Formation of Mullite and Spinel Phases from SiO₂−Al₂O₃ Gels Prepared by a Spray Pyrolysis Technique. A ²⁹Si and ²⁷Al MAS NMR Study

Abstract

Thermal behavior of alumino-silicate gels prepared by spray pyrolysis, with Al/Si ratios between 1.5 and 18, has been analyzed by differential thermal analysis (DTA), X-ray diffraction, transmission electron microscopy, IR, and 29Si and 27Al magic angle spinning NMR techniques. In samples heated at 800 °C, tetra-, penta-, and octahedral aluminum were detected. In samples heated at 900 °C, Al-rich mullite (Al/Si = 4−6) or Si-spinel nuclei (Al/Si = 10−18) are formed after the partial segregation of SiO2 from alumino-silicate gels. The strong rearrangement of aluminum atoms, with formation of tetra- and octahedral Al at the expense of pentahedral Al, is responsible for the exothermic peak detected at ∼980 °C in DTA curves. Thermal treatments between 1000 and 1300 °C produce the progressive incorporation of SiO2 into newly formed mullite nuclei. In Al-rich samples, with nominal Al/Si ratios between 8 and 18, the formation of mullite nuclei is only detected above 1100 °C. In this case, spinel and mullite phases coexist in the temperature range 1100−1250 °C.

Mineralogy and Microstructure of Hydrated Phases During the Pozzolanic Reaction in the Sanitary Ware Waste/Ca(OH)2 System

“…Earlier studies reported that (mullitetype) Al-rich aluminosilicates exhibit more intense octahedral than tetrahedral Al resonance, whereas in high Si samples, more tetrahedral than octahedral Al is present. 41,46 These findings are consistent with the XRF results (Table I), in which the clay-based waste was observed to be higher in Si than Al.…”

“…The fact that Al(VI) generated a much less intense signal than Al(IV) is an indication that the SW waste contained a large proportion of amorphous aluminosilicates higher in Si than in Al. Earlier studies reported that (mullite‐type) Al‐rich aluminosilicates exhibit more intense octahedral than tetrahedral Al resonance, whereas in high Si samples, more tetrahedral than octahedral Al is present . These findings are consistent with the XRF results (Table ), in which the clay‐based waste was observed to be higher in Si than Al.…”

“…Of the four narrow signals characteristic of mullite, whose theoretical chemical shifts (−82, −87, −90, −94 ppm) are reported in the literature to vary slightly depending on the conditions of synthesis and the Si/Al ratio, only three were observed on the spectrum for clay‐based waste. The signal at −89 ppm was attributed to one SiO 4 surrounded by three tetrahedral aluminate (Q 3 (3Al)), the signal at −93 ppm to one SiO 4 tetrahedron surrounded by another one plus two AlO 4 tetrahedra (Q 3 (2Al) units), and lastly the signal at −96 ppm, to one tetrahedral silicate surrounded by two SiO 4 tetrahedra and one AlO 4 (Q 3 (1Al)) …”

“…33,37 The signal at À104 ppm, in turn, was attributed to Q 4 (1Al) units, i.e., one SiO 4 tetrahedron surrounded by three tetrahedral silicates and one tetrahedral alumina, 38 for the absence on the FTIR spectrum of a band at 950 cm À1 , characteristic of the stretching vibrations in Si-OH groups, 39 ruled out the presence of Q 3 (0Al) units. 40 Of the four narrow signals characteristic of mullite, whose theoretical chemical shifts (À82, À87, À90, À94 ppm) are reported in the literature to vary slightly depending on the conditions of synthesis and the Si/Al ratio, 35,41,42 only three were observed on the spectrum for clay-based waste. The signal at À89 ppm was attributed to one SiO 4 surrounded by three tetrahedral aluminate (Q 3 (3Al)), the signal at À93 ppm to one SiO 4 tetrahedron surrounded by another one plus two AlO 4 tetrahedra (Q 3 (2Al) units), and lastly the signal at À96 ppm, to one tetrahedral silicate surrounded by two SiO 4 tetrahedra and one AlO 4 (Q 3 (1Al)).…”

Microstructural and Mechanical Properties of Alkali Activated Colombian Raw Materials

“…27 Al MAS-NMR spectra from the activated fly ash displayed four signals centred at +57.2, +49.4, +0.9 and −7.5 ppm. The former two signals were associated to Al T in the N-A-S-H gel and in the anhydrous fly ash, respectively, while the latter two signals to octrahedral aluminum in mullite [54]. The analysis of the activated mixtures confirmed that the amount of the anhydrous slag declined with an increase of the percentage of the fly ash, while the amount of the anhydrous fly ash and mullite grew, see Table 4.…”