Sinter-crystallization of a glass obtained from basaltic tuffs

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Abstract

Glass-ceramic materials, obtained by sinter-crystallization of melted alkaline-olivine basaltic tuffs, were investigated. The kinetics of bulk crystallization was evaluated by differential thermal analysis (DTA) at different heating rates. The phase formation and the sintering behavior of glass powders (<75 μm) were studied in air and in nitrogen atmospheres by DTA and dilatometry, respectively. The crystalline phases formed were identified by X-ray diffraction. The DTA traces showed an unusual phase formation behavior with a higher crystallization trend for the bulk samples. The crystallization activation energy was evaluated as 590 ± 20 kJ/mol in the range 1080–1110 K. A value of about 3 of the Avrami constant, corresponding to three-dimensional growth on a fixed number of nuclei, was evaluated by Ozawa and Augis–Bennet methods. The densification at low-temperatures is reduced by the intensive crystallization process in both air and nitrogen atmospheres. The sintering starts again at 1150–1250 K. In air atmospheres, due to the FeO oxidation, the sintering temperature increases and the percentage of formed crystal phase decreases by 15–20%.

Introduction

Due to the elevate melting temperatures, the specific thermal treatment and the high cost of pure raw materials, glass-ceramics attain outstanding properties with high production costs. In some cases, however, it is possible to produce glass-ceramics at low melting temperatures, by using cheap raw materials and short production cycles. Typical examples are the iron-rich silicate compositions.

The first iron-rich glassy material was developed before the First World War in the France ‘Compagnie Générale du Basalte’ [1] by re-melting rocks (petrurgy). Today, several petrurgical companies are active in Europe, producing building tiles, obtained by pressing or casting, pipes and bends by centrifuging, and glassy insulation fibers (rocks wool). Iron-rich glasses and glass-ceramics were developed for the nuclear waste disposal [2], [3], [4], [5] and more recently, for the vitrification of various hazardous industrial wastes [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20].

The iron-rich compositions often show a tendency to spontaneous crystallization. This phenomenon increases the apparent viscosity of the melt [12], [21], [22] and might generate problems during the forming process [1]; also, the crystalline structure may becomes inhomogeneous and coarse. In order to overcome these phenomena, the sinter-crystallization technique was successfully applied [8], [16], [17], [20].

The basaltic and basaltic related igneous rocks can be separated in two main groups: subalkaline and alkaline [23]; the first group is constituted by the tholeiitic and calcalkaline basalts, whereas the second – by the alkaline and alkaline-olivine basalts. Until now, the petrurgy has mostly used tholeiite basalts or diabasic rocks [24], [25], [26], [27], [28], which, due to their higher viscosity, are characterized by a reduced tendency to spontaneous crystallization. On the other hand, the low viscosity alkaline basalts appear more appropriated for the production of sintered ceramic and glass-ceramic materials.

The aim of present work is to obtain sintered glass-ceramics by using alkaline-olivine tuffs from Southern Anatolia. The kinetics of phase formation and the sintering behavior were studied in air and in nitrogen atmospheres by differential thermal analysis (DTA) and dilatometry, respectively. The crystalline phases formed were identified by X-ray diffraction.

Section snippets

Experimental

In a previous work [29], it was shown that, due to its initial porosity and elevated amount of amorphous phase (60–70 wt%) the basaltic tuff may be easily crushed and milled and that the crystal phases (mainly pyroxene, olivine and plagioclase) melt in the range of 1370–1480 K. These characteristics emphasize the possibility of applying simple and low-temperature melting procedure.

In the present study, the tuffs were crushed for few minutes in an agate mortar; then about 100 g of the obtained

Results

Fig. 1 shows the DTA results of bulk sample in air and powder samples in air and nitrogen atmospheres, recorded at 10 K/min, while Fig. 2 presents the traces of bulk samples at different heating rates. Fig. 3, Fig. 4 show the corresponding results for the activation energy of bulk crystallization, EC, by Eq. (1) and the Avrami parameter, m, by Eq. (2), respectively. Table 1 summarized the m values, obtained by Eq. (3).

The dilatometric sintering curves in air and nitrogen atmospheres are depicted

Discussion

The DTA results of bulk and powder samples (Fig. 1) show an unusual crystallization behavior: the bulk sample has a higher crystallization trend than the powder samples, both in air and nitrogen atmospheres. The bulk sample shows glass-transition temperature, Tg, at about 910 K, sharp and intensive crystallization exo-effect with peak temperature, Tp, at 1095 K, liquids temperature, Tl, at 1397 K and two melting endo-effects at 1424 and 1468 K. The two powder samples show similar Tg and Tl, while Tp

Conclusion

The studied alkaline basaltic glass shows an unusual crystallization behavior with an extremely high rate of phase formation in the bulk. A three-dimensional growth on a fixed number of previously formed nuclei is proposed as crystallization mechanism. The high value of the activation energy of crystallization, 590 ± 20 kJ/mol, can be related to the high viscosity during the phase formation.

The sintering of powder samples is reduced by the intensive crystallization, both in air and nitrogen

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