The Isothermal and Non-Isothermal Crystallization Kinetics La2O3 Doped, Sol-Gel Derived Mullite

The crystallization kinetic parameters of mullite formation from gels doped with La2O3 in 0.99, 1.96 and 2.91 mol% were determined by differential thermal analysis (DTA) under isothermal and non-isothermal conditions. The gels were prepared by sol-gel process from aluminum and lanthanum nitrate precursors and TEOS dissolved in ethanol (molar ratio Al/Si=3:1) Powder X-ray diffraction (XRD) and electron microscopy analysis methods (SEM, EDS) were used to characterize the present phases. Lanthanum forms no separate crystal phases and its incorporation ability in mullite lattice is neglectable, yet by promoting amorphous phase formation lanthanum evidently influences the mullite composition by making it rich in Al2O3. The obtained activation energies for the mullite crystallization process in non-doped sample are higher than 1300 kJmol-1. The increase of doping level brings the activation energies below 1100 kJmol-1 for the sample with highest amount of lanthanum, i.e. with the greatest deviation from stoichiometric mullite composition.

The Internal Structure of a Low-Calcium Fly Ash

ABSTRACTA low calcium fly ash, rich in iron oxide, was subjected to shaking in 1% HF for up to 20 hours, and measurements were made at intervals of the following parameters: total weight loss, composition of the dissolved material, x-ray peak intensities of crystalline components and residual glass, and internal structures present within the fly ash spheres as revealed by the progressive dissolution treatments. The same fly ash was subjected to similar but longer-term treatment with simulated cement pore solution (potassium and sodium hydroxide). The compositions of the magnetically separated fraction and the non-magnetic residue were also separately determined. As a result of these studies, it was suggested that: (a) the time at which all of the potassium is dissolved might be used as an indicator of the completion of glass dissolution; (b) the position of the x-ray band for residual (more resistant ) glass shifts to higher 2Θ angle before the glass is completely dissolved; (c) in this fly ash, as determined by chemical analysis, all of the iron is in the magnetically-separable fraction, a feature that permitted estimation of the overall glass composition; and (d) this fly ash contains certain poorly understood components including silica-rich spheres, small spheres yielding EDXA signals only for iron, and “patches” of silica and alumina bearing material not of mullite composition, but all of which are highly resistant to HF dissolution.