A Review on the Kaolinite-Mullite Transformation
TL;DR: In this article, the transformation of kaolinite to mullite is reviewed and the thermal effects observed during the transformation and the phases which accompany these effects are discussed in the light of the existing concepts.
Abstract: The transformation of kaolinite to mullite is reviewed. The thermal effects observed during the transformation and the phases which accompany these effects are discussed in the light of the existing concepts. The chemical, thermal, X-ray, electron-optical, infrared, orientational and crystallographic evidences are advanced to reveal the nature of the intermediate phases and to establish the course of the transformation reaction. Strong evidences exist in favour of Al-Si spinel and γ-Al2O3 as the precursor of mullite formation. The formation of γ-Al2O3 phase finds support in the recent ir, r.e.d. and X-ray fluorescence studies whereas the formation of Al-Si spinel phase is necessary to maintain the structural continuity of the phases from kaolinite to mullite as shown by the single crystal X-ray studies. The transformation follows the sequence:
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01 Oct 2021
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253 citations
TL;DR: In this paper, a study of the equilibrium relations of mixtures of pure alumina and silica at high temperatures is presented, expressed concisely in the form of an equilibrium, diagram and their bearing on ceramic problems is discussed.
Abstract: This paper deals with a study of the equilibrium relations of mixtures of pure alumina and silica at high temperatures. The results are expressed concisely in the form of an equilibrium, diagram and their bearing on ceramic problems is discussed.
The principal feature of the diagram is the absence of the compound Al2O3.SiO2, the only compound being 3Al2O3.2SiO2. Crystals of this latter compound occur in all alumina-silica refractories. The optical properties of these crystals have been determined and are compared with those of sillimanite, Al2O3.SiO2, which has hitherto been regarded as the crystalline compound occurring in refractories and clay bodies in general.
The behavior of natural sillimanite on heating is discussed.
213 citations
TL;DR: In this paper, the phases in the kaolinite-mullite reaction sequence were reexamined by ir absorption spectrophotometry, with particular attention paid to the controversial intermediate Al-containing phases.
Abstract: The phases in the kaolinite-mullite reaction sequence were reexamined by ir absorption spectrophotometry. Particular attention was paid to the controversial intermediate Al-containing phases. Amorphous materials were leached from fired kaolinite samples with NaOH to help identify crystalline phases. Metakaolinite partially decomposes, releasing amorphous γ-Al2O3 and SiO2, before the “950°C” exothermic reaction in which metakaolinite is completely decomposed. The resulting spinel-type phase, which is associated with amorphous SiO2 and some poorly crystalline “primary” mullite, is γ-Al203 (crystalline) rather than an Al-Si spinel. There is some evidence, however, that a fraction of the γ-Al2O3, may be an Al-Si spinel. At ≥1100°C secondary mullite therefore forms primarily from the γ-Al2O3/amorphous SiO2 reaction and the recrystallization of primary mullite, whereas excess amorphous SiO2 eventually crystallizes as cristobalite.
144 citations
TL;DR: In this article, the nature of the intermediate spinel-type phase is discussed in relation to the crystal chemistry of spinels, and the X-ray data show variation in the mullite parameters between 1200° and 1400°C; the composition probably approximates 3A12O3·2SiO2.
Abstract: X-ray diffraction data for the high-temperature phases show that a spinel-type structure develops with marked orientation at about 925°C. This phase is considered to be an aluminum silicon spinel with vacant cation sites. Mullite is thought to be formed by the decomposition of the spinel. Silica is eliminated progressively as metakaolin transforms to the spinel phase and thence to mullite. The X-ray data show variation in the mullite parameters between 1200° and 1400°C.; at 1400°C., the composition probably approximates 3A12O3·2SiO2. The nature of the intermediate spinel-type phase is discussed in relation to the crystal chemistry of spinels.
140 citations
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