In tests in which samples were exposed to a chloride environment, steel reinforcement embedded in concrete made with geopolymers exhibited far less corrosion than those embedded in concrete made with portland cement. Courtesy of Louisiana Tech University’s Trenchless Technology Center
A new form of concrete made with geopolymer binders rather than portland cement exhibits high strength, low permeability, and high resistance to corrosion and heat.
May 29, 2012—A novel form of concrete made from geopolymer binder technology developed by researchers at Louisiana Tech University is stronger and more resistant to corrosion and high temperatures than portland cement, potentially offering an ideal product for replacing the traditional construction material in harsh environments. For example, the geopolymer concrete recently performed well serving as a refractory surface at a facility used to test rocket engines. Because its main ingredient is an industrial by-product, the geopolymer binder technology requires much less energy to produce and results in significantly less carbon dioxide emissions relative to portland cement, making it a more environmentally sustainable option as well.
Geopolymers represent a class of cementitious materials that harden and gain strength without the use of portland cement. Although a variety of materials may be used to create geopolymers, the basic components are a fine-powdered material rich in aluminum and silica—including fly ash, clay powder, or rice husks—and sodium hydroxide, potassium hydroxide, or another alkali to cause the aluminum and silica to leach out of the powdered material. “We can use a variety of waste products or natural sources to get the powder that we are looking for,” says Erez Allouche, Ph.D., P.E., M.ASCE, the director of Louisiana Tech University’s Trenchless Technology Center. Fly ash has turned out to be the main ingredient of choice for Allouche and his colleagues. A by-product of coal combustion, fly ash represents a readily available material in the United States and throughout the world, as approximately 71 million tons of fly ash are produced annually in the United States, while roughly 800 million tons are generated worldwide.
Geopolymers differ from portland cement in that their setting mechanism depends on polymerization rather than hydration. As a result, geopolymers can achieve their significant maximum strength within 3 to 5 days, depending on the curing effort applied, whereas portland cement typically may require as much as a month to achieve most of its strength and an entire year to achieve full completion. While offering the same ease of application as portland cement, geopolymer technology provides such benefits as early high compressive and tensile strengths, low permeability, and high resistance to corrosive agents and elevated temperatures.
Although the concept of geopolymer binders has existed since the early 1950s, efforts to commercialize the technology have yet to succeed on a widespread basis. “It’s a very novel technology, with substantial challenges in taking the concept from the lab to the field,” Allouche says. Among the major challenges that he and his colleagues have had to overcome is the variability of fly ash, which, as a byproduct of coal combustion, can possess varying characteristics depending on such factors as the type and quality of the coal source and how it is burned and cooled.
To address this variability, the researchers collected approximately 60 fly ash samples from around the world. After characterizing the materials within each sample, the researchers created a geopolymer formulation suitable for each. Based on this work, the researchers developed a database summarizing the different properties of the various fly ash samples and created software that enables them to predict certain mechanical properties of geopolymer binders made from different fly ash samples. In this way, the researchers can account for the variability associated with the use of an industrial byproduct, enabling them to create an “engineered product that has predictable properties,” Allouche says.
Test samples embedded in concrete made with portland cement
exhibited far more corrosion in chloride environments, but due to
costs, portland cement will likely remain the preferred ingredient
in all but the most harsh environments. Courtesy of Louisiana
Tech University’s Trenchless Technology Center
Allouche and his colleagues have also developed various admixtures for use in manipulating the material so as to enable it to be applied in different ways or to achieve certain setting times or mechanical strengths. Geopolymer concrete can be used in structural and nonstructural applications and made either by precast, cast-in-place, or spray-applied methods. Meanwhile, the geopolymer technology can be produced using essentially the same equipment and processes as that used to create portland cement. For its part, the geopolymer concrete is a “more finicky material” than portland cement, Allouche acknowledges, as it requires greater precision during the process of mixing ingredients. However, geopolymer concrete offers certain clear advantages from an environmental standpoint. For example, portland cement production requires significant quantities of energy and generates billions of tons of carbon dioxide annually. By comparison, production of some formulations of geopolymer binder technology requires 90 percent less energy and 85 percent less carbon dioxide emissions.
In terms of mechanical properties, geopolymer technology can achieve a compressive strength of up to 16,000 psi in as little as 24 hours, while resisting corrosion by acids and sulfates as well as temperatures of up to 3,200°F. With its high strength and resistance to corrosion and extreme temperatures, geopolymer technology offers a potentially favorable alternative to portland cement in certain specialized applications. For example, geopolymer technology shows considerable promise in applications for which portland cement requires the use of an epoxy or other coating to protect against corrosion. If such coatings fail, they typically do so because of a problem with the bond, or the interface between the protective layer and the underlying portland cement, Allouche says. However, geopolymer technology is a single component that does not rely on an outer coating for protection. “There’s no interface to fail,” Allouche notes.
As part of a recent examination of its refractory capabilities, the geopolymer binder technology was applied as test patches within a concrete plume trench at the John C. Stennis Space Center, a facility in Hancock County, Mississippi, that is used by the U.S. National Aeronautics and Space Administration (NASA) to test rocket engines. Subjected to extremely high temperature and thrust loads during testing of the rocket engines, the plume trenches must be made of very strong materials. During the first test of the geopolymer technology in early May, the test patches were subjected to temperatures of roughly 2,700°F to more than 4,500°F, as well as up to 400,000 lb of thrust from the rocket engines. The geopolymer patches held well during the test, showing little wear and no sign of melting. “From an aesthetic prospective, the geopolymer surface looked even better following the test,” Allouche says. Future tests of the material at the site are planned beginning this summer.
Among a handful of other installations to date, the technology also has been used with success to construct an acid spill pad at an Arkansas manufacturing facility, Allouche says. The manufacturer needed to protect its loading dock against corrosion, which frequently resulted when acidic materials used as part of the production process leaked onto the dock. Since being installed on the loading dock in July 2011, the geopolymer technology has prevented corrosion. “So far, so good,” Allouche says. As for other potential applications in corrosive environments, the geopolymer technology likely would work well as bridge decking in marine environments, such industrial applications as petrochemicals and food processing, and pipelines, manholes, and storage facilities for wastewater, Allouche says.
As one of the most well-established construction products around, portland cement enjoys considerable economies of scale, helping to lower production costs. Therefore, geopolymer concrete is not intended to replace portland cement, Allouche says. Rather, the technology, with its unique mechanical characteristics, is expected to “supplement” its more conventional counterpart, he notes. Although geopolymer concrete is more expensive to make, it has been shown to come within 10 to 30 percent of the cost of portland cement on certain applications, Allouche says.
With its strength and resistance to extreme temperatures and corrosion, geopolymer technology could help engineers in their efforts to design and construct more enduring facilities. Because of the high cost associated with replacing critical infrastructure, care should be taken to construct such infrastructure using materials that will last much longer than what is currently considered possible, Allouche says. “We have to use materials that last hundreds of years, not tens of years,” he says.