The role of microstrain on the thermostructural behaviour of industrial kaolin deformed by ball milling at low mechanical load

The high-temperature structural behaviour of kaolin deformed by compaction and shear in a ball mill was investigated by extending the temperature range to 1500 degrees C. The deformation was induced for the first time with steps of low mechanical load through a specifically built planetary ball milling working in a controlled thermodynamic environment (25 degrees C and at a vacuum of 0.13 Pa). The investigated kaolin was made of about 65% of a well-ordered kaolinite, 10% of dickite and 25% of quartz. The mechanical energy transfer to the material was measured via the microstrain because we wanted to investigate the details of the evolution of the kaolin modification as a function of the microstrain. At the very early stage of milling (up to 1 h), no strain was accumulated in the kaolinite structure which, however, presented lamination, layer flattening and texturing. Further milling induced a progressive reduction of the stacking layer coherence and an increase of the microstrain, in both cases as a non linear function of the deformation time. The thermo-structural behaviour of kaolin was investigated by TG-DTA in a helium atmosphere at 10 degrees C/min of heating rate. In the medium temperature range (400-1000 degrees C), the mechanical milling affected the dehydroxylation reaction of kaolinite by a significant anticipation of about 300 degrees C of the temperature range, which usually occurs between 400 and 800 degrees C. Other reactions related to the formation of a Si-spinel and other intermediate phases were observed. The mechanical deformation severely influenced of the high-temperature reactions related to mullite and cristobalite formation. All thermal reactions linearly correlate to the microstrain accumulated in the kaolinite structure. The reported data are of particular usefulness in industrial applications involving grinding or milling of kaolin.