Mullite-based refractory castable engineering for the petrochemical industry

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 1.5 wt% of the evaluated sintering additive to the designed castables led to a remarkable increase of the hot modulus of rupture (maximum of 40.4 MPa at 800 °C for the CAC-containing refractory) and high erosion resistance (1.5–2.9 cm³) after pre-firing at 800 °C for 5 h. 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.     

Effect of different foam stabilizers on the setting behavior and pore characteristics of CAC-bonded Al2O3 foamed ceramics

Calcium aluminate cement (CAC) was used in this work as a novel binder of the foam slurries to prepare Al₂O₃ foamed ceramics. The different foam stabilizers employed in the slurries would not only impact the foam stability, but also affect the setting behavior of CAC-containing foam slurries, consequently influencing the pore size and the heat-insulating property of the Al₂O₃ foamed ceramics. In this paper, modified polyethoxylated silicone (MPS), sodium alginate (SA) and carboxymethyl cellulose (CMC) were selected respectively as the foam stabilizer. The effects of the different foam stabilizers on the setting behavior of foam slurries, and the dependence of the pore size and heat-insulating property of CAC-bonded foamed ceramics on the setting behavior of foam slurries were investigated. It is found that SA promotes the hydration of CAC in the foam slurries, while MPS and CMC postpone the hydration of CAC in the foam slurries; and the foamed ceramics with SA have better structural integrity, higher porosity, smaller average pore size and lower thermal conductivity than those with MPS or CMC.     

Preparation and pore-forming mechanism of MgO–Al2O3–CaO-based porous ceramics using phosphorus tailings

A simple strategy for preparing MgO–Al₂O₃–CaO-based porous ceramics (MACPC) with high strength and ultralow thermal conductivity has been proposed in this work based on the raw material of phosphorus tailings. The effects of phosphorus tailings content, carbon black addition and heat treatment temperature on the properties of MACPC were studied, and their pore-forming mechanism during sintering was revealed. The results showed that the main phase composition of MACPC was magnesia alumina spinel and calcium aluminate after sintering at 1225 °C. Furthermore, the MACPC exhibited excellent comprehensive properties when 60 wt% phosphorus tailings and 40 wt% alumina were added, whose apparent porosity was 62.8%, cold compressive strength was 14.8 MPa, and the thermal conductivity was 0.106 W/(m·K) at 800 °C. The synchronously enhanced strength and thermal insulation properties of MACPC were related to the formation of uniformly distributed micropores (<2 μm) and passages in the matrix, which originated from the decomposition of phosphorus tailings and the burnt out of carbon black during the sintering process. The preparation of MACPC with high temperature resistance and excellent mechanical and thermal insulation properties with the raw material of phosphorus tailings provided an effective method for the high-value utilization of phosphorus tailings.     

Calcium aluminate cement in castable alumina: From hydrate bonding to the in situ formation of calcium hexaluminate

Calcium hexaluminate (CA₆) is an intrinsically densification-resistant material, therefore, its porous structures are key materials for applications as high-temperature thermal insulators. This article reports on the combination of calcined alumina and calcium aluminate cement (CAC) in castable aqueous suspensions for the in situ production of porous CA₆. The CAC content (10–34 vol%) and the curing conditions ensure structural integrity prior to sintering and maximize the development of hydrated phases. Changes in physical properties, crystalline phases, and microstructure were investigated after isothermal treatments (120–1500 °C), and three sequential porogenic events were observed. The hydration of CAC preserved the water-derived pores (up to 120 °C), and the dehydroxylation of CAC hydrates (250–700 °C) generated inter-particles pores. Moreover, the in situ expansive formation of CA₂ and CA₆ (900–1500 °C) hindered densification and generated intra-particle pores. Such events differed from those observed with other CaO sources, and resulted in significantly higher pores content and lower thermal conductivity.     

Effect of carbon content on electrical,thermal, and mechanical properties of porous SiC ceramics with B4C and C additives

The electrical, thermal, and mechanical properties of porous SiC ceramics with B₄C-C additives were investigated as functions of C content and sintering temperature. The electrical resistivity of porous SiC ceramics decreased with increases in C content and sintering temperature. A minimal electrical resistivity of 4.6 × 10^?2 Ω·cm was obtained in porous SiC ceramics with 1 wt% B₄C and 10 wt% C. The thermal conductivity and flexural strength increased with increasing sintering temperature and showed maxima at 4 wt% C addition when sintered at 2000 °C and 2100 °C. The thermal conductivity and flexural strength of porous SiC ceramics can be tuned independently from the porosity by controlling C content and sintering temperature. Typical electrical resistivity, thermal conductivity, and flexural strength of porous SiC ceramics with 1 wt% B₄C-4 wt% C sintered at 2100 °C were 1.3 × 10^?1 Ω·cm, 76.0 W/(m·K), and 110.3 MPa, respectively.     

Electrical conductivity and phase composition of calcium aluminate cement containing air-cooled and water-cooled slag at 20, 40 and 60 °C

Calcium aluminate cement (CAC) pastes containing Egyptian air-cooled slag (AS) or water-cooled slag (WS) were prepared using different amounts of slag, namely, 5, 10, 15, 20 and 25 mass%. The pastes were prepared with deionized water using the required water of standard consistency to produce normal workability. The variations of electrical conductivity with the hydration time were measured at 20, 40 and 60 °C. The results demonstrate that electrical conductivity is a useful technique to study the change in the phase composition at different temperatures during the setting and hardening of calcium aluminate cement as well as reflecting the role of AS and WS, preventing the conversion occurring during the CAC hydration.     

Novel drying additives and their evaluation for self-flowing refractory castables

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).     

Gluconate action in the hydration of calcium aluminate cements: Theoretical study,processing of aqueous suspensions and hydration reactivation

Considering the production of stable CAC aqueous suspensions, this work addressed the action of gluconate anion as a Ca²⁺ complexing agent and retarder of CAC hydration. Using quantum simulations, the complexation energy of gluconate complexes was calculated. The pH range of stability for the complexes was estimated and aqueous suspensions containing CAC and sodium gluconate (NaG), stable up to 4 days at room temperature, were prepared. Afterwards, their hydration reactions were reactivated by adjusting the systems’ pH. Results of solidification kinetics and mineralogical characterisation highlighted that, after reactivation, calcium aluminate hydrates were formed. Thermodynamics simulations indicated that using NaG up to 1 wt% would not be deleterious to the systems refractoriness up to 1700 °C. These systems could be applied in less explored processing routes for CAC-based refractory compositions (e.g. slip casting, direct foaming and additive manufacturing), resulting in innovations to produce advanced refractory ceramics.     

Effect of Zn(OH)2 on properties of corundum based castables bonded with calcium aluminate cement

In this work, the effect of Zn(OH)₂ on properties of corundum based castables bonded with calcium aluminate cement (CAC) was investigated. The phase composition and microstructure of castable matrixes containing Zn(OH)₂ after firing 800 °C, 1100 °C and 1550 °C were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectra, respectively. The results indicate that a small amount of Zn(OH)₂ can dramatically improve the medium temperature strength of castables because the generation of zinc aluminate spinel increases the ceramic bonding of castables. In addition, the addition of Zn(OH)₂ also improves the volume stability of CAC-bonded castables due to the enhanced formation of pores from Zn(OH)₂ decomposition in castables.     

Binding additives with sintering action for high-alumina based castables

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 (CaCO₃) 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. CaCO₃ 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.     

Calcium aluminate cement-based compositions for biomaterial applications

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.     

Thermal properties of hydrating calcium aluminate cement pastes

As the hydration of calcium aluminate cements (CAC) is highly temperature dependent, yielding morphologically and structurally different hydration products that continuously alter material properties, a good knowledge of thermal properties at early stages of hydration is essential. Thermal diffusivity and thermal conductivity during CAC hydration was investigated by a transient method with a numerical approach and a transient hot wire method, respectively. For hydration at 15 °C (formation of mainly CAH₁₀), thermal diffusivity shows a linear decrease as a function of hydration degree, while for hydration at 30 °C there is a linear increase of thermal diffusivity. Converted materials exhibited the highest values of thermal diffusivities. The results on sealed converted material indicated that thermal conductivity increased with an increase in temperature (20-80 °C), while thermal diffusivities marginally decreased with temperature. The Hashin-Shtrikman boundary conditions and a simple law of mixtures were successfully applied for estimating thermal conductivity and heat capacity, respectively, of fresh cement pastes.     

Influence of potassium titanate whisker on the mechanical properties and microstructure of calcium aluminate cement for in situ combustion

This study investigated potassium titanate whisker-reinforced calcium aluminate cement (CAC)-based composites, and evaluated the influence of the quantity (0–5% of the weight of the binder) of potassium titanate whiskers on the mechanical properties of hardened cement mortar. X-ray diffraction analysis and Scanning electron microscopy (SEM) were employed to determine the phase compositions and micro-morphology of the cement composites, respectively. Experimental results indicated that the addition of potassium titanate whisker exhibits significant potential to improve the tensile strength and toughness of cement mortars. The compressive and tensile strengths of samples cured at 50 °C were increased by 46.90 and 74.10%, and the tensile strength samples under high-temperature treatment increased by 113.67%, with the addition of 4% potassium titanate whisker. Typical cement slurry properties, such as basic rheology, free water, and fluid loss could maintain stability when added with 0–5% dosages of potassium titanate whiskers. SEM analysis indicated that the whisker could increase the toughness of oil cement, which contributed to whisker pullout and whisker-cement coalition pullout in the cement matrix.     

MgO fumes as a potential binder for in situ spinel containing refractory castables

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.     

Thermal expansivity and conductivity of pure and silicon-alloyed pyrocarbons

Measurements were made of the thermal expansivity (22°–1000°C) and the thermal conductivity (22°–800°C) of isotropic pyrocarbons deposited in fluidized beds and containing up to 34 wt% silicon in the form of β-silicon carbide particles. The thermal expansivity of the pure carbons was proportional to their density, while that of the silicon-alloyed carbons decreased with increasing silicon content, falling from 6·3 × 10^?°C^?1 for material containing 4 wt% silicon to 4·6 × 10^?6°C^?1 for material with 34 wt% silicon. The thermal conductivity of both pure and silicon-alloyed pyrocarbons increased with increasing temperature up to about 500°C. The room-temperature thermal conductivity of the pure carbons increased with increasing apparent crystallite height (L_c), and the conductivity of the silicon-alloyed carbons was significantly lower than that of pure pyrocarbon with the same L_c. The results suggest that silicon entering substitutionally into the carbon lattice may reduce the conductivity.     

Supercritical carbonation of calcium aluminate cement

The microstructural changes occurring during supercritical carbonation (scCO₂) of calcium aluminate cement (CAC) and changes to its strength have been investigated. Cylindrical specimens of CAC cured at different temperatures were prepared and then subjected to scCO₂. It is shown that CAC carbonation in supercritical conditions is accelerated with a positive effect on the compressive strength. Due to the scCO₂ treatment, both conversion and alkaline hydrolysis are avoided. The best behaviour of the studied specimens was attained for samples cured at 25 °C. The residual compounds after the scCO₂ process, i.e. monocalcium aluminate, calcium carbonate and aluminium hydroxide are durable in normal ambient conditions. Complete carbonation of CAC is particularly important for the reinforcement of CAC with polymer fibres to improve its mechanical strength.     

Studies on thermal degradation kinetics and dielectric properties of polyether imide foam/nanosilica-based nanocomposites

ABSTRACT

In this paper, polyether imide (PEI) having properties such as a high glass transition temperature of 216°C, high heat resistance, high flame resistance, low smoke generation and a high melting point within the range of 400°C, having low thermal conductivity and low dielectric constant was chosen to be a polymeric foam. Water vapor-induced phase separation method was used to prepare PEI foams. PEI foams were reinforced with nano-silica (weight 1, 3 and 5%) in order to alter the dielectric properties, thermal conductivity and degradation kinetics of foamed polymer. The tested samples showed a reduction in dielectric constant than that of solid PEI but at a higher loading, it showed a higher value due to threshold percolation and a reduction in thermal conductivity was observed for foamed PEI. From thermogravimetric analysis, we can conclude that PEI with 3% filler loading showed better thermal stability compared to other PEI foam compositions.     

Reactions of fly ash with calcium aluminate cement and calcium sulphate

The hydration processes in the ternary system fly ash/calcium aluminate cement/calcium sulphate (FA/CAC/C$) at 20 °C were investigated; six compositions from the ternary system FA/CAC/C$ were selected for this study. The nature of the reaction products in these pastes were analysed by X-ray diffraction (XRD) and infrared spectroscopy (FTIR). At four days reaction time, the main hydration reaction product in these pastes was ettringite and the samples with major initial CAC presented minor ettringite but calcium aluminates hydrates. The amount of ettringite developed in the systems has no direct relation with the initial components.     

Effect of boron addition on microstructure,mechanical properties and oxidation resistance of TaC ceramics

TaC ceramics with 0.03–0.60?wt% of boron additions were prepared by hot pressing at 2100?°C for 1?h under a pressure of 40?MPa. Effects of boron content on densification, phase composition, microstructure, mechanical properties and oxidation resistance of the TaC ceramics were investigated. When the boron content was 0.12?wt% and above, full density was obtained due to reactions between boron and oxygen impurity at presence of TaC. Minor phases of TaB₂ and C were formed in the 0.24 and 0.60?wt% B compositions after gas-out of the oxygen impurity. Microstructure of the TaC ceramics was refined with increasing in boron content. The TaC ceramic with 0.24?wt% of boron showed the best mechanical properties with a Vickers hardness, flexural strength and fracture toughness of 17.7?GPa, 534?MPa and 4.6?MPa?m^1/2, respectively. When more boron was added, interfacial bonding of the TaC grains was strengthened causing a decrease in fracture toughness. Oxidation resistance of the TaC ceramics increased with boron content. Particularly, the 0.60?wt% B composition showed a weight gain of 0.0018?g/cm² after oxidization at 800?°C in air for 3?h.     

Effect of nano aluminum nitride filler on mechanical,thermal, and dielectric properties of the glass/mullite composites for low-temperature co-fired ceramic applications

Mullite/glass/nano aluminum nitride (AlN) filler (1–10 wt% AlN) composites were successfully fabricated for the low-temperature co-fired ceramics applications that require densification temperatures lower than 950°C, high thermal conductivity to dissipate heat and thermal expansion coefficient matched to Si for reliability, and low dielectric constant for high signal transmission speed. Densification temperatures were ≤825°C for all composites due to the viscous sintering of the glass matrix. X-ray diffraction proved that AlN neither chemically reacted with other phases nor decomposed with temperature. The number of closed pores increased with the AlN content, which limited the property improvement expected. A dense mullite/glass/AlN (10 wt%) composite had a thermal expansion coefficient of 4.44 ppm/°C between 25 and 300°C, thermal conductivity of 1.76 W/m.K at 25°C, dielectric constant (loss) of 6.42 (0.0017) at 5 MHz, flexural strength of 88 MPa and elastic modulus of 82 GPa, that are comparable to the commercial low temperature co-fired ceramics products.