Use of Glass Powders in Industrial Settings: A Short Overview
Introduction to Glass and Glass Powders
Quite simply, a glass powder (ground glass) is a powder of a glass. But its properties stem not mostly from the grind size but of the identity of the glass itself. A glass is a solid, non-crystalline, typically transparent, amorphous (meaning it lacks long range order in the solid phase) material. The most common type of glass is soda-lime glass, which comprises mostly of silicon dioxide, SiO2, along with sodium oxides, calcium oxide and alumina. Other minor components are added to fine tune properties to make the soda-lime suitable for use as plate glass or as container glass.
A large-scale source of ground glass is from municipal waste/recycling streams. Glass, typically bottles, are collected and finely ground for further use/processing. Most powdered glass is sourced from used glass and then ground down. This makes it a more cost-effective resource. There are however milled glass products which are sourced from new glass or reject glass containers for specialised applications. It is estimated that over 200 million tonnes of glass waste are sent to landfill annually (1). This however has changed dramatically in recent times due to many unique applications that glass can fulfil.
Glass Powders and Concrete
Concrete is a ubiquitous building material, essential in the construction of roads, bridges and buildings. It is made up of aggregate that is mixed together with a cement, typically Portland cement, and water. The cement and water react together forming a hard matrix that binds itself together and the aggregate, producing a stone-like substance.
Additives can be included in the concrete slurry to change the properties of the final concrete slab. Plasticisers can be added, for example, which slow the curing of the concrete by reducing the required water-to-cement ratio, yet maintain its pourability. The strength of the finished concrete is greater with a lower water content.
Another class of compounds commonly used as concrete additives are pozzolans - a group of siliceous and/or aluminous which can react in water with calcium hydroxide (present in the cement) to produce a material with concrete-like properties. The inclusion of pozzolans in the concrete mixture has numerous benefits; pozzolans can be cheaper than Portland cement and thus reduce the overall financial burden in manufacture, they can increase durability and longevity of the finished concrete, and their inclusion at the expense of some of the Portland cement reduces the environmental burden associated with the production of Portland cement in the first instance. It should be noted, however, that pozzolans cannot replace all of the Portland cement in a concrete mixture due to the requirement of calcium hydroxide. In addition, some pozzolans may offer the finished concrete other properties such as overall strength, and increased resistance to harmful compounds. Ground soda-lime glass, as a silicate, can be used as a pozzolan.
Why Ground Glass?
Research into the incorporation of ground glass dates back decades, though most relevant examples are fairly recent - spurred by the increasing desire to decrease cost and increase environmental stability(2). Contemporary studies have shown that the inclusion of waste glass with a grind size of less than 10 microns can be added to concrete without causing any detriment to strength or durability(3). A 2006 study looked into performance of mixtures of grind sizes (powdered/10 micron, fine grind/0.15-0.3 mm and coarse grind/0.6-2.36 mm) in a 40 MPa cement mix. The authors reported good performance with all mixtures reaching or exceeding the 40 MPa strength threshold within 404 days(4). They note that strength and durability of the resultant concrete increased in some cases to 55 MPa despite the overall reduction in cement content by 30% - attributing this to a powerful pozzolanic reaction between the ground glass and cement.
Using ground soda lime glass to replace some of both cement and aggregate in a mixture, no detrimental properties are observed in the production of self-levelling concrete(5), though the author did note a moderate change in the water to powder ratio was required in the self-levelling application, with an overall incorporation of up to 104 kg/m3 of ground glass.
Further studies have highlighted the 30% ‘sweet spot’ of inclusion of ground glass in concrete mixtures. Compared to concrete made with fly ash, concrete containing ground glass had comparable long-term strength properties; compared against concrete made with natural pozzolans, it was stronger. In addition, no degradation was observed after seven years immersed in water, and resistance to both chloride and sulphate attack was improved relative to fly ash and natural pozzolans(6).
Considerations on the Use of Ground Glass in Concrete
Overall, the benefits of utilising ground glass as an additive in concrete far outweigh the drawbacks. It has been reported that the overall pozzolanic activity of ground glass is related to the degree of hydration of the glass powder, therefore dependent on its surface area. In general a finer grind size is preferable to achieve optimal cement replacement(7). The major limitation to the use of ground glass in concrete manufacture is the alkali-silica reaction, which is when hydroxyl ions in the cement that can react with the silica from the glass in the presence of water(8). The net effect of this reaction is the production of a gel, “in situ” which absorbs water, then swells, causing cracking in the concrete. Mitigation is via using suitable grind sizes of glass(9), the sealing of concrete as it cures to eliminate atmospheric water and to use small amounts of other/natural pozzolans or fly ash(10). It is noted that the alkali-silica reaction can also be seen in conventional concrete(11).
Ceramic glazes are a common use of silica-type compounds. A glaze consists of a plastic, a non-plastic and additives. Of the non-plastics, these are mostly oxides, alongside pigments, feldspar and frits, which are also of a silica-type nature(12). A recent study has shown the use of recycled powdered glass (in this case from cathode ray tube televisions) as the oxide component in ceramic glazes(13). The ceramic glazes produced performed equally as well as their commercial counterparts, with a particularly good chemical resistance; in addition to providing a welcome use for waste glass.
A refractory material is a compound that is resistant to heat, but also has strong thermochemical and mechanical properties, and high levels of corrosion resistance. For this reason, refractory materials are used to line reactors in industrial processes, and especially in the production of iron and steel - this sector alone uses 70% of the global supply of refractories(14). Refractories are typically oxide materials and fall into classifications based upon their composition; acidic, basic, oxide-carbon or specialised. In the case of ground glass, it acts as a source of acidic refractory material, due to its primary component being silica. A refractory material will typically be formed into a block form, several of which can be linked together using a binder. Silica is, owing to its high melting point, the most popular of the refractory oxides(15). Materials such as silica, alumina and others occur naturally. Ground glass is a good source of silica, but ground glass contains other components too. Refractory materials are produced by a sequential process: raw material processing, forming and then firing. Fire brick is a prime example of a silica-containing refractory material, noted for its incredibly high resistance to temperature, it is found in steel-making furnaces and has contained ground glass(16) as a major component.
Ground Glass and Soil Improvement
Studies have shown that the introduction of 75 micron ground glass to clay-type soils can increase the engineering properties of the soil. For example, incorporation of 12% ground soda-lime glass decreased swelling values fivefold, increased the California Bearing Ratio (a measure of hardness used to determine the suitability of subgrade ahead of road laying) and increased its compressive strength(17). Such an addition could reduce the need to heavily reinforce road or rail subgrades in areas with clay-type soils. The addition of ground glass is added owing to its nature as a cohesionless material, and in one study decreased the compressibility of clay-type soil by over 50%(18). Improvement of soil by the addition of ground glass can prevent building damage during periods of seasonal temperature change(19).
Both powdered glass and limestone dust are waste products from several industrial processes worldwide, and the need to deal with these without sending them to landfill is a priority. One study shows that with the addition of a small amount of Portland cement, along with limestone dust and powdered glass can produce a new type of brick. The new brick is manufacturable without the need for firing in a kiln, and displays properties similar to contemporary concrete bricks. It is noted that the powdered glass enhances the compressive/flexural strength, abrasion resistance and thermal conductivity of the brick - whilst maintaining economic competitiveness(20). In traditional clay-type brick manufacture, the addition of 2.5 to 10% by mass 20 micron ground glass has been shown to decrease manufacturing losses and increase strength from 20 MPa to 29 MPa - due to the ground glass filling the internal pores of the clay with a glassy phase during firing(21).
From the production of plate glass, for example for use in windows, glass powder is a waste product. Researchers in Brazil have demonstrated the ability of this ground glass to be used in insulation products, as an enhancing filler in glass fibre-type products(22). Further applications as a component in heat storage have been investigated, with 150 micron ground glass being used as a support for n-octadiene in a vacuum-impregnated phase change material-type insulator inside walls of buildings. The use of ground glass prevented the leakage of the n-octadiene during phase transition(23).
- Ground glass is a prime source of silica, that happens to be incredibly inexpensive as it is often sourced as a waste product - and is used widely
- It has found use in concrete production, replacing some of the cement, in turn making the manufacture less environmentally damaging and affording the concrete enhanced properties
- Other uses include in refractory applications (where its high temperature tolerance is beneficial) in glazes, in building insulation, soil improvement and in brick manufacture
- Many of its applications are notably beneficial as they can divert large quantities of glass from landfill
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