Recycled glass as a source raw material for production of sintered board materials

Jiří Bydžovský, Tomáš Melichar

Brno University of Technology, Faculty of Civil Engineering

Introduction

In the last few decades of the past century there was a dynamic development in the building industry, namely in the area of utilization of waste, or more precisely secondary raw materials. The recycling cycle, whose outputs – raw materials – are, in many cases, considered to be full-value basic products, plays an important role concerning these issues and these raw materials are plentifully utilized in the building industry. Waste recyclability depends mainly on financial resources and their applicability to production of new materials which might be a limiting factor in this case. Despite of the fact that quite high volumes of waste are being processed, considerable volumes are still being disposed without any subsequent utilization.

Secondary raw materials include various types of recycled glass. Production processes connected with raw glass, or more precisely glass products, are focused mainly on their visual features (colour, transparency, etc.) which are significant or even crucial criteria for their competitive advantage in the market. With respect to this fact recycled raw glass cannot always be fully utilized in glass production, i.e. it cannot fully replace glass stone which is the basic raw material for glass production, but it can be used partially (this fact is closely connected with the high purity requirements for fragments and it depends on product types). Probably the best known types of raw glass in connection with recycling are clear and coloured bottles and containers and ordinary flat glass obtained mainly from demolished buildings or car wrecks. Some types of raw glass are not suitable for glassworks at all. Electrical products with expired lifespan, i.e. fluorescent lamps, spotlights, lamps, car headlamps, etc. are worth mentioning. A significant volume of glass waste is also obtained from disassembled useless TV screens and computer monitors. They are the older types of screens – CRT (Cathode Ray Tube) – which are currently being replaced by LCD screens. There are specialized lines designed for screen disassembling, cleaning and sorting raw glass whose technologies are patented. Therefore, there is the problem what to do with full-value raw glass which cannot be utilized in glass production because it contains some undesirable (toxic) elements. In the area of utilization of secondary raw materials obtained from glass recycling several researches have been carried out in many ways (cement composites, macromolecule-based plastering mixtures, glass ceramics, etc.) but no fully satisfying results which would solve these issues complexly have been achieved. Production of glass-silicate materials offers a wide range of ways how to utilize recycled glass which has not been utilized at all yet and, therefore, harms the environment due to its disposal.

Glass-based sintered material

Everybody knows glass in the common sense of the word, i.e. panels in building structures, cars and housing accessories. Glass-silicate material is known less among common people. This material consists mainly of raw glass which is supplied in the form of granulate of specific fractions. Borosilicate raw glass produced directly at glassworks is currently being used for production of glass-silicates, i.e. it is a source raw material. Raw glass creates most of the material batch in production of glass-silicates (approx. 95 - 99 %). The other substances, mainly the ones correcting production processes (sintering temperature) or some final features (volume weight, appearance – texture and colour), are mainly pigments and silica sand. The production process connected with glass-silicate materials consists of the following technological operations:

· Mechanical pre-treatment of the basic raw material (mainly borosilicate raw glass) in separation and refining machines (grinders and mills),

· Sorting the required fractions in order to obtain the required grain size composition (sieve analysis),

· Homogenization of all the batch components – i.e. raw glass granulate, silica sand, pigments, etc.,

· Piling up into refractory moulds which are usually equipped with a suitable, e.g. kaolin-based, separating layer,

· Heat treatment of the batch in electric furnace aggregates (where sintering is carried out) according to the defined temperature regime – controlled heating and cooling,

· Mechanical treatment in order to obtain the required form of products and polishing in order to obtain the final surface using specialized diamond tools formed into automated lines.

Fig. 1 : Piled up raw glass granulate with black pigmentation for production of glass-silicate board in a refractory mould

The typical feature of glass-silicates is their texture which is similar to crystalline materials (stone facing and paving) but their physico-mechanical and chemical features are based on the features of glass which makes dominant part of these materials. The characteristic texture appearance is caused by the suitable heat regime – sintering. In the course of this process particular grains are sintered together at continuous compaction of the system which avoids creation of pores and a coherent matrix is made. But the batch does not melt. Taking the pigment application into account is also very important. There are mainly metallic oxides which make the final colour spectrum of glass-silicate boards. Some types of pigment admixtures (even their fractional volume – i.e. approx. 1 %) regulate the boundary of the maximum isothermal persistence when sintering is carried out. The following figure shows a temperature regime curve which is commonly used for production of glass-silicate facing boards and paving in the building industry. This defined temperature mode is typical for products made of the above-mentioned borosilicate raw glass.

Glass-silicates have very good strength features (their bending strength is approx. 30 N.mm^-2), very low or no absorptive capacity, closed system of pores, high chemical resistance, frost resistance, colour stability, resistance to sudden temperature changes and they are environmentally friendly. The disadvantage of glass-based materials is their fragility. Moreover, glass-silicates are quite expensive which is connected mainly with their final mechanical treatment carried out to obtain the required form, shape and surface lustre. These materials are used mainly as facing and paving for both interiors and exteriors. Besides the building area these materials can be used e.g. for production of tombstones (nowadays there is a lack of Swedish black granite which is the basic raw material for tombstones) or building interior accessories (table boards, etc.). Glass-silicate boards can be used at food and chemical plants. Another way of their utilization is swimming-pools. Architectonical utilization can also be considered – in the form of aesthetic accessories for buildings (translucent facades or floors, etc.).

Experimental part

The above-mentioned text obviously states that production of glass-silicates is quite a difficult process in terms of energy and raw materials. Therefore, there is a concern to optimize the production process connected with glass-silicates and to possibly modify their composition using as many secondary raw materials as possible and, at the same time, keep their declared parameters. The following secondary raw materials were considered for the purposes of this research:

· Recycled raw glass obtained from disassembled screens and computer monitors – namely it was a mixture of funnel and screen parts – CRT samples,

· Recycled raw glass obtained from collected coloured bottles – i.e. common waste which has been recycled.

Borosilicate raw glass of specific fractions supplied directly by the producer was chosen as the reference raw material on the basis of the above-mentioned findings. It was necessary to pre-treat the grain size of the screen raw glass in a ball mill. Before the modification of the composition of the raw material batch started, the secondary raw materials as well as the primary raw materials had been analyzed – i.e. sieve analysis and determination of the chemical composition of the particular substances. Then the formulas were drawn up and laboratory firings were carried out according to the determined temperature regime curve. The important fact is that it was necessary to adjust the isothermal persistence in the sintering area (i.e. 960 °C – this temperature is typical for borosilicate raw glass) according to the features of the input raw material (i.e. the size of the maximum fraction and the chemical status) which is shown in the charts. Regarding the screen raw glass which contains lead monoxide and other melting compounds, the temperature dropped down but the container raw glass needed higher temperature.

Tab. 1 : Chemical composition of alternative raw materials used for production of glass-silicate materials

Component	Glass sample
	REF [%]	CRT [%]	COL [%]
SiO2	72.01	66.79	69.73
Al2O3	7.11	4.21	1.76
Fe2O3	-	0.28	0.41
BaO	1.98	10.70	0.26
CaO	105	0.28	9.96
B2O3	10.24	-	-
MgO	-	0.11	2.29
Na2O	6.22	7.57	12.20
K2O	2.02	6.91	0.88
PbO	-	0.91	-
SrO	-	0.25	-
TiO2	-	0.04	-
LiO2	-	0.38	-
MnO	-	-	0.02
Cr2O3	-	-	0.072
ZrO2	-	-	0.05
Organic compounds	-	-	0.33

The determination of presence of particular compounds in terms of chemical composition of the chosen types of raw materials shows a certain similarity. The dominant compound is silicon oxide which determines the basic physico-mechanical parametres of the glass. Other important compounds are alkalies combined with lead monoxide which play a role mainly in terms of reduction of the temperature necessary for sintering. No less important is the amount of heavy toxic metals in the glass matrix. In this particular case they are PbO and SrO. Although there is only a very small amount of these harmful compounds, it is very important to take them into account. With respect to the fact that they are secondary raw materials, presence of some organic impurities must also be taken into account. They might have a negative impact on the parametres of the final products as well as their appearance. By this reason the content of organic compounds in recycled glass obtained from collected coloured glass was determined. The chemical analysis also shows that the borosilicate (reference) raw glass contains approx. 10 % of B₂O₃. It is generally known that this oxide is added to raw glass matrices in order to ensure high chemical resistance.

The following table (Tab. 2) contains an assessment of the sieve analysis, i.e. the analysis of distribution of the particular granulate fractions. The values are included in the chart bellow for easier comparison and the following symbols are used:

· REF – reference borosilicate raw glass supplied directly by the producer which is unchanged in terms of mechanical and chemical treatment,

· CRT – recycled raw glass obtained from diassembled TV screens and computer monitors, namely it is a mixture of screen (front) and funnel parts supplied directly from a recycling line,

· CRT-ground – screen raw glass mechanically treated - refined in a ball mill,

· COL-fine – fine-grained raw glass obtained from recycled coloured bottles and other containers supplied directly from a recycling line,

· COL-coarse – coarse-grain raw glass obtained from recycled coloured bottles and other containers supplied directly from a recycling line.

Tab. 2 : Analysis of the size of elements of the granulated input material

Sieve [%]	Undersize [%]
	REF	CRT	CRT-ground	COL-fine	COL-coarse
0.00	0.0	0.0	0.0	0.0	0.0
0.50	9.0	0.9	12.5	41.6	0.9
1.00	21.2	12.0	24.6	63.4	2.5
2.00	732	31.7	45.0	89.3	5.9
4.00	96.2	54.8	72.6	100.0	24.5
8.00	100.0	74.0	100.0	100.0	59.0
16.00	100.0	956	100.0	100.0	98.5
32.00	100.0	100.0	100.0	100.0	100.0

Fig. 2 : Curve of the grain size of the screen raw glass supplied from a recycling line (a mixture of funnels and screens)

When the temperature compaction of the granulated batch was finished, the sizes of the samples and the roughness of their surfaces were modified to start an analysis of their physico-mechanical and chemical parameters. Bending strength and fracture power according to ČSN EN 10545-4, absorptive capacity according to ČSN EN 10545-3 and frost resistance according to ČSN EN 10545-11 were determined. For determination of particular parameters of the produced materials there were sets consisting of five pieces of test specimens each made in order to obtain reliable, statistically assessable sets of analyzed data. The following charts show only the average resultant values of the above-mentioned analyses and their mutual comparison. The following symbols are used in the charts:

R – bending strength [N.mm^‑2],

E – absorptive capacity [%].

Fig. 3 : Graphic assessment of the values of bending strength of a set of the test specimens under ordinary climatic conditions and a set of the test specimens after the frost test

Fig. 4 : Graphic assessment of the values of absorptive capacity of the samples before and after the frost test

Explanatory notes to the marking of the produced test bodies used in the charts above (Fig. 19 - 22):

· REF, CRT (ground), COL (fine and coarse) – a set of samples made of borosilicate (reference), screen and container raw glass (for the marking see Tab. 2),

· 700, 750, 775, 800, 900, 960, 1050 – temperatures of the maximum isothermal persistence in the course of the production process,

· 0-1, 0-4, 0-6, 0-8, 1-4, 1-8, 4-8, 0-2, 0-16 – used fractions of the secondary raw material.

The produced samples were also assessed visually, mainly with respect to their structure which is, thanks to the glass-silicates, very interesting for architectonical purposes. The following photographs show some representative samples and their textures.

Fig. 5 : Texture of a representative sample of the CRT 700 0-1 formula (left) and the CRT 800 0-8 formula (right)

Fig. 6 : Texture of a representative sample of the COL 800 0-16 formula (left) and the COL 960 0-16 formula (right)

Fig. 7 : Texture of a representative sample of the REF 960 0-8 formula (left)) and the CRT 775 0-8 formula (right))

Conclusion

The analyses above show a significant addiction of the bending strength of the final materials to the type of the secondary raw material used for the process. The maximum temperature in the area of the first isothermal persistence, whose drop raises the question of energy consumption and ecologic aspects, also closely corresponds with the features of the secondary raw material (i.e. its chemical status, the maximum grain size and mainly purity of the input raw material). The charts obviously show that the samples not using the fine fraction reported the highest strength which also minimizes creation of macro-pores. It is also obvious that most of the produced test specimens showed significantly better strength parameters than the reference samples which is a very positive result. There was a significant drop only in the formulas which did not undergo appropriately high isothermal persistence (CRT 700 4-8 and COL 800 0-16). By using the screen raw glass the volume of consumed electricity was significantly smaller.

The absorptive capacity significantly determines the openness, or more precisely the closeness of the pore system. An overwhelming majority of the samples of the proposed formulas showed very low values of their absorptive capacity, i.e. below 0.5 %, which was, in a certain way, foreseeable regarding the glass-silicate materials. Inversely proportional addiction of the absorptive capacity to the bending strength, the maximum fraction of the raw material and the isothermal persistence also proved there which can be demonstrated by the graphic comparison of the reached values. Regarding the container glass, purity of this input raw material, mainly the content of organic substances (residues of glues and labels) also played an important role – see Fig. 6 (presence of burnt out residues after decomposition of macromolecular substances which cause, together with a certain level of viscosity, so-called flatulence).

Frost resistance of facing elements is defined as a graphic comparison of the absorptive capacity values before and after the determined number of frost and defrost cycles is carried out. In this case there were 100 cycles carried out in total. The absorptive capacity comparison is shown in Fig. 4. We can say that all the produced samples showed the maximum frost resistance level according to ČSN EN 10545-12. Nevertheless, in order to make the frost influence on the strength parameters clear, the test specimens were a subject of determination of their bending strength where the influence of the temperature cycles was more evident. But there was no significant difference which would significantly restrict utilization of the materials under external climatic conditions.

At the conclusion I can state that recycled raw glass from different sources represents a very lucrative and potential raw material for production of glass-based board materials which are used mainly as façade facing, or possibly paving in residential buildings.

Acknowledgements

The text has been drawn up within the MSM 0021630511 research project – “Progressive Building Materials with Utilization of Secondary Raw Materials and their Impact on Structures Durability” and the FT-TA5/147 project – “Sintered products made of by-products for creation of walls and floor surface treatment”.

References

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