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).
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(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.
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