Refractory Applications of Glass Powders
A refractory material is one that has high tolerance of temperature, and at high temperature is resistant to cracking, degradation or other chemical attacks. 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 glass(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 (chiefly from wine bottles) it uses and so must export green glass cullet. It would be beneficial to use the cullet locally rather than export.
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.
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 contained ground 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 that of linings made from single, poured refractories(9) (less than five heats compared to 30 for the more monolith-type). An early tundish liner was comprised 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 a molten aluminum-killed steel, the ground glass replaces the need for calcium silicide needed to convert errant alumina particles. Calcium silicide is flammable and can ignite spontaneously in the air. Studies have shown that as little as 122g 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 1450°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 1000°C, with hardness and strength comparable to conventional porcelain(16) - researchers attribute the strength to interactions between glass and clay, producing several crystalline phases.
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(17). 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(18).
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(19). Data reported in the literature shows that if the waste glass is finely ground, especially to grades under under 75 μm, the alkali-silica effect does not occur and mortar durability is preserved or even enhanced(20). Preventing this effect prolongs the life of the refractory cement. In addition, utilising waste glass materials will lead to lower financial and environmental costs in cement production(21).
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(22). Alongside calcium oxide, ground glass - as a source of silica - has been used to form silica refractory bricks(23), and alongside limestone dust to produce bricks with enhanced compressive strength properties(24). Additionally, ground glass has been used in concert with biomass and clay to produce an effective fire brick from solely renewable materials(25).
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(26). Using silica sand in the manufacture of refractory bricks also provides enhanced mechanical properties, and high performance at typical fire brick-rated temperatures(27). Silica sand, like rice husk silica, is a fine powder that is broadly similar to ground glass.
- 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.
- 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.
- Overall, ground glass is a viable material for use in refractory applications.
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