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In this work, a novel and facile technique based on using KCl as space holders, along with partial sintering (at 1900 °C for 30 min), was explored to prepare porous ZrB2–SiC ceramics with controllable pore structure, tunable compressive strength and thermal conductivity. The as-prepared porous ZrB2–SiC samples possess high porosity of 45–67%, low average pore size of 3–7 μm, high compressive strength of 32–106 MPa, and low room temperature thermal conductivity of 13–34 W m−1 K−1. The porosity, pore structure, compressive strength and thermal conductivity of porous ZrB2–SiC ceramics can be tuned simply by changing KCl content and its particle size. The effect of porosity and pore structure on the thermal conductivity of as-prepared porous ZrB2–SiC ceramics was examined and found to be consistent with the classical model for porous materials. The poring mechanism of porous ZrB2–SiC samples via adding pore-forming agent combined with partial sintering was also preliminary illustrated.

To improve the properties of high alumina castables containing calcium aluminate cement (CAC) after firing at elevated temperatures, micro-sized basic zinc carbonate (BZC) was introduced as ZnO-based nano fragments into castables. To account for the influence of BZC on the evolution of castable properties, the phase composition and microstructure of castable matrices were examined with X-ray diffraction and scanning electron microscopy equipped with energy dispersive spectroscopy, respectively. Properties of castables having BZC were compared with those with Zn(OH)2. Results show that strength of castables added with BZC after firing at both medium temperatures and 1550 °C was improved. The volume stability and hot modulus of rupture at 1550 °C of fired castables containing BZC were also enhanced.

This study presents a novel strategy for fabricating high-strength mullite using Al3+/Ce4+-doped SiO2 and γ-Al2O3 powders. Doping of Al3+/Ce4+ ions in SiO2 results in the formation of a large amount of low-viscosity liquid phase in a narrow temperature range, which benefited the particle arrangement (closer) and reduced pores formation, thereby accelerating the mullitization rate and promoting the mullite grains growth. At elevated temperatures, almost all-SiO2 liquid phase were consumed due to a relatively high mullitization rate, and cerianite (CeO2) with high melting point gradually precipitated. Thus, the obtained ceramics with highly dense structure, and fully developed elongated columnar slab-shaped mullite grains form an interlocking structure, thereby leading to a high compressive strength (308.0 MPa) and excellent refractoriness under load (1672 °C).

To assess the impact of changes in slag type on the degradation of refractory linings of secondary copper smelters, the degradation behavior of direct-bonded and fused grain magnesia–chromite refractories in ZnO-containing ferrous calcium silicate slags was studied via dynamic refractory finger and static refractory crucible tests at 1200 °C under Ar atmosphere. The effect of ZnO level in the slag and refractory type on the degradation behavior was determined. A combination of microstructural and compositional characterizations of tested refractories, and thermodynamic calculations leads to comprehensive understanding of the degradation mechanism of the refractories. Based on the degradation mechanism, conclusions can be given about the prediction of lifetime of refractory lining when the slag type changes from fayalite-based to ferrous calcium silicate-based slags for secondary copper smelting.

Hydratable alumina (HA) is a superior Ca-free refractory binder, but the quick hydration rate restricts the working time of castables bonded with HA. In this work, HA was grounded for 1 h and 6 h by a rotational ball mill to study the effect of grinding on the hydration of HA and properties of HA-bonded castables. HA samples with and without grinding were cured at 30 °C and then terminated by freeze-vacuum drying. The phase composition and microstructure of the dried HA samples were then examined. Moreover, flow ability and mechanical strength of castables containing ungrounded and grounded HA were also investigated. The results indicate that the specific area of HA particles were decreased by grinding as the micro-pores and micro-cracks on the surface of HA particles were blocked by smaller HA particles, thereby decreasing the hydration rate of HA and increasing the flow ability of 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.

Calcium aluminate cement was premixed with hydromagnesite having different particle sizes, as a calcium magnesium aluminate cement precursor, to investigate the influence of particle size of hydromagnesite on the volume stability and thermo-mechanical properties of corundum based castables after firing at 1550 °C. The impact of particle size of hydromagnesite on the phase composition and microstructure evolution of fired castables matrix were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results demonstrate that hydromagnesite with smaller particle size has a better volumetric stability and thermo-mechanical properties of castables, because of the more homogeneously distributed micro-pores, and smaller size MA and CA6 resulted from the more uniformly distributed hydromagnesite in castables.

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.

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.

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.

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.

Porous mullite ceramics were fabricated by the transient liquid phase diffusion process, using quartz and fly-ash floating bead (FABA) particles and corundum fines as starting materials. The effects of sintering temperatures on the evolution of phase composition and microstructure, linear shrinkage, porosity and compressive strength of ceramics were investigated. It is found that a large amount of quartz and FABA particles can be transformed into SiO2-rich liquid phase during the sintering process, and the liquid phase is transient in the Al2O3-SiO2 system, which can accelerate the mullitization rate and promote the growth of mullite grains. A large number of closed pores in the mullite ceramics are formed due to the transient liquid phase diffusion at elevated temperatures. The porous mullite ceramics with high closed porosity (about 30%) and excellent compressive strength (maximum 105 MPa) have been obtained after fried at 1700 °C.

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.

The corrosion behavior of alumina-silica (Al 2 O 3 –SiO 2 ) refractories by liquid carbon steel was studied. This was performed by a refractory crucible test in a vacuum induction furnace at 1520–1545 °C under reducing atmosphere. The results indicate that Mn, Si and a minor amount of Fe in the steel were oxidized at steel/refractory interface, combing with refractory components to form a MnO–SiO 2 –Al 2 O 3 –based slag. This slag was liquid at the testing temperature, and infiltrated along the open-pore network and the grain boundaries into the Al 2 O 3 –SiO 2 refractories. Both Al 2 O 3 –rich and Al 2 O 3 –SiO 2 aggregates in the refractories dissolved into the infiltrating slag. As a result, the refractories were structurally weakened and residual Al 2 O 3 –rich and Al 2 O 3 –SiO 2 aggregates in the hot face of the refractories were washed away into the liquid steel, affecting steel quality.

This study evaluates commercially available magnesia–chromite, magnesia–carbon and magnesia–doloma bricks for their use in a Vacuum Oxygen Decarburisation ladle. The corrosion behaviour of these bricks by stainless steelmaking slags is, therefore, investigated through crucible tests in a vacuum induction furnace at elevated temperatures (1650 and 1750 °C) and low oxygen partial pressures (5.3 and 3.0×10 −11 atm). The results reveal that magnesia–carbon bricks are severely corroded due to the high dissolution of MgO, while magnesia–chromite and magnesia–doloma refractories exhibit an excellent corrosion resistance. The MgO enrichment in the slag is believed to be the reason of the low wear rate of the MgO–doloma refractories. Rebonded and direct-bonded MgO–chromite refractories show similar corrosion resistance against the slags because of the ‘secondary chromite inactivation’. Decreasing the slag basicity enhances the dissolution of MgO into the slag, thereby increasing the corrosion of the magnesia-based refractories.

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.

Forsterite was prepared via solid-state reaction by using hydromagnesite (basic magnesia carbonate) and fumed silica, aiming at completing the reaction 2MgO+SiO 2 →Mg 2 SiO 4 at low calcination temperature. The phase development and morphology evolution of the hydromagnesite and hydromagnesite–fumed silica mixture during heat treatment were characterized by means of scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) nitrogen-gas adsorption method, and X-ray powder diffraction (XRD). A diffusion distance of less than 300 nm for Mg 2+ and Si 4+ for forsterite formation was obtained because the formed high reactivity MgO with the nanosheet structure of its parental hydromagnesite and the fumed silica with particle size of less than 100 nm were homogeneously mixed. Monolithic forsterite was synthesized after calcination of the hydromagnesite–fumed silica mixture at 1100 °C.