Role of metakaolin dehydroxylation in geopolymer synthesis

• Impurities in the metakaolin influence the properties of the geopolymer.
• The reactivity induced by calcination counterbalance the influence of impurities.
• The calcination process influences the formation of Si- and Al-amorphous phases.
• The reactivity differences induce the formation of different networks.
• The wettability is an indicator of metakaolin reactivity.

The properties and structures of three commercial metakaolins were evaluated for their use in geopolymer materials. The dehydroxylation processes used on these three metakaolins (rotary, flash or oven processes) induce varying reactivities and purities according to the results obtained by FTIR spectroscopy, XRD, 27Al MAS-NMR, TEM characterization, and DTA. The flash-calcined metakaolin is the most reactive one. After geopolymerization, the impurities (such as quartz or micas) and the reactivities of the raw metakaolins induce the formation of different networks within the geopolymer material. This study demonstrates the main role of the metakaolin dehydroxylation process, the effect of impurities and the use of the wettability as an indicator of metakaolin reactivity.

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