Page 531 - Handbook of Thermal Analysis of Construction Materials

502 Chapter 12 - Clay-Based Construction Products

Thermomechanical Analysis (TMA) permits determination of length
changes occurring in a clay body that is heated from the ambient tempera-
tures to the temperature that the body is subjected to in actual practice. The
approximate shrinkage values for kaolinite, illite, and montmorillonite are
2–17%, 9–15%, and about 20% respectively. The total shrinkage in a clay
body is determined by the shape, type, amount of clay minerals, and their
constituents. For example, the presence of quartz will reduce shrinkage.
Shrinkage may increase in the presence of alkali, iron, and alkali earths. The
temperature-shrinkage curves not only are useful to identify the clay
mineral, but they also give information on the desirable temperature to
which clay should be fired. Ramachandran and Ramez applied various
thermal techniques for investigating the plastic clays. [18] The TMA showed
the following trends: Initial shrinkage occurred at 110–300°C due to the
loss of adsorbed water from clay minerals. Maximum shrinkage occurred
in samples containing montmorillonite. In the range 420–650°C, the
shrinkage was slight although dehydroxylation took place. A large shrink-
age, however, resulted in the range 680–800°C. This could be attributed to
the vitrification and crystallization effects. Some samples exhibited expan-
sion around 1000°C due to the bloating effect. Total shrinkage was higher
in samples containing sodium chloride and montmorillonite. Illitic clays
showed lower shrinkages. A TMA analysis of a clay to which 10%
limestone is added is shown in Fig. 8.
Dilatometry (length change measurement at temperatures up to
1400°C) can provide useful information on the relevant phase changes that
occur in fired clay products. This technique can be used to supplement
information obtained using other thermal methods. Clay minerals generally
shrink when fired. Shrinkage values (%) for kaolinite, illite, montmorillo-
nite, and halloysite vary between 2 and 20%. [19] The magnitude of shrink-
age is affected by the size and shape of the clay mineral particles as well as
the compositional variations. The amount of shrinkage for kaolinites is
dependent on particle morphology. Shrinkage is directly related to the
percentage of very small plate-shaped particles. The presence of large
masses of the mineral reduces shrinkage. A similar effect would be
expected for illite. Also, the shrinkage generally increases as the amount of
fluxing agent increases. There is, however, no apparent correlation between
the content of hydroxyls in the clay mineral structure and the firing
shrinkage.