Crystallization of (Na2O-MgO)-CaO-Al2O3-SiO2 Glassy Systems Formulated from Waste Products

L. Barbieri, A.M. Ferrari, I. Lancellotti, C. Leonelli, J.Ma Rincón, M Romero. Crystallization of (Na2O–MgO)–CaO–Al2O3–SiO2 Glassy Systems Formulated from Waste Products Journal of the American Ceramic Society, 83 (2000) [10], 2515–20; DOI: 10.1111/j.1151-2916.2000.tb01584.x

Crystallization of (Na2O–MgO)–CaO–Al2O3–SiO2 Glassy Systems Formulated

from Waste Products

Luisa Barbieri,* Anna Maria Ferrari, Isabella Lancellotti, and Cristina Leonelli*

Department of Chemistry, Faculty of Engineering, University of Modena and Reggio Emilia,

41100 Modena, Italy

Jesús María Rincón and Maximina Romero

Instituto E. Torroja de Ciencias de la Construccion (The Glass-Ceramics Laboratory), CSIC,

28033 Madrid, Spain

Abstract

Aluminosilicate and silicate glass-ceramics were obtained from controlled devitrification of

CaO–Al2O3–SiO2 glassy systems starting from Spanish and Italian coal fly ash or Italian

municipal incinerator slag mixed with other byproducts, such as glass cullet and dolomite. The

nucleation mechanism and the crystallization kinetics were investigated by thermal,

diffractometric, and microstructural measurements. Moreover, the experimentally observed

devitrification and the identification of the crystalline phases formed were compared with the

indications derived from Ginsberg, Raschin-Tschetveritkov, and Lebedeva diagrams used for

petrological glass-ceramics.

All the glasses showed a good crystallization tendency with the formation of dendritic pyroxene

and acicular wollastonite together with feldspar and iron spinels starting from the surface. The

activation energy values for crystallization ranging from 472 to 832 kJzmol21 were found to be

close to those typical for aluminosilicate glasses; moreover, the possibility to vitrify and

devitrify up to 100 wt% of slag and up to 40–50 wt% of ash mixed with glass cullet and

dolomite makes the vitrification treatment a suitable disposal procedure.

I. Introduction

HE controlled crystallization of amorphous materials is used to improve mainly mechanical


properties with respect to the parent glass. The most frequent crystallization mechanism, the one

at the surface, has proved to b e quite an effective method of enhancing the strength of glass,1

and the interest in these products also is due to the relatively low cost of fabrication, if

compared to advanced ceramics. The shaping of glass-ceramics can be conducted using a

variety of inexpensive glass-shaping processes, and, although high temperatures cannot be

avoided, savings can be made by simplifying the heat-treatment schedule. Moreover, some

glass-ceramics can be produced from inexpensive, natural raw materials,2 or from waste raw

materials.3 The design of glassceramics by recycling industrial waste in particular furnace slag

was started about 30 years ago in the former Soviet Union by Kitaigorodski4 and Pavlushkin.3

Recycling more recently has been extended to other types of byproducts, such as fly ash

generated from coal- and oil-fired electric power stations, and fly ash from power plant urban

solid waste incinerators.5–7 Some of these glass-ceramic materials became commercial products

(i.e., Slagsitall, Slagceram, Slagkyston)3,8,9 or were developed up to the preindustrial stage

(Silceram).10 Their main applications are in the field of abrasion-resistant materials, i.e.,

industrial floor coverings, wall facings, abrasion-resistant linings, and high-temperature

insulators.11 Moreover, the low cost and availability of the raw materials make them very

attractive from an economical point of view.

As far as the main chemical composition is concerned, most of the above-listed commercial

glass-ceramics belong to the CaO–Al2O3–SiO2 system, and sometimes include MgO, Na2O, and

transition-metal oxides. The crystalline phases present are mainly aluminosilicates or silicates,

such as wollastonite and anorthite (Slagsitall), or pyroxene (Silceram).

In this paper, different coal fly ashes and incinerator slags, belonging to the CaO–Al2O3–SiO2

three-component system, were used as raw materials to produce low-cost glass-ceramics. Their

glassy character makes them, in principle, good candidates for glass-ceramics production, as

demonstrated by the only industrial vitrification process for fly ash described in the literature:

the SOLUR glassmelting process, developed by the Sorg and Lurgi Companies.12

II. Experimental Details

According to previous studies,13,14 several compositions were formulated by mixing in different

percentages of Spanish (Teruel and Meirama) enriched silica and alumina, Italian (Reggio

Emilia) enriched calcium oxide coal fly ashes, Italian municipal incinerator slags, glass cullet,

and float dolomite from Spanish mineral extraction operations. Glass cullets and dolomite were


introduced to enhance meltability and stabilize the parent glass, which is evident in the average

chemical composition (Table I) of the 10 raw-material samples analyzed by inductively coupled

plasma spectrometry (ICP; Model Liberty 200, Varian, Palo Alto, CA).

The different raw materials were dry-mixed to obtain the binary and ternary compositions. The

optimized compositions are reported in Table II15–17 (the maximum amount of coal fly ash

permissible in batch with glass cullet and dolomite is ∼40–50 wt%, while the incinerator slag

melts by itself). The three main oxide components make all the compositions mainly members

of the CaO–Al2O3–SiO2 system. Also present are MgO, Na2O, and iron oxides. In Table II,

these compositions are compared to the compositional range for Silceram (CaO–MgO–Al2O3–

SiO2), or to Slagsitall (Na2O–CaO–MgO–Al2O3–SiO2) glass-ceramics.


The glasses, obtained by melting the homogenized batches at 1500°C with a thermal cycle of ∼5

h in aluminosilicate crucibles, were cast into a metal mold and annealed at 550°C for 2 h. The

samples were then cut with a diamond saw blade into equal pieces 1 cm x 0.5 cm x 0.5 cm in

size, and subjected to several thermal crystallization treatments. The tendency of the quenched

glass specimens to crystallize was evidenced by differential thermal analysis (Model DSC 404,

Netzsch, Selb, Germany) on glassy samples ground to ∼20 µm in size and heated from 20° to

1400°C with a heating rate of 10°C/min in static air. Evolution of crystalline phases was studied

with an X-ray powder diffractometer (XRD; Model PW 3710, Philips, Eindhoven, The

Netherlands), using CuKa radiation in the 2θ range from 5° to 60° on as-quenched and thermal-

treated specimens ground to 20 µm particle size. Microstructural analyses were performed on

pieces of the different glass-ceramics embedded in epoxy resin, polished with alumina paste

(0.3 µm), and coated with a thin Pd–Au film. Scanning electron microscopy (SEM; Model 505,

Philips), coupled with X-ray microanalysis using energy dispersive spectroscopy (EDS; Model

PV 9900, EDAX, Mahwah, NY) was used for these analyses.

3. Results and Discussion

Initially, the theoretical approach used for petrological glassceramics (i.e., glass-ceramics

obtained by slow cooling of the melt to room temperature) was applied in order to study the

capability to devitrify the different glasses. The results obtained from the diagrams of

Ginsberg,18 Raschin-Tschetveritkov,19 and Lebedeva18 were compared to the experimental

results.

By using the above-mentioned theoretical approaches to establish the capability of the different

glasses to produce glassceramics (Fig. 1), it was possible to have an idea of the role played by

the cations in the crystallization process (Ginsberg), and the nature of the crystalline phases

(Raschin-Tschetveritkov and Lebedeva).

All three approaches consider the chemical composition of the parent glass expressed in oxide

wt%. Thus, the Ginsberg diagram (Sal = SiO2 + Al2O3; Cafem = CaO + (FeO and Fe2O3) +

MgO; Alk = Na2O + K2O) reported in Fig. 1(a), shows that all the compositions are suitable for

producing glass-ceramics and are located in the 60%–70% Sal band, within which Ginsberg

predicts optimized glass formation. Below this zone, the high content of modifier oxides

weakens the glass network, making it very mobile. Above 70% SiO2, however, the network

becomes a rigid structure, making the processing operations too difficult.


In the triangular Raschin-Tschetveritkov diagram of Fig. 1(b), the corners are represented by Q

= SiO2 (i.e., the free silica not used in the formation of the crystalline phases), L = Al2O3 +

Na2O + K2O, and M = CaO + MgO + (FeO + Fe2O3) + TiO2 (i.e., the oxides group developing

plagioclase and pyroxene and/or olivine). All of the glasses are located close to the Fd (feldspar

theoretical composition)–P (pyroxene theoretical composition)– Tsch (Tschermark molecule, a

well-known calcium-excess pyroxene from the geochemical basalt processes, CaAlSi2O6)20

triangle. In particular, the most suitable compositions for producing glassceramics with

adequate crystalline phases, i.e., a pyroxene (R+,R2+)(R2+,R3+)(Si,Al)2O6 and a feldspar

(R+,R2+)(Si,Al)4O8, are those with the highest amount of waste, i.e., 5RE, 4M, S7, S9, and S10,

and are located nearer the triangle Fd–P–Tsch. Moreover, the two compositions 4RE and S5,

containing fly ash and slag, could present a higher difficulty in developing glass-ceramics

because of the higher free silica content, hence, an increase in melt viscosity. Finally, 4T and

4M compositions could precipitate olivine (R+,R2+)2SiO4 crystals during melting,21 with a

consequent increase of the melting temperature.

Fig. 1. (a) Gingsberg, (b) Raschin-Tschetveritkov, and (c) Lebedeva diagrams.


The Lebedeva diagram (MG = Mg2+, CA = Ca2+, FE = Fe2+ + Fe3+) of Fig. 1(c), derived from

the previous diagram by considering the role of the modifier cations in the crystallization

process, is divided into six regions according to the appearance of crystalline phases.

Consequently, glasses located under region I result in magnetite (Fe3O4) surrounded by

spherulitic pyroxene crystals. Inside region II, the magnetite is not the primary phase, resulting

in materials that develop pyroxene and magnetite at the same time. Regions III to V are similar

in microstructure composition, but if L > M, plagioclase crystallizes as primary phase;

conversely, if L < M, the pyroxene is the first phase. Nonetheless, region VI produces olivine

(MgO·SiO2) because it is near the magnesium corner. Because all of the compositions studied

fall into region V of the diagram, and taking into account that it is always true that L < M, no

relevant differences concerning the mineralogy of the final product can be expected, and a

pyroxene (i.e., diopside) should be the first phase to appear, immediately followed by a

feldspathic plagioclase (anorthite). This is particularly evident for the 4T and 4M glasses that

experimentally did not develop the olivine phase as suggested by the Raschin-Tschetveritkov

diagram.

Differently colored glasses (from green to black) were obtained depending on the fly ash or slag

content. The crystallization behavior was investigated by thermal and diffractometric

techniques.

The DTA curves conducted on the different coal fly ash and municipal incinerator slag-

containing compositions are reported in Fig. 2. In all cases, apart from the glass transition, a

single exothermic peak of crystallization was present, and it shifted to higher temperatures and

became sharper and sharper on increasing the coal or incinerator waste amount. This effect was

more relevant when dolomite, the CaO and MgO bearer in the batches, was added to coal ash,

and when the CaO, Al2O3, and Fe2O3 content was increased with respect to the alkaline oxides

as incinerator slag contribution. From the DTA data, a Tg (glass transition temperature)/Tm

(melting temperature) ratio . 0.6 was calculated (namely, a range of ∼0.62– 0.66) indicating a

heterogeneous nucleation and surface mechanism of crystallization.22


Fig. 2. DTA curves of the studied compositions containing (a) coal fly ash and (b) municipal incinerator slag; (c) comparison between a DTA and its DDTA curve.


As suggested by Marotta et al.23 and Barbieri et al.,24 the activation energy for the crystallization

of the glasses was evaluated by applying the derivative method (Fig. 2(c)) to the single

nonisothermal DTA run at the heating rate of 10°C/min. The equation used is

(1)

where E is the activation energy (J·mol-1), R the gas constant (8.314 J·mol-1·K-1), and Tf1 and Tf2

(kelvin) the DDTA peaks corresponding to the first and the second inflection points of the DTA

curve, and the factor C is a constant depending on the nucleation mechanism. In this case, a

value of 1.59 was used for the constant, corresponding to a surface-crystallization-dominant

mechanism, as was proved by SEM observations on fractured glasses and by the mathematical

processing on the DTA data. Table III shows the calculated activation energies related to the

main crystalline phases as identified by XRD on glass-ceramics. It is difficult to find a direct

dependence of E on the composition; however, the values for the incinerator-slag-containing

glasses are somehow always lower than those of the glasses containing coal fly ash.

The XRD patterns of the as-quenched glasses do not present peaks, which indicates a

homogeneous amorphous state. A progressive increase of the reflections, attributed to the

different crystalline phases listed in Table III, is dependent upon the composition and the

thermal treatment. Wollastonite is a common phase except for the ternary compositions made

with coal fly ash; the pyroxenes, diopside, and/or augite are present in all the glasses except

4RE. Feldspars, anorthite, or albite is present when percentage of slag is higher than 70 wt%


and in the coal fly ash compositions with dolomite; moreover, the iron spinels are a common

phase only of the coal fly ash glass-ceramics.

SEM observations on a polished section of an S9 glass-ceramic piece (Fig. 3), heat-treated at

1000°C for 8 h, revealed both locally oriented acicular crystalline growth (crystals variable in

size from 5 to 50 µm), and a significant number of flower-shaped crystals (35–45 µm). EDS

analysis showed that acicular crystals contain mainly Si and Ca confirming wollastonite,

CaSiO3, as identified by the XRD technique, while flower-shaped crystals showed the presence

of Mg, Al, and Fe, besides Si and Ca, which is characteristic of crystalline pyroxene phase

diopsidic, such as Ca(Mg,Al)(Si,Al)2O6, or augite-derived Ca(Fe,Mg)Si2O6. Finally, the

composition of the residual vitreous phase became poorer in Ca and Mg with respect to parent

glass, since such elements are removed by the above-mentioned crystalline phases. The

bidimensional crystallization percentage, determined by SEM photomicrograph image

processing, was ;60%–70% for the S9 composition, and in the 50%–80% range for the other

glass-ceramics studied.

Fig. 3. (a) SEM micrograph and EDS analysis of the S9 sample heat treated at 1000°C for 8 h, (b) flower-shaped crystal, (c) acicular crystal, and (d) residual vitreous phase.


The kinetics of crystallization of the glassy compositions investigated were followed

qualitatively by analyzing several XRD patterns recorded on the samples subjected to different

devitrification thermal treatments. Then, to establish the initial crystallization stages at a

different temperature, the time–temperature–transformation (TTT) curves were drawn for the

different compositions. The lines, which were traced corresponding to the time/temperature

coordinates for which the XRD patterns performed on the powder of the bulk treated samples,

showed sharp peaks at first, indicating crystalline phase formation. From a comparison of the

TTT diagrams of the compositions, (a) ternary with coal fly ash, (b) binary with coal fly ash

and, (c) binary with municipal incinerator slag (Fig. 4), it is evident that the kinetics of

crystallization are favored for those glasses in which the Al2O3 content decreases and,

correspondingly, the amount of silica and sodium oxide increase. It is well-known25 that an

increase of Na2O produces a decrease of glass viscosity that enhances the nucleation and

crystallization rates. The time/temperature coordinates of the TTT curve’s “nose” showed that it

is possible to attain the first crystal in a very short time (15 min) at moderate temperatures,

950°–1100°C. The 4M, 4RE, and S5 compositions are faster in crystallizing than 4T, 5RE, and

S10.

Fig. 4. Comparison of the TTT diagrams of the compositions (a) ternary with coal fly ash, (b) binary with coal fly ash, and (c) binary with municipal incinerator slag. These diagrams represent the onset of detectable crystallization by XRD.


IV. Conclusion

We have developed inexpensive calcium aluminosilicate glassceramics starting from waste raw

material containing significant amounts of iron oxides. The results obtained show that the

combined vitrification/devitrification technique represents a suitable methodology for the

recycling and exploitation of coal fly ash and, in particular, of municipal incinerator slag. A

large amount of waste materials (40%–50% of coal fly ash and up to 100% of incinerator slag)

are exploited by this technique, and it could be useful in solving the urgent disposal problem

related to these residues. The semicrystalline materials were developed by different heat

treatments that produced crystalline phases as predicted from the petrological approach used by

the Ginsberg, Raschin, Tschetveritkov, and Lebedeva diagrams. These results suggest that, with

a correct application of petrological diagrams, it is possible to design vitrifiable batches starting

from different types of waste materials, with the addition of other raw materials, by simply

knowing the chemical composition. The activation energy values for crystallization, lower for

the incinerator slag than for the coal-fly-ash-containing glasses, were in the range of other

aluminosilicate and silicate glass-ceramics.26 The role played by the different ions was

particularly evident in all of the experimental analyses performed to study the crystallization

tendency. The increase of the CaO, Al2O3, and Fe2O3, with respect to the alkaline oxides

resulting from the use of greater amounts of ash or slag, causes slight shifts of the glass

transition and crystallization temperatures to higher values, and is favorable to the precipitation

of different crystalline phases starting from the surface by a heterogeneous nucleation

mechanism. Surface nucleation combined with the possibility to attain the first crystal in a very

short time (15 min) at moderate temperatures, 950°–1100°C, make these compositions capable

of good devitrification.

Acknowledgment

The authors thank AGAC of Reggio Emilia, La Vetri Company of Villa Poma, Mantova, Italy,

and the Spanish Electric Power Plants of Meirama and Teruel for supplying the waste raw

materials.


References

1 E. D. Zanotto, “Crystallization of Liquids and Glasses: A Kinetic Approach”; pp. 25–39 in Glass-Ceramic Materials Fundamentals and Applications, Series of Monographs on Materials Science, Engineering, and Technology. Edited by T. Manfredini, G. C. Pellacani, and J. Ma. Rincón. Mucchi Editore, Modena, Italy, 1997.

2 J. Voldan, “Christallization Controllée dos Roches Fondues,” Verres. Refract., 19 [3] 230–33 (1965).

3 N. M. Pavlushkin, Principles of Sital Technology; pp. 203–35. Mashinostroenie, Moscow, U.S.S.R., 1970.

4 I. I. Kitaigorodski and K. T. Bondarov, “Glass Ceramics—A New Universal Material,” Soviet Daily Newspaper, Pravda, Mar. 22, 1963.

5 A. R. Boccaccini, M. Bucker, and J. Bossert, “Glass and Glass-Ceramics from Coal Fly-Ash and Waste Glass,” Tile Brick Int., 12 [6] 515–18 (1996).

6 A. Kipka, B. Luckscheiter, and W. Lutze, “Melting of Fly Ashes and Product Properties,” Glastech. Ber., 66 [9] 215–20 (1993).

7 A. R. Boccaccini, M. Petitmermet, and E. Wintermantel, “Glass-Ceramics from Municipal Incinerator Fly Ash,” Am. Ceram. Soc. Bull., 76 [11] 75–78 (1997).

8 M. W. Davies, B. Kerrison, W. E. Gross, M. J. Robson, and D. W. Wichall, “Slagceram: A Glass Ceramic from Blast-Furnace Slag,” J. Iron Steel Inst., 208, 348 (1970).

9 M. Pelino, C. Cantalini, and Hong-Tao Sun, “Glass-Ceramic Materials Obtained by Recycling Industrial Wastes”; pp. 233–42 in Glass-Ceramic Materials, Fundamentals, and Applications, Series of Monographs on Materials Science, Engineering, and Technology. Edited by T. Manfredini, G. C. Pellacani, amd J. Ma. Rincón. Mucchi Editore, Modena, Italy, 1997.

10 P. Rogers and J. Robertson, “Cheaper Glass Ceramic with Erosion Resistance,” Interceram, 38, 37 (1989).

11 M. Graison, Encyclopedia of Glass, Ceramics, and Cement; pp. 200–87. Wiley, New York, 1985.

12 H. Piper, H. Zschocher, G. Meyer-Schwinning, et al., “SOLURGlasschmelzverfahren”; pp. 116–18 in Mu¨ll Abfall, Beih. 29. Reststoffe aus der Rauchgasreinigung, 1991.

13 L. Barbieri, T. Manfredini, I. Queralt, J. M. Rincon, and M. Romero, “Vitrification of Fly Ash from Thermal Power Stations,” Glass Technol., 38 [5] 165–70 (1997).


14 L. Barbieri, I. Lancellotti, T. Manfredini, I. Queralt, J. Ma. Rincon, and M. Romero, “Design, Obtainment, and Properties of Glasses and Glass-Ceramics from Coal Fly Ash,” Fuel, 78 [2] 271–76 (1999).

15 P. S. Rogers and J. Williamson, “The Nucleation of Crystalline Phases in Silicate Glasses Containing Iron Oxides,” Glass Technol., 10, 5128–33 (1969).

16 P. S. Rogers, J. Williamson, J. F. Bell, and M. Cambell, “Erosion Resistant Glass-Ceramics Made by Direct Controlled Cooling from the Melt”; in Proceedings of International Seminar on Energy Conservation in Industry, Vol. 3, Applications and Technology. Commission European Community, 1986.

17 G. H. Beall, “Glass-Ceramics”; pp. 157–73 in Advances in Ceramics, Vol. 18, Commercial Glasses. Edited by D. C. Boyd and J. F. MacDowell. American Ceramic Society, Columbus, OH, 1986.

18 A. García Verduch, “Materiales Obtenídos a Partir da Rocas y Escorias Fundídas”; pp. 120–51 in Proceedings of the Jornadas Científicas Sobre Ceramica y Vidrio (Oviedo, 1980). Seccion de Ciencia Basica de la Sociedad Española Cerámica i Vidrio y Universidad de Oviedo.

19 G. A. Raschin and S. D. Tschetveritkov, “Me´todo Petroquìmico para Evaluar Materias Primas Petru`rgicas,” Izv. Rev. Inv. Geol. Prospec., 9, 71–79 (1964).

20 A. Morse, Basalt Phase Diagrams. Edited by Springer-Verlag, New York, 1980.

21 I. Queralt, “Dinamica Mineral de los Procesos de Devitrificación en Vidrios Basálticos”; Doctoral Thesis. Facultat de Geologia, University of Barcelona, Spain, 1988.

22 E. D. Zanotto and E. Mu¨ller, “A Simple Method to Predict the Nucleation Mechanism in Glass,” J. Non-Cryst. Solids, 130, 220–21 (1991).

23 A. Marotta, S. Saiello, F. Branda, and A. Buri, “Activation Energy for the Crystallization of Glass from DDTA Curves,” J. Mater. Sci., 17, 105–108 (1982).

24 L. Barbieri, C. Leonelli, T. Manfredini, M. Paganelli, and G. C. Pellacani, “Kinetic Study of Surface-Nucleated MgO–CaO–Al2O3–SiO2 Glasses,” J. Therm. Anal., 38, 2639–647 (1992).

25 J. M. Rincón, “Principles of Nucleation and Controlled Crystallization of Glasses,” Polym.-Plast. Technol. Eng., 31 [3– 4] 309–57 (1992).

26 N. K. Chakraborty, S. K. Das, S. K. Niyogi, and R. L. Thakur, “Crystallization of Some Glasses from CaO-MgO-Al2O3-SiO2”; p. 75 in Proceedings of the 10th International Congress on Glass, Vol. 14 (Kyoto, Japan, 1974).

Crystallization of (Na2O-MgO)-CaO-Al2O3-SiO2 Glassy Systems Formulated from Waste Products

Documents

Transcript of Crystallization of (Na2O-MgO)-CaO-Al2O3-SiO2 Glassy Systems Formulated from Waste Products