AL-27 AND SI-29 MAS NMR-STUDY OF KAOLINITE THERMAL-DECOMPOSITION BY CONTROLLED RATE THERMAL-ANALYSIS

The processing of kaolinite under controlled rate thermal analysis conditions (CRTA) provides homogeneously transformed samples at defined dehydroxylation rates, The kaolinite-to-metakaolinite transformation has been monitored by solid-state NMR and X-ray diffraction. The metakaolinite product is an amorphous material with no resolved XRD pattern and is based on a disordered polymerized silicon/aluminum network in which aluminum preferably occupies four- and five-fold structural positions in the ratio 60% Al-IV, 30% Al-V, and 10% Al-IV. This complex dehydroxylation process has been viewed at different scales, with the aluminum NMR tracing a site-by-site destruction of the octahedral sheet and the silicon NMR and XRD following a more rapid destruction of the long-range order.