Journal ofThermal Analysis. Vol. 51 (1998) 553-560

THERMODYNAMIC INVESTIGATION OF CRYSTALLIZATION BEHAVIOUR OF PYROXENIC BASALT-BASED GLASSES

A. W. A. El-Shennawi, M. M. Morsi, G. A. Khater and S. A. M. Abdel-Hameed Glass Research Department, National Research Centre, P. Code 12622, Dokki, Cairo, Egypt

(Received June 10. 1997; in revised form November 4. 1997)

Abstract

For aseries ofpyroxenic basalt-based glasses, DTA was used to elucidate the changes oc­curring on including specified oxidizing agents and rectifying oxides Na20 and/or CaO, or CaO+MgO to modify one or more of the ratios FeO:Fe203' CaO:Na20 and CaO:MgO that af­fect the monominerality and crystallization behaviour. From comparisons of the positions, characters and intensities of the DTA peaks, the effects of the rectifying component on the crystallization processes were readily demonstrated. The higher crystallizabilities exhibited when Mn02 was used as oxidizing agent and Na20 and/or MgO as rectifying oxides were re­lated to their effects in reducing the viscosity of the glass and in enhancing the nucleation rate of the glass.

Keywords: basalt glass-ceramic, crystallisation basalt glass, DTA glass

Introduction

DTA is a thermodynamic method of monitoring changes in energy relative to a thermally inert material, continuously and automatically, during heating or cooling. A DTA curve can yield three major items of information; the tempera­ture of an effect, the temperature difference, I1T, of this effect (i.e. peak height), and the amount of heat (peak area) liberated or consumed during the thermal process. DTA traces fumish detailed information on the thermal behaviour ofthe material, including the various transformation processes. In glass/glass-ceramic systems, endothermic effects relate to annealing, softening, nucleation and melt­ing temperatures and also reversible phase transformations; while the exother­mic effects indicate devitrification and irreversible phase changes [1].

A great deal of work has been carried out to investigate the preparation of glass-ceramics from natural basalt rocks [2-10]. In some of these studies [4-7], it was found that the oxidation state of iron in basalt melts is one of the most im-

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554 EL-SHENNAWI et al.: PYROXENIC BASALT-BASED GLASSES

portant factors affecting the nuc1eation and consequently the crystal size in ba­salt-based glass-ceramics.

The present paper, which comprises part of a research project aimed at the de­velopment from basalts of microcrystalline glass-ceramic materials with differ­ent crystalline phase assemblages and properties, reports the importance of the use of DTA to characterize glass-ceramic compositions and crystallization be­haviour. The compositions were formulated so as to possess relatively low melt­ing temperatures with good workability traits, and to crystallize into a single py­roxenic solid solution (ss) phase. In these basalt-based glas ses, a specified recti­fying component was inc1uded to modify the ratios FeO:Fez03, CaO:NazO and CaO:MgO necessary for the achievement of monominerality requirements.

Experimental

Seven glass compositions were formulated, and were derived essentially from Abu-Zaabal basalt rocks, which constitute more than 75% ofthe batch. The other complementary batch ingredients used to rectify the composition were

Table 1 Composition and mole ratios of some specified oxides of the designed basaIt-based pyroxenic glas ses

Oxide/ Glass## mol% PND PNLM PNL PD# PLM PL

Si02 48.39 48.39 48.39 45.54 45.54 45.54

Ti02 2.50 2.50 2.50 2.36 2.36 2.36

AIP3 8.36 8.36 8.36 7.87 7.87 7.87

Fe20 3 4.40 4.40 4.40 4.14 4.14 4.14

MgO 14.53 8.98 8.98 16.52 8.44 8.44

CaO 16.97 21.23 22.52 20.99 27.85 29.07

MnO 1.29 1.22

Na20 4.41 4.41 4.41 2.16 2.16 2.16

Kp 0.44 0.44 0.44 0.42 0.42 0.42

CaO/Na2O 3.85 4.81 5.11 9.72 12.89 13.46

CaO/MgO 1.17 2.36 2.51 1.27 3.30 3.44

CaO/MgO+MnO 1.17 2.07 2.51 1.27 2.88 3.44

# The mole percent of glass PD* without adding any oxidant is: SiOz 44.97, TiOz 2.33, AlzÜ3 7.77, FezÜ3 2.87, FeO 2.44, MgO 16.32, CaO 20.75, NazÜ 2.14 and KzÜ 0.41

## P = composition is based on a single Pyroxene phase; N = composition modified by adding extra NazO; L = the main modifier constituent was Limestone; D = Dolomite was the main modifier; when M was included it means that the oxidizing agent ammonium nitrate was replaced by Mn02

1. Thermal Anal., 51, 1998

EL-SHENNAWI et a\.: PYROXENIC BASALT-BASED GLASSES 555

soda ash and/or locallimestone or dolomite rocks, together with minor amounts of NH4N03 and Mn02 as oxidizing agents. The chemical compositions of these glasses are given in Table 1.

These glass compositions generaHy melt weH at about 13500e during 2.5 h, yielding homogeneous, blackish and bubble-free glasses with good workability traits. The melting regime adopted was given in detail by Abdel-Hammed [9]. These well-oxidized glasses were stable towards devitrification, and displayed high crystallizabilities on heat treatments. This was to be observed especially for those rectified by adding additional Na20 and/or dolomite, where a longer glass fibre could be drawn over relatively longer durations according to the viscosity test. This may indicate the effects of N a20 and/or MgO in reducing the liquidus temperature and viscosity of the melts.

DTA was carried out with the computerized Perkin EImer DTA 7 Series in a dynamic N2 purging gas atmosphere (at a constant rate of 50 cm3 min-1

). -70 mg ofthe powdered sampIe (0.09-0.25 mm), a heating rate of lOoe min-1 and corun­dum as reference material were applied in all DTA runs. The details of the other experimental techniques mentioned throughout the text will be given in a forth­coming paper [10] describing the composition - phase relations, sequence of crystallization and properties of glass-ceramics.

Results and discussion

The DTA traces of the investigated glasses are shown in Figs 1-3 and the DTA results are listed in Table 2.

Table 2 DTA results ofbasalt-based pyroxenic glasses

Glasses# Endo/oe Exote Phases

T~ Ti T~ T{ Area developed

PND 655 788 839 886 591 pyroxene

PNLM 651 801 837 865 496 pyroxene

PNL 653 808 894 941 593 pyroxene

PD 673 829 897 912 678 pyroxene

PD* 664 822 904 924 629 pyroxene

PLM 672 822 938 988 515 pyroxene

PL 676 822 939 1002 443 pyroxene

654 727 775 798 14 magnetite

Basaltic-glass frit 802 830 924 180 pyroxene

1102 1080 1041 37 plagioclase

# The same symbols are as illustrated under Tab1e 1; i-initial; p - peak;j - final

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556 EL-SHENNAWI et al.: PYROXENIC BASALT-BASED GLASSES

Almost all the DTA traces are somewhat similar in their general charac­teristics (except that of the unmodified basalt glass frit), exhibiting an endother­mic effect in the range 600-700°C and an exothermic peak in the range 800-1000°C. The onset of the baseline shift due to the endothermic effect before the crystallization is the glass transition temperature (Tg) and the peak tempera­ture may be the glass softening temperature (Ts) [1, 11]. This endothermic reac­tion is believed to be caused by an increase in heat capacity due to a transforma­tion of the glass structure [12]. The exothermic reaction due to the crystallization effect is accompanied by the release of heat due to the lower free energy; the maximum crystallization rate is attained near the summit of the exothermal peak.

Peak 897°C ~ 6

~ 5

4

3

2

TemperaturelOC

Fig. 1 DTA traces of sorne basaltic glasses, showing the effects of the presence and absence of oxidizing agent: a - PD glass oxidized by NH4N03; b - PD' glass without added oxi­dant; c - non-rnodified quenched basalt frits

! Peak 837°C ::

~ <l 5 '-.....::

:: : p_:::94~~ Peak'39~

2 ...... ~........ ................/ \, 1.1 ................. .

c

900 1000 1100

Tem perature I oe

Fig. 2 DTA traces of basalt-based glasses, showing effects of ratio CaO:Nap and type of oxi­dizing agent: a - PL glass rectified with lirne and NH4N03; b - PNL glass rectified with Nap+CaO and NH4N03; c - PLM glass rectified with lirne and Mn02;

d - PNLM glass rectified with Na20+CaO and Mn02

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EL-SHENNAWI et al.: PYROXENIC BASALT-BASED GLASSES

~ 12~--------------~----------~ ;:: t----<l 10 Peak 894'C :: Peak 839°C

8

6 ..... ~ ....................... ..-~--.._. ! ............ _ . 4 c

2 Peak 897°C

. 1 /. . I I i Peak 940"C ..... /

'-' ... 500 600 700 800 900 1000 1100

Temperaturel"C

557

Fig.3 DTA traces of basalt-based glasses, showing effect of ratio CaO:MgO: a - PL glass rec­tified with CaO (as limestone); b - PD glass rectified with (Ca, Mg) 0 (as dolomite); c - PNL glass rectified with Na20+limestone; d - PND glass rectified with Na20+dolo­mite

The DTA traces are compared here with the aim of elucidating the effects of oxidizing agents, and the incorporation ofNa20 and MgO at the expense ofCaO, on the crystallization behaviour. A broad exothermic effect indicates a sluggish crystallization propensity, a lower crystallization rate and/or a surface crystal­lization character, while a sharp exothermic peak signifies a higher crystal­lizability, a higher crystallization velocity and/or a bulk (volume) crystallization process [1]. On this basis, all the DTA traces (Figs 1-3) immediately indicate that almost all the modified pyroxenic basalt glas ses are characterized by a high crystallization propensity, but with different degrees of crystallizabilities, de­pending on the additive included in the composition. The positions, characters (shapes) and intensities ofthe DTA peak effects were considered during the com­parisons.

Effects of oxidizing agents

The effect on the crystallization behaviour of the ratio FeO:Fe20J, or more precisely the progression of the oxidation state of the iron oxides, as a result of adding the oxidizing agent NH4NOJ or Mn02 can readily be observed by com­paring the DTA traces in Figs 1 and 2. While the modified PD glass oxidized by NH4NOJ exhibits a single major exotherm, indicating the crystallization of py­roxene ss as the sole phase, the unmodified basaltic glass frit (curve lc) displays two other exotherms, indicating that the glass derived from basalt may give the same crystalline phase assemblage as the original basalt rock, i.e. pyroxene, pla­gioclase and magnetite (Table 2). However, the progression of the oxidation of ferrous into ferric iron on the addition of NH4NOJ greatly enhances the crystal-

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558 EL-SHENNAWI et al.: PYROXENIC BASALT-BASED GLASSES

lization process. This is mirrored by the sharper, higher-intensity and slightly lower-temperature exothermic peak of the PD glass as compared with its coun­terpart without the addition of oxidizing agent, viz. the PD' glass, as indicated in curves a and b in Fig. 1.

The type of the oxidant also has an effect on the crystallization behaviour. When NH4N03 was replaced by Mn02, the endo- and exothermic effects (Fig. 2) were shifted to lower temperatures and the exotherms became more intense, in­dicating an enhancement of the crystallization processes by Mn02. This effect was more explicit in glasses modified by Na20, PNL and PNLM (Fig. 2, curves b and d), indicating that the presence of this oxide had an additional powerful en­hancing effect on the crystallization process.

The results revealed that the presence of iron in the trivalent state in these complex glasses led to an enhancement of the crystallization process, which in­creased on the use of Mn02 as oxidizing agent and became more explicit when soda was introduced at the expense of CaO in the glass compositions. The rela­tively slight shift in the endothermic peak (Tg) to higher temperatures when oxi­dizing agents are used, and the slight decrease in these temperatures when Mn02 is used (Table 2), may be indicative of the viscosity changes occurring in these glasses; this is higher for higher oxidation states, and slightly lower when Mn02 and Na20 are incorporated into the glass. Accordingly, the higher crystal­lizabilities exhibited by these glasses may be explained in terms of the viscosity and/or the enhanced nucleation rate of these glasses.

The experimental SEM observations [9, 10] revealed that dropshaped segre­gation structures were enhanced when the present oxidized pyroxenic basalt­based glasses were reheated. Consequently, Fe3

+ may promote glass-in-glass phase separation which facilitates crystallization. Many authors considerthat the interfacial energy between two glassy phases is smaller than that between a glass and a crystalline phase [13]. Moreover, the viscosity ofthe glass may decrease in the presence of Na20 and/or MnO, and consequently the mobility and diffusion of the different ions forming glasses would increase, thereby explaining the higher crystallization rates in PNL and PNLM glas ses (Fig. 2).

Effect ofratio CaO:Na20

Substitution of a small extentofCaO by Na20 in the same glass (e.g. PNL and PL, Table 1) generally drastically modified the crystallization kinetics, as ex­pressed by modifications in the peak temperatures and shapes (Fig. 2). At a de­creased ratio CaO:Na20 the exothermic peak temperatures were sharply de­creased by about 50°C, and the peak shapes were sharpened and considerably in­tensified, as shown by curves a and b. These effects became more explicit and were highly increased in glasses containing MnO, e.g. glasses PNLM and PLM. The exotherm of PNLM glass is much more intense, sharper and has a lower peak temperature (-100°C lower) than its PLM analogue.

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EL-SHENNAWI et al.: PYROXENIC BASALT-BASED GLASSES 559

Consequently the crystallizabilities of these glasses were greatly stimulated when the ratio CaO:Na20 was decreased, and also increased in the presence of MnO, which may have a synergistic effect in speeding up the crystallization rate. Any interpretation of the crystallization behaviour described above must take into account the extremely rapid nuc1eation and growth processes induced in powdered glas ses, as in the DTA technique. Reduction to a fine powder has a cat­alysing effect on nuc1eation and growth processes in silicate glasses [1]. How­ever, the microscopic and X-ray analyses of heat-treated sampies [9, 10] indi­cated that the increase in crystallization velocity observed upon incorporation of Na20 in the glasses could be attributed to an increase in the nuc1eation rate.

Effect of ratio CaO:MgO

This effect can easily be observed in the DTA traces of glasses with different molar ratios CaO:MgO, viz. PL and PNL with glasses PD and PND, respectively (Fig. 3). The exothermic peaks intensified substantially and became more in­tense, sharper and lower by about 40-65°C than the equivalent glas ses with higher ratios CaO:MgO. In other words, the increase of MgO at the expense of CaO greatly enhanced the crystallization process, and therefore glasses PD and PND, with ratios CaO:MgO of -1.3 and 1.2, approaching that of pyroxene diop­side CaMgSh06 (I: I), have the sharpest exothermic peaks with the lowest peak temperatures. One possible explanation of the trigger effect of MgO on the crys­tallization of these complex glasses may be its lowering effect on the viscosity of these glasses, as indicated by the slight decrease in the endothermal peak tem­peratures corresponding to Tg• As a result of the decrease in the viscosity, the mo­bilities and diffusion rates of the different ions and ionic complex-forming glasses will be markedly increased during the crystallization process, leading to higher crystallizabilities. It is worth mentioning that this statement also holds for the enhancing effect of Na20.

ConcIusions

The simple dynamic DTA method was found to be very useful for charac­terization of the glass-ceramic compositions and adescription of the nature, character and temperature range of crystallization. In aseries of pyroxenic ba­salt-based glas ses, DTA was used to elucidate the changes occurring when a specified rectifying component was added to modify one or more of the ratios FeO:Fe203, CaO:Na20 and CaO:MgO affecting the monominerality and crystal­lization behaviour. Comparisons of the positions, characters and intensities of the DTA peaks when oxidizing agents and rectifying oxides Na20 and/or CaO, or CaO+MgO were incorporated readily revealed their effects on the crystallization

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560 EL-SHENNA WI et a1.: PYROXENIC BASAL T-BASED GLASSES

processes. The higher crystallizabilities exhibited with Mn02 as oxidizing agent and Na20 andlör MgO as rectifying agents were related to their effects in reduc­ing the viscosity of the glass and enhancing its nucleation rate.

* * * This work was carried out within the framework of research project No. 3/8/6/3 sponsored by

the National Research Centre. The authors are greatly indebted to Prof. Dr. Nabiel A. M. Saleh, Vice-President for Research.

References

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Technology. The American Ceramic Society. Plenum Press, New York 1962, pp. 382-95. 3 A. I. Berezhnoi, Glass-ceramics and Photositalls, Plenum Press, New York 1970. 4 A. W. A. El-Shennawi, 'Crystallization in some molten Egyptian basalts'. M. Sc. Thesis,

Cairo Univ., 1970. 5 A. A. Omar and A. W. A. E1-Shennawi, Nature Phys. Sciences, 231 (1971) 44. 6 G. H. Beal! and H. L. Rittler, Cer. Bull, 55 (1976) 579. 7 H. L. McCollister, United States Patent 19 (1977) 4009015. 8 A. A. Omar, S. M. Salman and H. I. Saleh, Ciramica Acta, abs., 196 (1992) 5. 9 S. A. M. Abdel-Hameed, 'Controlled crystal!ization of some Egyptian basalt-based

glasses'; M. Sc. Thesis, Zagazig Univ, 1995. 10 A.W. A. El-Shennawi, M. A. Mandour, M. M. Morsi, S. A. M. Abdel-Hameed, J. Am.

Ceram. Soc., accepted for publication. 11 R. C. de Vekey and A. J. Majumdar, Min. Mag., 37 (1970) 771. 12 K. Watanable and E. A. Giess, J. Non Cryst. Solids., 169 (1993) 306. 13 P. W. McMillan, Glass-Ceramics, 2nd Ed. Acad. Press, New York 1979.

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