Geopolymers made using organic bases. Part II: Crystallization and physical properties.

It was previously demonstrated that geopolymers can be synthesized using organic bases rather than alkali hydroxides, thereby removing the alkali cations from the composition and allowing other ceramics to be made. This work compared the microstructures of Al2O3·2SiO2 and Al2O3·4SiO2 bodies made using guanidine and tetramethylammonium hydroxide (TMAOH) with equivalent sodium geopolymers. The guanidine and sodium samples were similar, whereas TMAOH caused a clear difference in structure. Sodium and guanidine geopolymers with overall compositions of 3Al2O3·2SiO2 were also synthesized with the aim of developing a castable route to bulk mullite. The development of crystalline phases was followed by powder x‐ray diffractometry for all of these compositions after thermal treatment at temperatures up to 1600°C. The microstructures after firing at 1600°C were examined by scanning electron microscopy. When heated, the Al2O3·2SiO2 and Al2O3·4SiO2 compositions transformed from amorphous phases into a mixture of fine, needle‐shaped, mullite crystals and amorphous silica, while 3Al2O3·2SiO2 compositions became entirely mullite, with the shape of the mullite crystals being dependent on the type of precursors used. All samples exhibited substantial amounts of porosity, which could be reduced with improved processing techniques. This work demonstrated that alumina‐rich guanidine geopolymers are a potential route to casting bulk mullite bodies. [ABSTRACT FROM AUTHOR]