Reexamining calcination of kaolinite for the synthesis of metakaolin geopolymers - roles of dehydroxylation and recrystallization

Properties of metakaolins heated in a temperature range of 550-950 °C for 6 h are characterized by TGA and DSC for thermal analysis, NMR for Al coordination, and XRD for crystal structure. Dehydroxylation is the main reaction with heating temperature of 550-800 °C, and then recrystallization becomes important. Although recrystallization affects greatly the beginning of geopolymerization reactions even at a moderate heating temperature like 800 °C, a plateau of high compressive strength of geopolymers is obtained in the range of 650-850 °C. It is found that optimal mechanical property of geopolymers can be obtained with metakaolins of sufficient dehydroxylation and low recrystallization. In zeolite synthesis using metakaolin, the reactivity of metakaolin was at a maximum when the content of AlVI was at a minimum. However, in the case of metakaolin geopolymer synthesis, joint effects of dehydroxylation and recrystallization determine the reactivity of metakaolin in geopolymerization reactions.