Mineral Glass

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).

concrete with glass powder mix being poured
pre cast concrete slabs made with glass powder mix

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.

concrete being poured from machine

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.

concrete bricks
concrete slabs that could be made using glass powder additive

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).

Noteworthy is the concept of using blended pozzolans in the manufacture of concrete - i.e. where glass may be used as a pozzolan alongside other contemporary materials - which may result in enhanced longer term properties. Research has found that blended pozzolan use can produce a concrete that after 28 days is much more resistant to sulfate attack and chloride ion penetration(7).

Another reason why ground glass is used in cement is that it is a vitreous material. Such materials are known for their lack of structural defects and it is these which lead towards a superior quality interaction between the cement and the binder(8). Without such well defined and pure crystal structures, ground glass would no doubt not be a contender in this field.

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(9). 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 can react with the silica from the glass in the presence of water(10). The net effect of this reaction is the production of a gel in situ which absorbs water, then swells, causing cracking in the concrete. The gel is formed due to a weak interface allowing the reaction(11). Mitigation is via using suitable grind sizes of glass(12), the sealing of concrete as it cures to eliminate atmospheric water and to use small amounts of other or natural pozzolans or fly ash(13). Therefore, as a general rule, ground glass should not be routinely used as an aggregate(14). It is noted that the alkali-silica reaction can also be seen in conventional concrete(15). Ground glass can be used as an aggregate if so desired, however even at 850 μm grind size, the workability of the concrete is not as good, but can be remedied by the addition of more paste although this can overall lead to an undesired modulation of density and strength properties(16).

In the case of alumina cement, the effects of the addition of ground glass is not as clear cut as for conventional cement. Research has shown that including ground glass in alumina cement can lead to an increase in density whilst also increasing paste and mortar strength - but such benefits become lost at high temperatures - and overall adding glass is very much dependent on the alumina itself(17). As high aluminous cement is more geared for specialist applications, and therefore is used less than conventional cement, the need for ground glass for economic and environmental reasons is less pronounced.

concrete being poured

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(18), 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(19). 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(20). 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(21) when using refractory cement to form refractory bricks.

Carbide Lime Blends

Concrete and asphalt are two materials often considered alongside one another, especially when it comes to roads, paths and cycle tracks. It comes as no surprise that concrete analogues are being developed which use recycled reclaimed asphalt from old road installations. This old asphalt is ground up and mixed with carbide lime (a byproduct from acetylene production) as a cementing agent. As with concrete, a pozzolan is required and it comes in the form of ground glass(22). Using these three recycled/byproduct materials together can produce a sufficiently hard and durable material akin to concrete - all in an environmentally friendly fashion. In similar testing with different aggregate materials but the same carbide lime and ground glass set up, it was found that overall durability was a function of the amount of ground glass added(23). Higher amounts of ground glass present (up to 30% by weight) lead to lower porosity/binder content indices and therefore higher compressive strength values across multiple grind types and distributions.

Other Concrete Aspects

Despite modulating overall concrete density when employed as binder and/or pozzolan, ground glass can be highly effective in the production of aerated concrete, a popular material where significant weight bearing is not available or required. Ground glass at a surface area of between 500 and 550 m2 kg-1 is ideal, and overall particle size should not be incredibly low as doing so increases density and can lead to strength losses in this concrete type owing to interpore partitions being weak(24).

Further adding to enhanced environmental performance, ground glass can replace a majority of silica sand used when the cement itself is replaced in a large amount with electric arc furnace slag(25). The final material compared favourably with conventional concrete.

Glazes and Ceramics

Owing to its good performance at moderate to high temperature, glass powders find use in the manufacture of ceramics and glazes:

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(26). 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(27). 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.

glazed colourful pottery
glazed terracotta pots

Ceramics

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(28) - whilst also having superior chemical resistance(29). Synthetic wollastonite ceramics can be formed when kaolin, clay and ground glass are combined - and this can be used in a rockwool insulation application(30).

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(31). 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(32). 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.

Refractory Materials

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(33). 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(34). 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(35) 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(36). 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%(37). Improvement of soil by the addition of ground glass can prevent building damage during periods of seasonal temperature change(38).

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

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(39), which is often associated with the lowering of the overall melting temperature required, i.e. it behaves as a flux(40). 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(41).

Fibreglass

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(42) 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(43). Modern fibreglass made using ground glass cullet has performance equal to that made with fresh silica(44) - but with the added benefit of diverting waste from landfill and requiring less energy use to heat(45). 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(46). Mechanical strength is independent of temperature.

molten metal being poured in moulds made with filler sands
molten metal being poured

Other Applications

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(47). 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(48).

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(49). 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(50).

sand products for casting process made with glass powder
cores of moulds

Summary

  • 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
  • The use of ground glass may be environmentally useful, and may add resistance to chemical attack
  • 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.

glass_powder

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