The effect of water vapour atmospheres on the thermal transformation of kaolinite investigated by XRD, FTIR and solid state MAS NMR

The dehydroxylation of kaolinite and its subsequent transformation to cubic Al-spinel and mullite has been studied under water vapour atmospheres up t o 0.8 atm pressure. Dehydroxylation, as determined by XRD, FTIR and solid s tate MAS NMR, can be accomplished at low temperatures (ca 500 degrees C) un der water vapour atmospheres, the efficiency increasing with increasing wat er vapour pressure. The subsequent thermal transformations to crystalline p roducts are also facilitated by water vapour atmospheres, which also enhanc e the mechanical properties of the fired kaolinite (crushing strength and s oftening coefficient) by 3-4 times. These effects are discussed in terms of proton attack on the SCO bonds followed by polycondensation of the resulti ng silanol groups to siloxane, facilitating the nucleation of mullite from amorphous aluminosilicate and strengthening the chemical bonds between the grains. (C) 1998 Elsevier Science Limited. All rights reserved.