An unlikely pairing of materials that are claimed under the umbrella of ceramic materials — glass and concrete — appears to be working out well on several levels.
Both of these materials are staples of modern life and are produced in enormous quantities. Both carry some serious environmental baggage with them. But, work conducted over the last two years at Michigan State University is showing that incorporating ground-up waste glass into concrete actually improves the properties of the concrete. According to a university news story, “the concrete is stronger, more durable and more resistant to water.”
The research comes out of the MSU Department of Civil and Environmental Engineering and was supervised by Prof. Parvis Soroushian. Soroushian and his doctoral student, Roz-Ud-Din Nassar published two papers last fall on their work.
There have been past efforts to incorporate waste glass into concrete as the aggregate phase, but, according to their paper (pdf), “These efforts neglected the reactive nature of glass in concrete, which was slowed down due to the relatively large (millimeter-scale) size of the glass particles.” The paper was published in the Journal of Solid Waste Management and Technology.
Recognizing that waste glass is an excellent source of amorphous silica, they realized that it has the right chemistry and reactivity “to enter pozzolanic reactions with the lime released during hydration of cement,” and that “these reactions can yield highly stable end products with desired binding qualities,” that is, cement.
What is a pozzolanic reaction? According to the American Concrete Institute website, pozzolan is “siliceous or siliceous and aluminous material,” and by itself, is not cementitious. However, fine particles of pozzolans will react in the presence of moisture with calcium hydroxide to form cementitous compounds, hence the description, “pozzolanic reaction.”
Naturally occurring pozzolans include volcanic pumicites, opaline cherts and shales, clays and diatomaceous earths. Artificial pozzolans include fly ash, silica fume and, now, waste glass.
The researchers used a mix of colored waste glass ground to an average particle size of 25 micrometers (93 percent passing #325 sieve) and substituted it for cement in amounts of 15, 20 and 23 percents by weight. In all cases, the mixes were prepared at ready-mix concrete plants, and crushed limestone and nonreactive river sand were used in the mixtures. In their paper they report that incorporating glass particles is “compatible with conventional concrete production and construction techniques.”
Slump tests showed that the “fresh mix workability” of the glass-containing mixtures was a little lower for the two mixes with more glass. The 15 wt% mixture had the same slumping properties as the control batch of concrete. The authors suggest the nonspherical and rough geometry of the glass particles may contribute to the reduced workability.
In addition to lab tests, the mixes were field tested at several sites on the MSU campus, including driveways, heated pavements, sidewalks, gutters, curbs and parking stands. The 20% glass mix was used for the field tests.
Test specimens were prepared at the time that the concrete was poured in the field, and at several time intervals after pouring, cores were drilled from the field locations. Tests included compressive and flexure strength, water sorption, chloride permeability and abrasion resistance.
SEM work showed that the glass-containing cement had a relatively dense and uniform microstructure compared to the control composition. The authors attribute the microstructure differences to “the pozzolanic reactions of milled waste glass yielding secondary calcium silicate hydrate (C-S-H).
The authors report that after two years of exposure to Michigan weather and service loads (use by cars, trucks and pedestrians), the waste glass improves the durability and abrasion resistance of the concrete. They conclude, “the use of milled waste glass in concrete is a viable practice which would result in important energy, environmental and cost benefits, and would make important contributions towards reducing the carbon footprint of the construction industry.”
How important might those contributions be? They provide some interesting facts in the paper, which pretty much speak for themselves. Consider:
• 12.5 million tons per year of waste glass generated in US
• 77 percent goes to landfills
• 6 percent by weight of municipal solid waste is glass
• Every ton of cement manufactured generates a ton of CO2 emission
• U.S. cement production in 2007 was nearly 100 million tons, thus pumping nearly 100 million tons of CO2 into the atmosphere
• 90 percent of the energy used to make concrete is used to make the cement
• One ton of cement consumes about 5.5 million BTU of energy
• Two percent of global primary energy consumption is used by the cement industry
• Only aluminum and steel manufacture use more energy
A second paper on the work was published in the Journal of Construction and Building Materials and is available online (pdf).