Preparation of macroporous ceramic from metakaolinite-based geopolymer by calcination

Abstract Metakaolinite-based geopolymer was prepared using metakaolinite under activation of NaOH solution and sodium silicate solution. The geopolymer was calcined at 300 °C, 600 °C, 900 °C, 1200 °C and 1500 °C. IR, SEM and XRD were used to determine changes on composition, morphology and structure. It was found 900 °C was a key transition temperature. Calcined below 900 °C, the geopolymer did not show obvious structure change, although its bending strength was decreased. Calcined at 900 °C, the geopolymer began to melt partly and coagulate locally, and considerable interspaces existed and separated the partly melted geopolymer. When the calcining temperature was increased to 1200 °C, material with considerably high crystallinity came into being. XRD study indicated that main phase of the material was mullite. Increase of the calcining temperature between 900 °C and 1500 °C made the interspaces gradually shrink. When calcining temperature was increased to 1500 °C, these interspaces shrank into pores about 10 μm, thus resulted in the formation of one kind of macroporous mullite ceramic. Apparent density and flexural strength of the macroporous ceramic were 2.07–2.21 g/cm 3 and 47.69–49.62 MPa, respectively, which were higher than those of the metakaolinite-based geopolymer.