Conductimetric investigations on the dissolution of metakaolinite in dilute hydrofluoric acid. Structural implications

Abstarct Conductimetric curves based on dissolution in dilute HF of a series of metakaolinite samples prepared by thermal dehydroxylation of kaolinite in air from 500°C to 987°C, have been interpreted in terms of (i) short-time conductivity variation, and (ii) specific initial dissolution rate ( r si ). The first parameter has been correlated with a disorganization factor ( Fd , equal to unity for an entirely amorphous silica-alumina mix) which does not exceed 0·52 for metakaolinite and, like r si , reaches a maximum for samples prepared at about 720–750°C. These results, which agree with other data obtained by calorimetry and infrared spectroscopy, show that metakaolinite is never entirely amorphous and has a degree of disorder which varies with the conditions of preparation, and increases for samples which have been mechanically treated by grinding.