Glass Pozzolans: A Novel Path to a Sustainable Future

By Brian P. Lemay

Halletts Point - Queens, NY: Ground glass pozzolans significantly reduce the embodied carbon of concrete while also improving the efficiency of glass recycling.

As the construction industry becomes more focused on lowering carbon emissions, the concrete industry innovates to meet the expectations of a more eco-friendly future. Whether sourced from natural reserves or from industrial waste products such as fly ash, slag cement or ground glass, supplementary cementitious materials (SCMs) play a leading role in this movement towards low-carbon concrete. Indeed, by substituting SCMs for cement, the production of which accounts for the majority of the carbon emissions associated with concrete, producers can significantly reduce the embodied carbon of their mixtures while improving performance. Additionally, the use of waste byproducts disposes of industrial waste which could otherwise become harmful environmental pollutants. However, under present and foreseeable market conditions, waste SCMs such as fly ash and slag are becoming more scarce. In this economic climate, novel recycled glass pozzolans serve as a promising alternative to their older counterparts which are in short supply. Beyond their merit as an environmentally friendly SCM, ground glass pozzolans’ (GGP) performance-enhancing qualities in concrete are comparable to the most commonly used and accepted SCMs today.

Although they are not independently cementitious, pozzolans are used to replace cement because they display cementitious properties when finely ground and added to concrete mixtures. The hydration of cement produces the binding agent Calcium Silicate Hydrate (CSH) as well as Calcium Hydroxide (CH), an undesirable byproduct in concrete. Pozzolans function as substitutes for cement because they combine with CH to produce CSH, thus reducing the required quantity of portland cement in a given concrete mixture. Glass is highly siliceous and sometimes aluminous, which results in the material being an ideal pozzolan. Essentially, the chemical composition of GGPs is similar enough to that of other SCMs such as fly ash or natural volcanic ash and thus they function according to the same principles. GGPs are unique mainly due to the fact that they come from untapped reserves of waste glass and are beneficial to the sustainability of both the concrete and glass industries.

One of the most appealing attributes of GGPs is their potential to mitigate wastefulness in glass recycling. A 2017 EPA fact sheet asserts that out of 11.4 million tons of container glass waste discarded that year, roughly a quarter was genuinely recycled while the majority of the remainder was simply deposited in landfills. One reason for this inefficiency is the fact that most recycled container glass must be sorted by color in order to be processed into cullet that can be used to make additional products such as bottles. This is a labor intensive process and according to a 2020 ASTM summary of the context surrounding the official classification of glass pozzolans, about a third of waste glass is broken into pieces too small to be sorted by workers economically. In addition to container glass, this report also explains that between 1.5 and 2 million tons of plate glass and about 200,000 tons of e-glass primarily recovered from fiberglass waste are also generally recycled each year. However, It is worth noting that recycled glass is not exclusively available to the concrete industry, and that only a fraction of these overall reserves are available for use as pozzolans at any given time.

Container glass of any color and size can be processed into pozzolans for use in concrete, illustrating one way in which these products resolve inefficiencies in glass recycling.

Though there is not currently a supply of glass proportionate to the supply of fly ash, glass pozzolans have been used for nearly ten years on various projects and are now officially classified under ASTM C1866/C1866M-20, “Standard Specification for Ground Glass Pozzolan for Use in Concrete”. In addition to this mark of progress and legitimacy for recycled glass pozzolans, the ability of glass pozzolan producers to use any of the three aforementioned types of waste glass indicates that in time, the supply of waste glass to be used as pozzolans will increase.

It is also worth noting that glass pozzolans are already being used as an alternative to more mainstream waste pozzolans in regions where fly ash and slag cannot be sourced locally. For example, CEO Jack Lamuraglia of KLAW, a firm that produces glass pozzolans emphasizes that there are no longer any coal-fired power plants operating in New York State and only one steel foundry that produces slag that can be used as an SCM in concrete. To address this issue, KLAW claims to have an industrial process to harvest waste glass at any point during the recycling process, including glass that is mixed with other waste in landfills. Though KLAW’s product Pantheon has not been as widely marketed as some other glass pozzolans, this ability to effectively expand the available reserves of glass for use as pozzolans and eliminate large quantities of glass waste is promising for the future of the product in the concrete industry.

One demonstrative case of the promise of recycled glass as an SCM is the reuse of old window glass from the UN general assembly building which was processed into Pozzotive, a glass pozzolan product, and added to concrete pavers that were installed both in the United Nations Plaza and also in the parking lot of a nearby Brooklyn Whole Foods. This is perhaps the clearest example of the ubiquity of glass as a resource which can be sourced locally and easily with the proper organization and planning. Proponents of recycled glass pozzolans are of course also keen to emphasize the ecological impact of the product in addition to its availability as a substitute for more traditional pozzolans. Patrick Grasso, a managing partner at Urban Mining Industries, the firm that produces Pozzotive, maintains that recycling glass into pozzolans is a far more effective way to reduce greenhouse gas emissions than recycling glass to make more glass products. He even claims that it is five times more impactful than traditional glass-to-glass recycling. Most importantly, when comparing these new pozzolans to the benchmarks of fly ash and slag, cement can be replaced by glass pozzolans in concrete mixtures in the same proportions as mixtures using these conventional pozzolans. VitroMinerals, a glass powder manufacturer based in Georgia, states that most glass pozzolan concrete mixtures replace between 10% and 30% of cement with the crushed glass product.

VitroMinerals also claims that glass pozzolans are whiter than white cement, and also that they lessen the problem of efflorescence. The combination of these factors makes concrete incorporating these products highly aesthetically pleasing and thus ideal for decorative applications. However, the performance enhancing qualities and versatility of glass pozzolans go far beyond visual appeal. In fact, KLAW states that their pozzolan product has 11% greater compressive strength at a 20% replacement rate than slag concrete at the same rate of cement replacement. Further, researchers from the University of Sherbrooke write that the chloride-ion permeability of concrete containing GGPs can be reduced by 75% to 80% depending on the rate of cement replacement of the mixture.

One excellent example of high performance glass pozzolan concrete being used in a large construction project is the Halletts Point residential high rise in Astoria, Queens. The glass pozzolan concrete used in this project replaced cement at a rate of 35% and had a compressive strength more than 50% greater than that of conventional concrete. Additionally, glass pozzolan concrete resists chloride penetration to an even greater degree than concrete containing fly ash and slag, allowing for the protection of any steel reinforcements within the concrete. Further, concrete containing glass pozzolans are protected from sulfate attack compared to conventional concrete. Finally, glass pozzolan producers can be sure that the recycled glass that they use to make their product is pure and highly siliceous as the initial manufacturer of these glass products have had to ensure the quality of their own glass products for them to be deemed suitable for consumer use. The impressive performance of glass pozzolans as well as their contributions to the sustainability of concrete make them a welcome addition to the industry today.