From a geological point of view, clays are soft, fine-grained, and residual sedimentary rocks
resulting from the weathering of feldspars (e.g., orthoclases and plagioclases) and ferromagnesian silicates (e.g., micas, amphiboles) contained in igneous and metamorphic rocks.
Hence clays are always made of various hydrated aluminosilicates, mainly kaolinite but also
illite and montmorillonite, all exhibiting the typical structure of sheet silicates (i.e., phyllosilicates).
When a clay is fired, it loses its absorbed water between 100 and 200°C. Secondly, its
major phyllosilicate mineral, kaolinite [Al4(Si4O10)(OH)8
= 2Al2O3·4SiO2·4H2O], dehydrates
between 500 and 600°C, giving off its water to form metakaolin [Al2Si2O7
= 2Al2O3·2SiO2]:
Al4[Si4O10(OH)8] —> 2Al2Si2O7
+ 4H2O (500°C < T < 600°C)
Above 800°C an important chemical change takes place with the formation of one of the
three aluminosilicate polymorphs (Al2SiO5), i.e., andalusite, kyanite, or sillimanite, and free
silica according to the overall chemical reaction:
2Al2Si2O7
—> 2Al2SiO5+ 2SiO2
(at T > 800°C)
If firing is carried out above 1595°C, the highly refractory mineral mullite then forms (see
mullite) with an additional liberation of free silica that melts according to the following
chemical reaction:
3Al2SiO5
—> Al6Si2O13 + SiO2
(at T >1600°C)
