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INTERNATIONAL SYMPOSIUM Recycling and Reuse of Glass Cullet
University of Dundee 2001


ABSTRACT. The main problem in creating the technology of producing foamed glass from glass cullet is that physical and chemical properties of the initial glass may be different. Besides, using sulphite technology leads to the significant quantity of hydrogen sulphite in the product, which limits its application. The given problem was solved with the help of the system analysis of existing foamed glass technologies and the research of the physical and chemical processes proceeding in the system with the heat treatment. The offered technology includes grinding up glass to a powder with the size of the particles no more than 10 mkm. The powder is mixed with chemical additives and the resultant composite material is heated up to the temperature of softening and foaming of the glass in the furnace. The obtained blocks are processed up to the required sizes. Wastes are ground with the initial glass which provides complete wastelessness of the technology. The proposed manufacture allows not only to completely utilize glass cullet, but also to receive a high-quality building material. Manufacture of foamed glass according to the developed technology is organised at one of the Ural region factories in Russia. The obtained material has fire, ecological, sanitary and building certificates. Excellent heat insulating properties in combination with high strength provide high competitiveness of foamed material on the market of building materials. At present technological line provides the production of the material with the heat conductivity of 0,035-0,045 W/m·K at density of 160-200 kg/m3 and compression strength of 1,3-1,5 MPa. In future the start-up of the technological line for the manufacture of the material with the specified characteristics of 0,065-0,075 W/m·K at density of 350-370 kg/m3 and compression strength of 3,0-3,5 MPa is planned. This material has already passed the laboratory tests.

Keywords: Glass cullet, Foam material, Low heat conductivity, Compression strength, Glass-crystal structure, Technology.

INTRODUCTION

The subject of the given research is the production of foamed aluminosilicate material. Foamed glass is a light, closed-cell structure, non-toxic, rigid, easily treated, chemically inert, water- and vapor-resistant heat-insulating material. The combination of these properties with low heat conductivity, high strength, stability and manufacturability makes this material practically irreplaceable both in construction and in many other fields.

The decrease of power costs on the operation of housing accommodation is possible only with the use of effective and durable heat-proof materials. According to the literature data [[1], [2]], the complete loss of heat-proof and technical properties of foam polystyrene occurs in 10 years, foam urethane - also in 10 years, and glass- wool materials in 7 years.

Most durable in construction are heat-proof materials on the basis of inorganic foams: foam concrete and foam glass. The former has a wide application in construction in Russia, but the application of the latter is rather limited because of the technological problems in manufacture.

The existing technologies of foamed glass have the following drawbacks: - hydrogen sulphide formation; - a great deal of spoilage because of units cracking; - usage of specially welded glasses as raw material (impossibility of using only glass cullet); - low manufacturability (high temperature, long duration of the process, annealing in special furnaces).

The researches have shown that the structure of the silicate systems such as glasses, lies in the field of the liquidus at temperatures of 1200-1300 °С. Therefore, softening and foaming of glass proceeds on softening the amorphous and thermodynamically nonequilibrium aluminasilicate material. The addition of crystalline substances promotes crystallisation of the system and solidification of foam at the synthesis temperatures (780-800 °С).

The main difference of the proposed technology from the classic one is that in the classic scheme foaming and fixation of foam proceed according to the kinetic mechanism and depend on many parameters of the manufacture (time, structure of a glass etc.), but in the technology presented the fixation of foam proceeds by the thermodynamic mechanism. It occurs due to the transition of aluminosilicate to the crystalline form, infusible at the temperature of synthesis.

EXPERIMENTAL DETAILS

The substances and mixtures used in the research are usually those applied in building and certified according to the system of the state standards of Russia. Glass cullet applied is from the municipal wastes averaged over a week period. Initial substances were divided in a spherical mill. Dispersion of powders was determined by the sedimentation analysis [[3], [4]]. A scanning electronic microscope REM-100M was used.

RESULTS AND DISCUSSION

The usual foam glass has low stability to thermal impacts. The problem could be solved by partial crystallisation of glass in the volume of foaming. The analysis of the SiO2-Na2O-CaO diagram, characteristic of the household glasses, shows that the temperature of the liquidus is about 1000 °С. Therefore, if there is enough germs of a new phase, the crystallisation in powder glass blocks will go spontaneously at the temperatures of foaming and softening of glass. Moreover, the process of growth of foam will be controlled by a decrease in viscosity of the system because of crystallisation.

The additives of various substances promoting the crystallization of glasses allowed us to combine the processes of foaming of a glass bulk and its partial crystallisation. According to X-ray analysis, the phases of SiO2, Na2Si2O5, and small amounts of CaO·Al2O3 and 5CaO·3Al2O3 were formed.

Partial crystallization of glass does not result, nevertheless, in the change of the specific volume of the substance and the material crackingl. In Figure 1, the picture of the surface obtained foam glass-crystal material is shown. The solid phase forms the walls of separate cells several microns thick, which are, in turn, formed by the walls of cells of a smaller size. It is necessary to pay special attention to the closed character of "bubbles" and absence of micropores on the surface of the material.

The picture of an ordinary gas foamed concrete surface is shown in Figure 2. It is obvious, that the differences in the character of the materials surface result in essential differences in the operational characteristics. First of all, the micropores in the structure of a gas foamed concrete lead to the low moisture resistance and especially to the low frost resistance. So, the investigated 400 kg/m3 sample of the gas foamed concrete has lost more than half of the initial weight after 5 cycles of freezing and thawing. At the same time, the sample of the obtained foam glass-crystal material has lost only 2 % of its initial weight after 50 cycles. Besides, due to the micropores and microcracks in the structure gas foamed concrete is less durable than the foam glass-crystal material.

So, the compression resistance of the gas foamed concrete with the density of 300 kg/m3 is 0,7-0,8 MPa. The same durability of the obtained glass-crystal material is achieved with the material having the density of only 140-160 kg/m3. Besides, the natural sorptive capacity of heat resistant materials plays an important role in the practice of construction in severe climatic conditions of Russia. According to the building standards [[5]] at high humidity of air, the characteristic sorptive capacity to water of the gas foamed concrete varies from 12 to 15%. This factor causes the growth of heat conductivity up to 50-56% in comparison with the heat conductivity of an absolutely dry material. The obtained foam glass-crystal material practically does not absorb moisture. Therefore it maintains high heat resistant properties under various conditions.

The theoretical and practical questions of “classical” foam glass technology are considered in detail in the monographs [[6], [7]]. However, the above mentioned problems limited the manufacture of the given material in Russia. The proposed technology gives a number of decisions directed, first of all, onto crystallisation of items in the course of heat treatment. The major factor effecting the main parameters of the obtained material is the size of the initial particles of glass. So, in Figure 3 the dependence of density of foam glass-crystal material on the prevailing size of initial particles of glass is shown.

Decrease in the prevailing size of particles in the initial powder reduces the density of the obtained material. This influence is very important for powders with the size of particles less than 25·10-6 -30·10-6 m. Therefore, for the technology discussed, special attention is given to the quality of the smashing equipment and mills.

The influence of other parameters is less evident. So the dependence of density on the amount of coal used as a porous former has a wide minimum in the range of 0,30 to 0,55 % (mass) as it is shown in Figure 4.

The experiments showed the dependence of the "classical" foam glass density on the time of heat treatment. This process is natural to foam. The redundant superficial energy of system is decreased by the reduction in the specific surface of the unit of phases that externally is shown as coalescence and "falling" of foam. The typical dependence of the change of density of foam glass on the time of isothermal endurance is illustrated in Figure 5 (curve а). After the process of gases release finishes, the gradual process of destruction and falling of foam begins. This effect can be prevented by the transition of foam into a solid state.

Usually, for the formation of foam glass it is necessary to calculate precisely the time of heat treatment and to take foam away from the hot zone prior to the beginning of the coalescence process. In case of a crystal structure formation inside the foamed silicate, the system forms a rigid skeleton at temperatures of softening glass which prevents its destruction. The dependence of densities of glass-crystal material on time is shown in Figure 5 (curve b). It is possible to observe the absence of a sharp increase of densities after its minimal value is achieved.

This feature of the proposed materials is extremely important from a technological point of view; since it allows us to achieve high security of the process from the parameter of time and considerably reduces the influence of temperature on the process.

The research allowed us to organise the manufacture of the effective heat resistant material, and the municipal waste glass cullet is used as a raw material. All the wastes of the production are supplied to raw-material storage and are reused in the manufacturing process.

CONCLUSIONS

The main difference of the proposed technology from the classic one is that in the classic scheme foaming and fixation of foam proceed according to the kinetic mechanism and depend on many parameters of the manufacture (time, structure of a glass etc.), but in the technology presented the fixation of foam proceeds by the thermodynamic mechanism. It occurs due to the transition of aluminosilicate to the crystalline form, infusible at the temperature of synthesis.

The offered solution allowed us to successfully solve a problem both in the level of the chemical mechanism of the obtaining of the material, and in the technological level, and to achieve the purposes of the research.

ACKNOWLEDGEMENTS

The author would like to acknowledge the TermoEc Ltd (RU), Perm State Technical University and Dr. N.Rudina (Boreskov Institute of Catalysis, RU).

 

REFERENCES:

  1. Orlov, D. Foam glass as an effective heat resistant material. Glass of the World, No 4, 1999. pp 66-68.
  2. Ludikov, V. Supply of heat resistant materials. Materials Wastes Raw stuff Engineering, Vol 12, 1997. pp. 12-17.
  3. Kouzov, P. The analysis of dispersion of industrial dusts and powders of materials. Chemistry, St-Peterburg, 1987, pp 264.
  4. Frolov, Yu. Colloid chemistry. Chemistry, Moscow, 1982.
  5. GOSSTROI OF RUSSIA. Building norms and rules II - 3-79*. GUP CPP, Moscow, 1998, pp 29.
  6. Demidovich, B. Foam glass. Science and Engineering, Minsk, 1975.
  7. Demidovich, B. Production and useing of foam glass. Science and Engineering, Minsk, 1972.
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