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.

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, allencompassing 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.