Aluminum‐27 and Silicon‐29 Magic‐Angle Spinning Nuclear Magnetic Resonance Study of the Kaolinite‐Mullite Transformation

Abstract

The "Al and "Si magic-angle spinning nuclear magnetic resonance (MAS-NMR) study of the kaolinite-mullite transformation has shown the presence of A1 in tetru-and pentacoordination in dehydroxylated kaolinite. The 29Si NMR signal analysis of samples heated above 400 "C demonstrates that the tetrahedral sheet of kaolinite begins to break down near 600°C and continues to do so to 900°C. From the 27Al NMR signal evolution, it can be deduced that the exothermic peak at 980°C in DTA curves is associated with the mod$ication of the coordination of Al, which changes from the tetra-or pentacoordination to the more stable octahedral coordination. Heating the sample at 880°C ,for 36 h produces the same transformation in the coordination of A1 ions and the elimination of the exothermic peak at 980°C in the DTA diagram. After this transformation, all spectra show two tetrahedral lines characteristic of mullite, indicating that nuclei of mullite with low crystalliniv are generated during the exothermic process which are not detected by XRD. At higher temperatures tetrahedral NMR peaks increase in intensity, yielding, at 1200"C, the 3:2 mullite NMR spectrum.

Solid-state NMR characterisation of the thermal transformation of a Hungarian white illite

“…There are two AlO 6 peaks, one of which at 4 ppm is probably from mullite. This mullite formation appears to occur before the 1200°C found for halloysite [25] and kaolinite [16,20,22]. However there is certainly another AlO 6 -containing phase which could be a γ-alumina or spinel based phase present in this illite at these temperatures corresponding to the peak at ~12…”

“…There have been various multinuclear NMR reports (especially 29 Si and 27 Al) of the thermal decomposition of clay minerals. The most comprehensive NMR data set from clays has been for 1:1 clay minerals, such as kaolinite [14][15][16][17][18][19][20][21][22][23][24] and related phases such as halloysite [25]. For 2:1 type clay minerals pyrophyllite is a cation-free clay that has no iron and it shows different thermal breakdown characteristics from the cation-free 1:1 clay minerals, with much more AlO 5 in the intermediate dehydroxylate state [26][27][28].…”

Firing transformations of an argentinean calcareous commercial clay

“…In this exothermic reaction, amorphous silica, the spinel-like phase, and nuclei of mullite are produced. This spinel like phase can difficultly be detected by XRD [24], only the three broad peaks can be detected at 37º, 46º, and 67º associated with a cubic phase. From this analysis it can be conclude that the studied formula corresponds to a kaolinitic-illitic clay with important alkaline earth metals (Ca and Mg) carbonates presence.…”

“…The exothermic peak at 980 °C has previously been assigned [24] to the transformation of the Al coordination from the pentahedral or tetrahedral form of the metakaolinite to the octahedral form. In this exothermic reaction, amorphous silica, the spinel-like phase, and nuclei of mullite are produced.…”

Aluminum-rich belite sulfoaluminate cements: Clinkering and early age hydration

“…The strong endotherm at ~800ºC (point 3), with an associated weight loss of 27.2 wt%, is due to the decomposition of CaCO 3 , (the theoretical value was 27.7 wt%). The sharp exothermic peak at ~940ºC (point 4) corresponds to the coordination change of aluminum in meta-kaolin (amorphous) during its transformation into a spinel-like transient phase prior to the formation of mullite [60,61].…”