Reaction and Sintering for the Mixture of Kaolin and Calcium Carbonate below 1000°C

Reaction and sintering characteristics of kaolin and CaCO3 mixtures were investigated using New Zealand kaolin and finely ground calcite powders as starting materials. Depending on the content of CaCO3, dense ceramics consisting of single phase of anorthite or composite of gehlenite, wollastonite and anorthite were obtained by firing below 1000°C. With the heat processing, CaO reacted with decomposed kaolin from 800 to 850°C and formed metastable amorphous materials. Thus formed active amorphous materials sintered rapidly between 850 and 950°C by the viscous flow to form glass-like dense ceramics. According to the increase in amount of CaO, the sintering temperature lowered from 950 to 900°C. With the further heat processing, gehlenite, anorthite and wollastonite instantly crystallized, and the dense polycrystals were obtained. Their thermal expansions could be controlled between 4.5 and 6.5×10-6/°C and the bending strengths were about 200MPa. Thus obtained alkaline-free silicate ceramics could be utilized as the electrical insulators or the multilayered ceramic substrates.