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Effect of Different Binders on the Properties of Mullite-Silicon Carbide Castables
An analysis of the refractory damage to the inclined-chute support pillars in dry quenching furnaces reveals that enhancing the flexural strength and thermal shock resistance of the refractory materials constitutes an effective strategy for extending their service life. Consequently, the incorporation of steel fibers into mullite-silicon carbide castables is employed to provide reinforcement and toughening, thereby prolonging their operational lifespan. Furthermore, the choice of binder is critical to both the installation and service performance of refractory castables; this study investigates the impact of three distinct binders—pure calcium aluminate cement (Secar 71), silica sol, and alumina-silica gel powder—on the microstructure and properties of the castables, with the objective of identifying the most suitable binder for this application.
This experimental study investigates the effects of various binders—specifically pure calcium aluminate cement (Secar 71), silica sol, and aluminosilicate gel powder—on mullite-silicon carbide castables.
After drying at 110°C and subsequent heat treatment at 1000°C, specimens bonded with calcium aluminate cement exhibited the lowest apparent porosity and the highest bulk density. This indicates that the cement-bonded castable possessed the best flow properties, which facilitates the molding of the specimens. At 850°C, the calcium aluminate cement-bonded specimens underwent significant dehydration, resulting in an increase in apparent porosity and a decrease in bulk density. Following heat treatment at 1000°C, the specimens underwent sintering shrinkage, leading to an increase in density.
For specimens utilizing different binders, both the room-temperature flexural strength and room-temperature compressive strength increased as the heat treatment temperature rose. After drying at 110°C, the calcium aluminate cement-bonded specimens demonstrated the highest flexural strength, reaching 7.5 MPa, while the specimens bonded with aluminosilicate gel powder exhibited the lowest strength. This suggests that the chemical reaction between the cement and water—which leads to setting, hardening, and strength development—yields the highest strength, thereby contributing most significantly to the construction safety of refractory castables. After heat treatment at 850°C, the differences in room-temperature flexural strength among the specimens with the three different binders were not significant; however, the calcium aluminate cement-bonded specimens maintained the highest room-temperature compressive strength, at 53.6 MPa. After heat treatment at 1000°C, the calcium aluminate cement-bonded specimens achieved the highest room-temperature flexural strength (14.3 MPa), while the aluminosilicate gel powder-bonded specimens attained the highest room-temperature compressive strength (70.2 MPa). This demonstrates that the phases formed during the hydration of calcium aluminate cement—such as monocalcium aluminate (CA), dicalcium aluminate (CA2), and dodecacalcium hepta-aluminate (C12A7)—possess high bonding strength. Furthermore, within the aluminosilicate gel powder, the reaction between nano-sized Al2O3 and SiO2 generates a mullite bonding phase, which serves to enhance the strength of the castable.
Pore Size Distribution
Following heat treatment at 1000°C, the calcium aluminate cement-bonded specimens (Group A) exhibited an average pore size of 0.23 μm and a median pore size of 0.74 μm, displaying the most concentrated pore size distribution (ranging from 0.01 μm to 2 μm). The silica sol-bonded specimens (Group B) had the smallest average pore size—0.13 μm—and a median pore size of 0.40 μm, with a relatively broad pore size distribution (ranging from 0.01 μm to 4 μm). Conversely, the alumina-silica gel powder-bonded specimens (Group C) possessed the largest average pore size—0.28 μm—and a median pore size of 0.77 μm; while their pore size distribution spanned a range of 0.01 μm to 6 μm, the pores were predominantly concentrated within the 0.01 μm to 1 μm range.
High-Temperature Flexural Strength
Specimens bonded with silica sol exhibited the highest high-temperature flexural strength, reaching 13.7 MPa; in contrast, specimens bonded with cement and those bonded with aluminosilicate gel powder demonstrated lower high-temperature flexural strengths, measuring 7.8 MPa and 8.3 MPa, respectively. This phenomenon is attributed to the fact that the nano-SiO₂ present in the silica sol forms a silica-oxygen network structure within the specimen and possesses extremely high chemical reactivity. At 1000°C, this nano-SiO₂ readily reacts with active α-Al₂O₃ micro-powders to form a mullite network structure, thereby enhancing the specimen’s strength. The aluminosilicate gel powder, containing a lower proportion of SiO₂, forms a mullite network structure at 1000°C that is less robust than that formed by the silica-sol-bonded specimens, resulting in lower high-temperature flexural strength. Calcium aluminate cement, containing a certain amount of CaO, tends to react with the SiO₂ and Al₂O₃ present in the material at high temperatures to form low-melting-point phases—such as 3CaO·Al₂O₃ and 2CaO·Al₂O₃·SiO₂. These phases transform into a liquid state at elevated temperatures, consequently reducing the specimen’s high-temperature flexural strength.
Thermal Shock Stability
Specimens bonded with silica sol exhibited the highest residual flexural strength, at 7.8 MPa, while those bonded with aluminosilicate gel powder showed the lowest residual flexural strength, at 5.3 MPa. Specimens bonded with calcium aluminate cement demonstrated relatively high residual flexural strength as well as a high retention rate of flexural strength. The superior thermal shock resistance observed in the calcium-aluminate-cement-bonded specimens and the silica-sol-bonded specimens can likely be attributed to their respective characteristics: a concentrated pore size distribution in the former, and the presence of a silica-oxygen network structure in the latter. Within heterogeneous, multi-phase refractory materials, discrepancies in the coefficients of thermal expansion among the various constituent phases lead to the generation of numerous microcracks during the process of thermal expansion mismatch. These internal microcracks not only absorb elastic strain energy—thereby reducing the driving force for the propagation of major cracks—but also serve to disperse the stress concentrated at crack tips. By dissipating the energy associated with crack propagation, these microcracks effectively enhance the material’s resistance to thermal shock.
Wear Resistance
Wear resistance tests were conducted on specimens bonded with various agents after firing at 1000°C. The results indicate that specimens bonded with calcium aluminate cement and those bonded with aluminosilicate gel powder exhibited the lowest rates of material wear. Among the samples tested, the specimen bonded with aluminate cement exhibited the lowest wear volume (3.75 cm³), while the specimen bonded with silica sol showed the highest wear volume (7.58 cm³). In the case of heterogeneous refractory materials—which consist of both aggregates and a matrix—erosion wear typically begins by scouring away the matrix, thereby causing the protruding, isolated aggregate particles to become the primary focal points of wear. Once these particles dislodge, cracks form, which in turn exacerbate the further deterioration of the surrounding matrix. The aluminate cement-bonded specimens possessed a relatively high density; specifically, the formation of Si-O-Al bonds between the SiO₂ micro-powder and the cement hydrates resulted in a tightly bonded matrix with superior wear resistance. In the specimens bonded with aluminosilicate gel powder, the reaction between nano-Al₂O₃ and SiO₂ generated a mullite-based matrix, thereby enhancing their wear resistance. Conversely, the matrix of the silica sol-bonded specimens contained a significant number of microcracks, a characteristic that rendered them less resistant to erosion wear.
Microstructural Analysis
Following heat treatment at 1000°C, the bond between the matrix and the aggregates in the calcium aluminate cement-bonded specimens was found to be exceptionally tight; this accounts for the specimens’ high density, superior strength, and excellent wear resistance. Simultaneously, the matrix contained a significant number of microcracks, resulting in a concentrated pore size distribution and favorable thermal shock stability for the specimens. In the silica sol-bonded specimens, numerous voids and microcracks were present; this explains the specimens’ high apparent porosity, broad pore size distribution, and poor wear resistance. Conversely, the presence of an extensive silicon-oxygen network structure within these specimens contributed to their high flexural strength at elevated temperatures and robust thermal shock resistance. In the aluminum-silica gel powder-bonded specimens, the bonding between the aggregates and the matrix was robust; furthermore, the matrix featured an extensive network structure of columnar mullite, endowing the specimens with superior mechanical properties and wear resistance.
What are the optimal particle size and addition level for silicon carbide in high-strength wear-resistant self-flowing castables?
Silicon carbide possesses characteristics such as corrosion resistance, high-temperature resistance, high strength, excellent thermal conductivity, and impact resistance; consequently, it is widely utilized in high-temperature industries such as ceramics, refractories, and metallurgy. For the cement-bonded, high-strength, wear-resistant self-flowing castables commonly used in the industrial furnaces and kilns of the aforementioned sectors—where application sites often feature complex structures and harsh environments—the material is required to exhibit superior strength, self-flowing properties, and wear resistance. Therefore, during the formulation design of high-strength, wear-resistant self-flowing castables, in addition to controlling the aggregate structure to ensure the castable’s strength and fluidity, an appropriate amount of silicon carbide is incorporated to enhance the material’s wear resistance and erosion resistance.
(1) Silicon carbide can significantly enhance the wear resistance of castables. Fine silicon carbide powder effectively fills the material matrix, thereby improving density and structural strength.
(2) Excessive addition of silicon carbide leads to the generation of an excessive liquid phase during secondary mullitization at high temperatures, which negatively impacts both the installation performance and mechanical properties of the castable.
(3) The castable achieves its most ideal comprehensive performance and cost-effectiveness when a total of 20% silicon carbide—comprising a blend of 1–0 mm particles and 180-mesh powder (in a ratio of 1:2)—is added to the mixture.
High-strength wear-resistant self-flowing castables have been successfully applied in the industrial furnaces and kilns of various enterprises, including refineries, demonstrating consistent product quality. The material exhibits excellent installation and operational performance, boasting an average service life of over eight months and effectively meeting the diverse operational requirements of various industrial furnaces and kilns.
