Hazard and Precautionary Statements (GHS)

ALUMINO-SILICATES Chemical Properties,Usage,Production

Preparation

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).

Reactions

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)