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Abstract The replacement of calcium aluminate cement (CAC) and/or the addition of specific agents (i.e., mineralizers) have been proposed as alternative routes to develop refractory products suitable for intermediate working temperatures (700–1000 °C). In this sense, the use of calcium-free binders, such as hydratable alumina or silica-based compounds, has gained some attention. Additionally, the incorporation of sintering additives to castables can speed up the expected sintering reactions, due to the generation of liquid phase in the designed microstructure during the first heating treatment of these products. Therefore, this work evaluates self-flowing high-alumina castables bonded with 4 wt.% of calcium-free binders: hydratable alumina, a novel powdered microsilica-alumina-based one or their blend. Besides that, it was also analyzed the influence of adding 0 or 1 wt.% of boron carbide to the castables, as this compound can act as a transient liquid sintering agent. The designed compositions were characterized via flowability, thermogravimetric tests, hot elastic modulus, erosion resistance, cold and hot mechanical strength and other techniques. The castable bonded with the blend of the selected binders showed shorter setting time and lower drying profile performance, as the samples' explosion for these compositions could be detected even for small samples (50 × 50 mm) when using a heating rate of 5 °C/min. On the other hand, the refractory containing the microsilica-alumina-based binder presented enhanced thermo-mechanical behavior. Furthermore, boron addition favored faster sintering of the castables, resulting in optimized performance around 815 °C or even at 1100 °C for the mixture containing only the silica-based additive. [ABSTRACT FROM AUTHOR]

In recent years, nano-binders (mainly colloidal suspensions) have been proposed as alternative materials for applications that require CaO-free refractory lining or improved mechanical behavior at intermediate temperatures (700 °C< T < 1200 °C). Despite the benefits of these suspensions, nano-bonded castables usually present limited green mechanical strength and different on site logistics to handle the liquid. Considering the availability of novel alumina-silica-based powdered binders, this work investigated the role of submicron alumina and SioxX®-Zero (both supplied by Elkem company) on rheological and mechanical properties of vibratable high-alumina castables, aiming to identify whether they can be suitable options to replace colloidal silica suspensions. Cold and hot mechanical strength and apparent porosity in the range of 110–1400 °C, cyclic thermal shock resistance, creep tests and hot elastic modulus of the designed formulations were evaluated. According to the results, SioxX®-Zero-bonded compositions presented good flowability levels and their sintering process started around 800 °C. Adding boron carbide to the same formulations resulted in transient liquid sintering of the silica-containing refractories, which allowed the development of compositions with improved thermo-mechanical performance in the 600–1400 °C temperature range. Furthermore, the submicron alumina-bonded samples presented fast sintering, resulting in E values close to 200 GPa (the highest value so far registered in our lab for a coarse grain size formulation) after one heating-cooling thermal-cycle up to 1400 °C. [ABSTRACT FROM AUTHOR]

Sintering additives (SA) have great potential to be used in refractories to enhance their thermo-mechanical properties at intermediate temperatures (600–1200 °C). Boron-based compounds are pointed out as suitable options for this purpose as they undergo phase transformations (i.e., decomposition, oxidation, etc.) at relatively low temperatures, which may lead to boron-rich liquid phase generation and transient liquid phase sintering. Considering these aspects, the present study aims to evaluate the role of five different SA [boron oxide (B 2 O 3 ), boric acid (H 3 BO 3 ), sodium borosilicate (BS), magnesium borate (BM) and boron carbide (B 4 C)] when 0.5, 1.0 and 2.0 wt% of these materials were added to alumina-based castable compositions bonded with hydratable alumina. Flowability, apparent porosity, XRD, hot elastic modulus, thermal shock resistance and mechanical tests were carried out in order to identify whether (i) transient liquid phase sintering would be favored in these refractories, and (ii) at which temperature range such compositions might be submitted during service and still present an improved performance when compared to an additive-free castable (reference). Based on the results, incorporating boron oxide into the alumina-based castables was not recommended as this hygroscopic material reduced the flowability of the compositions, preventing a suitable preparation of the samples. The other boron-containing compounds could be directly added to the high-alumina mixtures without resulting in major changes in their processing steps. Nevertheless, the paper highlights the importance of selecting a proper SA source and optimizing its content in the castable composition as these parameters may affect its overall rheology and thermo-mechanical properties. The formulations containing 0.5 wt% of H 3 BO 3 or B 4 C, 1 wt% of sodium borosilicate and 2 wt% of magnesium borate presented enhanced erosion and thermal shock resistances and hot mechanical strength at 1000 °C and 1200 °C, which makes them potential refractory castables to be used in petrochemical and aluminum industries. [ABSTRACT FROM AUTHOR]

Refractoriness under load (RUL) tests are usually indicated to evaluate the maximum working temperature that refractory materials can withstand by measuring their linear dimensional change when subjected to compressive stress. Nevertheless, ASTM C832 and ISO 1893 standards do not point out which sort of samples (pre-fired or not) should be used during the measurements of this property. Thus, this study addresses the evaluation of different refractory castable systems, focusing on analyzing calcined (fired up to 600 °C for 5 h for the decomposition of the hydrated phases) or pre-fired (1550 °C for 5 h) samples in order to identify the effect of the material's thermal treatment on the RUL results. Additional properties (i.e. linear expansion change, apparent porosity and cold flexural strength after firing at different temperatures) of the designed compositions were also evaluated. Based on the results, it can be concluded that the maximum working temperature of monolithics should not be assessed via one single test (i.e. RUL), but only a set of techniques (RUL, dilatometry, assisted sintering, creep and others) can provide useful data to better understand the refractories performance. RUL measurements should be carried out after a preliminary sintering step of the castables (where the selected firing temperature might be as close as possible to the expected T 0.5 ) to enable most of the microstructural changes resulting from heating up and sintering could already have taken place. Furthermore, it is most likely that numerical solutions (modeling) could help to better understand and interpret the multivariable aspects of this complex issue. However, reliable and accurate values of the refractory properties (based on well-selected tests) must be fed to the software to permit calculations that yield meaningful and accurate results, and conclusions. [ABSTRACT FROM AUTHOR]

Phosphate refractories have a great potential to be applied in petrochemical industries as they present suitable properties at the temperature range used in fluid catalytic cracking units. This study addresses the development of high-alumina self-flowing castables bonded with H 3 PO 4 solution (48 wt% concentration) or a mixture of phosphoric acid and monoaluminum phosphate (MAP) solutions, using MgO as a setting agent. Two polyphosphates (Budit 3H and 6H) and citric acid were evaluated as dispersant additives for these castables. The compositions were characterized by measuring their free-flow and temperature evolution over time, working and setting times, cold and hot mechanical strengths, drying behavior and explosion resistance, eroded volume and thermal shock resistance. The results indicated that high flowability (free flow >100%) could be attained when adding the selected polyphosphates to the mixtures, whereas citric acid acted mainly as a retarder agent for the castables’ setting. Moreover, free-flowing compositions with a suitable working time were obtained when combining H 3 PO 4 +MAP solutions as main binders. The thermo-mechanical tests pointed out that the most promising designed refractory (containing mixture of H 3 PO 4 +MAP and 0.5 wt% of Budit 3H) presented similar or even a better performance than a benchmark commercial vibratable product used in petrochemical units. [ABSTRACT FROM AUTHOR]

Phosphate-bonded refractories may be applied as repairing materials due to their short setting time and good thermo-mechanical properties in the 30–1000 °C temperature range. This works addresses the development of vibratable high-alumina castables containing commercial monoaluminum phosphate (MAP) solutions (Fosbind 151 and Fosbind 50) and dead-burnt magnesia ( d <212 µm, setting agent) as the binder systems. Flowability, setting time, X-ray diffraction, cold erosion, thermal shock resistance, cold and hot modulus of rupture, thermogravimetric measurements and hot elastic modulus tests were carried out in order to understand the phase evolution and thermo-mechanical behavior of the refractories. Furthermore, the effect of adding a boron source (sintering additive) to the phosphate-bonded compositions was also investigated. According to the attained results, the reaction of MAP with MgO and the reactive aluminas of the compositions resulted in setting times of the mixtures around 90–120 min at 30 °C, which was associated with the in situ generation of magnesium and aluminum phosphates [MgHPO 4 ·3H 2 O, Mg(H 2 PO 4 ) 2 (H 2 O) 2 and AlPO 4 .2H 2 O]. The boron-containing castables presented Al 18 B 4 O 33 around 650–800 °C and this phase favored the increase of the samples' stiffness, mechanical strength, erosion and thermal shock resistance. The refractoriness under load measurements indicated that the maximum working temperature for the evaluated refractories was in the range of 1400–1500 °C. [ABSTRACT FROM AUTHOR]

This work addresses the main aspects related to the use of alternative binders [hydratable (HA) or colloidal alumina (ColAlu)] in castables containing different spinel sources (pre-formed or in situ generated), in order to point out: (i) the features that control the corrosion behavior of these materials, and (ii) the key factors to better select a refractory composition. Thermodynamic calculations, corrosion cup-test and SEM analyses were carried out in order to evaluate the slag attack of the designed refractory compositions. According to the attained results, the alumina-based binders (HA or ColAlu) induced a more effective sintering process due to their high specific surface area, improving the physical properties and the binding level of the generated microstructure. The spinel grain size also played an important role in the corrosion behavior of these refractories, as the finer the particles, the greater their dissolution was into the molten liquid, leading to further precipitation of spinel in the solid–liquid interface as a continuous and thick layer. Among the evaluated compositions and considering the presence of silica fume, the most suitable formulation with optimized corrosion resistance was the one with in situ spinel generation and HA as a binder. [ABSTRACT FROM AUTHOR]

Monoaluminium phosphate [Al(H 2 PO 4 ) 3 or MAP] is pointed out as the most applied chemical binder in the refractory field due to its solubility in water, high bonding strength and reaction with basic and amphoteric raw materials at low temperatures. Nevertheless, different routes can induce the in situ generation of Al(H 2 PO 4 ) 3 in castable compositions. This work evaluates the effect of two Al(OH) 3 sources added to refractory formulations or H 3 PO 4 solutions (with distinct concentrations) in order to highlight their influence on the phase evolution and thermo-mechanical properties of the prepared samples. Various experimental tests (flowability, setting time, cold and hot mechanical strength, thermogravimetric measurements, X-ray diffraction and hot elastic modulus) were carried out. According to the attained results, the most promising binding system was obtained by the mixture of H 3 PO 4 solution with 48 wt% of concentration+Al(OH) 3 particles prior to preparing the castables. Furthermore, the work suggests that a systemic analysis of various parameters (acid concentration, liquid solution amount, type and amount of setting and sintering agents, etc. ) should be conducted in order to attain improved phosphate-bonded products. [ABSTRACT FROM AUTHOR]

This work focuses on the evaluation of the corrosion behavior of Al 2 O 3 –MgO cement-bonded castables based on thermodynamic calculations, in order to understand the reaction steps and some scientific and technological issues that still need to be better explained when such materials come into contact with basic industrial slags at high temperatures. Special attention was given to analyzing the role played by the calcium hexaluminate phase (CA 6 ) during the first heating stage of the samples and their further chemical attack. Experimental measurements (cup tests at 1500 °C) and thermodynamic simulations were carried out considering a basic slag (CaO/SiO 2 =4.2) and four refractory castables with different spinel (MgAl 2 O 4 ) sources (pre-formed or in situ) and with or without silica fume (0 or 1 wt%). Considering the attained results, a novel corrosion mechanism is proposed pointing out that a suitable design of the castable compositions can induce the formation of CA 6 at the border of the coarse aggregates, preventing their further dissolution and increasing the overall wear resistance of these refractories. In Addition, incorporating optimized contents of silica fume and cement to the spinel-containing castables helps to master the generation of the CA 6 and CA 2 protective layers, resulting in a lower chemical attack at high temperatures. [ABSTRACT FROM AUTHOR]

Abstract: Even though many investigations have focused on describing the kinetics of both MgO hydration and Mg(OH)2 decomposition, few studies have addressed the understanding of the mechanisms to control or to modify the reaction of magnesia with water. Silica is the most applied anti-hydration additive in MgO-containing castables. The use of silico-phosphates (SiO2 rich and presenting higher reactivity and solubility) seems to be a promising alternative to induce a faster generation of magnesium–silicate–hydrated gels on the MgO grain surface, inhibiting brucite formation. In the present work, the performance of a commercial phosphate chemical binder (combined or not with silica fume) in Al2O33MgO refractory castables and MgO aqueous suspensions was evaluated. Thermodynamic simulations, thermogravimetric measurements, X-ray diffraction, splitting tensile and hot elastic modulus tests were carried out to understand the additive performance and the characterization of the castables properties. According to the simulated Pourbaix diagrams, two different gels [Mg3Si2O5(OH)4 or Mg3Si4O10(OH)2] can be formed in the structure of the samples (depending on the Mg:Si molar ratio), halting the MgO hydration. Due to the high amount of MgO (6wt%) contained in the designed Al2O33MgO castables, the chrysotile-like phase [Mg3Si2O5(OH)4] should be the main compound formed during the refractories' processing. Moreover, the blend of silica fume and the phosphate-based additive consisted of the most effective route to stop the Mg(OH)2 formation so that castables with high flowability, mechanical strength levels and elastic modulus values can be prepared. [Copyright &y& Elsevier]

• The performance of fused silica-based semi-insulating castables was evaluated. • A Na-metakaolin-based geopolymer (GP) was investigated as an alternative binder. • CAC-bonded castables with and without a porogenic agent (PA) were also evaluated. • A CAC- and PA-free castable with enhanced properties could be obtained. • The GP-bonded castable exhibited a thermal conductivity of 0.735 W/m.K at 800 °C. Semi-insulating refractories are recognized for their capability to minimize thermal losses and potentially save energy. Encouraging the advancement of cement-free compositions, cold-setting binders like geopolymers have emerged as captivating alternatives. This study focused on evaluating the incorporation of a metakaolin-based geopolymer binder in semi-insulating fused silica-containing castables. As reference materials, calcium aluminate cement (CAC)-bonded compositions with and without polymeric microspheres were also investigated. Three refractory castables were processed, and the prepared samples underwent characterization, including mechanical strength, apparent density and porosity, thermal shock resistance, thermal conductivity measurements, and SEM analysis. The geopolymer-bonded castable presented improved performance, exhibiting a thermal conductivity of 0.735 W/m.K at 800 °C, flexural strength of 6.24 MPa and apparent density of 1.68 g/cm3 after firing at 815 °C. The results underscore that utilizing geopolymer as a replacement for CAC enabled the design of a semi-insulating castable with enhanced properties and without the need for using porogenic agents. [ABSTRACT FROM AUTHOR]

The current trend to replace shaped refractories for monolithics is related to various technical aspects, i.e., lack of joints, easier installation and free format. However, the main drawback of these non-shaped products is their longer first heat-up demand, otherwise mechanical failures or even explosive spalling may take place due to steam pressurization within the well packed consolidated structure. As a consequence, over-conservative drying schedules are commonly used in industrial equipment lined with such ceramics, which increase the energy consumption and the production down-time. Based on this scenario and considering the recent advances in numerical simulations, this article discusses the implications of applying different heating rates during the drying of a refractory castable lining of a steel ladle. Additionally, a novel methodology based on the correlation of the maximum vapor pressure and the mechanical strength of the ceramic material was also proposed. Using some of the castable properties measured as a function of temperature, simulation results pointed out that higher vapor pressures and a displacement of the maximum pressure peak are attained by applying high heating rates and thicker linings. This behavior is associated with the water migration to the inner regions of the refractory structure. It was observed that these conditions displace the maximum resistance ratio position towards the hot face, where most common spallings are observed. The work also emphasizes the importance of carrying out accurate measurements of physical properties as a function of the temperature, otherwise the outcome can be far from the reality of the working conditions. [ABSTRACT FROM AUTHOR]