Concrete International

November 14, 2020

The construction sector is continually seeking new sources of supplementary cementitious materials (SCMs) to augment the portland cement, fly ash, slag cement, and silica fume used in modern concrete mixtures. Extensive research and testing have shown that several types of ground glass will perform well as a pozzolan in concrete. Supported by those results, ASTM Subcommittee C09.24, Supplementary Cementitious Materials, has drafted ASTM C1866/C1866M-20, “Standard Specification for GroundGlass Pozzolan for Use in Concrete.” The new specification was published earlier this year, after 3-1/2 years of balloting by the committee. This article provides much of the background information and industry context that accompanied the balloting.

Motivation

Glass production is a major source of greenhouse gases. While recycling can reduce the environmental impact,1 8.4 million tons (7.6 million tonnes) of container glass is landfilled annually in the United States (almost triple the amount that is recycled).2 A significant resource is therefore being discarded. A preliminary, third-party life-cycle assessment of one ground-glass pozzolan (GGP) producer’s output3 indicates that the global warming potential (GWP) impact for 1 ton (0.9 tonne) of GGP is 56 kg (123 lb) CO2e. For comparison, the U.S. industry average GWP for portland cement is 1040 kg (2293 lb) CO2e. Thus, the GWP calculated for a recent New York City project concrete mixture with 50% cement replacement with GGP would be about 40% less than the GWP for a concrete mixture with cement only.

Glass Sources and Chemistry

Much of the glass produced in the world is one of the following types:

• Container glass (used in packaging)—This material is generally soda-lime glass produced in flint (clear), green, blue, or amber colors and formed by air pressure in molds;

• Plate glass (used as glazing in buildings and automobiles)—This material is also generally soda-lime glass produced in clear or tinted colors and formed by floating on molten tin; or

• E-glass (used as reinforcement in fiber-reinforced polymers)—This material is low-alkali glass formed by extrusion through a bushing to form filaments that are rapidly drawn to a fine diameter before solidifying.

Table 1 summarizes the chemistry of these glass types and other pozzolanic or cementitious materials used in concrete, and Fig. 1 contextualizes GGP versus ordinary portland cement (OPC) and other SCMs. Although the chemistry of E-glass is quite different from the chemistry of container or plate glass, all three glass types have been shown to be suitable for use as a pozzolan in portland cement concrete. Also, because of the controlled processes used to manufacture these glass types, each has a very uniform chemistry worldwide, as demonstrated by the standard deviation reported in Table 2 for container glass chemistry.

The subcommittee members agreed that the three glass sources listed in ASTM C1866/C1866M are produced in sufficient quantities to provide viable resources for concrete production. The subcommittee also agreed that ground glass could be used safely. Glass production is regulated to limit toxic materials content, and the glasses listed in the standard are not included on the U.S. Environmental Protection Agency (EPA) Resource Conservation and Recovery Act (RCRA) lists of hazardous wastes. Further, the glass pozzolan sources are composed of amorphous silica. Unlike crystalline silica, amorphous silica has not been found to produce cancer in lung tissue.10,11 However, as with all nonhazardous dusts, the U.S. Occupational Safety and Health Administration (OSHA) provides permissible exposure levels (PEL) for amorphous.

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