Properties and characterization of alumina platelet reinforced geopolymer composites

Geopolymers are porous, amorphous, alkali-aluminosilicate hydrate materials formed at room temperature via a solution process. Geopolymer based on metakaolin had a relatively homogeneous microstructure that offered consistent behavior but suffered from dehydration cracking and large densification shrinkages when heated. It was found that by reinforcing a metakaolin geopolymer of composition (K2O center dot Al2O3 center dot 4SiO(2)center dot 11H(2)O) with 50 mu m diameter alumina platelets, dehydration cracking could be prevented, and shrinkage could be reduced by an order of magnitude. Samples were reinforced with 30, 50, and 70 wt% of alumina platelets. Although the properties of the 30 and 50 wt% conditions were better than those of unreinforced geopolymer, those samples still showed warping, cracking, and strength losses on heating. The 70 wt% samples did not warp or crack when heated to temperatures of up to 1500 degrees C. The room-temperature 4-point flexural strength of these samples remained at around 20 MPa regardless of heat treatments. The in situ measured flexural strength increased to almost 40 MPa at 600 degrees C, and remained higher than 20 MPa until 1200 degrees C. Samples subjected to propane-torch thermal shock heating and subsequent quenching did not crack or fragment. Dilatometry, X-ray diffraction, and scanning electron microscopy were used for additional characterization. Given these properties, this material showed promise as a castable refractory.