Geopolymers

AFRL scientists are developing a new class of inexpensive ceramic-like materials.

The use of ceramics in an ancient culture is one measure of that society's overall technical sophistication. Combining the study of ancient ceramics with modern science and technology has led to the creation of new ceramics with superior properties. Victor Glukhovsky, a scientist working in the Ukraine half a century ago, researched differences in the durability of ancient cements and more modern concretes. His work ultimately led to the synthesis of various aluminosilicate binders from clays, feldspars, volcanic ashes, and slags. These binders exhibited properties superior to those of the cementitious materials in common use at that time. Decades after Glukhovsky's discoveries, Ukrainian builders continued to employ aluminosilicate binders in construction applications, confirming their outstanding durability.

In the 1970s, French Professor Joseph Davidovits investigated the construction of the Great Pyramids of Egypt. He concluded (almost assuredly incorrectly) that they could not have been built from quarried rock, surmising that the large aluminosilicate-based blocks that form the pyramids must have instead been synthesized in place. Prof Davidovits' endeavor to explore the possible mechanisms for producing such blocks led, by 1979, to the practical invention of geopolymers. While these aluminosilicates share much in common with Glukhovsky's building cements, they constitute a new class of materials. They are formed by condensation polymerization and, unlike calcium-based cements, do not incorporate waters of hydration within a crystal structure. Consequently, geopolymers are stronger than cementbased materials, impermeable to water, and far more resistant to degradation by acids or high temperatures.

Figure 1. Foundry workers fill geopolymer composite molds with molten, 1428°C Fe-Si.

Although geopolymers possess many useful properties and are relatively inexpensive to make, they have not found widespread acceptance as substitutes either for current cements, mortars, or pastes or for conventional ceramics. Materials scientists offer many explanations for the comparatively slow emergence of geopolymers into the marketplace, but from the perspective of AFRL scientists, the geopolymer knowledge base is not yet sufficient to support widespread industrial use of these materials. To expand the use of this promising technology, AFRL is using Small Business Technology Transfer (STTR) contracts and basic research sponsored by the laboratory's Asian Office of Aerospace Research and Development to fund three different research teams in their efforts to explore and advance geopolymer science and technology. Specifically, the goal of these efforts involves developing a better understanding of the chemistry and physics associated with geopolymers, generating geopolymer performance data, and exploring improved geopolymer-based materials.

Prof Jannie van Deventer, of the University of Melbourne, Australia, leads one of the three AFRL-supported teams. He and his colleagues work with a private Australian company, Siloxo; other international groups; and fellow researcher Prof Trudy Kriven, of the University of Illinois at Urbana- Champaign. Recent noteworthy results stemming from this collaboration include an explanation of the role of calcium in geopolymerization, the production of more uniform geopolymers through the use of comparatively inexpensive synthetic metakaolin, a new understanding of the effects of high temperatures on geopolymers, and the fabrication of new geopolymer-based composites that exhibit excellent properties to at least 800°C. A second AFRL-funded team, headed by Mr. Doug Comrie (Catawba Resources, Inc.), is focused on the fabrication of fibrous and particulate geopolymer composite molds for use in metals casting. During Phase I of this team's STTR effort, researchers tested various mold materials in an industrial setting by pouring molten, 1428°C ferrosilicon (Fe-Si) into the molds (see Figure 1). The STTR Phase II project will optimize the molds for commercial use.

These two research teams have achieved other results as well; most notably, they have provided insight into the structure of geopolymers. The accepted structural model is based on a charge-compensated glass, wherein silicon oxygen tetrahedra (SiO4) are joined to other SiO4 tetrahedra or to aluminum oxygen tetrahedra, AlO4, with an alkali atom associated with each aluminum tetrahedron to provide the additional positive charge required. However, analytical studies (including high-resolution transmission electron microscopy, synchrotron-radiation diffraction, and nuclear magnetic resonance) have cast doubt on that model's accuracy. Scientists have consequently postulated a new model based on nanoscale zeolite-like crystals embedded in an amorphous matrix. If validated, this revised model will constitute the foundation of new geopolymer research and development efforts, since it is crucial that scientists be able to apply adequate knowledge of structure and its mechanism of formation in designing any new material.

Figure 2. Various geopolymer-based composite panels fabricated with aluminum oxide, carbon, and steel fiber reinforcements

Scientists can form geopolymers from many raw materials—from pure metakaolinite to industrial fly ash and blastfurnace slag—and can tailor geopolymer properties based on their composition. As Dr. Grant Lukey (University of Melbourne) notes, "There is no single, all encompassing geopolymer composition that produces a material with a long list of exceptional chemical and mechanical properties. A fundamental understanding of the chemistry and reaction mechanisms enables the material to be nanostructurally designed or tailored to possess specific properties for desired applications." AFRL researchers learned this lesson years ago when they unsuccessfully attempted to insert a geopolymer as an ablative material in a rocket engine nozzle. Furthermore, just as no single geopolymer composition is suitable for all applications, geopolymers themselves are not conducive to all applications.

Perhaps one of the most exciting possibilities for geopolymers is their potential use as oxide matrices for structural ceramic composites. Geopolymers offer significant advantages associated with the processing of ceramic matrix composites (CMC), including low-cost, near-net-shape processing, a tailorable coefficient of thermal expansion, and corrosion resistance in acids. The third team working on an AFRL STTR Phase II project operates under the leadership of Dr. Balakrishnan Nair, of Ceramatec, Inc. Dr. Nair and his collaborators at Brown University (Providence, Rhode Island) and Rutgers University (New Brunswick, New Jersey) are developing an innovative, technologically simple, net-shape processing method for the fabrication of CMCs. They demonstrated the formation of a hydrothermally stable matrix phase using the geopolymer processing approach. To obtain improved mechanical properties and facilitate graceful failure in the geopolymer CMCs, the scientists then incorporated a distinct functional layer (interphase) at the fiber/matrix interfaces. The team reported the development of a novel geopolymer steel mesh composite with enhanced strength and ductility at the 2004 Annual Meeting of the American Ceramic Society (see Figure 2).

Current basic research efforts focus on refining and confirming the structural model, studying reaction mechanisms and kinetics in greater detail, exploring new composite systems, and examining additional high-temperature properties. The research team on which Prof Kriven participates has observed that geopolymers form pollucite, a highly creep-resistant oxide, when they decompose at elevated temperatures. This and other insights have prompted the group to investigate various hightech applications, such as refractory adhesives, versatile ceramic binders, and ceramics with tunable thermal expansion coefficients. Potential Air Force geopolymer materials applications range from lightweight fireproof aircraft insulation to strong, rapidly setting runway repair materials.

Geopolymer research has greatly benefited from the funding supplied by AFRL and other governmental agencies. Increasingly rapid advancement in this area will, however, require wider commercialization of geopolymers and investment of the resulting profits to expand research and development. According to Prof van Deventer, "With the recent development of durable products [constructed] from a range of low-cost waste materials, geopolymers are on the verge of commercialization." As with all structural materials, cost remains a key selection factor, and geopolymers possess many inherent cost advantages, from their availability as plentiful and inexpensive raw materials to their reliance on simple, near-net-shape processing methods. Consequently, they may soon become another option for designers of future aerospace structures.

Dr. Ken Goretta, Dr. Joan Fuller, and Ms. Erin Crawley, of the Air Force Research Laboratory's Air Force Office of Scientific Research, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn_index.asp . Reference document OSR-H-05-05.