TG/DTA analysis of kaolinite showed the characteristic profile described in the experimental
section. TEM investigations confirmed that a layer structure was retained in metakaolinite
even after heat treatment at 800°C for 48 h, as shown in Figure 2. The distancebetween layers in kaolinite increased from 0.71 nm to 1.1 nm. These results are in concordance
with Bergaya et al. [22]. Heating of kaolinite powder at 1000°C for 20 h resulted only
in traces of mullite formation.
According to data obtained from the N2-sorption experiment, there was a strong correlation
between changes in micropore volume, Vmp, and heat treatment at 1000°C. A plot of
Vmp(before heat treatment) vs. Vmp(after heat treatment) resulted in a correlation coefficient
of 0.998. The relative decrease in micropore volume was about 4%. The same correlation was
obtained for the Langmuir surface areas, also with a decrease by 4% (Table 2).