Use of Glass Powders in Industrial Settings: A Short Overview
Glass powder is a material with a plethora of uses and is available at almost any grind size imaginable from African Pegmatite - the go-to industrial partner for a wide array of minerals. refractories and aggregates.
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).
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).
Glass Powders In Refractory Cement
Refractory cement is a cement designed to perform at higher temperatures than regular cement. To achieve this, some or all of the Portland cement is replaced by various calcium aluminates(12), with a popular additive being ground glass cullet. In addition to the aforementioned pozzolanic effects, the addition of ground glass to the refractory cement reduces alkali-silica expansion(13). Research shows that when ground glass that has been powdered to less than 75 μm diameter, the alkali-silica expansion phenomenon does not occur while at the same time the durability of the final refractory concrete is improved. Furthermore, using more recycled glass in production leads to lower environmental costs(14). Research has also shown that increasing glass content is associated with increases in both bulk density and compressive strength properties, whilst shrinkage during firing decreased(15) when using refractory cement to form refractory bricks.
Glazes and Ceramics
Owing to its good performance at moderate to high temperature, glass powders find use in the manufacture of ceramics and glazes:
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(16). 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(17). 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.
The large scale production of ceramics is a field that is constantly looking for ways to use less of the more expensive minerals such as kaolin, and alongside kaolin and clay, ground glass can be added to a ceramic mixture making the ceramic more tolerant to rapid temperature change or shock(18) - whilst also having superior chemical resistance(19). Synthetic wollastonite ceramics can be formed when kaolin, clay and ground glass are combined - and this can be used in a rockwool insulation application(20).
Stoneware is known for its ability to be tolerant of elevated levels of ground glass, with hardness and flexural strength being comparable to porcelain, whilst sintering at 1,000 °C. Like the pozzolanic effect in cements, researchers have proposed that strength increases are due to a better developed network of interactions between glass and clay across several crystal phases(21). Frits are able to be produced using recycled ground glass, and when combined with ceramic sludge, engobes can form which can then be used in highly thermally stable tile products(22). Engobes are layers of clay-based materials deposited on the surface of a ceramic to enhance mechanical properties.
A key and distinct advantage to using ground glass in the production of ceramics is that it behaves as a flux in the manufacturing process, reducing the melting temperature and thus the energy requirement needed - representing cost and time savings.
Glass Powder In Fibrous Scenarios
An often overlooked use case for ground glass is a prime source material for the manufacture of glass fibre-type materials. Not only does the use of ground glass and/or cullet provide an inexpensive and reliable source of glass starting material, but the outcomes of using ground glass are the same as if a manufacturer were to make the product from fresh sand or silica. Glass fibre products are typically used for insulation.
Mineral wool - also known as rock wool - is a fibrous product formed from the melting down of natural materials and a source of silica which is then subjected to spinning and drying (akin to candy floss) until a wool-like material is formed. Of the natural materials, igneous rocks such as basalt and sedimentary rocks like limestone are used. In addition, other materials with lower melting points are used in quantities up to 30% by weight, with the remaining material being resins (as binders) and mineral oil to prevent sticking.
Of these ‘other’ materials, an increasingly common additive is ground glass powder(29), which is often associated with the lowering of the overall melting temperature required, i.e. it behaves as a flux(30). Not only does using ground glass reduce expenditure - both by reducing material and heating costs - but reports suggest that the addition of ground glass can lead to an added dimension of strength, which thus gives rise to greater durability over time(31).
Fibreglass is formed by taking a liquid glass that has been produced by heating sand to approximately 1,500 °C and then forcing it through a fine mesh by centripetal force, at which point the molten glass cools and solidifies upon contact with air. Binders are added to agglomerate and stick the fibre strands together and add further mechanical strength. Finer fibre diameters may be achieved using faster rates of spinning(32) with the resultant fibres and binders heated to induce polymerisation, trapping air in the process. Typical compositions bear around 70% by weight of glass fibres, ca. 0.5 to 0.7% resin and 0.5% mineral oil to prevent sticking. The mass balance is completed by quartz and limestone(33). Modern fibreglass made using ground glass cullet has performance equal to that made with fresh silica(34) - but with the added benefit of diverting waste from landfill and requiring less energy use to heat(35). This is not only due to cullet’s lower melting temperature but also its ability to behave as a flux. Isotropic fibres are produced with fibreglass produced from cullet, i.e. thermal expansion characteristics are the same in all directions(36). Mechanical strength is independent of temperature.
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(37). 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(38).
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(39). 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(40).
- 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
- Ground glass is an attractive additive in the production of refractory cement, glazes for ceramics and even ceramics themselves
- Fibrous glass materials may use moderate to large quantities of cullet in lieu of virgin material, offering substantial cost savings with some enhanced performance
- 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
Ground glass cullet is an attractive product for a variety of uses, and is available from African Pegmatite, a leading supplier of glass products, refractories, minerals and more. Ground glass cullet offers the broadest applicability, often matching or surpassing the performance of virgin silica or fresh glass in a range of use cases, whilst being an environmentally sustainable alternative.
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