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Significant interest in waste-cement recycling has been stimulated because of the high contents of calcium and silicon in waste cement. The reactivity of calcium and silicon in the raw material is one of the important factors for the hydrothermal synthesis of xonotlite. Therefore, the effect of the reactivity of calcium and silicon in the waste cement on the hydrothermal synthesis of xonotlite was studied in this paper. Portland cement that was hydrated for 6 months, with the aim of simulating the waste cement, was used for the first time as the calcium and silicon source in the hydrothermal synthesis of xonotlite. As calcination would raise the reactivity of the hydrated cement, the effect of calcination of the Portland hydrated cement on the hydrothermal synthesis of xonotlite was investigated. The hydrated cement was calcined at 900 °C, and the hydrothermal synthesis was carried out at 220 °C for different times. The phases of the hydrothermal products were analyzed by XRD and TG-DSC, and it was noted that the calcination of hydrated cement affected the formation rate of xonotlite. The content of xonotlite increased from 18% (synthesized with hydrated cement without calcination) to 74% (synthesized from hydrated cement with calcination at 900 °C) during a reaction time of 24 h. Furthermore, the micromorphologies of xonotlite using calcined and hydrated cement were compared and discussed from the perspective of the reactivity of the starting materials.

In order to verify the thermal expansion behaviour of andalusite aggregates with different mullitization degrees and its influence on the properties (thermal shock resistance and especially volume stability) of mullite-corundum refractories, the properties of mullite-corundum refractories with uncalcined and different pre-calcined andalusite aggregates (1–3 mm) are investigated in terms of permanent liner change, thermal shock resistance, cold modulus of rupture, modulus of elasticity and apparent porosity. The experimental results show the volume stability of samples increases when the pre-calcining temperature of andalusite is improved (1300–1500 °C), as less residual andalusite in the pre-calcined aggregates leads to smaller volume expansion in the refractory samples during firing at high temperatures. Meanwhile, mullite-corundum refractories with andalusite aggregates (1–3 mm) pre-calcined at higher temperature have better cold modulus of rupture, because the increase of pre-calcining temperature could promote mullization degree and thereby bring down the length of microcracks and also enhance the densification of these as-prepared refractories. Moreover, mullite-corundum refractories with uncalcined and pre-calcined andalusite aggregates present good thermal shock resistance; the corresponding residual strength ratio of mullite-corundum refractories containing 1500 °C or 1600 °C pre-calcined andalusite aggregates is still higher than 90%. Overall, the volume stability and cold modulus of rupture of the mullite-corundum refractories could be significantly enhanced through the incorporation of 1500 °C pre-calcined andalusite aggregates (1–3 mm), and all mullite-corundum refractories with uncalcined and pre-calcined andalusite aggregates (1–3 mm) exhibit good thermal shock resistance.

Our work previously found that the addition of andalusite aggregate enhanced the oxidation resistance of Al2O3–SiC–C castable. Therefore, it is speculated that the addition of andalusite can reduce the dosage of anti-oxidant. In this work, decreasing amounts of anti-oxidant Si (1.5 wt% - 0 wt%) are employed to further reveal the influence of andalusite aggregate on the oxidation resistance of Al2O3–SiC–C castable. The oxidation resistance of the specimens is evaluated by the oxidation index after the thermal shock resistance test. The phases composition of the castables were characterized via XRD, and its microstructures were tested via SEM. The Al2O3–SiC–C castables containing andalusite aggregate and reduced amount of antioxidant Si demonstrate better oxidation resistance as the SiO2-rich liquid glass from andalusite transformation enters the pores under capillary force, blocks the pore channels and thus prevents the inward diffusion of oxygen. Additionally, partial silica-rich liquid glass produced by the converted andalusite particle would have a reaction with Al2O3 in the castables to form secondary mullite, contributing to a denser structure, which is also conducive to blocking oxygen entry and consequently improving the oxidation resistance of the castables.

In this work, the effect of Zn(OH)2 on properties of corundum based castables bonded with calcium aluminate cement (CAC) was investigated. The phase composition and microstructure of castable matrixes containing Zn(OH)2 after firing 800 °C, 1100 °C and 1550 °C were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectra, respectively. The results indicate that a small amount of Zn(OH)2 can dramatically improve the medium temperature strength of castables because the generation of zinc aluminate spinel increases the ceramic bonding of castables. In addition, the addition of Zn(OH)2 also improves the volume stability of CAC-bonded castables due to the enhanced formation of pores from Zn(OH)2 decomposition in castables.

Mullite (3Al₂O₃·2SiO₂) is commonly used as the basic component to prepare refractory saggars for the calcination of Li-ion battery (LIB) cathode materials. Coal gangue, basically treated as aluminosilicate solid waste/tailings, could be used as the starting material to synthesize mullite at elevated temperatures. However, high operation energy and additional high-quality alumina sources are required in this process. In addition, mullite is prone to corrosion by LIB cathode materials, which remains a major challenge. The present work reports the design and fabrication of refractory powders with a core–shell structure comprising calcined coal gangue (CCG) and a layer of in situ-generated KAlSi₂O₆ (KAS₄) around CCG grains. The obtained CCG–KAS₄ powders with a core–shell structure exhibit superior corrosion resistance against the Li(NiₓCoyMnz)O₂ precursor/material during calcination compared with CCG or mullite. Materials prepared with CCG–KAS₄ powders containing 4.2 and 8.4 wt % KAS₄ possess better thermomechanical properties than those synthesized from mullite powders. This makes coal gangue a good candidate to prepare refractory saggars for the calcination of LIB cathode materials.

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.

In the present work, the effects of the pre-firing temperature of andalusite aggregates (5–3 mm) on the conversion of andalusite as well as the volume stability and oxidation resistance of the Al2O3–SiC–C castables were investigated. The phases of the andalusite aggregates were tested via X-ray diffraction, and their microstructures as well as those of the castables were characterized via scanning electron microscopy. The linear expansion of the castables decreased with increase in the pre-firing temperature of andalusite, as less residual andalusite in the pre-fired aggregates lowered its transformation in the castables during firing at high temperatures. Moreover, the higher amounts of SiO2-rich glass produced by andalusite pre-fired at high temperatures and secondary mullite generated between andalusite and the matrixes were both favorable to the production of castables with denser structure. This prevented the diffusion of O2 into the castables and improved their oxidation resistance.

The formation of CA2 and CA6 in calcium aluminate cement bonded castables lead to a large volume expansion, which probably results in the generation of cracks, thereby increasing slag penetration and decreasing mechanical strength. In this study, the influence of curing time on the dimensional stability of calcium aluminate cement bonded castables is considered to improve the volumetric stability. Phase compositions and morphologies evolution of matrix samples and castables are characterized by XRD and SEM, respectively. The results indicate that the volume stability of castables is greatly improved by increasing curing time from 0.5 days to 6 days after firing at 1600 °C. This can be attributed to two main reasons. First, the improvement in migration of CaO through extending curing time favors more uniform distribution of in situ CA6. Second, more hydrates after dehydration creates more micro-pores, thereby providing space for the generation of in situ CA6.

This study investigates the strength progression of calcium aluminate cement (CAC)-bonded castables with a CAC content ranging from 2 to 16 wt% in the temperatures between 300 and 1000 °C. Focusing on the phase changes and microstructure evolution of the hydrates due to heating, we discovered that castables with higher CAC contents have increased strengths between 300 and 800 °C because the morphology and structure of the dehydrated C3AH6 and AH3 remain similar. At 1000 °C, a higher strength of the 16 wt% CAC castable results from the higher degree of coalescence/sintering between the particles formed from the decomposition products of CAC hydrates. The CAC-bonded castables were prepared using tabular alumina aggregates and α-Al2O3 micro-powder as starting materials, and CAC as the binder. The phase compositions and the microstructures of the castable matrices were characterised by X-ray powder diffraction and scanning electron microscopy, respectively.

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.

There have been reports on the strength decrease of CAC-bonded corundum-based castables without silica fume due to dehydration and decomposition of CAC hydrates when temperature is raised from 110 °C to 900 °C. It is found in this work that there are two steps of strength decrease of the CAC-bonded castables at temperatures of 300 °C and 700 °C respectively. In this paper, corundum-based refractory castables were prepared using tabular alumina and α-Al2O3 micro-powder as starting materials and calcium aluminate cement (CAC) as binder. The phase composition and microstructure of the castable matrix after heating at temperature between 110 °C and 1200 °C were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The strength decreases of the castables at temperatures of 300 °C and, in particular, 700 °C is illustrated using the phase and microstructure evolution of CAC-bonded castable matrixes after heating treatment at different temperatures. For the first time, investigated are the strength increase of the CAC-bonded castables and its causes at temperatures between 900 °C and 1000 °C with emphasis on the structure evolution of the hydrates examined by SEM using high magnification.

Al2O3–MgAl2O4 castables are prepared using tabular alumina, fused spinel, α-Al2O3 micro-powder, calcium aluminate cement and nano-Al2O3 particles as starting materials. The effects of trace nano-Al2O3 particles (0.2–0.8 wt%) on the thermal shock resistance (TSR) and microstructure of Al2O3–MgAl2O4 castable are investigated. The results show that trace addition of nano-Al2O3 particles contributes to CA6 crystal development and microstructure optimisation. The platelet CA6 crystals form an interlocking network structure creating a toughening effect, which leads to a significant improvement in TSR. In addition, CA6 formation accompanied by volumetric expansion generates many microcracks, which are beneficial for relieving thermal stress during the thermal shock process and enhancing the TSR of the Al2O3–MgAl2O4 castable.

This work investigates the effect of andalusite with different particle sizes (8-5 mm, 5-3 mm, 3-1 mm) on the properties of Al2O3–SiC–C castables. It has been found that the addition of andalusite aggregates not only improves high-temperature strength and thermal shock resistance of the materials, but additionally favors the oxidation resistance of the castables. The phases and microstructure of the castables were characterized by X-ray diffraction and scanning electron microscopy, respectively. The microstructural observation suggests that the enhanced oxidation resistance of Al2O3–SiC–C castables with andalusite aggregates addition results from the obstruction of the pore channel and improved densification of the castables caused by andalusite mullitization, which prevent oxygen diffusion inside the castables.

Calcium aluminate cement (CAC) was ground for 1 and 2 h to investigate the impact of mechanical grinding on CAC hydration at 30 °C and CAC-bonded castable strength. Phase composition and microstructure of unground and ground cements after hydration for predetermined times and terminated by the freeze-vacuum drying were compared. The results indicate that the particle size and particle size distribution of CAC were reduced and narrowed, respectively by grinding, thereby favoring the hydration rate and the conversation of C2AH8 to C3AH6. Then enhanced cement hydration also increases the strengths of castables bonded with milled CAC after drying and firing.

The conventional refractory sagger materials, e.g., mullite, cordierite, and magnesium aluminum spinel, employed to calcine Li-ion battery (LIB) cathode materials, are easily attacked by the LIB material and thus have short lifetimes. To improve the corrosion resistances of refractory saggers to LIB materials, andalusite–KAlSi2O6 (KAS4) materials with different contents of in-situ KAS4 were synthesized using andalusite and KAS4 precursor calcined at 1300 °C. The microstructure evolution, phase development, corrosion resistances to Li(NixCoyMnz)O2 (LNCM) cathode materials and thermomechanical properties of the designed andalusite-KAS4 materials were characterized. The results show that the corrosion resistances of the prepared andalusite–KAS4 material were superior to those of the LNCM materials owing to the in–situ KAS4 wrapping andalusite grains. The prepared andalusite–corundum–KAS4 materials also exhibited improved strengths and appropriate thermal shock resistances compared with mullite-based refractories.

Guided by the corrosion mechanism in contact with Li(NixCoyMnz)O2 materials, a method was developed by wrapping refractory grains with a KAlSi2O6 (KAS4) layer for corrosion protection during the calcination of Li-ion battery (LIB) cathode materials. Preformed or in-situ KAS4 surrounds around refractory grains as a corrosion barrier by physically separating their contact with LIB materials. Because KAS4 did not react (or reacted little) with LIB materials, materials having in-situ or preformed KAS4 exhibit superior corrosion resistance, and in-situ KAS4 improves the corrosion resistance more. Additionally, the in-situ or preformed KAS4 also improves the thermomechanical properties of the prepared materials.

The distribution of in situ CA6 depends on the amount of the residual calcium aluminate cement (CAC) anhydrous phase and the distribution of CAC hydrates in CAC-bonded alumina-based castables and can influence the properties (especially the volume stability) of the castables. Because the CAC hydration process is strongly related to the curing temperature, CAC-bonded alumina-based castables were cured at 5 °C, 10 °C, 25 °C, 40 °C, and 50 °C to investigate the influence of curing temperature on the volume stability of CAC-bonded alumina-based castables. The castables after curing, drying, and firing were examined by SEM to determine the distribution of the residual CAC anhydrous phase and CA6 grains. Phase compositions of the castable matrixes after drying at 110 °C and firing at 1450–1600 °C were analyzed by X-ray diffraction to identify the hydration degree and generation of CA6. The contribution of curing temperature to the volumetric stability of the castables is discussed in terms of hydration products after curing and CA6 distribution in the castables after firing at 1600 °C.

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.

Additions of andalusite aggregates (19 wt%) were shown in previous literature to enhance the antioxidation of Al2O3-SiC-C (ASC) castables. This work aims to investigate whether micronized andalusite has a greater influence on antioxidation improvement than andalusite aggregates. Various low contents (5 wt% and below) of micronized andalusite (≤5 μm) were introduced as a substitute for brown fused alumina in the matrix of ASC castables. The antioxidation of castable specimens was estimated by the oxidized area ratio on the fracture surface after a thermal shock test. The microstructure and phases of micronized andalusite and the castable specimens were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The results suggest that the antioxidation effects of ASC castables with a low addition of micronized andalusite are effectively enhanced. The heat-induced transformation of andalusite produces SiO2-rich glass, favoring the sintering of the castable matrix and impeding oxygen diffusion into the castable’s interior. Therefore, the castable antioxidation is enhanced without deteriorating the hot modulus of rupture.

To assess the feasibility of potassium aluminosilicates (KAlSiO4, KAS2 and KAlSi2O6, KAS4) as the refractory sagger material for the calcination of Li-ion battery cathode materials, the interaction behavior between KAS2 (KAS4) and the Li(NixCoyMnz)O2 (LNCM) material was investigated at temperatures ranging from 800 to 1100 °C. The phase composition and microstructure of calcined KAS2 (KAS4)-LNCM precursor mixtures were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results indicate that potassium aluminosilicates exhibit superior resistance to corrosion by LNCM materials compared with mullite and MgO-based refractories, and rarely react with LNCM materials during calcination.

We found in our research that andalusite aggregate fired in a reducing atmosphere exhibits a lower mullitization rate than that fired in an air atmosphere. For investigating the effect of atmosphere on the transformation of andalusite and the properties of andalusite-containing refractories, andalusite powder (≤0.074 mm) and refractories containing andalusite aggregate (3–1 mm) were fired in air and carbon embedding, respectively. The phases and microstructure of the andalusite fired in both atmospheres were characterized by X-ray diffraction and scanning electron microscopy, respectively. The correlations of the properties of the andalusite-bearing refractories with the firing atmospheres were investigated in terms of volume stability, mechanical strength, and thermal shock resistance. The difference in the properties of the refractories was discussed with respect to the varied transformation rates of andalusite, and in terms of the different viscosities of the silica-rich glass caused by the different atmospheres.

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.

Micro-sized MgCO3 and Al(OH)3 were used as inorganic porogenic agents for increasing porosity in alumina bubble lightweight castables. The influence of 0–4 wt% micro-sized MgCO3 and 0–8 wt% micro-sized Al(OH)3 additions on the porosity, thermal conductivity and strength of castables after firing was investigated. The results indicate that porosities in castables containing micro-sized Al(OH)3 or MgCO3 after heat-treatment were raised by augmenting micro-sized Al(OH)3 or MgCO3 contents. It is found that MgCO3 favors the porosity formation more than Al(OH)3 in castables because of the different formation behavior of calcium aluminates and MgAl2O4 spinel. The dependence of properties of castables on the micro-sized Al(OH) or MgCO3 addition was discussed with respect to the pore evolution.

To assess the viability of micro-sized hydromagnesite as a precursor in refractory castables bonded with calcium aluminate cement, the volumetric stability and thermo-mechanical properties of high-alumina castables containing different micro-sized hydromagnesite amounts (0–1.6 wt%) after firing at 1550 °C were investigated. Phase composition and microstructure evolution in fired castable matrices with different micro-sized hydromagnesite contents were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Dimensional stability, hot modulus of rupture and thermal shock resistance of castables were improved by adding micro-sized hydromagnesite. The microstructure evolution in castable matrix added with micro-sized hydromagnesite was discussed to understand the mechanism of enhanced volume stability and thermo-mechanical properties.

Used mullite-based saggars from a Li(NixCoyMnz)O2 (LNCM) materials-making plant were collected to characterize the degradation occurring during application. Based on the post-mortem analysis, laboratory scale experiments were designed using mixtures of mullite powders and commercial Li-ion battery cathode precursors. The mixtures were calcined at temperatures ranging from 800 °C to 1100 °C, and characterized with X-ray diffraction and scanning electron microscopy. The influence of temperature on the interactions between mullite and LNCM materials was determined. The combination of the post-mortem study and the laboratory scale tests allowed a more comprehensive understanding of the evolution of the saggar microstructure during degradation.

In this work, mullite-Al2TiO5 composites were fabricated by natural andalusite with TiO2 as an additive. The densification characteristic, phase composition and mullitization process of andalusite with TiO2 addition was investigated by the Archimedes’ principle, dilatometry, X-ray diffraction and scanning electron microscopy (SEM-EDS) techniques. The results showed that the incorporation of TiO2 not only enhanced the thermal stability of in-situ Al2TiO5 in the silica liquid yielded from the mullitization of andalusite, but also accelerated andalusite decomposition and retarded mullite formation, which facilitated the sintering and densification of mullite-Al2TiO5 composites.

Calcium aluminate cements (CAC) with 0.21 wt% B2O3 and without B2O3 were used as binders of corundum-based castables. The properties of the castables with and without B2O3 after heat treatment at 110 °C, 1,100 °C and 1,450 °C were investigated, with emphasis on studying the effect of B2O3 in CAC on the volume stability and high temperature strength of the castables. It is found that a very small amount (about 0.01 wt%) of B2O3 introduced by the cement alleviates the expansion of the castables after firing at 1,450 °C and decreases the high-temperature strength of castables as the presence of B2O3 should generate liquid phase.

Additions of andalusite aggregates (19 wt%) were shown in previous literature to enhance the antioxidation of Al2O3-SiC-C (ASC) castables. This work aims to investigate whether micronized andalusite has a greater influence on antioxidation improvement than andalusite aggregates. Various low contents (5 wt% and below) of micronized andalusite (≤5 μm) were introduced as a substitute for brown fused alumina in the matrix of ASC castables. The antioxidation of castable specimens was estimated by the oxidized area ratio on the fracture surface after a thermal shock test. The microstructure and phases of micronized andalusite and the castable specimens were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The results suggest that the antioxidation effects of ASC castables with a low addition of micronized andalusite are effectively enhanced. The heat-induced transformation of andalusite produces SiO2-rich glass, favoring the sintering of the castable matrix and impeding oxygen diffusion into the castable’s interior. Therefore, the castable antioxidation is enhanced without deteriorating the hot modulus of rupture.

The influence of impurities such as SiO2 and F2O3 on the hydration behavior of calcium aluminate cement (CAC) was studied in this work. The hydration of the cement was characterized by the exothermic profile of cement pastes, and the dissolution-precipitation rate of the cement aqueous suspension was evaluated by electrical conductivity measurement. In order to analyze the resulting hydration products, the cement hydration was terminated via the freeze-vacuum drying method at the designated times. Then the phase development and microstructure evolution during cement hydration were investigated by XRD and SEM, respectively. The results indicate that the hydration rate is faster when the cement contains a higher amount of impurities. It is proposed that the higher hydration rate of the cement is probably related to the lattice imperfection of the main calcium aluminates in CAC.

In this work, the effect of different micro-sized alumina powders on the hydration products of calcium aluminate cement (CAC) during hydration at 40 °C is studied. The cement hydration at the designated times is terminated by the freeze-vacuum method. The phase development and microstructure evolution during the cement hydration are investigated by XRD and DSC, and SEM, respectively. It is found that 3CaO·Al 2 O 3 ·6H 2 O (C 3 AH 6 ) is the dominant product of the pure CAC after hydration at 40 °C for 3.5 h. But 2CaO·Al 2 O 3 ·8H 2 O (C 2 AH 8 ) is the dominant hydrate and C 3 AH 6 is not found in the mixtures of CAC and micro-sized alumina powder under the same condition. The results indicate that the addition of alumina powders promotes the formation of C 2 AH 8 and retards the conversion of C 2 AH 8 to the C 3 AH 6 phase. Moreover, such phase development with alumina addition is discussed.

In this paper, MoSi2/WSi2 composite powders with different phase compositions were fabricated via a self-propagating high-temperature synthesis (SHS) method. The influence of initial raw material compositions on the formation and the phase evolution of the MoSi2/WSi2 composite powders were investigated. Thermodynamic analysis indicated that the incorporation of W in the starting materials decreased the adiabatic temperature during the SHS reaction, and self-sustainable reaction can be hindered when the W content reached up to 40%. For samples with raw material composition of (\(1-x){\rm Mo}{\cdot} {\rm xW}{\cdot} {\rm 2Si}\) (X = 0.10, 0.20, 0.30, 0.40), composite MoSi2/WSi2 powders can be prepared, which were confirmed by the existence of two fitted peaks that can be assigned to tetragonal MoSi2 (103) and WSi2 (103) crystallographic planes in the XRD fine scan patterns. SEM images also confirmed the existence of two maxima in particle size distributions for samples with high W addition contents. The formation of MoSi2/WSi2 composite powders and the evolution of composite phases reported in this paper may provide an effective strategy for the synthesis of silicide based composite materials.

Calcium aluminate cement (CAC) is extensively applied as castable binder, and the drying process is a crucial step before castables come into service. Limited researches have focused on the hydration and phase transformation of CAC in the castables during the evaporation stage of drying process, in which the free water is gradually discharged. This work involves the investigation of the strength development of CAC-bonded alumina castables and phase evolution of the hydrates during drying at 110 °C. After curing for 24 h at 30 °C, the CAC-bonded alumina castables and matrices were dried at 110 °C for different periods of time to explore the effect of drying time on the mechanical property of CAC-bonded castables and on the phase assemblages and microstructure of the castable matrices. The results demonstrate that the initial drying period prompts the rapid conversion from metastable C2AH8 to stable C3AH6 as well as the continued hydration of the residual CA and CA2. The drying strength of the castables is significantly enhanced in comparison with demolding strength.

This work aims to investigate the effect of impurity content of andalusite on the oxidation resistance of Al2O3–SiC–C castables with andalusite as part of the aggregates. Two different grades of andalusite were employed in the Al2O3–SiC–C castables, and their oxidation resistance performances are evaluated. The results show that after firing at 1450 °C × 3 h and 10 times of thermal shock at 950°C-air cooling, the castable containing low-grade andalusite exhibits better oxidation resistance property than that containing high-grade andalusite. The phase composition, microstructure and physical performance analysis indicate that the lower grade andalusite contains more impurities, which would facilitate the liquid phase formation at high temperature. The more generated silica-rich liquid phase would fill the pores in the matrix and have a reaction with the alumina micro-powder in the matrix to form more secondary mullite. Consequently, the addition of the lower grade of andalusite would block the entry of O2 and thus improve the oxidation resistance of the castable to a higher degree.

Brucite-fumed silica and hydromagnesite-fumed silica mixtures were used to investigate the influence of MgO precursors on mechanically activated forsterite synthesis. The changes in morphology, chemical bond and phase composition of the ground and calcined mixtures were examined with scanning electron microscopy (SEM), Si 2p X-ray photoelectron spectroscopy (XPS) and 29 Si magic angle spinning nuclear magnetic resonance (MAS-NMR), and X-ray diffraction (XRD), respectively. The XPS and MAS-NMR analyses show that high-energy milling generates more Mg–O–Si chemical bonds in the brucite-fumed silica mixture than in the hydromagnesite-fumed silica sample. This is because brucite has a higher concentration of Mg–OH bonds than hydromagnesite. However, single-phase forsterite forms at a higher temperature of 1000 °C in the milled brucite-fumed silica mixture than that of 800 °C in the ground hydromagnesite-fumed silica sample after the same grinding. The different forsterite completion temperature is probably due to the longer Mg 2+ and Si 4+ diffusion distance of over 500 nm in the former milled mixture than that of less than 300 nm in the latter ground sample.

For preparing wurtzite and cubic boron nitride (wBN, cBN) under moderate conditions, here we developed a solvothermal hot-press method, by which the phase transformation of sp3-bonded BN in boron nitride (BN) nanoparticles was realized at quite low temperature and pressure. The results reveal that the transformation of aBN/rBN→cBN is highly dependent on the temperature, pressure and solvent promoting effect; and chlorobenzene seems to have a stronger effect on facilitating the phase transformation of sp3-bonded BN as compared to pyridine and aniline. Besides, the decrease in particle size and crystallinity as well as the existence of structural defects plays great roles on the phase transformations of sp3-bonded BN. On the basis of analyzing the experimental results, we proposed a simple model used for explaining the phase transformation process.

Mg-O-Si chemical bond formation in a light burned magnesia (MgO) and fumed silica (SiO2) mixture during mechanical activation was investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), magic angle spinning nuclear magnetic resonance (MAS-NMR), and X-ray photoelectron spectroscopy (XPS). Crystallinity and intrinsic structure changes of the starting mixture during high-energy milling were examined by XRD. The formation of new Mg-O-Si chemical bonds of the ground mixture was illustrated by the incorporation of Mg2+ in Si-O-Si linkages, the appearance of new resonance in the 29Si NMR spectrum and the decrease of the Si 2p binding energy. The formation of Mg-O-Si chemical bonds created during grinding partly contributed to the lowered temperature of complete forsterite formation from 1400 to 1100°C.