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The elucidation of fracture mechanisms is helpful for the optimization and design of toughness and microstructure of refractories. Fracture behavior of ultra-low cement bonded Al2O3–SiO2 castables was researched using the wedge splitting test coupled with digital image correlation technique (WST-DIC). Britleness of Al2O3–SiO2 castables is reduced by introducing andalusite aggregates into the castables. The characteristic length LCH, a parameter used to assess flexiblity of materials, was observed to reach 287.2 mm in andalusite-containing Al2O3–SiO2 castables, more than 5 times that of reference castables. Microcracks toughening is the main toughening mechanisms for flexibility improvement of the Al2O3–SiO2 castables containing andalusite. Microcrack network in microstructure of the Al2O3–SiO2 castables could be designed by exploiting the volume expansion caused by mullitization of andalusite and the the coefficient of thermal expansion (CTE) mismatch between the andalusite aggregate and the matrix. Unlike andalusite-free castables, castables containing andalusite possess a distinct fracture process zone (FPZ), the crack branching and deflection phenomena can be observed around the main crack during the fracture process, which leads to the prolong of the crack propagation path, the increase of the dissipation energy during the fracture, and the enhancement of resistance to crack propagation.

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.

To evaluate the andalusite role on the fracture behavior of Al2O3–SiO2 castables and reveal the improvement mechanism of flexibility, Al2O3–SiO2 castables with various andalusite additions (0, 10, 20, 30, 40, and 50 wt%) were prepared adopting homogenized bauxite, andalusite, calcium aluminate cement, microsilica and ultrafine α-Al2O3 powder as raw materials. Wedge splitting test was used to determine fracture behavior of Al2O3–SiO2 castables. The improvement mechanism of toughness was discussed according to the analysis of microstructure. The results show that incorporation of andalusite is an effective way to change fracture behavior and improve the flexibility of the Al2O3–SiO2 castables. The force-displacement curve of the Al2O3–SiO2 castables without andalusite addition is featured with a linear fracture behavior, while the force-displacement curves of the Al2O3–SiO2 castables containing andalusite are characterized by a nonlinear fracture behavior. The flexibility parameter (GF/σNT ratio) rises markedly from 35.3 μm of the castable without andalusite to 197.8 μm of the castables with 40% of andalusite addition, which increases more than twice. Microcracks toughening plays one major role in the improvement of the flexibility of Al2O3–SiO2 castables containing andalusite. The formation of microcracks can be attributed to mismatch of coefficient of thermal expansion between andalusite particles and matrix, caused by anisotropic behavior of andalusite aggregates and mullitized andalusite aggregates. The interlocking microstructure of in situ formed mullite grain in the matrix is the second reason for the toughness improvement of Al2O3–SiO2 castables containing andalusite.

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.

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.

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.

Bauxite-SiC refractories as the working lining of cement rotary kiln are subjected to severe alkali attack due to alternative fuel combustion. Herein, the present work aims to enhance their alkali attack resistance via incorporation of different types of andalusite (aggregates and powder). Results show that addition of andalusite reduces penetration of alkali vapor by decreasing their pore size. In particular, the presence of andalusite aggregate traps alkali vapor inside its pore network, retarding the corrosion of refractory matrix. By comparison, the presence of andalusite powder contributes to formation of liquid corrosion products to block the pores, which prevent alkali vapor from further penetration. Incorporation of andalusite into BSRs is straightforward and sought-after, providing an effective approach for high-performance refractory materials with excellent alkali attack resistance.

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.

Colloidal silica bonded bauxite-andalusite based castables were prepared using homogenized bauxite and andalusite as aggregates, andalusite fines, corundum fines, ultrafine Al 2 O 3 as matrixes and colloidal silica as binders. Effects of microsilica addition on the green strength, physical properties, hot strength and thermal shock resistance of castables were investigated. Moreover, phase composition and morphological evolution of specimens were characterized by XRD and SEM analysis. Green strength after demoulding, cold strength and hot strength as well as thermal shock resistance of the castables are enhanced with microsilica addition, which attribute to generating more chemical bond (–Si–O–Si–) after demoulding and heating at intermediate temperature (up to 1100 °C), and creating a stronger mullite bonding at higher temperature (1400 °C) compare to the specimens without microsilica.

For increasing working stability of cordierite-spodumene composite ceramics for solar heat transmission pipeline, andalusite was utilized as modified additive to improve mechanical strength and thermal shock resistance of the composite ceramics. The effects of andalusite on densification, mechanical strength, thermal stability, phase composition and microstructure were studied. The experiment results showed that andalusite significantly influenced bending strength and thermal shock resistance of the composite ceramics. Especially, specimen B1 with 5 wt% andalusite sintered at 1400 °C achieved the best performances. The linear shrinkage, water absorption, apparent porosity, bulk density and bending strength were 5.62%, 0.02%, 0.06%, 2.19 g cm−3 and 104.94 MPa, respectively. After 30 thermal shock cycles (wind cooling from 1100 °C to room temperature), the residual strength of the specimen increased to 110.65 MPa, accompanying with −5.44% strength loss rate. The XRD and SEM analysis illustrated that mullite grains with short rod-like shape could prevent crack growth of inter-granular fracture to enhance bending strength of the specimens. Furthermore, the generation of β-spodumene grains with low thermal expansion coefficient after thermal shock improved thermal shock resistance of the composite ceramics. It is considered that the cordierite-spodumene composite ceramics with high densification, good mechanical strength and excellent thermal stability can be a potential material for high temperature thermal transmission pipeline in solar thermal power generation.

Silicon carbide reticulated porous ceramics (SiC RPCs) with three-layered struts were fabricated by polymer replica method, followed by infiltrating alumina slurries containing silicon (slurry-Si) and andalusite (slurry-An), respectively. The effects of composition of infiltration slurries on the strut structure, mechanical properties and thermal shock resistance of SiC RPCs were investigated. The results showed that the SiC RPCs infiltrated with slurry-Si and slurry-An exhibited better mechanical properties and thermal shock resistance in comparison with those of alumina slurry infiltration, even obtained the considerable strength at 1300 °C. In slurry-Si, silicon was oxidized into SiO2 in the temperature range from 1300 °C to 1400 °C and it reacted with Al2O3 into mullite phase at 1450 °C. Meantime, the addition of silicon in slurry-Si could reduce SiC oxidation of SiC RPCs during firing process in contrast with alumina slurry. With regard to slurry-An, andalusite started to transform into mullite phase at 1300 °C and the secondary mullitization occurred at 1450 °C. The enhanced mechanical properties and thermal shock resistance of SiC RPCs infiltrated alumina slurries containing silicon and andalusite were attributed to the optimized microstructure and the triangular zone (inner layer of strut) with mullite bonded corundum via reaction sintering. In addition, the generation of residual compressive stress together with better interlocked needle-like mullite led to the crack-deflection in SiC skeleton, thus improving the thermal shock resistance of obtained SiC RPCs.

The corrosion chemistry of SiC-modified andalusite refractories has been studied using a post-mortem approach that considers both K + and Ca 2+ ions as the main corrosive agents existing in the cyclone preheater applied in the cement production line. The transient progress of corrosion reactions was investigated as the function of distance from the face exposed to the corrosive media (hot face) to the relatively little altered middle zone of Al 2 SiO 5 -SiC brick. Chemical alteration of the Al 2 SiO 5 and SiC grains was examined and discussed on the basis of phase equilibria in binary (CaO-SiO 2 , K 2 O-SiO 2 ) and ternary (K 2 O-Al 2 O 3 -SiO 2 , CaO-Al 2 O 3 -SiO 2 ) systems. Ca and K-Si-oxide phases along with Ca-aluminosilicate – gehlenite Ca 2 Al 2 SiO 7 and K-aluminosilicates – kalsilite KAlSiO 4 and leucite KAlSi 2 O 6 were formed as a consequence of the reactions between both SiO 2 and Al 2 SiO 5 with CaO and K 2 O. The heating microscopy thermal analysis was used to evaluate the behavior of SiC-/ZrSiO 4 -andalusite refractory bricks in contact with a partly decarbonized material.

Corundum-mullite composite ceramics have high hardness, small plastic deformation and other excellent performances at high temperature. Corundum-mullite composite ceramics were fabricated from andalusite and α-Al 2 O 3 by in-situ synthesis technology. Effects of mullite/corundum ratio and sintering temperatures on the water absorption, apparent porosity, bulk density, bending strength, thermal shock resistance, phase composition and microstructure of the sample were investigated. Results indicated that the in-situ synthesized mullite from andalusite combined with corundum satisfactorily, which significantly improved the thermal shock resistance as no crack formed after 30 cycles of thermal shock (1100 °C-room temperature, air cooling). Formula A4 (andalusite: 37.31 wt %, α-Al 2 O 3 : 62.69 wt %, TiO 2 in addition: 1 wt %, mullite: corundum=6:4 in wt %) achieved the optimum properties when sintering at 1650 °C, which were listed as follows: water absorption of 0.15%, apparent porosity of 0.42%, and bulk density of 3.21 g⋅cm −3 , bending strength of 117.32 MPa. The phase composition of the sintered samples before and after thermal shock tests were mullite and corundum constantly. The fracture modes of the crystals were transgranular and intergranular fractures, which could endow the samples with high thermal shock resistance.

One of the most important aspects to be evaluated when predicting the performance of refractory materials is their thermal shock resistance, especially considering the thermomechanical loads to which they are subjected. The methods currently used to predict thermal shock resistance behavior are based on Hasselman's studies, which take into account various experimental parameters. Thermal shock resistance can also be evaluated experimentally by applying thermal shock cycles and measuring the residual elastic modulus after each cycle. To predict and evaluate this resistance, test specimens of a commercial castable were cast, cured, dried and fired at 1000 °C and 1450 °C for five hours. Using Hasselman's parameters, the refractory fired at 1450 °C was predicted to present lower thermal shock resistance, which was confirmed by experimental thermal shock tests. The results indicated that the modulus of elasticity and modulus of rupture decreased by about 72% in specimens fired at 1000 °C, and by 82% in those fired at 1450 °C. Therefore, the specimens fired at 1450 °C showed lower thermal shock resistance. This finding underscores the importance of evaluating the behavior of castables with different microstructures resulting from different firing temperatures.

Cordierite-spodumene composite ceramics used for solar heat transmission pipeline were prepared via pressureless sintering from kaolin, talc, γ-Al 2 O 3 and spodumene. Effects of andalusite and zircon on densification, mechanical properties, phase composition, microstructure and thermal shock resistance of the composite ceramics were investigated. The results showed that andalusite and zircon additives effectively affected mechanical strength and thermal stability of the composite ceramics. Expecially, sample B5 with 15 wt% zircon additive sintered at 1360 °C achieved the highest bending strength and thermal shock resistance. The bending strengths of the sample before and after 30 thermal shock cycles (wind cooling from 1100 °C to room temperature) were 100.17 MPa and 101.90 MPa, respectively. XRD analysis indicated that the main phases of the sample before 30 thermal shock cycles were α-cordierite, spinel, corundum, α-quartz and zirconium silicate, while β-spodumene grains generated after thermal shock. SEM micrographs illustrated that the existence of zirconium silicate and β-spodumene at the grain boundaries after 30 thermal shock cycles endowed the composite ceramics with a high residual strength. It is believed that the cordierite-spodumene composite ceramics can be a potential material to apply in the field of solar thermal transmission pipeline.

Roles of hydratable alumina and firing temperatures on the physical properties, hot modulus of rupture and thermal shock resistance of the bauxite–andalusite based castables were investigated. Cold strength, hot strength and thermal shock resistance of castables are improved with hydratable alumina content increasing from 2% to 6%, and also noticeably improved with the temperature rising from 1300 °C to 1450 °C, which can be ascribed to the increase in in-situ mullite content and its crystal size as well as the decrease in the silica-rich glass phase in the castables.

Andalusite composite ceramics used for solar heat transmission pipeline were fabricated via pressureless sintering from andalusite, PSZ, Suzhou kaolin and talc. The effects of dolomite and spodumene on densification, mechanical properties, phase transformation, microstructure and thermal shock resistance of the composite ceramics were investigated. Results demonstrated that dolomite and spodumene used as flux materials promoted densification greatly, and the lowest sintering temperature of the composite ceramics was decreased significantly by 90–150 °C, comparing with the previous works with the usage of potassium feldspar and albite as flux materials. Introducing an appropriate amount(7 wt%) of spodumene improved thermal shock resistance of the composite ceramics since no cracks formed after 30 thermal shock cycles (air cooling from 1100 °C to RT) with a bending strength increasing rate of 22.41%. XRD patterns indicated that the dominant phases of samples before and after 30 thermal shock cycles were consistently mullite, ZrSiO 4 and m-ZrO 2 , while the content of mullite increased after thermal shock, resulting in the enhancement of bending strength. It is believed that the andalusite composite ceramics can be a promising candidate material for heat transmission pipeline in the solar thermal power generation.

In the article the results of investigations into the mullitization process of two andalusite concentrates are presented. XRD, XRF and ICP investigations revealed nearly identical phase and chemical composition of both concentrates. However, the andalusites considerably differ in the rate of the mullitization process. XPS and LM investigations revealed that the factors responsible for mullitization include not only the content of mineral inclusions and grain size distribution but also impurities dispersion and the content of naturally occurring carbon in andalusite grains.

In this work, the effects of mechanical activation and microwave heating of andalusite on mullite formation have been investigated. XRD results revealed that andalusite peaks disappeared after 60 h of milling and the peaks of alumina were observed. The formation of mullite from activated and as-received andalusite occurred at 800° and 1250 °C, respectively, while mullitization was completed at 1100° and 1400 °C in the former and latter, respectively. Mullite samples prepared from activated andalusite showed better densification with an elongated morphology.

The chemical attack of alumina refractories by sodium vapours is far from been completely understood. In order to contribute to a better knowledge of this attack, a laboratory test was developed to simulate the sodium gaseous corrosion of different raw materials and refractories. Corrosion of alumina raw materials by sodium vapours is due to a dissolution–precipitation process by a Na 2 O rich liquid phase. The gaseous corrosion strongly depends on the microstructures and the assemblage of phases in alumina raw materials. Fire clay and andalusite raw materials exhibit very high corrosion by Na vapours. In spite of an initial high silica glass content, and as a result of trapping of the main part of the glass in the capillary network of the mullite composite crystal, mullitised andalusite leads to excellent corrosion resistance which is close to monocrystalline fused mullite. Consequently, The use of mullitised andalusite particles in the matrix of alumina refractories limits the liquid phase formation during corrosion by sodium gas. These experimental results are in agreement with thermodynamic calculations.

In this paper, the corrosion behaviour of aluminosilicate type refractories in frit melts is studied in an isothermal corrosion test setup. A refractory brick of largely andalusite and sillimanite composition was compared to another refractory brick of mullite and sillimanite composition, both of which were made by different manufacturers for use in different frit furnaces. The industrial frit used for corrosion tests was a commercial product used in a wall tile glaze formulation. Corrosion tests conducted under isothermal conditions provide quantitative and reproducible data about the corrosion performance of refractories. In this study, tests were performed by partially immersing a 15 mm × 15 mm × 115 mm refractory specimen into a frit melt at temperatures between 1404 and 1504 °C. The effects of temperature, duration of exposure and the refractory brick type were investigated using a statistically designed set of experiments. The ANOVA (analysis of variance) table indicated that temperature and test duration were the most important factor effects, as expected. Increasing both temperature and exposure duration led to an increased amount of corrosion as measured by the cross-sectional area loss of the corroded refractory specimen. Postmortem microstructural analysis was also done on the specimens, with extensive amount of ZnO·Al2O3 precipitation observed along the frit-refractory interface, where crystals of mullite and alumina were also found to precipitate. Increasing the amount of exposure time and temperature produced more ZnO·Al2O3 precipitation. As identified by SEM-EDS analysis, mullite crystals were in the needle-like morphology, while alumina crystals were generally cubic. Additional experiments were conducted by rotating the specimens in the melt at 50 rpm of rotational speed. Due to the reduction of boundary layer thickness, more dissolution was observed from the rotated specimens. In all specimens, corrosion was more pronounced in the bond phase than through the large filler grains of mullite and andalusite., TÜBİTAK project number 104M403

Mullitization and microstructure of mullite grown from fired andalusite grains has been studied in the literature. Mechanisms of mullite formation are well known: during heating, andalusite single crystal grains are transformed into a composite made of a 3:2 mullite single crystal with a capillary network filled with silica rich liquid. But very few works studied the influence of the impurities on the kinetics of mullitization. This paper shows that the presence of impurities (mainly iron and alkalis) in the andalusite increases the amount of the liquid phase and decreases the liquid viscosity. The mullitization kinetics depends on the andalusite grain size, the chemical composition, the origin of the andalusite deposit and the amount and the properties of the liquid phase. Because of the specific microstructure and the presence of the liquid phase, during thermal shocks in air, the mullitized andalusite crystals exhibit a behaviour typical of composite materials: microcracks are deflected by mullite/glass interfaces and stopped in glass zones. During a further heating, cracks generated by a thermal shock are healed.

The kinetics of the high-temperature transformation of andalusite (