Role of the morphology and the dehydroxylation of metakaolins on geopolymerization

Two commercial metakaolins were tested during partial geopolymerization with potassium silicate in order to emphasize the different surface reactivities. Both manual and short-term mechanical stirring were used for slurry preparation, while radiation, infrared, and microwave heating were used for curing. The metakaolins had similar compositions and specific surface areas, but different morphologies and dehydroxylation degrees due to different calcination kiln technologies. The degree of geopolymerization was checked by SEM and N2 adsorption (BET), FTIR and 27Al MAS NMR spectroscopy. While the dehydroxylation degrees were different, the metakaolins had similar reactivity. The metakaolin powder with rounded agglomerates and lower water demand was more sensitive to the various geopolymerization conditions than the fine dispersed lamellar one, thus giving rise to very different micro- and macrostructures of the partially geopolymerized samples. IR heating seemed to increase the geopolymerization degree slightly, while MW heating induced the fast evaporation of the water, forming porous samples.