Refractory Applications of Glass Powders
Glass powder isn’t just good for making new glass - it has applications in the furnaces used to make glass and beyond - for example in refractory materials, high quality insulation, cements and more. African Pegmatite is a leading supplier of ground glass in a range of gauges and qualities for the broadest array of refractory applications.
A refractory material is one that has high tolerance of temperature, and at high temperature is resistant to cracking, degradation or other chemical attack. Powdered glass can act as a component in the manufacture of refractory materials. The iron and steel production sectors consume 70% of the global supply of refractory materials (1). Refractory materials come in several classes; acidic, basic, oxide-carbon or specialised. As a prime source of silica(2), ground glass is therefore an acidic refractory material; silica is the most commonly used refractory(3). It is estimated that in excess of 200 million tonnes of glass waste is generated annually(4), so reusing as much of this waste glass is imperative. As it is a waste product, it is very inexpensive, and thus attractive for industrial uses.
The vast majority of ground glass is sourced from waste container and plate glasses(5). Waste glass is typically collected at the municipal level and experiences an amount of cleaning before it is crushed into pellets referred to as cullet. Cullet itself can be melted directly to manufacture more glass, or it can be ground into a fine powder. Crucial to the process is the identification of the glass itself - glass with high metal or metalloid content can cause a lack of uniformity in the cullet itself, the powdered cullet if it is processed via that route, or in any future glass product.
The re-use of glass cullet is important due to the recycling imbalances in a particular country’s waste system. The UK, for example, cannot process all of the green glass (that chiefly arises from imported wine bottles) it uses and so must export green glass cullet. It would be beneficial to use the cullet locally rather than export - for both greater financial and environmental efficiency.
Ground glass has found use in the manufacture of fibreglass insulation and as a flux in the manufacture of bricks. These, however, are not refractory materials in their own right. A subset of bricks, however, so-called ‘fire bricks’ are.
It is noteworthy that glass powder and cullet require less heat input to melt them down in order to be reused or incorporated into another material - this is a significant concern especially when compared to using fresh silica. Lower energy requirements mean a reduced electricity bill and a more efficient process - both effectively contributing to help maintain a foundry or plant’s bottom line.
Refractory materials are produced by a sequential process: raw material processing, forming and then firing. Fire bricks are an example of a silica-containing refractory material. They are prized for their high resistance to temperature. Such bricks are found in steel- and glass-making furnaces and over time included ground container glass(6) as a major component. Other uses beyond bricks have been demonstrated, such as a tundish lining and in ceramics and cements.
A tundish is a device used in the continuous manufacture of steel products that acts as a buffer between the furnace and the casting moulds. Crucial to the efficient production of steel is that the molten steel does not begin to cool and solidify in places other than the mould. Tundishes are, therefore, insulated with several layers of refractory materials(7). In addition to insulators, tundishes are increasingly used to aid in purification of the molten steel to absorb non-metallic impurities and prevent oxidation(8).
When selecting refractory materials to form tundish layers, it is critical to consider not only the properties of the material itself, but the properties inferred from the form it is in. Tundishes lined with layers of refractory bricks tend to last for less time than do linings made from single, poured refractories(9) (less than five heats compared to 30 for the more monolith-type). An early tundish liner was composed of sodium silicate, itself formed from the reaction of silica and sodium hydroxide.
A particularly useful feature of silica - and thus ground glass - is that it is able to remove iron oxide from molten steel, both in the tundish and when directly injected into the melted steel(10). When used directly in molten aluminum-killed steels, the ground glass replaces the need for calcium silicide, itself needed to convert errant alumina particles. Calcium silicide is flammable and can ignite spontaneously in air. Studies have shown that as little as 122 g per tonne of molten steel is required. As part of the liner, cullet can be used and acts as a flux, assisting in purifying the molten metal(11) and by potentially providing downstream benefits such as improving ductility and machinability.
Ground cullet has found application in the production of ceramics alongside kaolin and clay, these ceramics are highly tolerant to temperature change, ostensibly due to the presence of the ground glass-derived silica refractory(12), and are considered as candidates for use in harsh-condition environments due to their chemical resistance(13). With the treatment of kaolin and clay in this way, the ground glass can be transformed into a synthetic wollastonite ceramic(14), itself utilised as an insulator in rockwool, and for further incorporation into ceramics with a requirement for high thermal performance.
As a component in the manufacture of renewable ceramic frits from waste materials, ground glass has proven useful alongside ceramic sludge to form engobes, which can then be transformed into thermally stable tile products(15). Not only was the composition resistant to heating to 1,450 °C, but it was significantly cheaper than producing frits and engobes from new materials. An engobe is a thin layer of mostly clay-based materials used on and with ceramics to provide mechanical properties. One of the many advantages of employing ground glass in the engobe is a saving of energy required for production - as it behaves as a flux in the manufacturing process. In production of stoneware, ceramics can be made with a large percentage component of ground glass, sintering at 1,000 °C, with hardness and strength comparable to conventional porcelain(16) - researchers attribute the strength to interactions between glass and clay, producing several crystalline phases.
Glazes are thoroughly interesting compound materials used in a whole host of situations and provide strong and resistant coverings for many ceramics. Glazes consist of a plastic, a non-plastic (often these are metal oxides, feldspar, frits and other materials) and some additives which are often predominantly pigments. The non-plastics tend also to be of a silica-type classification(17). Ground glass is often used as a ‘plastic’ phase and is a high performing and economically attractive choice. Furthermore, it melts at a lower temperature than pure silica and therefore can be said to have the fluxing effect with which it has become so commonly associated. Experimental evidence using recycled glass and recycled glass from cathode ray tubes in the production of glazes to be highly effective(18) - matching performances of glazes based on fresh silica and other commercial materials. The authors noted that the glazes produced with recycled glass powder had exemplary chemical and thermal resistance.
Cements and Bricks
In the manufacture of cements, ground glass has long been used as an aggregate. In powdered form, researchers have noticed that ground glass affords some pozzolanic activity. Simply put, the finer the ground glass powder, the stronger the pozzolanic activity. In some scenarios, an amount of Portland cement in the concrete mix can be replaced with ground glass(19). Refractory cement is analogous to conventional cement, but the Portland cement is replaced with various calcium aluminates. It is used when strength under high temperature is required, such as in furnace linings(20).
A further benefit to using ground glass in refractory cement is its ability to reduce alkali-silica expansion, though it is noted that particle size plays a substantial role(21). Data reported in the literature shows that if the waste glass is finely ground, especially to grades under 75 μm, the alkali-silica effect does not occur and mortar durability is preserved or even enhanced(22). Preventing this effect prolongs the life of the refractory cement. The alkali-silica effect is more commonly known as ‘concrete cancer’ and occurs when the alkaline compounds in cement react with acidic silica over time, given enough moisture content. The reaction leads to the production of sodium silicate in gel form, which is highly hygroscopic. Expansion of this gel due to water being present causes cracking in the concrete followed by break up of the material. In addition, utilising waste glass materials will lead to lower financial and environmental costs in cement production(23).
In one study, it was found that the inclusion of waste ground glass enhanced the performance characteristics of the refractory brick produced: firing shrinkage decreased, whilst bulk density and compressive strength increased with greater glass content(24). Alongside calcium oxide, ground glass - as a source of silica - has been used to form silica refractory bricks(25), and alongside limestone dust to produce bricks with enhanced compressive strength properties(26). Additionally, ground glass has been used in concert with biomass and clay to produce an effective fire brick from solely renewable materials(27).
Throughout the glass-added concrete and cement sector, there is a ‘sweet spot’ of ground glass composition at around 30% by weight. Studies have shown that when compared to concrete made by conventional means, glass-added concrete was stronger in longer term exposure testing especially compared to concrete made using natural pozzolans, with superior resistance to chloride and sulfide attack(28). This is also the case for self leveling concrete(29), such as those which may be used to form industrial insulation for reactors.
Rice husk silica is a natural product derived from the milling of rice husks, it has a high silica content and is similar in properties to ultra-fine ground glass. It has been used to produce cordierite for thermal insulation purposes(30). Using silica sand in the manufacture of refractory bricks also provides enhanced mechanical properties, and high performance at typical fire brick-rated temperatures(31). Silica sand, like rice husk silica, is a fine powder that is broadly similar to ground glass.
Fibreglass And Mineral Wool
Considering a perhaps broader definition of a refractory, it can be suggested that fibreglass and mineral wool materials are refractories: they are materials that do not decompose or otherwise weaken upon the application of heat. Ubiquitous in home and buildings insulation, fibreglass and mineral wool are valuable and widely produced materials that often contain a significant proportion of glass powder in their manufacture. Both materials rely on the idea of molten glass being formed into some kind of fibre-like structure. Rockwool is a common term - and genericised trademark - for mineral wools.
Mineral wool manufactured today typically contains on the order of 30% by weight glass powder(32). Mineral wools are made by melting together the constituent parts, which are predominantly slag and waste ceramics into a molten glass-like material (a “melt”) which is then spun and dried forming a wool-like material. Production temperatures for commercial mineral wools tend to be around 1,500 °C - and the ground glass is added as it behaves like a flux, reducing energy demand(33). Additionally, replacement of some of the overall material mass with low-cost ground glass is economically attractive. Further benefits to using ground glass in mineral wool manufacture include increased strength and overall durability over time(34) and an improved resistance to alkali attack(35) - in a similar vein to the earlier mentioned alkali-silica reaction in concretes.
In terms of fibreglass, compositions using up to 70% recycled glass powders are not uncommon(36) with no decrease in performance to using fresh silica(37). The major attractiveness to using ground glass, like in mineral wool, is the significant fluxing benefit afforded by using the ground glass as part or full replacement for fresh silica(38,39).
- Ground class (cullet) is an inexpensive product that largely goes to landfill if it is not recycled. It is a viable source of silica.
- It is used in steel and glass production for the lining of furnaces, often as part of a refractory cement.
- Ground glass is utilised in tundishes in steel manufacture, ensuring stable flow of molten steel whilst removing impurities.
- In refractory ceramics and stoneware, ground glass is used to replace some of the clay material and in doing so increases thermal capacities and strength.
- Additionally, ground glass can be used as part of a glaze for the post-firing treatment of a wide range of ceramics.
- As part of refractory cements, ground glass can be added to replace some of the aggregate, cement or both - providing enhancements in strength, durability and heat resistance.
- In refractory bricks, the addition of ground glass enhanced some mechanical performance characteristics.
- Ground glass finds extensive use in the manufacture of fibreglass and mineral wools - materials prized for their insulation qualities.
- Overall, ground glass is a viable material for use in refractory applications.
African Pegmatite is a leading supplier of ground glass cullet for a broad array of refractory and other uses - milled and processed to a customer’s exacting specification. Combining in house milling services, the widest reach and a highly experienced team, African Pegmatite is the go-to industrial partner for a wealth of refractory materials, including fine ground glass.
1 A. Muan and E. F. Osborne, Phase Equilibrium among Oxides in Steelmaking, Addison Wesley, Reading, United States, 1965
2 M. H. Rahman et al., Int. J. Sust. Built. Env., 2017, 6, 37
3 A. Muan and S. Somiya, J. Am. Ceram. Soc., 1959, 42, 603
4 Q. Ma, et al., Constr. Build. Mater., 2015, 93, 371
5 R. K. Dhir OBE et al., Sustainable Construction Materials: Glass Cullet, Woodhead, Duxford, United Kingdom, 2018
6 US Patent US3360387A, 1967, expired
7 US Patent US3963815A, 1974, expired
8 S. Aminorroya et al., Basic Tundish Powder Evaluation for Continuous Casting of Clean Steel, in AIS Tech - The Iron & Steel Technology Conference and Exposition, Cleveland, 2006
9 Y. V. Materikin and V. A. Molochkov, Refractories, 1983, 24, 108
10 E. T. Turkdogan, Ironmaking and Steelmaking, 2004, 31, 131
11 US Patents US5366535A, 1992, expired and US617437B1, 1996, expired
12 M. R. Sahar et al., Int. J. Mining Metallurgy Mat., 2018, 25, 350
13 K. Okada, Eur. J. Ceram. Soc., 2004, 24, 2367
14 W. Zhang and H. Liu, Ceram. Int., 2013, 39, 1943
15 A. P. N. Oliveira et al., J. Cleaner Prod., 2015, 86, 461
16 E. Bernardo, J. Am. Ceram. Soc., 2008, 91, 2156
17 K. Bonk et al., Tile Brick, 1992, 1, 14
18 F. Andreola et al., J. Eur. Ceramic Soc., 2007, 27, 1623
19 C. Shi et al., Cement Concr. Res., 2005, 35, 987
20 N. Black et al., Adv. Appl. Ceram., 2010, 109, 253
21 W. Li et al., Int. J. Concrete Struct. Mater., 2018, 12, 67
22 A. Monterro et al., Waste Management, 2005, 25, 197
23 I. B. Topçh and M. Canbaz, Cement Concr. Res., 2004, 34, 267
24 H. H. Abdeen, Masters thesis, The Islamic University-Gaza, 2016
25 G. Almarahle, Am. J. Appl. Sci., 2005, 2, 465
26 P. Turgut, Mater. and Struct., 2008, 41, 805
27 M. Vlasova et al., Sci. Sintering, 2011, 43, 81
28 M. Liu, Constr. Build. Mat., 2011, 25, 919
29 M. Carsana et al, Cem. Concr. Res., 2014, 45, 39
30 S. Sembling et al., Ceram. Int., 2016, 42, 8431
31 C. Sadik, J. Mater. Env. Sci., 2013, 4, 987
32 R. Gellert, Inorganic mineral materials for insulation in buildings, in: M. R. Hall (ed.) Materials for energy efficiency and thermal comfort in buildings, CRC Press, Boston, 2010
33 R. Farel et al., Resources Conserv. Recycl., 2013, 74, 54
34 K. Sonsakul and W. Boongsood, IOP Conf. Ser.: Mater. Sci. Eng. 2017, 273, 12006
35 V. I. Onishchuk et al., Glass and Ceramics, 1999, 56, 5
36 R. Gellert, Inorganic mineral materials for insulation in buildings, in: M. R. Hall (ed.) Materials for energy efficiency and thermal comfort in buildings, CRC Press, Boston, 2010
37 A. H. Goode et al., Glass Wool From Waste Glass, Bureau of Mines, United States Department of the Interior, Washington DC, 1972
38 Remade-Scotland, Glass recycling handbook: Assessment of available technologies, Remade-Scotland, Glasgow, 2003
39 R. Farel et al., Resources Conserv. Recycl., 2013, 74, 54