Self-flowing Refractory Castables Research Articles

Use of rice husk as a source of silica for the production of self-flowing refractory castables

The objective of this study was: i) to obtain rice husk ash (RHA) with high purity silica for the production self-flowing refractory castables with different percentages of RHA as a partial substitute for microsilica; ii) analyze the mechanical, physical, thermal, and microstructural properties of the formulations studied in relation to the effect of firing temperatures. The rice husks were washed with acetic acid in autoclave, dried at 105 °C for 24 h and calcined at 600 °C for 4 h. The ashes obtained were analyzed by particle size distribution, X-ray fluorescence spectrometry (FRX), X-ray diffraction (XRD) and scanning electron microscopy with X-ray microanalysis (SEM-EDS). The presence of amorphous silica of high purity was verified with irregular grains and sizes between 40 and 145 μm. Refractory castables were produced by replacing 0%, 5%, 10% and 15% (mass) of the microsilica by rice husk ash. After curing, the specimens were dried at 110 °C for 24 h and a part was sintered at 800 °C for 5 h, while the other was sintered at 1430 °C for 8 h. The samples were submitted to tests of free creep; apparent density; linear variation; resistance to bending, compression and thermal shock. The formulation with the best mechanical performance, modulus of rupture (55.10 MPa) and compressive strength (119.62 MPa), was 10% replaced by rice husk ash and sintered at 1430 °C. This result was due to the formation of higher strength alumino-silicate phases and stronger bonds between the matrix and the aggregates. In addition, all analyzed materials using rice husk ash showed high density and good resistance to thermal shock, which is desirable for self-flowing refractory castables.

Anti-spalling fibers for refractory castables: A potential application for recycling drinking straws

Disposing plastic after using is an urgent worldwide issue, and recycling is usually pointed out as the best solution. Recycled thermoplastic polymers, however, in most cases, present inferior properties when compared to their as-produced equivalent grades. Re-exposure to processing temperatures and intense shear results in significant thermo-mechanical-oxidative degradation of the polymeric material. Typical consequences lead to a reduction in the average molecular mass and melting temperature, and modifications in the composition, microstructure and rheological behavior. Although such effects usually hinder using these materials in their original application, other technological uses can be found. In the present paper, a methodology used to produce thin continuous fibers from recycled polypropylene (PP) and high-density polyethylene (HDPE) drinking straws is presented. The processing parameters for melt-extrusion and chopping the fibers were set to produce staples of similar length and diameter as the conventional PP fibers currently employed as anti-spalling additives during the first heating of self-flow refractory castables. Because of their lower melting temperature and high melt flow index, recycled polymers cannot be used to produce new straws, however the fibers produced with them increased the castables' permeability at lower temperatures than those observed for the conventional ones. Therefore, besides helping the environment, the castables’ overall risk of explosive spalling and total drying time can be successfully reduced.

Novel drying additives and their evaluation for self-flowing refractory castables

The drying step of dense refractory castables containing hydraulic binders is a critical process, which usually requires using slow heating rates due to the high explosion trend of such materials during their first thermal treatment. Thus, this work investigated the performance of alternative additives to induce faster and safer drying of self-flowing high-alumina refractory castables bonded with calcium aluminate cement (CAC) or hydratable alumina (HA). The following materials were analyzed for this purpose: polymeric fibers, a permeability enhancing compound (RefPac MIPORE 20) and an organic additive (aluminum salt of 2-hydroxypropanoic acid). The drying behavior and explosion resistance of the cured samples were evaluated when subjecting the prepared castables to heating rates of 2, 5 or 20 °C/min and the obtained data were then correlated to the potential of the drying agents to improve the permeability and mechanical strength level of the refractories at different temperatures. The collected results attested that the selected additives were more efficient in optimizing the drying behavior of the CAC-bonded compositions, whereas the HA-containing castables performed better when the aluminum-based salt was blended with a small amount of CAC (0.5 wt%), which changed the binders hydration reaction sequence and optimized the permeability level of the resulting microstructure. Consequently, some of the designed compositions evaluated in this work showed improved drying behavior and no explosion was observed even during the tests carried out under a high heating rate (20 °C/min).

Thermomechanical evaluation of self-flowing refractory castables with and without the addition of aluminate cement

Much has been discussed about the advantages regarding the thermomechanical performance of self-flow refractory castables (SFRC), with very little or no addition of calcium aluminate cement (CAC) when compared to conventional composites with cement. Previous works demonstrated that a 100% alumina SFRC with optimized particle size distribution simultaneously results in high-flowability fresh paste and high mechanical strength.In this work, the thermomechanical behaviour of zero-cement ceramic matrix is compared to that of the equivalent composite with 1% CAC content.The results show that the differences in performance can be related to differences in hydration mechanisms, namely the formation sequence of calcium aluminate hydrates, which results in increased open porosity. With lower dried strength, the all-alumina castable presents lower porosity, higher mechanical strength and uncompromised thermal shock resistance, being a valuable option when fast drying or rough green handling can be avoided.

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.

Influence of Surface Area on the Flowability Behaviour of Self-Flow Refractory Castables

Alumina, with high melting point (2050°C), high hardness and mechanical strength, and excellent abrasion resistance, is one of the most common raw materials used in self-flow refractory castables (SFRC) for monolithic linings and is commercially available in various fine to coarse size classes. However, the performance of the refractory lining depends not only on the properties of its ingredients but also on its easy installation (good flowability). The aim of this work was to evaluate the relationship between the flowability index (FI) of fresh castable and the specific surface area (SSA) of its particles, which is mostly determined by the finer particles content. The results obtained showed that, by controlling the proportion between matrix and aggregate, it is possible to control the SSA of the refractory castable and find a mathematical relationship between the specific surface area and the minimum flowability index required to obtain a self-flow refractory castable. It is, thus, possible to optimize the refractory castable size composition and obtain an estimate for FI as a function of SSA. Using a minimum 45 wt.% matrix content in the castable mixture, a SSA value above 2.215 m2/g is obtained, which leads to FI ≥ 80%, the recommended value for self-flow.

MPT Influence on the Rheological Behaviour of Self-Flow Refractory Castables

The success of a refractory castable is largely due to the quality of its properties and ease of application. Self-flow refractory castables (SFRC), with high flowability index (>130%), can be easily accommodated in a mould without the application of external energy, being ideal for the manufacture of monolithic linings. SFRC castables without cement require a matrix of very fine particles, which guarantees improved rheological behaviour and performs the role of the binder in the absence of the refractory cement. The presence of the aggregate (coarse particles) hinders the flowability index, but improves the castable mechanical strength and reduces firing shrinkage, and also contributes to the reduction of the castable costs. The control of the maximum paste thickness (MPT) allows the reduction of the coarse particles interference, minimizing the number of contact points among the grains and avoiding the formation of an aggregate skeleton that impairs the flowability of the mixture. In the present work, 100% alumina SFRCs without cement were produced with a fixed matrix of fine particles, whose particle size distribution was optimized using statistical techniques (mixtures design and triangular response surfaces). Different aggregate particle size distributions were used, with several MPT values, with the objective of evaluating which was the mean distance that maximized the flowability index, simultaneously ensuring good mechanical strength for the refractory castable. Ensuring a minimum surface area of 2.22m2/g, the mixtures reach the self-flow turning point with a minimum water content and the maximum flowability is obtained for an aggregate particle size distribution modulus of q=0.22, and consequently an optimized MPT value. SFRC with high mechanical strength (>60MPa) were obtained.

Statistical Modelling of the Particle Size Composition of an Alumina Matrix for No-Cement Self-Flowing Refractory Castables

In the processing of any particulate system (including refractory castables), the finer sized particle fraction (matrix or filler) is mostly responsible for the final system properties. Alumina fine powders were used in this work as raw materials, namely two tabular alumina fractions (–500 mesh and –230 mesh) and a commercial reactive alumina. Statistical modelling and the Response Surface Methodology (Statistica, Mixtures Designs and Triangular Surfaces module) were used to optimise the particle size composition of the three-component mixtures leading to matrix maximum flowability. The mixing methodology, aimed at minimising the water content, was kept constant. No-cement alumina castables produced with the various fine size powder mixtures, were used to prepare test-pieces. After drying and sintering, their mechanical resistance (MoR), density, porosity, water absorption, thermal shock behaviour and microstructure were evaluated. The results obtained evidenced the relevance of variables such as the water content per unit specific surface area, needed to reach the self-flow “turning point”, and validated the statistical optimisation method used. Moreover those results showed the existence of a matrix composition range, which favours the formation of a flow-bed that enables the aggregate self-flow.

Modelling the microstructure of concrete with spherical grains

The modelling of the geometrical structure of a special kind of concrete is discussed. It is a matter of self-flowing refractory castables with spherical grains. The system of hard spheres is compared to several well-established models of spatial statistics and statistical physics as well as to a model used by concrete engineers. It turns out that the structure can be well described by the latter model and a further newly developed model, which is similar to a marked Gibbs process. The result is based on the comparison of common summary statistics like the pair correlation function and the spherical contact distribution function.

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.