Search

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]

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]

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]

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]

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]

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]

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: 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]

Environmental issues regarding Cr6+ formation lead to replacing chrome-containing refractories with greener alternatives. MgO-containing compositions have been extensively investigated for this purpose, however, few studies evaluated the likelihood of using other chemical elements as inducers of spinel-like phase formation in refractory castables. In this study, the addition of zincite in alumina-based castables was evaluated and compared with its MgO-counterpart. In-situ elastic modulus, assisted sinterability and differential scanning calorimetry pointed out that the gahnite (ZnAl 2 O 4) formation took place at lower temperatures (∼ 1100 °C) than MgAl 2 O 4 (∼ 1300 °C). On one hand, this feature induces anticipated strengthening of the Zn-containing compositions, giving rise to the possibility of firing these compositions at lower temperatures. On the other, the faster kinetics of gahnite formation led to a significant Kirkendall effect, changing the morphology of the pores created during sintering, which became preferentially located at the interface of alumina aggregates, negatively affecting some mechanical properties of the castable. [ABSTRACT FROM AUTHOR]

This paper evaluated the role played by citric acid and gallic acid in high alumina colloidal silica-bonded castables. The addition of citric acid to three different types of colloidal silica-containing compositions led to distinct flow behavior. Vibrating flow measurements were carried out in order to define the castables׳ workability. Zeta potential and specific conductivity tests were also used to analyze the effect of the selected carboxylic acids in alumina-colloidal silica suspensions. Furthermore, other important parameters considered during the evaluation of such additives were: a) pH of the colloidal silica binder and b) pK a values of the carboxylic acid. The higher the pH of the nano-binder solution, the less adsorption of carboxylic acid on the alumina surface will take place. As a result, molecules presenting acidic pK a values such as citric acid are effective additives for colloidal silica binders with pH≤10, whereas others with alkaline pK a values (e.g. gallic acid) can be used with colloidal silica sources presenting higher pH. The use of the mentioned carboxylic acids is required to provide enough workability to the system due to a retarding effect on the setting agent. [ABSTRACT FROM AUTHOR]

Abstract: The use of magnesia sources with high specific surface area and small particle size in the Al2O3–MgO system can induce faster in situ spinel (MgAl2O4) formation in castable compositions, improving the slag corrosion resistance. However, the higher reactivity of these raw materials lead to an intensive brucite formation (followed by volumetric expansion), spoiling the castable''s properties during the curing and drying steps. Considering these aspects, a systemic analysis of three magnesia sources (dead-burnt and caustic ones) was carried out in order to evaluate: (1) their hydration impact on the refractory castables properties, and (2) their bonding ability in cement-free compositions. Mechanical strength, thermogravimetric and Young''s modulus tests were conducted during the castables’ curing and drying steps. According to the results, the elastic modulus measurement is an efficient tool to evaluate the magnesia hydration. The addition of proper amounts of calcium aluminate cement and/or silica fume to the castables can inhibit the crack formation and provide suitable mechanical properties. The results also show that under certain conditions, MgO can be used as a binder, replacing calcium aluminate cement and leading to a significant reduction in the castables costs with no drawbacks to their refractoriness. [Copyright &y& Elsevier]

Developing MgO-bonded castables is still an important subject for refractory producers and end-users based on the expansive character of the in-situ Mg(OH) 2 formation. Considering that magnesia undergoes hydration when exposed to water and the generated hydrated phase needs to be properly accommodated in the resulting microstructure to inhibit the generation of cracks, it is very important to find out alternatives to control/change the MgO hydration reaction rate, which may help to optimize the permeability and green mechanical strength of the castables. Therefore, fast and safer drying of such refractories can be carried out when adjusting Mg(OH) 2 generation with proper additives. This research investigated the use of aluminum lactate (AL) as a likely additive to change the hydration and drying behavior of vibratable castables bonded with different MgO sources (dead burnt, caustic or fumed one). Firstly, XRD, TG and DSC measurements of magnesia-based aqueous suspensions were evaluated to identify the AL effect on changing the hydration reaction products during the curing and drying steps. After that, Al 2 O 3 –MgO refractories were prepared and their flowability, curing behavior, cold flexural strength, apparent porosity, permeability and explosion resistance were evaluated. The results indicated that, instead of Mg(OH) 2 , Mg 6 Al 2 (OH) 16 (OH) 2.4.5H 2 O/Mg 6 Al 2 (OH) 16 (CO 3).4H 2 O was the main hydrated phase identified in the AL-containing compositions. Due to this change in the hydration behavior of the refractories, the mixtures prepared with dead-burnt or magnesia fumes plus organic salt presented a longer setting time. Besides that, crack-free samples with improved permeability and green mechanical strength could be obtained when adding 0.5 wt% or 1.0 wt% of aluminum lactate to the tested castable compositions. Consequently, 1.0 wt% of the selected additive favored the design of refractories with enhanced properties and greater spalling resistance, as no explosion could be observed even when subjecting the prepared samples to severe heating conditions (20 °C/min). [ABSTRACT FROM AUTHOR]

Refractory producers face many challenges in terms of producing MgO-containing castables due to the high likelihood of magnesia to hydrate in contact with water, resulting in Mg(OH) 2 generation. The expansive feature of this transformation affects the performance of such refractories, as (i) if this hydrated phase is not accommodated in the formed microstructure, ceramic linings with cracks and low green mechanical strength will be obtained; and (ii) if crack-free pieces are prepared, they should present low porosity and reduced permeability, which require special attention when heating these materials. In both cases, spalling/explosion may be favored during the drying step of MgO-containing compositions. Hence, this work investigated the ability of various additives in the optimization of the drying behavior of Al 2 O 3 –MgO castables. Vibratable compositions were tested after incorporating polymeric fibers (PF), an organic salt (OAS), SiO 2 -based additive (SM) or permeability enhancing active compound (MP) into the dry-mixtures. Various experimental measurements were performed to infer the role of the drying agents to prevent the samples' explosion and whether they would also influence other properties of the castables. As observed, OAS and MP helped to inhibit the MgO-bonded samples' explosion even under severe heating conditions (2–20 °C/min) and increased their green mechanical strength and slag infiltration resistance when compared to the additive-free composition. On the other hand, the addition of polymeric fibers (PF) or silica-based compound (SM) to the formulations was not able to prevent the castables' explosion when using a high heating rate and other side effects (samples' cracking during drying at 110 °C, high linear expansion and increased slag penetration during corrosion tests) could also be observed when testing these materials. Thus, the selection of suitable drying agents is a key issue, as they may allow the development of MgO-bonded castables with enhanced properties and lower spalling risk during their first thermal treatment. [ABSTRACT FROM AUTHOR]