On the Formation of Closed and Interconnected Pore Structures in Nepheline-Based Foam Glass
Abstract Generated abstract
This study examines why foam glass synthesized from nepheline-containing glass develops interconnected pores and high water absorption despite otherwise favorable properties. Nepheline glass was prepared from Uzhur deposit rock with additives, foamed under varied temperatures, times, heat treatments, and gas-forming agent contents, and evaluated by property measurements and petrographic analysis. The results indicate that open porosity increases most strongly near 900 to 920°C, where crystallization is greatest, and that excess gas-forming agent can further rupture glass films and increase communicating pores. Reducing the foaming agent content and modifying the thermal regime to weaken crystallization made it possible to obtain closed-pore thermal insulation foam glass with water absorption below 3% and low thermal conductivity.
Full Text
On the Formation of Closed and Interconnected Pore Structures in Foam Glass Based on Nephelines
S. S. Nesterenko, A. V. Petrov
(Presented by the scientific-methodological seminar of the ChTF)
The production of a cellular product from nepheline-containing glass was accompanied by increased water absorption of the material. Taking into account that, by its other indicators, the foam glass was not inferior—and in a number of properties exceeded—the quality of cellular glass produced by industry, the objective of the study included establishing the causes of the formation of through pores in the synthesized product.
Summarizing the literature on the foaming of glass melts, the factors influencing the ratio of open and closed pores in the material may be reduced to the following.
Interconnected pores in the material are caused by the properties of the glass melt itself: its tendency toward crystallization, the high surface tension of the melt, and a sharp change in its viscosity within the pore-formation interval [1, 2].
Insufficient annealing, excessive pressure developed by the gas phase of the system, and an incorrectly chosen degree of dispersion of the glass powder and gas-forming agent may also be causes of increased water absorption of the porous material [3].
For the synthesis of foam glass based on nephelines, a glass melt of the following chemical composition was used: SiO₂ — 55; TiO₂ — 0.43; Al₂O₃ — 14.11; F₂O₃ — 5.89; MgO — 0.94; CaO — 10.06; Na₂O — 14 (percent by weight). It was melted from nepheline rock from the Uzhur deposit with additions of quartz sand, chalk, and sodium sulfate in an electric furnace with silite heaters at 1350°C for one and a half hours. Refining was carried out by casting the melt into water. The resulting granulate, preliminarily dried at 200°C, was ground to a powder specific surface area of 6000 cm²/g. The reaction mixture, composed of glass and the standard amount of gas-forming agent (3% cryptol above 100 parts by weight of glass), was introduced into a furnace preheated to 700°C, where it was foamed in the temperature range 780–980°C with different sintering durations. Heating of the furnace from 700°C to the experimental temperature was carried out at a rate of 12 deg/min. Annealing took place for one hour at 620°C, followed by cooling of the foam-glass blocks to room temperature at a rate of 0.8–1.1 deg/min.
On the basis of analysis of the properties of the synthesized foam glass, it was established that pore formation in high-alumina nepheline glass obeys the general regularities characteristic of other industrial glasses during foaming, up to temperatures of 880–920°C. With an in-
the latter show an increase in the number of open pores in the material and in its bulk density, with a simultaneous reduction in cell size.
Petrographic analysis indicates intensive crystallization, under these conditions, of the walls of the pores of the foam glass, with the formation of a fine-grained structure (the crystal size did not exceed 1 µm).
It is known that uniform fine-grained crystallization during the foaming process does not hinder the production of a porous material; moreover, an increase in the amount of the crystalline phase is not accompanied by growth of communicating channels [4].
When nepheline glass is foamed, it is precisely the number of crystalline inclusions that plays the decisive role in the number of open pores in the material. Thus, the maximum crystallization properties of the glass batch under study occur at 900–920°C; in this interval the greatest water absorption of the foam glass is also observed.
Additional heat treatment of the porous material for one hour at 750–1000°C further increases the degree of crystallization of the glass, which is reflected in its properties: the coefficient of thermal expansion, the temperature at the onset of deformation, and the number of communicating pores in the material increase, while its strength decreases somewhat (Table 1).
Table 1
Effect of additional heat treatment on the properties of foam glass
| Bulk density before treatment, g/cm³ | Heat-treatment temperature, °C | Water absorption, % | $\sigma_{\mathrm{comp}}$, kg/cm² | CTE × 10⁻⁷, deg⁻¹ (20–600) | Temperature at onset of deformation, °C |
|---|---|---|---|---|---|
| 0.274 | — | 36.6 | 29.4 | 80.3 | 625 |
| 0.270 | 750 | 37.3 | 25.3 | 80.9 | 630 |
| 0.280 | 800 | 40.7 | 26.8 | 82.8 | 650 |
| 0.278 | 900 | 58.8 | 20.6 | 85.2 | 680 |
| 0.281 | 1000 | 40.0 | 26.5 | 83.4 | 660 |
The greatest change in all properties, as in the synthesis of foam glass, occurs at 900°C; in particular, water absorption reaches 58.8%. Evidently, in the process of crystallization the residual glass batch becomes enriched in alkalis and cannot withstand the pressure developed by the gas-forming products under the specified foaming regimes.
In addition, the excess partial pressure of the gaseous phase of the system may exceed the tensile strength of the uncrystallized glass films. Table 2 illustrates the results of changes in the properties of foam glass obtained by foaming a batch for
Table 2
Change in the properties of foam glass as a function of the amount of gas-forming agent
| Amount of cryolite, % | Bulk density, g/cm³ | Water absorption, % | Pore diameter, mm | Foaming coefficient |
|---|---|---|---|---|
| 1 | 0.334 | 1.23 | 0.5 | 1.6 |
| 2 | 0.270 | 14.5 | 0.8 | 3.34 |
| 3 | 0.234 | 36.2 | 1.3 | 5.08 |
20 min at 900°C without preliminary heating, as a function of the amount of gas-forming agent. The amount of foaming agent selected by us, as can be seen, is excessive for the nepheline glass batch, despite the almost complete absence of its crystallization.
By reducing the amount of cryptol to 2% and weakening the crystallization properties of the glass, which is achieved by brief sintering of the batch followed by forced cooling of the foam-glass blocks to the annealing temperature, it is possible to obtain a high-quality thermal-insulation material with a thermal-conductivity coefficient of 0.079 kcal/m·h·deg. with closed pores (water absorption does not exceed 3%).
REFERENCES
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- E. Z. Zhitomirskaya. Glass and Ceramics, No. 2, pp. 19–20, 1963.
- I. I. Kitaigorodskii, T. N. Keshishyan. Foam Glass. Promstroiizdat, Moscow, 1953.
- I. I. Kitaigorodskii, T. L. Shirkevich. Doklady AN SSSR, vol. 162, No. 6, pp. 1339–1341, 1965.