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Alternative binders offer an avenue for advancing ultra-low-cement and cement-free castables while reducing carbon emissions and energy consumption. In this study, geopolymer binders partially or fully replaced calcium aluminate cement (CAC) in high-alumina castables. The formulations underwent diverse testing, including flowability, physico-thermo-mechanical measurements, and XRD, ATR-FTIR, and SEM analyses. The findings highlighted the feasibility of utilizing geopolymers to craft innovative refractories. The formulation comprising 2.7 wt% CAC and 1.3 wt% geopolymer (AT-3 C-1 G*) exhibited the most successful performance. At 1400 °C, these samples displayed shrinkage of 1.18%, elastic modulus of 135 GPa, flexural strength of 39.1 MPa, and high thermal shock resistance (∆T∼1000 °C). The interaction between CAC and geopolymer inhibited cement hydration while enhancing the geopolymerization process. Moreover, firing the samples at 800–1400 °C led to liquid phase and nepheline formation from the geopolymeric gel, resulting in viscous sintering and improved ceramic densification. These developments produced refractories with remarkable overall performance, surpassing the reference material. [ABSTRACT FROM AUTHOR]

This study explored the feasibility of utilizing varying proportions of metakaolin – MK (4, 6, or 8 wt%) and two types of alkaline liquid reagents – LR (incorporating nanosilica and NaOH or KOH) to induce in situ geopolymer formation in high-alumina refractory castables. The impact of geopolymeric binders on processing behavior, as well as on the physico-mechanical and structural properties of the designed refractories, was investigated within a temperature range of 40–1250 °C. Optimal physico-mechanical performance and thermal shock resistance were achieved with compositions 8 MK-Na (prepared with 8 wt% MK and NaOH-based LR) and 6 MK-K (containing 6 wt% MK and KOH-based LR). The liquid generation during viscous sintering enhanced castable densification at 1100–1250 °C and promoted the formation of nepheline, kalsilite or leucite phases. This resulted in microstructures featuring a suitable interfacial bond strength between the glassy phase and alumina grains, leading to an interlocking structure and improved castable properties. These achievements underscore the potential of geopolymer-bonded high-alumina castables for applications involving intermediate temperatures (800–1200 °C), such as those found in the petrochemical and non-ferrous metal industries. [ABSTRACT FROM AUTHOR]

• An alkaline solid reagent (SR) based on NaOH and colloidal silica suspension was synthesized. • The processing stages and physico-mechanical properties of one-part MK-geopolymers were evaluated. • Optimal results were achieved with a mixing time of 20 min and a formulation containing 50 wt% of SR. In recent years, there has been a growing interest in one-part geopolymers due to their simplified preparation process, which involves adding water instead of highly alkaline and corrosive solutions. This study aimed to synthesize a solid reagent (SR) based on NaOH and colloidal silica and investigate its impact on the processing stages and resulting physico-mechanical properties of one-part geopolymers based on metakaolin. After curing the samples at 40 °C, various properties, including elastic modulus, cold crushing strength, apparent porosity, and density, were evaluated. Optimal results were achieved with a mixing time of 20 min and a formulation containing 50 % by weight of SR. Under these conditions, the cured samples exhibited an elastic modulus of 8.2 GPa, a crushing strength of 14.04 MPa, an apparent porosity of 16 %, and a density of 1.51 g/cm3 after one day. These findings highlight the potential application of the developed solid reagent in the design of geopolymers for thermal and acoustic insulation in modern construction projects. [ABSTRACT FROM AUTHOR]

In situ spinel (MgAl 2 O 4 , ZnAl 2 O 4 , etc.) formation in alumina-based castables has been pointed out as a key issue in the refractory area, as it may enhance the mechanical properties and corrosion resistance of such materials. Nevertheless, the generation of these compounds is usually accompanied by a large volume expansion, which may induce crack formation in the resulting microstructure. Aiming to investigate the influence of zinc and magnesia spinel generation on the properties of alumina-based castables, three vibratable compositions, containing calcium aluminate cement or hydratable alumina as binders, were designed and characterized in this work. Different experimental tests were carried out to analyze the produced samples, such as: cold flexural strength, apparent porosity, hot elastic modulus, assisted sinterability, corrosion cup-tests, etc. According to the results, ZnAl 2 O 4 was mainly formed above 800 °C, favoring an earlier sintering of the samples. The addition of high amounts of ZnO or MgO (3 wt% to 9.4 wt%) to the castables resulted in the expansion of the samples during their first thermal treatment up to 1500 °C, reaching dL/L0 values equivalent to 1.16% up to 2.35%. Thermodynamic simulations indicated that spinel phase presented higher chemical stability when in contact with the evaluated synthetic slag at 1500 °C. Depending on the corrosion cup-test procedure (free or under constraint), different results of the slag infiltrated area were obtained (13.7%–20.5%) and, the combination of two different spinel phases (MgAl 2 O 4 and ZnAl 2 O 4) in a castable composition resulted in a refractory with enhanced corrosion behavior when the tests were carried out under constraint. [ABSTRACT FROM AUTHOR]

Drying is the most critical process of the first heating cycle of monolithic dense refractories, as the reduced permeability of the resulting microstructure may lead to explosive spalling and mechanical damage associated with dewatering. The first part of this review series pointed out the various drying stages, the role of the binder components and the techniques that can be used to follow the water release in as-cast refractory materials, when they are exposed to heat. Although defining a suitable heating schedule is a great challenge, some tools can be applied to minimize the spalling risks associated with steam pressurization. In this context, this second review article points out (i) the main drying agents and how they affect the resulting castables' microstructure (organic fibers, metallic powders, permeability enhancing active compounds, silica-based additives and chelating agents), and (ii) the effects related to the procedures commonly applied during the designing of heating routine (i.e., the role of the heating rate, ramp versus holding time), as well as the influence of the castable's dimension on the overall drying behavior. Considering the recent advances regarding the design of refractory formulations and their processing, one may expect that incorporating suitable drying additives into the prepared composition should lead to a suitable and safer water release in such dense consolidated structures. Besides that, novel engineering opportunities, such as the use of in-situ based experimental techniques (i.e., neutron and X-ray tomography) to obtain more accurate data and the development of numerical models, might help in simulating and predicting the steam pressure developed in refractory systems during their first heating. Consequently, instead of designing conservative drying schedules based on empirical knowledge, the novel optimized heating procedures should be based on technical and scientific information. [ABSTRACT FROM AUTHOR]

Polymeric fibers can be used as drying additives for refractory compositions as their softening and decomposition at the beginning of the heating procedures of those ceramics induces the generation of permeable pathways, providing safer steam release out of the dense microstructure. Thus, it is required to select fibers that favor the preparation of castables with suitable permeability levels at temperatures as low as possible to enhance their explosion resistance during drying. Therefore, this work addresses the production of low-melting-point polymeric fibers (T m ∼ 70–150 °C) and the evaluation of their performance when incorporated into dense high-alumina calcium aluminate cement- or hydratable alumina-bonded refractory castables. EVA (ethylene vinyl acetate copolymer), PCL (polycaprolactone) and PLA (polylactic acid) fibers were produced and added to the designed compositions. Based on the collected results, changes in fiber morphology during their softening and melting process had a great impact on the drying behavior of the castables under different heating rates. Hydratable alumina-bonded compositions were more susceptible to explosions due to such transformations, whereas the CAC-bonded ones exhibited explosion resistance at the hardest heating schedule (20 °C/min). Micrographic analysis of the fiber morphology during heating provided relevant information that helped to better understand the drying process and the permeability evolution of the evaluated dense refractory castables. [ABSTRACT FROM AUTHOR]

Despite the continuous evolution on the performance of refractory ceramic products, monolithic materials still require special attention during their processing steps as various phase transformations may take place during the curing, drying and firing stages. Drying is usually the longest and the most critical process observed during the first heating cycle of refractory linings, as the enhanced particle packing and reduced permeability of the resulting microstructure may lead to recurrent explosive spalling and mechanical damage associated with dewatering and the development of high steam pressure at the inner regions of such dense materials. In this context, this review article mainly addresses (i) the theoretical aspects related to the drying process of dense refractories, (ii) the influence of the phase transformations derived from the binder additives, and (iii) the usual and advanced experimental techniques to assess the water removal from consolidated castable pieces. Many studies have pointed out that due to the complex nature of this phenomenon (i.e., considering combined thermal stresses and pore pressure, heterogeneous microstructure, evolving pore structure with temperature, etc.), the mechanisms behind the water withdrawal and castables' explosive spalling are lacking further understanding and, consequently, it has been difficult to save time and energy during the first heating of industrial equipment lined with ceramic materials. On the other hand, different methods are used for refractory spalling assessment and many efforts have been carried out in applying in situ imaging techniques (such as NMR and neutron tomography) to follow the moisture evolution during such thermal treatments. These novel techniques, also addressed in this review, might be of particular importance to provide more accurate data for the validation of many state-of-the-art numerical models, which can be used to predict the steam pressure developed in refractory systems and help in the design of proper heating schedules for such products. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

Great efforts have been made recently to totally or partially replace calcium aluminate cement (CAC) by alternative materials in refractory castables, in order to attain an enhanced thermomechanical performance of these ceramic linings at intermediate temperatures (600–1200 °C). Besides that, using additives that induce earlier sintering/densification of the refractory microstructure may also reduce the energy costs derived from the production of pre-formed pieces. Based on these aspects, this work investigated the viability of replacing CAC by calcium carbonate (CaCO 3) or calcium hydroxide [Ca(OH) 2 ] to ensure a suitable binding action and effective sintering/densification of the designed compositions at intermediate temperatures. Six high-alumina castables containing these alternative additives or their blend were prepared and their green mechanical strength, apparent porosity and Young's modulus evolution with temperature were evaluated within the 30–1400 °C range. After that, the most promising compositions were characterized via X ray diffraction and thermomechanical tests, such as cold and hot modulus of rupture, thermal shock resistance, etc. Although the selected binders did not result in specimens with green mechanical strength values as high as the ones for the cement-bonded materials (2–8 MPa versus ∼18 MPa, respectively), they could be demolded and handled without any problems. CaCO 3 and/or Ca(OH) 2 -bonded compositions presented a sintering effect at intermediate temperatures (600–1000 °C) due to the so-called "sintering-coarsening-coalescence" phenomenon. These transformations favored the faster sintering/densification of the tested castables, resulting in samples with improved cold and hot mechanical strength at 900 °C, reaching values within the range of 28–30 MPa instead of 10–13 MPa for the CAC-bonded one. After firing the evaluated compositions at higher temperatures (up to 1500 °C), all compositions presented similar results regarding their modulus of rupture or thermal shock resistance. [ABSTRACT FROM AUTHOR]

Abstract New challenges have been identified over recent decades regarding the increasing complexity of the dry out step of refractory castables, mainly due to the advances on the particle size distribution design (and consequent reduction in the ceramic linings' permeability) and growing demand for shorter equipment maintenance time, which generally results in pressures to speed up the drying. Some efforts focused on the design of novel permeability enhancing active compounds (PEAC) have been receiving great attention, as such products may be an interesting alternative for producers and end-users. This work evaluates the addition of a permeability enhancing active compound (AC) to high-alumina based castables and also analyzes the effect of different parameters (i.e., particle packing models, hydraulic binders, drying temperature, etc.) which could affect the dry out behavior of such refractories. Initially AC-containing castables, additive-free and mixtures with a polymeric fiber with low melting point were analyzed. Further investigations of the selected AC action mechanism were carried out in a second moment, where novel castables containing distinct binders (Secar 51, Secar 71 or Alphabond 300) and with or without this drying agent were characterized regarding their flexural strength, apparent porosity, permeability and explosion resistance at different temperatures and heating rates. The obtained results indicated that adding this active compound into the alumina-based castables favored a change in the cement hydration path, giving rise to gel-like hydrates (instead of AH 3 and C 3 AH 6 phases) that underwent decomposition at low temperatures (100–140 °C). Consequently, such transformations resulted in the samples' permeability increase, which inhibited their explosion during the heating up step. However, other important properties such as flowability, setting time and green mechanical strength of the AC-containing compositions were also affected, due to the interaction of this additive with water and the fine refractories' raw materials and the generation of a CAH-gel phase in the castables' microstructure. [ABSTRACT FROM AUTHOR]

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]

Commercial (Rs) and laboratory synthesized resoles (LSRs) containing 3–5 wt% ferrocene have been used to study the development of thermosetting resin compositions as binder materials for carbon-containing refractories (CCRs) production. The LSRs were developed based on formaldehyde to phenol 1.5 mol.% (1.5Rs) and 2.0 mol.% (2Rs). Microstructure and oxidation resistance of carbons derived from plain and modified resins have been compared. Chemical structure of the resins was studied using Fourier Transform InfraRed-Attenuated Total Reflection (FTIR-ATR) spectroscopy. X-ray Photoelectron spectroscopy was used to examine chemical changes during heat treatment of the modified products. Flow behavior of plain and formulated resins has been measured under isothermal condition. Rheology measurement results show that the selected amount of ferrocene did not affect the resin near-Newtonian fluid behavior and processability during refractory production. X-ray diffraction (XRD) analysis and transmission Electron Microscope (TEM) were used to characterize the derived carbon samples after thermal treatment of the resins. The results demonstrate that ferrocene is an excellent additive for crystallizing resoles carbon. The presence of fringes in TEM images indicates the graphitized carbon microstructure. The carbon reactivity in an oxidizing environment was also determined via thermogravimetric analyzer (TGA). Several factors, such as bond strength and atoms arrangement, have been found to control the carbonized modified-resin oxidation resistance. fx1 • Synthesis of resole resins. • Graphitization of carbons derived from resole resins by ferrocene. • Oxidation resistance of carbons from the plain and modified resins. • Chemical changes during carbonization of resin formulation containing ferrocene. [ABSTRACT FROM AUTHOR]

MgO is pointed out as an alternative binder for refractory materials, mainly for systems where the presence of CaO might not be desired. Selecting the most suitable magnesia source is an important step as its purity and reactivity should influence the hydration reaction, leading to binding effect or cracks. This work investigated the design of vibratable high-alumina compositions bonded with MgO fumes [which is a very fine powdered oxide (d < 3 µm) resulting from the production process of electrofused magnesia] and/or dead-burnt magnesia (d < 212 µm). Acetic and formic acids were added to the castables during their processing steps in order to adjust the density of active sites for Mg(OH) 2 formation and control the crystal growth of this phase. The green mechanical strength and thermomechanical performance (cold and hot mechanical strength, thermal shock, refractoriness under load, corrosion, etc.) of designed MgO-bonded compositions were analyzed. Improved green mechanical strength and crack-free samples were obtained when adding up to 6 wt% of MgO fumes to the refractories and processing them with aqueous solutions with 3 wt% of formic acid. The compositions with 6 wt% of magnesia fumes resulted in samples with flexural strength in the range of 12.0 MPa after curing at 50 °C/24 h and similar green mechanical strength (12.9 MPa) as the ones bonded with 4.0 wt% of calcium aluminate cement after drying at 110 °C for 24 h, which highlights the great potential of this MgO source. Despite the enhanced green mechanical strength, alumina-based castables containing 6 wt% of MgO (fumes, dead-burnt or their blend) showed low mechanical strength at intermediate temperatures and high linear expansion, as a consequence of the in situ spinel phase formation above 1200 °C. Thus, better densification, improved HMOR, thermal shock resistance and corrosion behavior were obtained for the castables prepared with less MgO fume contents. [ABSTRACT FROM AUTHOR]

Calcium hexaluminate (CA 6 ) presents interesting properties and morphology, which can be readily changed depending on specific additives and refractory processing conditions. Aiming to investigate the role of calcium carbonate in inducing the formation of elongated CA 6 grains and also identify its sintering effect during the thermal treatments of high-alumina castables, this work focused on evaluating compositions containing calcium aluminate cement, CaCO 3 or their blend with the help of in situ techniques (hot elastic modulus, assisted sintering) and other traditional methods (mechanical strength, thermal shock resistance, etc.). A sintering effect derived from CaCO 3 addition to alumina castables could be identified during the hot E measurements, pointing out the ability of this compound to undergo a sintering-coarsening-coalescence transformation at 500–900 °C. This process also enhanced the mechanical strength and thermal shock resistance of the designed refractories at intermediate temperatures. Acicular CA 6 grains were formed in all analyzed compositions after firing at 1400 °C. [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]

Slag engineering (i.e. conditioning and foaming practices) is becoming an essential issue in many companies due to the increasing need felt by the end-users to cut costs and produce high quality steel. Slag foaming has been mainly used in electric arc furnaces (EAF) in order to protect the refractory materials from the high energy intensity (radiation) generated by electrodes, decrease the noise level, improve productivity and the energy efficiency of this equipment. Nevertheless, the correct control and optimization of the foam generation are still limited and some of the main factors (basicity, viscosity, surface tension, presence of suspended solid particles, FeO content and injection rate of carbon particles and oxygen, and others) that affect this complex phenomenon are discussed in this review. Considering the data presented in the literature, there are various conditions and opportunities to be exploited, and a standard procedure for the experimental evaluation of the foam formation is still required. Thermodynamic calculations can be an alternative tool to help understand the phase transformations related to slag foaming at high temperatures. [ABSTRACT FROM AUTHOR]

This study investigated how to induce graphite generation from the carbonization process of novolac resins using conditions that can be adopted for carbon-containing refractories (CCRs) production. The effect of boron oxide or boric acid (graphitizing agents), cross-linking additive (hexamethylenetetramine) and some processing parameters (mixing technique, vacuum degassing, heating rate and thermal treatments) on carbon graphitization from a commercial novolac resin were evaluated. The X-ray diffraction (XRD) technique was selected to measure the graphitization level and crystal parameters of the prepared samples. Based on the attained results, adding graphitizing agents prior to the pyrolysis of resin resulted in carbon crystallization. The best graphitization level was obtained when the mixtures containing 6 wt% B 2 O 3 or 10 wt% H 3 BO 3 were fired up to 1000 °C for 5 h using a heating rate of 3 °C/min. Although the reproducibility of the obtained results was ascertained, heterogeneous graphitization could be observed based on the XRD profiles, as well as some discrepancies in the calculated graphitization level values. This phenomenon was attributed to the additives susceptibility to agglomeration, preferential graphitization starting from lower binding energy sites and heat treatment temperature, among others. [ABSTRACT FROM AUTHOR]

Among the available routes to enhance the mechanical behavior and toughening degree of ceramics, in situ formation of acicular or platelet grains in the resulting microstructure has gained special attention. Various studies focused on investigating the self-reinforced role of these elongated phases (acting as bridging sites in the wake of a crack), as well as the use of very reactive raw materials to favor faster and more effective sintering/densification, have been carried out for compositions comprised mainly by fine raw materials. Nevertheless, there is a lack of studies considering more complex ceramics, such as refractory castables, which commonly present heterogeneous microstructures with coarse and fine components. Based on that, this work addresses the evaluation of self-reinforced high-alumina refractories containing reactive aluminas (AloxX spheres and/or submicron alumina) and calcium aluminate cement as binders. Boron carbide and microsilica were also added to some of the prepared compositions to favor the in situ formation of Al 18 B 4 O 33 or CA 6 phases with needle-like morphology. Various experimental evaluations (cold and hot mechanical strength, hot elastic modulus, SEM, XRD, thermal shock resistance, refractoriness under load, etc.) were carried out to identify the influence of the new phases’ formation in the overall performance of such materials. According to the attained results, both mineralizing agents (B 4 C and SiO 2 ) induced the formation of Al 18 B 4 O 33 or CA 6 grains in the designed compositions. However, a remaining liquid content was still present in these samples at a high temperature, resulting in some drawbacks concerning their hot properties. Additionally, submicron alumina proved to be an interesting binder as it induced improved sintering and led to a refractory with high stiffness (~200 GPa), high thermal shock resistance and mechanical strength. [ABSTRACT FROM AUTHOR]

This work revisits the proposed mechanisms presented in the literature for CA 6 formation in Al 2 O 3 -MgO and Al 2 O 3 -MgAl 2 O 4 castables bonded with calcium aluminate cement. New experimental tests, thermodynamic simulations and re-evaluation of the chemical composition and microstructural aspects observed for samples fired in the temperature range of 1150 °C to 1500 °C were carried out. Based on these data, a new interpretation of the CA 6 generation process, as well as the features which influence the location and morphology of this phase were proposed. CA 6 formation via solid and liquid states are suggested to take place in all evaluated compositions, where the former (solid-state) is the main reaction predicted for the silica-free refractories (AM0MS and AS0MS), whereas the liquid-state one prevails in the AM1MS and AS1MS materials. The CA 6 crystal morphology should be affected by these different reaction mechanisms. According to the experimental results, it was also discussed the role of the calcium hexaluminate features in the overall corrosion behavior of the designed refractories when they were placed in contact with molten slag at high temperatures. Such aspects have not been previously reported in published papers related to this subject. [ABSTRACT FROM AUTHOR]

Based on refractory end-users’ requirements, continuous efforts have been made to design engineered products able to withstand high temperatures (800–1500 °C) and severe thermal gradients. One alternative to enhance the mechanical properties of alumina-based compositions consists of inducing in situ generation of phases with platelet or acicular shape within their matrix fraction, which may improve crack deflection and grain bridging mechanisms. Mullite and Al 18 B 4 O 33 are some compounds that present such interesting features. Thus, this work addresses the evaluation of alumina refractory castables bonded with SioxX-Zero and/or microsilica, containing 0 or 1 wt% of B 4 C (sintering additive), aiming to: (i) induce transient liquid sintering, (ii) point out which silica source could favor a more effective mullite formation at high temperatures, and (iii) analyze the influence of B 4 C in the phase transformation and thermo-mechanical properties of the designed compositions. Comparing SioxX-Zero and microsilica-bonded refractories, the latter showed more likelihood to give rise to the mullite phase during the samples’ thermal treatments. Moreover, adding B 4 C to the castables containing 3 wt% of SiO 2 induced the generation of a boron-rich liquid phase with transient features during the samples’ firing step, favoring earlier sintering and faster mullite and Al 18 B 4 O 33 formation. These transformations resulted in refractories with enhanced creep, thermal shock resistance and HMOR behavior in a broader temperature range (600–1550 °C), which may enable them to be used in various industrial applications (petrochemical, steel-making, etc.). [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]

The chemical resistance and thermo-mechanical properties of refractories bonded with resole or novolak resins depend on the presence of crystalline carbon phases (preferentially with features close to graphite ones) in their compositions. Although thermosetting resins are commonly classified as non-graphitizing carbon sources, many efforts have been made in recent years in order to find effective routes to induce the in situ graphitization of such components in refractory products during service. This work evaluates the role of processing parameters (mixing, curing and firing temperature) and additives (ferrocene, boric acid and exfoliated graphite) in the graphitization process of two commercial resins (resole and novolak) and a synthesized one (modified-novolak). X-ray diffraction, Raman spectroscopy and thermogravimetric analyses were carried out to identify the microstructural evolution of the compositions. According to the results, carbon graphitization was already detected after firing the samples at 1000 °C for 5 h under reducing atmosphere. Ferrocene addition favored a more effective graphitization of the selected resins, but H 3 BO 3 also induced the rearrangement of the carbon derived from the commercial novolak product. The mixing and curing procedures used when preparing the compositions proved to be very important steps as they affected, to a greater extent, the resulting graphitization degree of the fired samples. [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]

Adding carbon to refractory products (i.e. MgO-C) usually results in unique properties that allow these materials to attain the performance level required for the steel-making process. Nevertheless, C oxidation is still a big concern and using antioxidant additives has become an important alternative to prevent carbon loss and induce the generation of further compounds (carbides, nitrides, etc.) in the fired microstructure, which will affect the overall thermo-mechanical behavior of the refractories. Aiming to better understand the main phase transformations derived from the interaction of polymeric binders with antioxidants and other refractory components in the MgO-C system, this work addresses the evaluation of mixtures containing novolak resin + antioxidants (Al, Si, and/or TiO 2 ) and: (i) fine MgO, (ii) ferrocene (catalyst for carbon graphitization), and (iii) coal-tar-based binder. X-ray diffraction, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analyses were carried out in order to identify the main phases comprising the fired samples (kept at 1000 °C and 1400 °C for 5 h in a reducing environment) and observe their morphology and distribution in the resultant microstructure. Based on the attained results, a great variety of whiskers with different shapes, dimensions and distributions were also observed on the fractured surface of the prepared samples containing antioxidants. Furthermore, not only the carbon graphitization, but also the presence of MgO and other binders (Carbores® P) affected the phase evolution at a high temperature, which highlights the complexity of phase transformations and the many likely interactions derived from the combination of various raw materials. [ABSTRACT FROM AUTHOR]

Using statistical methods (such as Design of Experiments – DOE) is an important issue due to the increasing demand to reduce costs and optimize properties of different processes and products. In order to assess the feasibility of these techniques in the refractory field, the Response Surface Method (RSM) and Tukey test were applied to optimize the modulus of rupture of high-alumina calcium aluminate cement (CAC)-bonded castables after curing and drying, considering three continuous factors (the amount of water, cement and dispersant) and a categorical one (the drying process). In addition to the cold mechanical strength, the flowability and apparent porosity of all the castable samples were also measured. The Tukey test pointed out the composition with the highest strength after curing and after drying. Based on the attained RSM models, the first one (considering all four factors) indicated the composition with the highest modulus of rupture after curing, but it could not properly predict the castables' properties after drying. The other two RSM models analysed different processing conditions (post curing and post drying, separately) highlighting that the complexity of the drying stage related to the hydration of the cement phases invalidated the models used for the post drying condition. The major difficulty in using statistical techniques comes from the transient and non-equilibrium conditions experienced by refractory ceramic castables during their processing and working conditions. [ABSTRACT FROM AUTHOR]

Metals and alloys (such as Al, Si, Al–Mg and Al–Si) are commonly added to MgO–C refractory bricks as antioxidants due to their effectiveness to prevent carbon oxidation (in the 600–1400 °C range) and their low cost. These additives act at different temperatures and react with refractory components and gases in the environment, inducing significant changes in the resultant microstructure and affecting the overall thermo-mechanical performance of these products. This work addresses the evaluation of physical properties, cold and hot mechanical resistance, as well as in situ hot elastic modulus (E) measurements in the temperature range of 30–1400 °C for MgO–C bricks containing antioxidants (Al, Si or Al–Mg alloy) in a reducing atmosphere. Cured and fired samples of the designed formulations were evaluated throughout 1 or 2 heating-cooling cycles. Despite the improved mechanical behavior (higher cold crushing strength and hot modulus of rupture) of the antioxidant-containing formulations, compared to the additive-free MgO–C sample, the interaction of the selected additives with the refractory components and CO (g) led to a generation of phases (i.e., Al 4 C 3 , Al 2 O 3 , SiC, SiO 2 , MgAl 2 O 4 ) that could not be well accommodated in the microstructure. Consequently, the in situ E drop was observed during cooling (mainly below 600 °C) of the antioxidant-containing sample due to crack and flaw formations. Si and Al–Mg were the most promising antioxidants, whereas the Al-containing composition showed the highest E damage level after two heating/cooling cycles up to 1400 °C for cured samples. Based on the elastic modulus profiles with the temperature, the results also indicated the best working conditions for these ceramic materials. [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]

The chemical resistance and thermo-mechanical properties of carbon-containing refractories are highly dependent on the presence of carbonaceous phases with structures similar to graphite ones. However, most of the thermosetting resins (used as binders) are classified as non-graphitizing carbon sources. Consequently, there is a high degree interest in finding alternative routes to induce an effective graphitization of such polymeric components at temperatures and conditions similar to the ones that refractories are submitted to in service. This research evaluates the effect of different catalytic agents (ferrocene, hematite and nano-Fe 2 O 3 powder) and some processing parameters (i.e., temperature, dwell time, heating rate, amount of cross-linking additive) on the graphitization process of a novolak resin. X-ray diffraction, SEM/EDS and thermogravimetric analyses were carried out to identify the phase and microstructural evolution of the prepared compositions. According to the attained results, carbon graphitization was observed after firing the samples at 1000 °C and 1400 °C for 5 h under a reducing atmosphere. The addition of ferrocene (>2 wt%) favored the generation of a higher amount of iron oxide particles in the microstructure at 1400 °C and led to a more effective graphitization, whereas the greater tendency for nano-Fe 2 O 3 powder to agglomerate during the initial samples’ mixing step inhibited carbon crystallization. Regarding the effect of the processing parameters, the use of a heating rate of 3 °C/min and dwell time of 5 h at the maximum temperature of 1400 °C, led to the generation of a maximum amount of graphitic carbon, 33.1%, in the composition with novolak resin +3 wt% ferrocene. [ABSTRACT FROM AUTHOR]

In situ spinel (MgAl 2 O 4 ) generation in Al 2 O 3 –MgO refractory compositions at high temperature is pointed out as one of the most important reactions to improve their thermo-mechanical performance in service. However, this transformation effectively takes place above 1200 °C. Magnesium borates are alternative compounds that can be used as mineralizer agents to speed up such a reaction. Aiming to develop improved spinel-containing castables and identify the impact of adding boron-based additives to the mixtures, this work evaluated the phase evolution of aqueous suspensions, matrix (fine components) and overall Al 2 O 3 –MgO refractory compositions containing 0% or 1%-p of a magnesium borate source. According to the XRD results of the castables׳ matrix, spinel formation took place at relative low temperatures (above 850 °C) due to reactions involving 1 wt% of the added magnesium borates and the available alumina sources (reactive alumina or Al 2 O 3 containing in the cement). In general, these samples also presented lower elastic modulus values in the range of 30–1400 °C, which was mainly related to the presence of the liquid phase, volumetric expansion associated to spinel and CA 6 formation. Faster MgAl 2 O 4 and CA 6 generation observed in the sinterability measurements indicated that the consequent drawback of the selected mineralizer addition was the greater volumetric expansion of the Al 2 O 3 –MgO refractory, leading to flaws nucleation and affecting its porosity level. [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]

Applying colloidal silica suspensions as a binder source for refractory castables has been of great interest in recent years. Nevertheless, the resultant low green mechanical strength of these castables has hindered their application in relevant areas. The current paper provides a novel engineering route to improve the mechanical properties of colloidal silica bonded castables before firing. In order to attain this target, small amounts of calcium aluminate cement (CAC) and/or hydratable alumina (HA) were used as gelling agents. A splitting tensile test showed that although using HA resulted in a more significant increase in the modulus of rupture of the samples when compared to CAC, mixing both (CAC+HA) had the most positive impact on the green mechanical properties of the castables, leading to strength levels as high as the reference cement-bonded system (CAC-Ref). XRD and DTG tests were carried out to evaluate the hydration behavior of the selected additives. The results indicated that CAC enhanced the hydration of HA based on the accelerated dissolution of the gel layer formed on HA particles, which was induced by the Al(OH) 4 − common ion effect. The proposed mechanism was confirmed by a specific conductivity test. Additionally, 29 Si solid state NMR spectroscopy was used to determine the effect of the dual additive system (HA+CAC) on the siloxane bond formation. Quantification of Q 2 , Q 3 and Q 4 species in alumina–colloidal silica suspensions containing HA, CAC or HA+CAC highlighted that the improved green strength is mainly related to a more effective hydration of HA. Based on the NMR and XHR SEM analyses, HA can be considered as an alternative gelling agent for nano-bonded refractory castables. [ABSTRACT FROM AUTHOR]

Elastic modulus evaluation is a simple and very accurate technique to investigate refractory castable behavior during curing, drying and firing steps. Therefore, analyzing this property might be useful to assess microstructural changes in Al2O3–MgO systems, where in situ transformations play an important role at high temperature. Considering this aspect, dead-burnt magnesia containing refractory castables with 0wt%, 2wt%, 4wt% or 6wt% of calcium aluminate cement and 1wt% of fumed silica, as well as an additional cement-free composition containing a caustic magnesia source (with higher reactivity) were evaluated using hot elastic modulus measurements, thermodynamic calculations and XRD analysis. The cement-free castables attested that the greater the brucite content, the higher the drop in the elastic modulus. On the other hand, MgAl2O4 formation increased the sample′s stiffness, but the presence of caustic magnesia did not significantly affect the elastic behavior at high temperatures. For the cement bonded systems, reactions between CaO, SiO2 and Al2O3 resulted in a remarkable increase in the elastic modulus above 800°C, whereas at higher temperatures liquid phase generation spoiled the castable′s properties. Thermodynamic calculations indicated that reducing the cement content, a lower amount of glassy phase was generated in the matrix, leading to a higher E value after the cooling stage. The elastic modulus profile for the second thermal cycle also suggested that a glassy phase transition occurred above 1000°C for the evaluated castables. [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]

Refractory castables used in fluid catalytic converter (FCC) risers should present suitable particle erosion and thermal shock resistances at temperatures below 900°C. Considering that calcium aluminate cement (CAC)-bonded refractories usually start their densification above 1200°C, the use of sintering additives to induce faster densification is a promising technological alternative. Therefore, this work addresses the evaluation of mullite-based castables containing a boron-based sintering additive and CAC and/or hydratable alumina as the binder sources. Hot elastic modulus, cyclical thermal shock, hot modulus of rupture and cold erosion resistance measurements were carried out to evaluate the compositions. According to the attained results, adding 1.5wt% of the evaluated sintering additive to the designed castables led to a remarkable increase of the hot modulus of rupture (maximum of 40.4MPa at 800°C for the CAC-containing refractory) and high erosion resistance (1.5–2.9cm3) after pre-firing at 800°C for 5h. Moreover, the combination of CAC and hydratable alumina gave rise to an improved refractory (M–2CAC–2HA–S) showing a transient liquid formation at an increased temperature, high thermal shock resistance (no E decay after 8 thermal cycles, ΔT=800°C) and high mechanical strength at 800°C and 1000°C. [ABSTRACT FROM AUTHOR]

Abstract: The slag foaming process is currently applied in the steel production due to its greater benefits, i.e., reduction of energy losses, enhancement of the refractory service life and productivity, etc. Lime/dolomite, oxygen and carbon are usually added during an electric arc furnace (EAF) operation to adjust the liquid basicity and saturation and also provide suitable conditions for foam generation. Considering the high concentration of MgO and C contained in spent MgO–C refractories and dolomite fines (derived from rotary kilns), one good alternative consists of recycling those materials as slag conditioners. In this work, experimental tests and thermodynamic calculations (phase equilibrium, viscosity and isothermal solubility diagrams) were carried out in order to define the impact of some mixtures (comprised mainly of ground MgO–C spent bricks, dolomite fines and coke) on the foaming behavior of a molten synthetic slag at 1700°C. Based on the attained results, recycling MgO–C bricks and dolomite fines led to faster slag saturation and provided suitable conditions for foam formation. Furthermore, the increase in FeO content in the liquid portion of the slag compositions presented a clear correlation with the measured foam height, indicating that this oxide plays a major role in the foaming phenomenon. [Copyright &y& Elsevier]

Abstract: Slag conditioning practice provides important benefits to the steelmaking processes including ladle refractory life improvement, lower flux cost/additions, better control of metal recovery, among others. Furthermore, with the advent of thermodynamic and mass balance calculations it is possible to predict the slag saturation and define the most suitable conditioner (oxide components) that should be added during the operational process. Thus, the aim of this work was to analyze the interaction of industrial slags and a MgO–C refractory composition by thermodynamic simulations (FactSage™). Based on the attained results, a conditioned slag was designed and corrosion tests in the laboratory were carried out (cup-tests and induction furnace), for two MgO–C materials containing distinct magnesia sources. According to the calculated slag penetration area and refractory corrosion rate, a remarkable enhancement of the MgO–C bricks wear resistance was observed when such materials were placed in contact with the conditioned molten slag. Therefore, proper slag engineering can be a suitable replacement for purchased synthetic slags and the conditioning practices can directly improve refractory life, with more competitive costs for both, refractory producers and end-users. [Copyright &y& Elsevier]

Abstract: When refractory castables are submitted to continuous thermal changes, crack nucleation and/or propagation can take place resulting in a loss of mechanical strength and overall degradation of such materials. This work evaluates the thermal shock damage cycling of high-alumina and mullite refractory castables designed for petrochemical application. Hot elastic modulus analyses were carried out after 0, 2, 4, 6, 8 and 10 thermal cycles (ΔT=800°C) in order to investigate the microcracking evolution due to the temperature changes. Additionally, apparent porosity, hot modulus of rupture, erosion and work of fracture measurements were also performed. According to the attained results, it was detected at which temperature range the stiffening or embrittlement took place in the mullite-based refractory (M-SA) microstructure. Nevertheless, the damage induced by the thermal shock tests was not permanent, as further increase of the elastic modulus results was observed for all evaluated samples after annealing. On the other hand, the alumina-based composition containing a sintering additive (TA-SA) presented enhanced mechanical strength, high thermal stability and E values. Simulations indicated that refractories with high E values (∼140GPa, such as those attained for alumina-based castable) showed a reduced amount of stored elastic strain energy even under severe thermal stresses, which seems to be a key aspect for the engineered design of thermal shock resistance materials. [Copyright &y& Elsevier]

Abstract: In this work, properties from colloidal silica containing foam systems were analyzed and correlated to the fundamental concepts of the particle adsorption at the gas–liquid interface, specifically for the adsorption energy and its relationship to the thermal energy of the particle. Based on the presented data, it can be pointed out that hydrophilic colloidal particles should give rise to unstable foams due to their low adsorption energy, which is below the minimum threshold required evaluated for the particle containing foam stabilization. [Copyright &y& Elsevier]

Abstract: Carbon-containing refractories are widely used in the steelmaking process due to their outstanding properties and, in order to improve their oxidation resistance, the so-called antioxidants have often been used. Al4SiC4 is pointed out as a novel additive that presents suitable properties such as Al, but without its drawbacks. Therefore, the effect of Al4SiC4 addition to Al2O3 yph name="sbnd" />SiO2 es designed for lining blast furnace troughs was investigated in this work. Apparent porosity, oxidation, thermogravimetric, X-ray diffraction, hot elastic modulus tests and thermodynamic calculations were carried out in order to better understand the antioxidant effects and reaction mechanisms. Additionally, the collected results were compared with those from the compositions containing other commonly used antioxidants (Si, B4C and sodium borosilicate glass). The performance of the novel additive proved to be limited as most of the carbon source used reacted earlier than the Al4SiC4 action. As a consequence, intense carbon oxidation, along with the thermal expansion mismatch among the phases during the cooling step, intensified the deterioration of the evaluated refractory material. [Copyright &y& Elsevier]

Abstract: Although the in situ spinel formation in alumina–magnesia refractory castables induces an expansive behavior, many investigations highlight its positive role in the corrosion resistance of such materials. Thus, this work addresses the slag attack evaluation of four designed in situ spinel-containing castables (containing hydratable alumina or calcium aluminate cement as a binder source and 0 or 1wt% of silica fume) when in contact with a Fe x O rich industrial slag. Corrosion cup-tests, microstructural characterization and a two-step thermodynamic simulation model were used in order to investigate the reactions taking place during the slag–refractory interactions. According to the attained results, hydratable alumina seems to be a suitable binder to improve the corrosion resistance of such castables, as it induces densification and the formation of an alumina-rich spinel phase at the slag–matrix interface. Moreover, the thermodynamic calculations matched to the experimental observations, attesting the efficiency of the proposed simulation model for the evaluation of the in situ spinel-containing castable corrosion behavior. [Copyright &y& Elsevier]

Abstract: Calcium aluminate cement (CAC) hydrates conversion can be inhibited by adding CaCO3, leading to C3A·CaCO3·11H (3CaO·Al2O3·CaCO3·11H2O) formation. However, despite its benefits, the stability of this monocarbonate hydrate is not fully understood, especially when the samples are kept in contact with liquid during the curing step. Thus, taking into account the increasing interest in the CAC application as a biomaterial in the endodontic area, this work addresses the evaluation of the mechanical strength and phase transformations of a commercial cement (Secar 71) containing 15 or 20wt% of CaCO3. Compressive strength, apparent porosity, dimensional linear changes, X ray diffraction and thermogravimetric tests were carried out to evaluate samples immersed in water and kept at 37°C between 1 and 30 days of curing. According to the collected results, CAH10 and C2AH8 formation were inhibited in CaCO3 containing compositions and the presence of the C3A.CaCO3.11H phase led to a significant cement mechanical strength increase. Nevertheless, the partial decomposition of this monocarbonate hydrate was detected at 37°C in the range of 1–7 days and the continuous hydration of CA and CA2 also affected the compressive strength behavior of the evaluated samples. [Copyright &y& Elsevier]

Abstract: Alumina–magnesia refractory castables usually present silica fume in their compositions, due to their ability to induce better flowability and to compensate the expansion related to the in situ spinel formation. In this paper, four compositions containing distinct silica fume content (0–1wt%) were designed and analyzed by creep resistance and hot mechanical strength. The θ-projection concept coupled with the thermodynamic simulations were used in order to predict the creep behavior and to identify the main mechanism leading to the deformation of the samples. Based on the collected results, a linear correlation between the creep parameters (θ i ) and the silica fume content was attained by analyzing the experimental data, resulting in reliable data and the likelihood to simulate the performance of other compositions in the same system. Moreover, particle sliding assisted by viscous flow was suggested as the dominant creep mechanism in the studied castables. [Copyright &y& Elsevier]

Abstract: Calcium aluminate cement reactions with water result in anhydrous phases dissolution, followed by nucleation and crystal growth of hydrate compounds. Due to the dynamic characteristics of this process and in order to evaluate the phase transformation kinetics of such materials, suitable methods to halt hydration are required. In this work, the use of acetone and microwave drying, aiming to withdraw free water and inhibit further reactions, was evaluated. X ray diffraction was used to quantify the phases generated in the cement samples kept at 37°C for 1–15 days. The advantages and drawbacks of these procedures are presented and discussed. Using microwaves to halt the hydration process in shaped samples seems to be effective to withdraw the cement free water, if a previous grinding of the material is carried out. The procedure suggested can be used in researches in the refractory castable area, endodontic cements and others. [Copyright &y& Elsevier]

Abstract: The chemical corrosion of two Al2O3–MgO castables (containing distinct binder sources: hydratable alumina or calcium aluminate cement) were evaluated in this work via thermodynamic calculations. Two simulation models were proposed according to the following procedures: (1) firstly the matrix and later the aggregates of the castables were placed, separately, in contact with an industrial basic slag, and (2) the overall chemical composition of the design castables was directly reacted with the molten slag. The theoretical results were further compared with experimental data collected after corrosion cup tests. Although the thermodynamic evaluation of the overall castable compositions was able to identify the phase transformations correctly, a two-step analysis of the matrix components and aggregates particles seems to be the best alternative to evaluate the binder source effect on the corrosion performance of the two Al2O3–MgO refractory materials. [Copyright &y& Elsevier]

Abstract: This work discusses an investigation of the hot elastic modulus and crack generation of two Al2O3–SiC–SiO2–C castable compositions throughout two thermal cycles in an oxidizing atmosphere. A high temperature ultrasonic technique carried out using a long bar mode, thermogravimetric, X-ray diffraction, apparent porosity analyses and thermodynamic calculations were evaluated in order to understand the results. Significant changes in the castables’ elastic modulus values were observed with temperature, which were related to the decomposition of hydrated phases, antioxidant reactions, changes in the liquid phase viscosity, and formation and closure of microcracks in the castable microstructure. The results attained are fundamental for providing data for thermo-mechanical computing simulations by finite element analyses and for the design of large refractory structures, such as blast furnace runners for the steel industry. [Copyright &y& Elsevier]

Abstract: This work addresses the thermodynamic evaluation of different spinel-containing refractory castable compositions in contact with a basic steel ladle slag (CaO/SiO2 ∼9). The main differences among the castable compositions were the amount of silica fume (0 or 1wt%), the binder source (calcium aluminate cement or hydratable alumina) and the spinel incorporation route (in situ or pre-formed). The interaction of the liquid slag with the refractory was carried out with the help of thermodynamic software (FactSage) and the applied methodology considered the changes in the slag composition due to the interaction with the castable. The theoretical results were compared with the experimental data attained by corrosion cup-tests, pointing out that the thermodynamic calculations were suitable for predicting various aspects observed in the corroded samples by SEM. Therefore, the equilibrium simulations led to parameters that indicated the corrosion resistance trends, complementing the experimental evaluation and reducing further experimental testing. [Copyright &y& Elsevier]

Abstract: The literature suggests that MgAl2O4 can accelerate SiC oxidation in Al2O3–MgAl2O4–SiC–C refractory castables. Thus, in this work thermodynamic calculations have been carried out using FactSage® software in order to explore, search for and understand the role of MgAl2O4 on the SiC oxidation. According to the thermodynamic predictions, at 1500°C and under a reducing atmosphere, there is no evidence that spinel might directly affect SiC oxidation. The increase of SiC content in an Al2O3–SiC–C (AL) castable composition was mainly related to the reaction between mullite and carbon. On the other hand, the SiC generation in the Al2O3–MgAl2O4–SiC–C (SP) composition was a result of the reaction involving liquid SiO2 and carbon from the refractory. Therefore, the lower SiC content in the SP castable resulted from the refractory''s phase transformations. It was also suggested that the samples thermally treated 15 times at 1500°C did not reach the equilibrium condition, which explains the differences between experimental and thermodynamic results. [Copyright &y& Elsevier]