Microstructure and phase evolution of alumina-spinel self-flowing refractory castables containing nano-alumina particles

The microstructure and phase composition of alumina-spinel self-flowing refractory castables added with nano-alumina particles at different temperatures are investigated. The physical and mechanical properties of these refractory castables are studied. The results show that the addition of nano-alumina has a great effect on the physical and mechanical properties of these refractory castables. With the increase of nano-alumina content in the castable composition, the mechanical strength is considerably increased at various temperatures. It is shown that nano-alumina particles can affect formed phases after firing. The platy crystals of CA₆ are detected inside the grain boundaries of tabular alumina and spinel grains in samples fired at 1500 °C. CA₆ phase can be formed at lower temperatures (1300 °C) with the addition of nano-alumina particles. As a result of using nanometer-sized alumina particles with high surface area, the solid phase sintering of the nano-sized particles and CA₆ formation can occur at lower temperatures.     

Use of alumina dispersant in alumina-spinel castable: Comparison between in situ and preformed spinels

The role of alumina dispersant as a flow modifier, completely replacing fume silica, on the properties of preformed and in situ spinel-containing low cement high alumina castables was investigated with variations in spinel content and granulometry. Both 10 and 20 wt.% spinel-containing compositions were prepared using preformed spinel or magnesia in alumina castable compositions with vibratable and self-flowing consistency. The castables underwent conventional processing methods and accessed for various refractory-related parameters post-heat treatments. No glassy phase was observed in the castables upon firing. The presence of self-flowing consistency led to improved characteristics. Preformed spinel-containing compositions showed improved density, strength, hot strength, and thermal shock resistance properties, whereas slag corrosion and penetration resistances were found to be higher for in situ spinel-containing castable.     

Microwave and conventional treatment of low-cement high-alumina castables with different water to cement ratios; Part I. Drying

The aim of this study was to determine the effect of the microwave and conventional drying method on the strength, porosity and composition of low-cement alumina castables with various water to cement ratios (wcr). High-alumina low-cement castable samples were prepared with different w/c ratios: 0.64, 0.75, 0.82 and 1.13. Changes in wcr were effected through volumetric replacement of cement with 0–0,045-mm tabular alumina having a comparable particle size. Water content in all the composition was constant (4,5%). After curing, the samples were dried conventionally in a laboratory electric drier or in a laboratory microwave drier. After drying open porosity and modulus of rupture were determined. The pore size distribution, pore median and tortuosity of the samples were measured by the mercury porosimetry method. Phase composition was determined using X-ray diffraction. The Rietveld method was used for quantitative analysis. It was found that at low wcr (0.62) the main hydrate formed in the castable was C₃AH₆, which caused a release of a smaller amount of water during the drying process, mainly pore water, resulting in lower open porosity and lower pore size than in the castables with a high wcr (1.13). At a low wcr, the strenght of castable was higher due to a higher amount of hydrates, low porosity and small pore size. On the other hand, at a high wcr, the strength of castable was lower owing to a higher amount of water released in the drying process, which led to loosening the structure of castable. With an increased water-to-cement ratio the degree of CA₂ hydration decreased. The temperature rise due to cement hydration probably influenced the kinetics of this process.     

Comparisons Between Vibration and Self-Flowing Ultra-Low Cement Castables Properties in Al2O3-SiC-C System

In this study, the properties of self-flowing ultra-low cement castables in Al2O3-SiC-C system have been investigated and compared to vibration castables. The major physical and mechanical properties, microstructure and corrosion behavior of these castables against slag have been evaluated. The results showed that the microstructure of Al2O3-SiC-C self-flowing castable is more uniform than the vibrated structure. Also self-flowing castable has smaller pore size and more uniform pore size distribution. Hence, density, strength, oxidation and slag resistance of the self-flowing castables is higher than that of vibration castables. Therefore, besides other benefits such as noise free, easy installation, fewer mold defects and reduced installation costs, Al2O3-SiC-C self flowing ultra low cement castables will have longer service life in comparison with vibration casables.     

The physical and mechanical properties of alumina-based ultralow cement castable refractories

In this study, the effects of the type of alumina on the physical, chemical and mechanical properties of the ultralow cement castable (ULCC) refractories were investigated. Brown fused alumina, tabular alumina and rotary bauxite-based ULCC refractories were prepared by mixing each type of alumina with silicon carbide, carbon, cement, metallic silicon and microsilica. The density, porosity and cold crushing strength (CCS) of the refractory castables were measured after drying at 110 °C for 24 h and firing at 1450 °C for 5 h. The slag penetration resistance of the refractory castables was determined using slag corrosion tests. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffractometry (XRD) were used to characterize the castables. It was found that all three refractory castables had strong slag penetration resistance and that the tabular alumina-based refractory castable had the largest specific cold crushing strength with an acceptable percent of porosity among the refractory castables.     

Heat resistance and thermomechanical behaviour of ultralow and zero cement castables

Abstract

Knowledge of relative heat resistance as well as thermo-mechanical behaviour of refractory castables is very important for their use as linings in high temperature furnaces and refining vessels in the metallurgical, cement, and petrochemical industries. The present work aims at studying these properties for different types of refractory castable. Two classes of castable were prepared, namely ultralow and zero cement, containing either high alumina cement or hydratable alumina as bonding agent. For each class, two different castable systems were prepared, one containing an alumina-silica mixture in its matrix and the other containing magnesia-alumina. In all castables studied, calcined alumina was used as aggregate. The prepared castable samples were subjected to firing temperatures up to 1500°C. Relative heat resistance, bending strength before and after thermal cycling, hot modulus of rupture, and creep deformation were measured according to international standard specifications. It was concluded that a limited content of cement (ultralow cement castables) is beneficial with the magnesia-alumina mix in the matrix owing to the formation of calcium hexaluminate-magnesium aluminate-corundum (matrix advantage system) that results in excellent relative heat resistance as well as thermome-chanical properties. Zero cement castables on the other hand are recommended for use with the alumina-silica mixture, since the absence of cement improves the chances of mullite formation without glassy phases, thereby enhancing the properties of such refractory castables.     

Does a tiny amount of dispersant make any change to refractory castable properties?

A growing demand for refractory castables with specific behaviors has given rise to a continuous technological evolution, mainly due to the broad knowledge of hydraulic binders available nowadays. The high alumina cements remain as the most important hydraulic binders for castables. Nevertheless, calcium aluminate bound castables still show a characteristic drop of strength at intermediate temperatures, which could also be affected by the castable chemical additive. Thus, this paper aims to highlight the influence of dispersants on the refractory castable properties with the firing temperature. It was noticed that the hydrates formed during the curing process of castable depends on the dispersing additive used. The FS60, a polycarboxylate ether, induced the AH₃ formation and its decomposition resulted in a more stable hydrate (AH), which increased the splitting strength with the thermal treatment temperature. At a high temperature, the CA₂ and CA₆ formation is also favored in the presence of this additive. However, it did not bring benefits to the castables creep behavior, resulting in a less tough structure.     

水泥含量对高纯铝尖晶石自流浇注料性能的影响

以致密刚玉和尖晶石为主要原料,铝镁尖晶石的含量为19%,在颗粒组成(5～0 mm)不变的情况下,研究了水泥含量分别为3%、8%和13%对高纯铝尖晶石自流浇注料性能的影响.结果显示1)随着水泥含量的增加,浇注料的加水量、干燥后致密度和强度均增加,但烧后致密度和强度都下降;2)水泥含量的变化对自流浇注料热态强度的影响不明显;3)随着水泥含量的增加,自流料烧后线变化增加,但水泥含量达到8%以后变化不明显;4)对于LF精炼炉渣,不管水泥的含量多少,自流料都不被渗透,但随水泥含量增加,自流料的抗熔蚀性变差;5)对于转炉钢包渣,随着水泥含量的增加,自流料的抗渗透性增强,抗熔蚀性变差,而渗透与熔蚀量之和减少;6)对于转炉末期渣,随着水泥含量增加,浇注料的抗渗透性增强,抗熔蚀性变差;7)应用相图分析了铝尖晶石浇注料的侵蚀机理,认为渣中Fe2O3或低熔点相CF的增加是渣渗透的主要原因.     

Microstructural and mechanical investigation of high alumina refractory castables containing nano-titania

In this study, mechanical and structural properties of two high alumina refractory castables, i.e. without nano-titania and with 0.4?wt% TiO₂, are investigated. Bulk density, apparent porosity, cold compressive strength, cold modulus of rupture, permanent linear changes, hot modulus of rupture and shock resistance tests were performed on the samples. The microstructure and phase analyses were conducted by field emission scanning electron microscope equipped with EDX analyzer and X-ray diffraction, respectively. Results showed that hibonite phase formation in refractory is dependent on the temperature which could be decreased by addition of nano-titania, leading to the improvement of refractory strength. On the other hand, the decrease of hot modulus of rupture and shock resistance was observed for the refractory castable containing nano-titania. Also, Ca₃Ti₈Al₁₂O₃₇ phase was produced after firing at 1000?°C in the presence of nano-titania. Probable mechanisms are proposed for phase and microstructural changes in high alumina refractory castable containing nano-titania.     

Thermomechanical and thermophysical characteristics of alumina-carbon monolithic refractory containing surface-modified graphites in matrix

The thermophysical and thermomechanical behavior of graphite-containing refractory castable matrix with 20.0?wt% of graphite had been compared with graphite-free high alumina based castable matrix analogous to it. The thermomechanical properties of similar type of castables with and without 5.0% of graphite had also been evaluated. Graphite was incorporated both as coated and as-received forms, the former having a thin sol-gel derived calcium aluminate coating on graphite flakes. The influence of thermal conductivity, thermal diffusivity and pore size distribution had been critically estimated. The changes in flexural strength, porosity and density of the matrices had also been determined to interpret the castable performance, namely refractoriness under load (RUL), spalling resistance and conventional physical properties. The role of coated graphite on improved densification and thermal tolerance of refractories was further explored by microstructure and phase evolution studies of all kinds of fired samples at 1500?°C. It was further supplemented with the scanning electron microscope (SEM) studies of surface-modified graphites at uncalcined and calcined condition.     

Ceramic Castables — Final Stage in the Evolution of Low-Cement Refractory Castables. Part 3

Technological and service characteristics of ceramic castables and low-cement refractory castables (LCRCs) are compared. Unlike LCRCs prepared from dry-ground powders, the ceramic castables contain a nanoparticle phase (5 – 100 nm), which to a significant extent determines their operational properties. The initial mechanical strength of ceramic binders can be increased substantially by adding water-soluble resins to their composition. A way toward developing oxide-carbon ceramic castables is outlined. Compositions for binding agents with enhanced high-temperature strength are developed.     

The influence of Al2O3 nanoparticles addition on the microstructure and properties of bauxite self–flowing low-cement castables

In this research, the impact of Al₂O₃ nanoparticles addition on microstructure, mechanical, and physical properties of bauxite self–flowing low-cement castables were investigated. Also, the optimum amount of Al₂O₃ nanoparticles is determined. For this propose, up to 3 wt% Al₂O₃ nanoparticles were added to the bauxite castable compositions. The physical and mechanical properties of castable compositions such as bulk density (BD), apparent porosity (AP), self-flow values (SFV), and cold crushing strength (CCS) were examined. Also, the X-ray diffraction (XRD) and scanning electron microscopy (SEM/EDX) techniques were used for detection the ceramic phase's formation and microstructural analysis of the castables compositions, respectively. Results show that addition of Al₂O₃ nanoparticles up to 1 wt% improved the properties of bauxite self–flowing low-cement castables. As well as, the use of Al₂O₃ nanoparticles led to the formation of the platy and needle crystalline phases such as hibonite (CaO·6Al₂O₃), calcium dialuminate (CaO·2Al₂O₃), and mullite (3Al₂O₃·2SiO₂), between the grain boundaries of the bauxite particles. Also, Al₂O₃ nanoparticles addition led to aforementioned phase formation occur at the lower temperatures.     

Microwave and conventional treatment of low-cement high-alumina castables with different water-to-cement ratio; Part II. Dehydration

Modern refractory castables contain between 3.5 and 5?wt.-% water that is necessary for sufficient flow during emplacement and for the formation of hydrate phases, necessary for the green strength of the material. Prior to the high temperature use of this material, it must be dried very carefully to avoid explosive spalling.This paper will demonstrate that beside conventional drying of pre-shaped materials in resistance furnaces microwave radiation is an energy saving and rapid method to remove pore water as well as hydrate bond water from the castable. In comparison to resistance furnaces, the use of microwave radiation does not affect the castable properties as there are mechanical strength (MOR, CCS), open porosity and pore size distribution. This study proved microwave radiation as valuable alternative with a series of tabular alumina based low cement castables (LLC) in which the water-to-cement-ratio (wcr = 0.64, 0.75, 0.82 and 1.13) was systematically altered by changing the cement concentration at constant mixing water concentration of 4.5%.     

In situ spinel bonded refractory castable in relation to co-precipitation and sol–gel derived spinel forming agents

This paper deals with the preparation and characterization of two types of in situ spinel bonded low cement high alumina based castable refractories. Semidried magnesium aluminate mass was prepared from cheaper precursors via coprecipitation and sol–gel routes for application in a refractory castable composition in different concentrations. The pH, average particle size, solid content, DTG analysis and XRD patterns of those two additives were observed. After being fired at elevated temperatures those two kinds of in situ spinel bonded castables were characterized and compared in terms of bulk density, apparent porosity, cold crushing strength, flexural strength, volume shrinkage, spalling resistance, and XRD phase analysis. Scanning electron microscopy of some selected fired samples was done to analyse the mode of interaction of in situ spinel bonds in castable microstructure. The corrosion resistance of the castables was estimated by heating with blast furnace and converter slags.     

Phase composition of bauxite-based refractory castables

The bauxite-based refractory castables are composed of 90 wt.% well-graded (coarse, medium, and fine) bauxite aggregate, 10 wt.% binding matrix and adequate amount of distilled water. The binder mixture was calcium aluminate cement (CAC) containing 80% alumina and magnesium-aluminate spinel (MA-spinel) or mullite either preformed or obtained in situ.The phase composition of the prepared castable samples was investigated by using X-ray diffraction (XRD) analysis, scanning electron microscopy, energy dispersive X-ray analysis, differential thermal analysis (DTA) and thermal gravimetric analysis.The XRD patterns indicated the formation of the corundum phase along with mullite phase in all of bauxite-based castable samples. The scanning electron microscopy (SEM) micrographs show densely packed microstructures with an abundant of corundum grains of comparable sizes, rounded and sub-rounded, homogeneously embedded in the matrix. Some needle-shaped mullite crystals are distributed from place to place. The DTA curve of the castable samples showed three endotherms due to the release of free or the physically absorbed water, the dehydroxylation of the chemically bound water and due to some dehydration and dehydroxylation reactions.     

Basic slag attack of spinel-containing refractory castables

The better performance of spinel-containing refractory castables when in contact with basic slag is mainly associated with their higher corrosion resistance. Although the literature has shown various studies related to this subject, only few of them evaluated the overall microstructural effect on the corrosion resistance. Considering this aspect, four different compositions were produced, in order to evaluate the binder source influence (calcium aluminate cement or hydratable alumina), the silica fume addition and spinel incorporation method (in situ or pre-formed) effects. Based on the physical properties (apparent porosity, linear thermal expansion and pore size diameter distribution) and also on the phases generated (detected by SEM, before and after corrosion), a basic slag corrosion mechanism, for the set conditions, is proposed. The results pointed out that lower penetration can be attained by reducing the pore size diameters, whereas chemical corrosion resistance is a consequence of higher content of fine alumina, lower amount of calcium aluminates and the lack of liquid phase in the castable matrix. This study stresses that the castable formulation design and the proper raw material selection are of utmost importance to understand and master the performance of this class of refractory castables.     

Improving the structure and properties of a refractory castable containing porous chamotte fillers

The potential use of a porous (20%) chamotte filler in a low-cement castable with an integrated matrix is considered. Results of a study of the properties of conventional, medium-cement, and low-cement castables are presented.Translated from Novye Ogneupory, No. 7, July, 2004, pp. 46–49.     

Selection of binders for in situ spinel refractory castables

The expansive behavior of alumina–magnesia refractory castables is usually associated with in situ spinel formation. Nevertheless, when bonded with calcium aluminate cement (CAC), this class of materials can present additional expansion reactions due to CA₂ and CA₆ formation. Considering that these reactions impart a further contribution to the material's overall volumetric change, the objective of this work has been to analyze the effect of partial or complete replacement of CAC by hydratable alumina (HA). Taking into account that this substitution would affect various castable processing steps, properties such as the mechanical strength (during curing, intermediate or high temperatures), linear change behavior during heating, creep and thermal shock resistance were evaluated. In general, CAC-containing castables led to better mechanical strength and thermal shock resistance, whereas HA-containing castables presented higher creep resistance, lower apparent porosity and better volumetric stability. Due to the substantial reduction of the overall expansion of alumina–magnesia castables, the addition of hydratable alumina was pointed out as an interesting alternative to attain designed expansion levels.     

Advantageous features of sodium silicate as the plasticizer for low-cement refractory castables

The plasticizing effect of an aqueous sodium silicate solution of low concentration (0.3 – 0.4%) on the properties of a low-cement refractory castable is studied. The merits of using sodium silicate, rather than the conventional sodium tripolyphosphate, for plasticizing sintered refractory castables are emphasized.Translated from Novye Ogneupory, No. 9, pp. 48 – 51, September, 2004.     

On a qualitative relationship between degree of inhomogeneity and cold crushing strength of refractory castables

The influence of the spatial arrangement of aggregates on the cold crushing strength (CCS) of refractory castables is studied by means of methods of point process statistics. Two series of self-flowing refractory castables differing in the kind of aggregates used (tabular alumina, corundum balls) are analysed using quantitative image analysis and methods of spatial statistics. Their spatial variability is described by means of the pair correlation function (pcf) and the density function of the nearest neighbour distance (nnd) distribution. It is shown that both characteristics, pcf and nnd, are in close correlation with CCS.