Search

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]

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]

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]

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]

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: 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: 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]

Based on a kinetic model that describes the hydration reaction via nucleation and growth mechanisms, formic, acetic or propionic acids were analyzed as hydration site activators on magnesia particles with different reactivity and granulometry used as a main binder source in high-alumina refractory castables. These additives induced the hydroxide generation in MgO suspensions and the hydration degree was proportional to the size of the carboxylate chain. Moreover, a more controlled growth of the brucite crystals could be achieved when formic acid was added to the refractory castables. By adjusting the amount of formic acid in the castable mixtures, large crack-free samples could be prepared, which pointed out the role of this additive to prevent the drawbacks associated with MgO hydration. Therefore, the suitable kinetic control of the nucleation and growth of brucite crystals on the magnesia surface allowed for the development of high-alumina magnesia-bonded castables. [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]

A growing interest in designing high-alumina MgO-bonded refractory castables has been identified in recent years due to the magnesia ability to react: (i) with water at the initial processing stages of these materials (inducing the precipitation of brucite phase) or (ii) with alumina, giving rise to in situ MgAl 2 O 4 generation at high temperatures. Nevertheless, despite the great potential of caustic magnesia to be used as a binder in such systems due to its high reactivity, it is still a challenge to control the hydration reaction rate of this oxide and the negative effects derived from the expansive feature of Mg(OH) 2 formation. Thus, this work evaluated the incorporation of different contents of aluminum hydroxyl lactate (AHL) into caustic magnesia-bonded castables, aiming to control the brucite precipitation during the curing and drying steps of the prepared samples, resulting in crack-free refractories. The designed compositions were characterized via flowability, setting behavior, X-ray diffraction, cold flexural strength, porosity, permeability and thermogravimetric measurements. According to the results, instead of Mg(OH) 2 , hydrotalcite-like phases [Mg 6 Al 2 (OH) 16 (OH) 2.4.5H 2 O and Mg 6 Al 2 (OH) 16 (CO 3)·4H 2 O] were the main hydrated phases identified in the AHL-containing compositions. The addition of 1.0 wt% of aluminum hydroxyl lactate to the designed castable proved to be, so far, the best option for this magnesia source, resulting in the development of a crack-free refractory with enhanced properties and greater spalling resistance under heating. [ABSTRACT FROM AUTHOR]

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]