3]. These are the Japanese "Neoparies"[4 and 5] with y^-wollastonite (CaOSi02) as the mainphase and a similar material developed by a Bulgarianteam [6], the major crystalline phase in this case beingdiopside (CaO-MgO-2Si02). These two kinds of materi-als have an appearance similar to that of natural marble,and physico-mechanical and thermal properties thatconsiderably surpass the properties of natural wall-covering materials. The absence of nucleating agents al-low surface crystallization to take place, which producesthe excellent appearance of these products.Received 2 February, revised manuscript 9 June 2002.The purpose of the present paper is to demonstratethe possibility of obtaining wall-covering glass-ceramicsby crystallization of frits prepared from raw materials.2. ExperimentsThe glass-ceramic compositions (table 1) were formu-lated through a modification to the CIPW chemico-min-eral norm [7] to yield compositions of the y^-wollastoniteand diopside. This norm is a hypothetically calculatedcomposition from the chemical composition of the rockor glass; CIPW refers to the initials of the researchersCross, Iddings, Frisson and Washington who created thenorm in 1903.Egyptian limestone, dolomite and sand (—95.8 % intotal with Na2C03 (^4 %) were used as starting materi-als. The chemical compositions of the starting raw mate-rials are listed in table 2. Well mixed 100 g of the batchmaterials were melted in a platinum crucible in an elec-trically heated Globbar furnace at 1500°C for 3 h withoccasional swirling every 30 min to ensure homogeniz-ation. The homogeneous glass obtained was cast into2 cm diameter discs with  1 cm thickness and(10x1x1) cm^ rectangular rods and transferred to theelectrically heated furnace at 650 °C which was thenswitched off.The effect of heat treatment on crystallization wasstudied in glass samples isothermally cured at tempera-tures ranging from 900 to 1000°C at --50 K intervals for2 h. The nucleation effects were studied in glass samplesheated for 2 h at 700 °C followed by heat treatment at1000°Cfor 2h.--Table 1. Chemical compositions (in mol%) of the initial glassesglass samples CaO MgO SÍO2 Na20G l 26.20 21.70 47.89 4.21G2 28.47 19.42 47.87 4.24G3 30.77 17.10 47.86 4.27G4 33.10 14.76 47.85 4.29G5 35.46 12.38 47.84 4.32Table 2. Chemical compositions (in wt%) of the starting materi-als usedoxide limestone dolomite(Samalout (Egypt)) (Ataqua (Egypt))SÍO2 0.27 1.02AI2O3 0.39 0.95MgO 0.18 39.61CaO 99.16 58.42X-ray powder diffraction analyses of the heat-treatedglass were carried out using a Philips PW 1390 X-raydiffractometer with nickel-filtered CuKa radiation. Theinstrument settings were maintained for all the analysesby using an external standard sihcon disc.The linear thermal expansion coefficients were deter-mined for glasses and their crystalline products usingspecimen rods of -1.5 to 2 cm length that were heatedat a rate of 5  K/min from room to softening temperaturefor glass and to 700 °C for glass-ceramics.The indentation hardness measurements were per-formed with a load of 100 g using a Shimadzu micro-hardness tester for glasses and glass-ceramics. HV inN/mm^ was calculated from the equation:HV=2P sm(e/2)lP where  P is the load in N, 9 the angle between oppositefaces  136°, and / the length of the diagonal.Bending strength of the glass-ceramics was measuredusing a 3-point bending tester. The bending strength, B, (in MPa) of the tested samples was calculated from therelationB  3 PLI{2bD^) where  P is the load of rupture,  L the span,  b the widthand  D the thickness of the sample, respectively.3, ResultsThe samples obtained have a smooth surface, and theyare very appropriate for use as covering materials.D  DiopsideW  Wollastonite\ I I 1 60° 50  A0 30 2020Figure 1. X-ray diffraction analysis of sample G3 heat-treatedat various temperatures.Figure 1 shows the results from X-ray phase analysisof glass G3 heat-treated at 900 to 1000°C with 50 K in-terval and at 700 and lOOO^C for 2 h each. The crystal-line phases are: ^-wollastonite and diopside. It is noticedthat ^-wollastonite is the first phase crystallized at lowertemperature (900°C/2 h) (note the J-values in nm: 0.382,0.350, 0.331, 0.323 and 0.307 (given in  A in figure 1))with minor diopside. Increasing the temperature to1000°C for 2 h leads to an increase of the diopside over^-wollastonite. The intensity of X-ray peaks becomessharper by using double-stage heat-treatment.Al l glasses investigated are similar in their crystalliza-tion behavior but differ in the percentage of crystallizedphases, where ^-wollastonite is increased from 10 % inG l to 50 % in G5 at the expense of diopside, as the com-positions are designed. For all glass compositions heat-treated at the same temperature  as expected  increasing the content of MgO at the expense of CaO results in a decrease of the peak intensity of wollastoniteand an increase in diopside phase.Studying the microstructure of the crystalline glassesobtained from isothermal heat treatments at 1000°C/2 h shows that all glass-ceramics have a coarse-grained tex-ture due to surface crystallization but increasing the Ca-0:MgO ratio from G l to G5 (i.e., by increasing thewollastonite over the diopside phase) results in muchmore relatively fine-grained textures. For example, G2heat-treated at 1000°C/2h shows  a coarse-grainedmicrostructure of fair-sized stellate pyroxenic growthswhile G4 shows an interlocking fine-grained anhedralintergrowths of diopside stippled by light granules ofwollastonite. G5 with 50 % wollastonite exhibits a uni-form fine-grained texture of minute shapeless growths.The measurements of thermal properties, i.e., tran-sition (Tg) and softening (TJ temperatures as well as thethermal expansion coefficient (o:), of the studied glassesand their corresponding glass-ceramics are given in table3 and figures 2 and 3. = =° ° ° ° - - ­-= =Salwa A. M. Abdel-Hameed:Table 3. Thermal properties of y^-wollastonite-diopside glasses and their corresponding glass-ceramicsthermal expansion coefficient  a in 10~^ K ., , glass no. treatment Tg in °C 7̂  in °C ^̂ ^̂ ^̂ ^̂ ^̂at°C/2h 20 to 200 20 to 300 20 to 400 20 to 500 20 to 600 20 to 700 ^^^^^^none 8.1 8.2 8.4 8.6 - 611.7 636.3 amorphous1000 10.2 10.4 10.4 10.2 11.8 12.4 - D+Wnone 8.8 9.0 9.2 9.4 - - 617.7 648 amorphous1000 9.1 9.1 9.3 9.7 11.4 11.4 - D+Wnone 9.0 9.0 9.2 9.2 - 617.9 654.8 amorphous1000 8.8 8.9 8.8 8.7 10.0 10.3 - D+Wnone 8.7 8.9 9.0 9.1 - 607.3 635.3 amorphous1000 8.4 8.6 8.8 9.0 12.9 14.2 - - D+Wnone 9.0 9.1 9.2 9.3 - - 613.4 646.4 amorphous1000 8.2 8.4 8.6 8.9 10.7 11.4 D+WNote: D  diopside, W  woUastonite.Figure 2. Thermal expansion coefficient of some selected glasssamples.Temperature in °C • Figure 3. Thermal expansion coefficient of some selected glass-ceramic compositions heat-treated at 1000°C/2 h.In general, the thermal expansion coefficient ofglasses increases slightly by increasing the y^-wollastonitepercentage. The same behavior is noticed for 7̂  whichincreases from 636.3 C in G l to 654.8 C in G3 thendecreases slightly to 646.4 °C in G5.Table 4. Microhardness and bending strength measurements ofsome selected glass-ceramic samples heat treated at 1000°C/2 h glass microhardness bending strengthsamples in N/mm^ in MPaGl 504.8 36.412G2 - -G3 588.6 -G4 623.2 62.984G5 70.029Thermal expansion is one of the properties which arehighly sensitive to structural cohesiveness. It is foundthat in silicate glasses, the expansion coefficient valuesincrease with the decrease of the electrostatic bondstrengths of the cations present in the glass structure [8].The small cations (i.e. Mg^"^), with higher field strengths,seem to produce stronger bond structures than largercations (i.e. Ca^^), and consequently, the smaller cat-ions-based glasses are generally characterized by lowerexpansion coefficients [9]. Though the thermal expan-sion coefficient is increased from G l to G5 where Mg^^is decreased and Ca^^ increased.The opposite observation is made for glass-ceramicswhere the thermal expansion coefficient is decreased byincreasing the y^-wollastonite percentage. A transform-ation from P ^ a wollastonite appears in the range 496to 518 °C. Such transformations are shown in figure 3 where the thermal expansion coefficient is increased sud-denly by about 2 x 10~^/K.The temperature for the ^ a wollastonite trans-formation in solid state reaction is 1125°C [10], while inglass-ceramics the transformation is decreased to about506 °C. Ions of high field strength such as Mg^^ canpossibly act as surface-active agents to lower the interfa-cial energy (tension) between the crystal and the glass[11 and 12].Tests on the bending strength for samples heat-treated at 1000°C/2h have shown that for none of thesamples it is below 30 MPa (table 4). This value is twice— =  =-° ° Development of wall-covering glass-ceramics from raw materialsas high as that of natural marbles and granite [3]. Thebending strength values are increased from G l  36 MPa to G5 70 MPa by increasing the ^-wollaston-ite percentage.The microhardness values (table 4) are in most caseshigher than those of granites, which leads to a higherwear resistance. It should be noted that the increasingmicrohardness is a result rather of the total amount ofthe crystal phases formed than of the nature of thephases separated in the materials [3]. According toMoh's scale these j^-woUastonite-diopside glass-ceramicsshow values of —5.5.It is noticed that the increase of MgO content gener-ally results in lower hardness and bending strength val-ues. This is related to the pore growth process owing tothe difference in density between crystals (—2.73 g/cm^)and residual glass (—2.66 g/crm?) and also to the phaseseparation in the glass phase which increases the vis-cosity [13] at high content of MgO retarding the escapeof gases during glass melting and casting [2]. In order toensure zero porosity, the molten batch is held for 2 to3 h at higher temperature (1550°C) with occasionalswirling to achieve a homogeneous and air-bubble freemeU.4. ConclusionWall-covering glass-ceramic materials based on crystalli-zation of ^-woUastonite and diopside were producedfrom raw materials such as limestone and dolomite. ^wollastonite is the first crystalline phase followed bycrystallization of diopside. Increasing the crystallizationof ^-wollastonite over diopside, i.e. the CaOiMgO ratio,leads to more relatively fine-grained textures. This alsoleads to a decrease in thermal expansion coefficient andan increase in bending strength and microhardness. Thebending strength of all the glass-ceramics is between 36to 70 MPa, which is higher than twice the value of natu-ral marble and granite. The microhardness is also higherthan that of granite.5. References[1] Baldi, G; Generali, E.; Leonelli, C: Effects of nucleatingagents on diopside crystallization in new glass-ceramics fortile-glaze application. J. Mat. Sei. 30 (1995) p. 3251-3255.[2] Alizadeh, P.; Marghussian, V. K.: The effect of compo-sitionl changes on the crystallization behavior and mech-anical properties of diopside-wollastonite glass-ceramics inthe Si02-CaO-MgO(Na70) system. J. Eur. Ceram. Soc. 20(2000) p. 765-777.[3] Karamanov, A.; Gutzow, I . ; Chomakov, I . et al.: Synthesisof wall-covering glass-ceramics from waste raw materials.Glastech. Ber. Glass Sei. Technol. 67 (1994) no. 8, p.227-230.[4] Strand, Z.: Glass-ceramic materials. Amsterdam et al.: Elsevier, 1986.[5] Nakamura, S.: Crystallized glass article having a surfacepattern. U.S. pat. no. 3 955 989. Publ. date 11 May 1976.[6] Karamanov, A.; Penkov, I . ; Gutzow, I . et al: Diopside marble-like glass ceramics. Bulg. pat. no. 50879. Publ. date 13July 1990.[7] Abdel-Hameed, S. A. M.: Controlled crystallization ofsome Egyptian basalt-based glasses. Zagazig Univ., M . Sc.thesis 1995.[8] Takahashi, K.: Calculating the expansion coefficient ofsilicate and borosilicate glasses. Bull. Foe. Eng. YokohamaNot. Univ 2, 145.[9] Uhlmann, D. R.: The energetics of nucleation. Ind. Eng.Chem. 57, p. 19-31.[10] Kotsis, I . ; Balogh, A.: Kinetics of isothermal solid statereaction of ß-wollastonite synthesis from natural raw mate-rials. Ceram. Int. 20 (1971) no. 2, p. 119-125.[11] Hillig, W. B.: A theoretical and experimental investigationof nucleation leading to uniform crystallization of glass.In: Proc. Symp on Nucleation and Crystallization inGlasses and Melts. Columbus, OH: Am. Ceram. Soc. p. 77.[12] Doremus, R. H.: Glass science. 2nd ed. New York: Wiley,1994.[13] Mazurin, O. V.; Porai-Koshits, E. A.: Phase separation inglass. Amsterdam: North-Holland, 1994.E6O2F003Contact:Dr. Salwa A. M. Abdel-HameedGlass Research DepartmentNational Research CenterP Code 12622Dokki, CairoEgyptE-mail: Salwa_NRC@hotmail.com~~ ---

Development of wall-covering glass-ceramics from raw materials

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Development of wall-covering glass-ceramics from raw materials

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