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Because calcined alpha alumina (α-Al2O3) inherits the morphology characteristic of milled precursors, it is expected that the α-Al2O3 morphology could be improved by controlling the precursor morphology through the use of different milling processes. The microstructure evolution of the boehmite precursor under different milling treatments (planetary ball milling [PBM] and high-energy ball milling [HEBM]) and its influence on the microstructure of as-synthesized α-Al2O3 were investigated. The experimental results indicate that HEBM has a stronger modification effect in crystallinity, particle size and dispersibility of the boehmite precursor than PBM, which is of great importance to inhibit the formation of the typical worm-like structure of α-Al2O3. The microstructure of α-Al2O3 was further improved by the introduction of NH4BF4, NH4F and NH4Cl as additives. In particular, polygon-like α-Al2O3 particles with a size of 0.5 μm and a good dispersibility were prepared by calcination of the precursor with 30 h of HEBM and 20 wt.% NH4BF4.

Herein, nano-sized α-Al2O3 powders were successfully synthesized at low temperatures via unidirectional ball milling of Al(OH)3 and addition of an AlF3 additive. The effects of the rotational direction in planetary ball milling on the α-phase transformation and morphology of α-Al2O3 were studied. Unidirectional rotation was observed to be significantly more helpful than bidirectional rotation with regard to improving the α-phase transformation and microstructure of α-Al2O3. Further, α-Al2O3 was obtained at a lower temperature (800 °C) by employing unidirectional ball milling; this was because gibbsite agglomerates (20–100 μm) could be refined into micro-sized curled flocs with a high specific surface area (124.95 m2 ∙ g−1) by performing unidirectional ball milling for 9 h. The addition of 1 wt% AlF3 accelerated the gas-phase mass transfer, which could effectively promote α-phase transformation and decrease the temperature at which α-Al2O3 was obtained. In comparison with the pyramidal α-Al2O3 polyhedron that exhibited a size range of 0.2–2 μm, homogeneous α-Al2O3 polyhedron with the size of approximately 100 nm was prepared at the temperature range of 910–940 °C via unidirectional ball milling.

As water-soluble calcium chloride (CaCl2) could be uniformly distributed in the castable compared with commercial CAC particles (25–55 μm), the effect of CaCl2 addition (0–2 wt%) on the distribution of CA6 and the properties of the alumina-based castables (especially thermal shock resistance and permanent linear change) after calcination at 1450 °C were investigated. It was found that the alumina-based castable with 1.5 wt% CaCl2 had the minimum PLC value of 0.18% and the maximum value of residual CMOR ratio (25%). The castable matrix fired at 800–1450 °C was examined by XRD to identify the phase evolution of castable with the addition of CaCl2. The 1450 °C fired castable matrix without and with 1.5 wt% CaCl2 additive was analyzed by SEM to determine the morphology evolution of CA6. The improved volume stability and enhanced thermal-shock resistance of the alumina-based castables containing CaCl2 additive has been illustrated through the formation of CA6 with smaller size and more uniform distribution.

Hexagonal α-Al2O3 platelets with about 1-μm diameter and 0.2-μm thickness are prepared by the calcination of a milled Al(OH)3 precursor with NH4F and NH4Cl additives. The influences of the ball milling treatment and the composite additive on the microstructure of α-Al2O3 micro-powders are studied. The results indicate that the particle size distribution of α-Al2O3 is highly dependent on the aggregate size of the Al(OH)3 precursor and that the morphology of α-Al2O3 can be significantly favored by the addition of 5 wt.% NH4F and 5 wt.% NH4Cl as additives. The growth velocity of the (0001) plane is significantly reduced because of the accelerated growth rate of other crystal faces caused by the increase of gas phase mass transfer. The combined effect of composite additives and precursor ball milling treatment leads to the generation of well-developed α-Al2O3 platelets with uniform size distribution.

The change in demoulding strength of colloidal silica-bonded castables with and without magnesia is investigated with emphasis on the relationship between the demoulding strength and chemical bond changes. It was confirmed that the demoulding strength was raised with the presence of magnesia in colloidal silica-bonded castables because of the increased chemical bonding between the sol particles. The X-ray photoelectron spectroscopy (XPS) and the Fourier transformation infrared spectroscopy (FTIR) results indicate the formation of new Si–O–Mg chemical bond from the decreased O 1s and Si 2p binding energy, and the appearance of weak vibration peaks at 668 and 419 cm−1 in the spectrum of colloidal silica with the addition of MgO after curing at 30°C for 24 hours. The reaction between colloidal silica and magnesia could promote the formation of –Si–O–Mg–O–Si–bonds, which is the primary reason for the demoulding strength improvement.

The distribution and size of in-situ CA6 is related to CAC particle size in CAC-bonded corundum-based castables and would influence the properties of the castables. In order to investigate the influence of CAC particle size on the properties of CAC-bonded castables, CAC was milled for 2 h to reduce particle sizes. The properties of CAC-bonded castables with different CAC particle sizes were investigated in terms of demoulding strength after curing at 30 °C, drying strength after drying at 110 °C and, in particular, volumetric stability and thermal shock resistance after firing at 1450 °C, respectively. The results indicate the volumetric stability and thermal shock resistance of the CAC-bonded castables were improved by replacing unground CAC (D50 = 12.39 μm) with ground CAC (D50 = 1.36 μm) because the incorporation of ground CAC produced smaller-sized CA6 with more uniform distribution after firing at 1450 °C.

In comparison with the typically worm-like α-Al2O3 powders formed from an unground Al(OH)3 precursor calcined at 1450°C, spherical α-Al2O3 powders with ~1 μm in size were prepared by the calcination of a ground Al(OH)3 precursor with 5 wt.% [NH4]+BF4 − under the same calcination conditions. The influence of a high-energy ball milling pretreatment as well as of the additives on the morphological evolution of α-Al2O3 powders was studied using the commercial precursor Al(OH)3 as raw material. The results indicate that the morphology of α-Al2O3 powders is closely related to the morphology of the Al(OH)3 precursor and to the introduction of different additives. The refinement of the Al(OH)3 precursor in aggregate size and of the primary crystal size by high-energy ball milling has effectively suppressed the neck growth of α-Al2O3 grains. In contrast to the findings made previously with the introduction of 5 wt.% [NH4]+Cl−, the morphology of the α-Al2O3 particles could be significantly improved from a ground Al(OH)3 precursor with the addition of 5 wt.% [NH4]+BF4 −, which resulted in the formation of spherical α-Al2O3 powders with 1 μm size at 1450°C.