What is glass cullet?
Glass cullet is recycled container glass (previously used for bottles, jars and other similar glass vessels) prior to processing. The material is typically collected via bottle banks, kerbside collection schemes and from premises handling large quantities of containers, with the primary aim of processing it for returning to the glassmaking process to manufacture glass containers or other products. The term ‘cullet’ also refers, however, to waste glass produced as a result of breakage and rejection on quality control grounds during the manufacturing process.
Container glass manufacture
Container glass is manufactured by combining quantities of minerals (and usually glass cullet) to obtain a feedstock rich in silica (SiO2), soda (Na2O) and lime (CaO).
Whilst a wide range of minerals can be used, the most common ingredients are high purity silica sand, soda ash (Na2CO3) and limestone (CaCO3). The mixture is heated to temperatures between 1300°C and 1650°C where it is melted. During its time in the glassmaking furnace, the molten material is ‘fined’ (allowed to release all gas bubbles within its volume) and homogenised via mechanical stirring and convection mixing. It is then brought to a suitable temperature for forming, and released from the furnace. Forming is carried out automatically, usually by a combination of blowing and pressing.
Presence of foreign matter
By the time cullet has been recovered for recycling, it is typically broken into smaller fragments and may contain other substances which have been introduced and carried with it during its use and disposal. These substances can include paper, adhesives, aluminium foil, plastics, and residues of the substances originally held in the containers. The quantity of foreign matter present with cullet varies depending on the method of collection, storage conditions and any pre-processing carried out on the material.
How glass cullet is used
Glass cullet is a hard, granular material and this has led many engineers to consider using it as a construction aggregate.
In recent years, glass cullet has been used in the UK as an aggregate in bituminous highway materials. The extent of the material’s use as an aggregate in concrete is currently small, although the production of precast concrete containing glass aggregate is being carried out on a small-scale in the UK at present.
The use of powdered glass in concrete has been rare in the UK, but it has been used routinely for a number of years in Sweden as filler.
Properties of glass cullet
In terms of its physical and mechanical properties, crushed glass cullet behaves in a very similar way to sand, being a hard, granular material with a similar particle density. In many circumstances there may be economic benefits in using glass cullet in place of natural sand. However, the material possesses other beneficial properties which may be exploited.
Concrete containing glass cullet as aggregate typically displays higher resistance to abrasion than equivalent materials made using quartz sand or similar materials as fine aggregate. Reduced drying shrinkage is also often observed for mixes containing glass cullet aggregate.
Powdered glass cullet is pozzolanic, meaning that it will react with lime to form products that contribute towards strength development. As with all pozzolanas, the contribution to strength development is less than that of Portland cement, meaning that cement combinations comprising these two materials will produce lower compressive strengths than Portland cement alone. However, there are clear economic benefits of using cullet as a cement component. Abrasion resistance of concrete containing powdered cullet is also improved.