Analysis of refractory damage in dry quenching coke oven chute pillars indicates that improving the flexural strength and thermal shock resistance of refractory materials is an effective way to extend their service life. Introducing steel fibers into mullite silicon carbide castables provides reinforcement and toughness, thereby extending their service life. The type of binder is crucial to the construction and performance of refractory castables. This article examines the effects of three binders-pure calcium aluminate cement (Secar 71), silica sol, and alumino-silica gel powder-on the structure and properties of castables to determine the appropriate binder.
General Physical Properties
After drying at 110°C and heat treatment at 1000°C, the calcium aluminate cement-bonded sample had the lowest apparent porosity and the highest bulk density, indicating that the cement-bonded silicon carbide castables has the best flow properties, facilitating sample forming. The calcium aluminate cement-bonded sample experienced significant dehydration at 850°C, resulting in increased apparent porosity and decreased bulk density. After heat treatment at 1000°C, the sample sintered and shrunk, increasing its density.
The room-temperature flexural strength and compressive strength of samples with different binders increased with increasing heat treatment temperature. After drying at 110°C, the sample bonded with calcium aluminate cement had the highest flexural strength, at 7.5 MPa, while the sample bonded with alumina-silica gel powder had the lowest strength. This indicates that the chemical reaction between cement and water solidifies and hardens, resulting in the highest strength, which is most conducive to the construction safety of refractory castables. After heat treatment at 850°C, the room-temperature flexural strength of the samples with the three binders did not differ significantly. The sample bonded with calcium aluminate cement had the highest room-temperature compressive strength, at 53.6 MPa. After heat treatment at 1000°C, the sample bonded with calcium aluminate cement had the highest room-temperature flexural strength, at 14.3 MPa, while the sample bonded with alumina-silica gel powder had the highest room-temperature compressive strength, at 70.2 MPa. This indicates that the calcium monoaluminate (CA), calcium dialuminate (CA2), and calcium dodecaluminate (C12A7) phases produced by the hydration of calcium aluminate cement possess high bonding strength. The nano-Al2O3 and SiO2 in the alumino-silica gel powder react to form a mullite bonding phase, which can enhance the strength of the silicon carbide castable.
Pore Size Distribution
After heat treatment at 1000°C, the average pore size of the samples bonded with calcium aluminate cement (Group A) was 0.23 μm, with a median diameter of 0.74 μm. The pore size distribution was the most concentrated (0.01 μm to 2 μm). The samples bonded with silica sol (Group B) had the smallest average pore size, 0.13 μm, with a median diameter of 0.40 μm, and a wider pore size distribution (0.01 μm to 4 μm). The samples bonded with alumino-silica gel powder (Group C) had the largest average pore size, 0.28 μm, with a median diameter of 0.77 μm. The pore size distribution ranged from 0.01 μm to 6 μm, but the pore size distribution was concentrated within the range of 0.01 to 1 μm.
High-Temperature Flexural Strength
The silica sol-bonded sample had the highest high-temperature flexural strength, at 13.7 MPa. The cement-bonded and alumina-silica gel powder-bonded samples had lower high-temperature flexural strengths, at 7.8 MPa and 8.3 MPa, respectively. This is because the nano-SiO2 in the silica sol forms a silicon-oxygen network within the sample and is highly reactive. At 1000°C, it readily reacts with the active α-Al2O3 micropowder to form a mullite network, enhancing the sample's strength. The alumina-silica gel powder contains less SiO2, so the mullite network formed in the sample at 1000°C is not as strong as that of the silica sol-bonded sample, resulting in lower high-temperature flexural strength. Calcium aluminate cement contains a certain amount of CaO, which readily reacts with SiO2 and Al2O3 in the material at high temperatures to form low-melting-point phases such as 3CaO×Al2O3 and 2CaO×Al2O3×SiO2. These phases then become liquid at high temperatures, reducing the sample's high-temperature flexural strength.
Thermal Shock Stability
The silica sol-bonded sample exhibited the highest residual flexural strength, at 7.8 MPa. The sample bonded with alumina-silica gel powder exhibited the lowest residual flexural strength, at 5.3 MPa. The sample bonded with calcium aluminate cement exhibited both high residual flexural strength and flexural strength retention. The superior thermal shock resistance of the calcium aluminate cement-bonded and silica sol-bonded samples may be due to their concentrated pore size distribution and silicon-oxygen network structure, respectively. Within heterogeneous multiphase refractory materials, the differences in thermal expansion coefficients between the phases cause numerous microcracks to form in silicon carbide castables during thermal expansion mismatch. These microcracks not only absorb elastic strain energy, reducing the driving force for primary crack growth, but also disperse the stress concentrated at the crack tip, dissipating the energy required for crack propagation and improving the thermal shock resistance of the material.
Wear Resistance
Abrasion tests were conducted on samples with different binders after sintering at 1000°C. The results showed that the aluminate cement-bonded and alumino-silica gel powder-bonded samples exhibited less wear, with the aluminate cement-bonded sample exhibiting the lowest wear, at 3.75 cm³, and the colloidal silica-bonded sample exhibiting the highest wear, at 7.58 cm³. For heterogeneous refractory materials consisting of aggregate and matrix, erosion wear typically first removes the matrix, leaving protruding, isolated island-like particles as the primary wear target. These particles then fall off, forming cracks and further damaging the surrounding matrix. The aluminate cement-bonded samples exhibited a higher density, forming Si-O-Al bonds between the SiO₂ powder and cement hydrate, resulting in a tight matrix bond and better wear resistance. In the alumino-silica gel powder-bonded samples, nano-Al₂O₃ reacted with SiO₂ to form a mullite matrix, enhancing wear resistance. The colloidal silica-bonded samples exhibited numerous microcracks in the matrix, making them less resistant to erosion wear.
Microstructural Analysis
After heat treatment at 1000°C, the calcium aluminate cement-bonded specimens exhibited the tightest bond between matrix and aggregate, contributing to their higher density, strength, and wear resistance. Furthermore, the matrix contained numerous microcracks, resulting in a concentrated pore size distribution and excellent thermal shock resistance. The silica sol-bonded specimens exhibited numerous voids and microcracks, contributing to their high apparent porosity, wide pore size distribution, and poor wear resistance. Furthermore, the presence of a large silica-oxygen network structure contributed to their high high-temperature flexural strength and excellent thermal shock resistance. The alumina-silica gel powder-bonded specimens exhibited a better bond between aggregate and matrix, with a large columnar mullite network in the matrix, resulting in superior mechanical properties and wear resistance.
