According to a U.S. Environmental Protection Agency report, the total amount of waste material has increased from 88 million tons in 1960 to 262 million tons in 2015. In 2015, 26% of the waste materials was recycled, 9% was composted, 13% was combusted with energy recovery and 52% was disposed of in landfills.
Among all the waste materials that have been recycled, batteries and steel are the most recyclable. Although glass is a material that can be recycled, just 34% of waste glass has been recycled. The main reason why collected mix-color waste glass isn’t recycled is due to its chemical composition.
Typically, glass containers consist of sand, soda ash, limestone and cullet. Sand is glass’s main component and is its source of silica. Soda ash is added to the mixture to decrease the melting temperature. The chemical compositions of different glass colors are slightly different which affect their melting temperature. Therefore, glass containers can’t be properly recycled unless they’re sorted based on their color.
Another reason why glass containers can’t be recycled efficiently is the source of the waste glass. Usually, waste glass such as bottles or containers, are mixed during the collection process. To recycle collected waste glass, it needs to be sorted based on colors and types which is a very time-consuming and inefficient process.
Contamination is another reason why some waste glass containers aren’t recycled. Any chemical residue, medicine or any other hazardous material needs to be removed before the recycling process.
Using glass as a pozzolan
Pozzolans are cementitious materials added to a concrete mixture to enhance its mechanical and durability properties. When grinded to a powder, glass is one of the natural pozzolans that improves concrete’s fresh and hardened properties.
While concrete is in its plastic phase, glass powder will increase its workability, so less energy, cost and time are required to place and consolidate the concrete. While the concrete is in its hardened phase, concrete containing glass powder exhibits better strength, freeze-thaw resistance and sulfate resistance.
Glass powder can be used to replace portland cement, typically at a 10%-30% replacement level by weight. Portland cement production is one of the main sources for CO2 emission. Incorporating waste glass in concrete as a portland cement replacement not only helps the environment by reducing the amount of CO2 emission but also reduces the amount of waste glass disposed in landfills.
The size of the glass particles plays a crucial role in the pozzolanic performance of the glass powder. Concrete containing finer glass particles shows more promising mechanical and durability performance than that of concrete containing coarser particles.
Glass aggregates and ASR
Besides powder, waste glass can also be used as an aggregate in concrete in the form of fine or coarse aggregates. Since aggregates occupy approximately 70% of the concrete, using glass cullet as aggregate in concrete not only results in incorporating more waste glass in concrete but it also decreases the energy and time required to ground the glass particles into powder.
However, a mitigation method should be considered to suppress alkali-silica reaction (ASR) when glass aggregates are present in the concrete mixture. ASR is a chemical reaction between alkalis from portland cement and reactive aggregates within the concrete. The product of this reaction is ASR gel.
If moisture is present, ASR gel expands and causes the concrete to deteriorate. Glass aggregates are considered highly reactive aggregates due to both high alkali and silica content embedded in their microstructure (glass has approximately 70% silica and 15% sodium). Typically, concrete containing green glass exhibits less distress due to the presence of chromium.
Eliminating ASR reaction
There are several ways to eliminate the likelihood of deleterious ASR reaction in concrete containing waste glass aggregates. Using low-alkali portland cement and/or incorporating supplementary cementitious materials (SCMs) such as silica fume, fly ash, slag and metakaolin can mitigate ASR distress in concrete containing waste glass aggregate.
Among the conventional SCMs, silica fume and metakaolin are the most effective ones. Previous studies showed that using approximately 10% of either silica fume or metakaolin can mitigate the ASR distress in concrete containing 100% glass aggregates by almost 90%.
Glass powder can also be used as a SCM in concrete (i.e., replacement for portland cement) to mitigate ASR distress; however, it’s not as effective as the conventional SCMs. For example, 30% of portland cement and 100% of aggregates can be replaced by glass powder and glass aggregate, respectively, and no ASR distress is encountered.
The strength of concrete containing glass powder as a cement replacement material is approximately equal to that of concrete containing only portland cement. However, its durability properties outperform that of conventional concrete.
The strength of concrete containing glass aggregates is 10%-20% less than that of concrete containing mineral aggregates. This reduction in mechanical properties is due to the lower strength of aggregates as well as less bonding between the glass aggregates and the paste compared to the mineral aggregates.
Having said that, concrete containing waste glass can be used for indoor applications (where no exposure to aggressive materials and moisture are expected), sidewalks, pavement and curbs (where less strength is required). Using this type of concrete in indoor applications eliminates the presence of water and, consequently, ASR distress.
It should be considered that using waste glass aggregates significantly reduces the concrete mixture’s workability. The glass cullets’ angular shapes negatively affect the concrete mixture’s workability. Therefore, a superplasticizer is required to ease concrete placement and consolidation.
Research has shown that concrete containing waste glass aggregates has better abrasion resistance than conventional concrete containing mineral aggregates. Even if all the mineral aggregates aren’t replaced with glass aggregate, the glass particles rise to the concrete’s surface due to their lower density compared to mineral aggregates and the paste.