This course is part of the Concrete Academy

Standards and GGP Characteristics

GGPs are made from 100% glass, which is readily available, and harvesting it doesn’t require total purity: High-performance ground-glass pozzolans can use post-consumer glass—ground up and milled for use in concrete—regardless of color and even when mixed with ceramic materials. In addition, the production of GGP requires less energy and emits fewer greenhouse gases compared to the production of cement and other pozzolanic materials.

A high-performance GGP concrete has greater resistance to sulfate attack than some other types of concrete. Sulfate attack tends to be a problem in coastal areas and in waters containing sulfur compounds. It can also have extreme resistance to chloride penetration that results from the application of road salts to melt ice and snow, and it reduces moisture penetration which, as discussed earlier, significantly suppresses the negative effects of freeze-thaw cycles. These attributes make it particularly suited for transportation infrastructure.

Nevertheless, GGP must be manufactured with particular specifications in mind. The interest in GGPs and a need for industry standards has led to the emergence of ASTM C1866, which went into effect in March 2020. The standard references two chemistries of glass: Type GS and Type GE. Type GS includes container and plate glass while Type GE, known as VCAS, is E Glass powder derived from fiberglass reinforcements.

According to ASTM C1866, glass pozzolans must be powder finer than 95% passing 325 mesh (44 microns). The glass powder pozzolans have a d50 particle size of 10-15 microns and a Blaine fineness of 4000 or finer. The primary difference between the two glass chemistries is alkali content. Container and plate glass Type GS have 13% Na2O, and Type GE has less than 1%. In practice, Type GE will mitigate alkali-silica reaction (ASR), while Type GS is more suited for indoor locations and non-reactive aggregates.

The standard provides information on chemical and physical specifications, which are important for GGPs in particular. One example is the need to keep below 0.5% the total non-glass residue (NGR), such as paper, plastic, metal, and liquid and organic content. A higher level of NGR results in loss of ignition (LOI), which impacts the air content of fresh cement.

Whereas LOI in fly ash and some other types of SCMs comes from unburned carbon and actually lowers the air content, with glass more LOI leads to more trapped air. Trapped air in this case can increase the porosity of the concrete and reduce its strength and durability. It can also lead to the formation of undesirable compounds that further degrade the concrete’s properties, or even affect the manufacturing process in other ways. The new standard helps clarify many properties needed for GGPs to be effective. NGR levels, which can be reduced in the manufacturing process, as well as physical specifications such as fineness, are two important specifications.

The Future of GGPs and Carbon Reduction

The demand for ground-glass pozzolan continues to rise in response to several conditions. The most common pozzolan used in the concrete industry is fly ash—the byproduct of coal-burning power plants. But recent environmental regulations placed on coal-fired plants and the emergence of new, more cost-effective alternative energy sources are leading to the shutdown of plants. At the same time, glass manufacturing is going up, especially in packaging. Yet because the economy for glass recovery is so poor, glass waste is going into the landfill at a higher rate than it is being recycled.

Currently, 31% of glass produced in the U.S. is recycled, while 62% is often trucked long distances just to be landfilled. From an environmental point of view, the loss of so much glass to the landfill illustrates a negative impact. But for the concrete industry, it provides an opportunity to help solve a waste problem and tap into a cheap, available material supply. Working with a material that reduces the carbon footprint of concrete has also inspired high-performance GGP manufacturers to dial in their processing even more by creating tightly controlled closed-loop circuits that yield highly durable products.

Some high-performance GGPs can reduce the CO₂ of the replaced cement in concrete at an almost ton-for-ton basis. They can be used in gray or white cement to help reduce the “heat island” effect in urban areas, and minimize energy and lighting costs. The U.S. Environmental Protection Agency, U.S. Green Building Council, and Northeast Recycling Council are just a few agencies and organizations that honor high-performance ground-glass pozzolan for its environmental benefits.