MIT researchers confirm recycled glass pozzolanic performance

Sources: MIT Concrete Sustainability Hub, Cambridge, Mass.; CP staff

PHOTO: Fairfax County, Virginia

The newest Research Brief from the Massachusetts Institute of Technology-hosted Concrete Sustainability Hub illustrates how concrete practitioners can realize voluminous recycling and pozzolanic value extraction from municipal waste streams. “Open-loop Recycling of Glass in Concrete Provides Upcycling Opportunities” authors also posit a new metric for architectural/engineering/construction professionals: Mid- to high-rise buildings with floor-to-ceiling windows can use three times the volume of recycled glass in their structural concrete mixes than their façades.

“Crushed glass has been considered a material choice that should be avoided in concrete mixtures as it could act as a concentrated source of alkalis, therefore increasing the risk of the alkali-silica reaction—deleterious chemical reaction that causes internal swelling and ultimately cracking in concrete,” authors observe. “Over the past two decades, researchers have found that the use of glass powder can reduce the risk of alkali-silica reaction and, more importantly, result in a pozzolanic reaction in concrete.”

CSHub Deputy Director Hessam AzariJafari, along with Yiwei Lyu and Ipek Bensu Manav, examine emerging practice in light of ASTM C1866, Standard Specification for Ground-Glass Pozzolan for Use in Concrete, issued in 2020. In “Open-loop Recycling of Glass,” they conclude: 

• Demonstration projects reveal the positive effect of glass powder on the mechanical performance and durability of concrete;
• When measured on carbon dioxide emissions scales, the environmental performance of a mixture replacing 20 percent of portland cement with glass powder is similar to  that of the industry average;
• The annual generation of 12 million tons of glass powder can provide a sustainable source of concrete-grade pozzolan; and, 
• Due to its availability in urban areas, glass can partially meet the growing demand for supplementary cementitious materials.

“Glass powder appears to be a reliable source of pozzolan, but producers should be educated and made aware of the variation in glass quality and properties. As an example, a larger fraction of available waste glass pozzolans comes from container glass that has a high alkali content (compared to other types, such as e-glass). Although the incorporation of glass powder from recycled containers into concrete provides durability and strength benefits, it requires manufacturers to use this SCM with other low-alkali SCMs to ensure the mitigation of alkali-silica reaction,” conclude AzariJafari, Lyu and Manav.

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