Conference, Proceedings

3 Results
During heating, the structure and composition of kaolin‐quartz samples is changed. These
changes determine the mechanical and dilatometric behavior of the samples. The results of
thermodilatomery of green kaolin‐quartz samples at the linear heating up to 1200 °C are shown
in Fig. 1.
relative expansion / %
2
1
0
-1
-2
-3
-4
-5
-6
0 200 400 600 800 1000 1200
temperature / °C
Figure 1: Thermodilatomeric curves of green kaolin‐quartz samples. The content of quartz (from the
bottom): 0 wt.%, 9 wt.%, 20 wt.%, 30 wt.%, 40 wt.%, 50 wt.%, 60 wt.%, 70 wt.%, 80 wt.%, 100 wt.%
The thermodilatometric curve of green samples can be divided into typical parts corresponding
to processes in kaolin and quartz. The liberation of physically bound water at temperatures
from 20 °C to 250 °C leads to more tight contacts between kaolinite crystals and, subsequently,
to the shrinkage of the sample. On the other side, thermal expansion takes place at the same
time which increases the size of the sample. The curve between 20 °C and 250 °C follows from
these two competitive mechanisms. After liberation of the physically bound water, only the
thermal expansion takes place. Dehydroxylation at temperatures from 450 °C to 650 °C is
accompanied by the creation of metakaolinite. Its crystals are of the same type as kaolinite
crystals, but have a smaller c‐parameter, so the shrinkage of the sample can be observed. The α
→ β transformation of quartz takes place at 573 °C. If the quartz grains are free and can change
their dimensions without any obstruction, then their relative volume change is +0.68 %.
However, the quartz grains are not free but surrounded by the kaolin structure. So, the quartz
grains increase their volume less than 0.68 %. The beginning of the solid‐phase sintering also
gives rise to more shrinkage which continues untill the end of heating. The collapse of
metakaolinite and the creation of alumina spinell (metastable phase) and primary mullite takes
place above 950 °C. This new structure significantly increases the shrinkage.
Initially, it is noted that there are significant differences between thermodilatometric curves of
the samples with different content of quartz (see Fig. 1). If the sample contains a larger amount
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