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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]

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]

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]

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]

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]

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]

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: 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: 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]

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

Calcium aluminate cement (CAC) Calcium aluminate cement (CAC) is classified as a hydraulic binder presenting various advantages, such as fast hardening at room temperature and suitable rheological properties, when compared to traditional materials. Based on this, CAC has been investigated as an alternative biomaterial in order to overcome some drawbacks presented by commercial products usually applied in the dentistry (mineral trioxide aggregate=MTA and glass ionomer) and orthopedics (polymethyl methacrylate=PMMA) fields. In this work, the properties of CAC-based compositions containing different amounts of additives (i.e., alumina, zirconia, zinc oxide, hydroxyapatite, tricalcium phosphate, chitosan and collagen) were evaluated and the attained results were compared to those of MTA, PMMA and two glass ionomers (Meron and Vidrion F). The characterization of the selected materials comprised their particle size distribution, as well as the cold crushing strength, apparent porosity, pore size distribution and radiopacity. Plain CAC presented higher crushing strength than the commercial products used in dentistry and the blend of this cement with 4 wt% of additives (alumina, zirconia, zinc oxide, tricalcium phosphate or hydroxyapatite) resulted in improved mechanical performance when compared to PMMA (cement for bone repair). The addition of zinc oxide and hydroxyapatite to CAC also gave rise to samples with low porosity levels and smaller pore sizes after their contact with simulated body fluid solution over 7 days at 37 °C. Conversely, collagen and chitosan-containing compositions showed higher porosity and lower mechanical strength. Regarding the radiopacity results, the evaluated compositions presented better results than the commercial products, except for MTA. [ABSTRACT FROM AUTHOR]

Abstract: The better performance of spinel-containing refractory castables when in contact with basic slag is mainly associated with their higher corrosion resistance. Although the literature has shown various studies related to this subject, only few of them evaluated the overall microstructural effect on the corrosion resistance. Considering this aspect, four different compositions were produced, in order to evaluate the binder source influence (calcium aluminate cement or hydratable alumina), the silica fume addition and spinel incorporation method (in situ or pre-formed) effects. Based on the physical properties (apparent porosity, linear thermal expansion and pore size diameter distribution) and also on the phases generated (detected by SEM, before and after corrosion), a basic slag corrosion mechanism, for the set conditions, is proposed. The results pointed out that lower penetration can be attained by reducing the pore size diameters, whereas chemical corrosion resistance is a consequence of higher content of fine alumina, lower amount of calcium aluminates and the lack of liquid phase in the castable matrix. This study stresses that the castable formulation design and the proper raw material selection are of utmost importance to understand and master the performance of this class of refractory castables. [Copyright &y& Elsevier]

Hydroxyapatite (HA) is a calcium phosphate and the main mineral component of bones and teeth. However, its use as a biomaterial is limited due to the poor mechanical strength presented by this compound. Various techniques have been investigated for HA production, considering its good biocompatibility and chemical stability in the implanted area. In this paper, two methods for hydroxyapatite synthesis (precipitation and mechanosynthesis) were evaluated and compared, in order to select the most viable HA processing route before adding this component to calcium aluminate cement (CAC). The blend of HA+CAC aimed to attain a biomaterial presenting the biological activity of calcium phosphate and the optimized green mechanical strength derived from the selected cement. The obtained HA powders were characterized via XRD, SEM and particle size distribution measurements. The mechanical strength, apparent porosity and pore size distribution of the prepared HA+CAC samples were analyzed. Both studied routes induced hydroxyapatite generation, but the mechanosynthesis was the most suitable one due to the reduced reaction time required. The main advantage of adding HA powders to CAC was associated with the production of a compound with higher mechanical strength, when compared to the original materials. Besides that, the HA favored the bioactivity of the prepared compositions, as observed by the Ca 2+ ions release and pH increase of the analyzed aqueous medium, leading to apatite precipitation on the HA+CAC samples׳ surface. This latter transformation also resulted in the decrease of both, apparent porosity and pores diameter, of the samples. [ABSTRACT FROM AUTHOR]