Influence of degree of dehydroxylation on the pozzolanic activity of metakaolin

Three kaolins were heated between 500 and 850 °C. The samples were tested by X-ray diffraction (XRD), differential thermal analysis (DTA), and Infra-Red (IR) spectroscopy in order to determine their mineralogical composition and degrees of crystallinity and dehydroxylation. The reactivity of the heated samples was evaluated by the determination of the residual quantity of Ca(OH)2 by differential thermal analysis (DTA) performed on hydrated mixtures of 50% metakaolin and 50% Ca(OH)2. There was no direct relationship between the pozzolanic activity of metakaolin and the degree of dehydroxylation. Highest activity was obtained when the degree of dehydroxylation was > 95%.