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Basic Properties of Three Different Low-Cement Refractory Castables
The basic properties of low-cement refractory castables are determined by carefully designing and screening tests according to formulation principles, and preparing samples using optimal process conditions. The samples are then cured under natural conditions at an ambient temperature of 10–25°C. After curing, the 3-day room temperature compressive strength is tested, typically reaching 15.45 MPa. The samples are then dried and their properties are further examined.
Low-Cement Aluminosilicate Refractory Castables
Low-cement aluminosilicate refractory castables use Grade I clay clinker and Grade I and extra-grade bauxite clinker as aggregates, generally with a maximum particle size of 10mm and a dosage of 70%. Refractory powder uses extra-grade or Grade I bauxite clinker powder or is blended with brown corundum powder, with a dosage of 18%~24%. Ultrafine powder uses α-Al₂O₃ powder and silica fume, with a dosage of 6%~12%. An appropriate amount of composite water-reducing agent is also added. With increasing heating temperature, the strength of the castable increases significantly; the strength at 1500℃ is approximately twice that of the oven-dried strength. The high-temperature flexural strength at 1400℃ is 1.5~2.3MPa, nearly twice that of CA-50 cement refractory castables. The linear change after firing at 1400℃ or 1500℃ generally exhibits an expansion state; the absolute value is smaller, but relatively stable, meaning the volume change is relatively small, which is beneficial for the use of the castable. The CaO content meets the requirements for low cement castables.
The refractoriness of low-cement refractory castables is greater than 1790℃, and their softening temperature under 0.2MPa load (4% deformation) is 20-100℃ higher than that of CA-50 cement refractory castables. Therefore, the service temperature of low cement castables is generally about 100℃ higher than that of CA-50 cement refractory castables of the same material.
The thermal expansion rate of low-cement refractory castables exhibits an expansion characteristic throughout the heating process. It can be seen that the thermal expansion rate increases with increasing heating temperature, reaching a maximum of approximately 0.68% at 1250℃. Afterward, it begins to shrink slowly, with a thermal expansion rate of 0.43% at 1400℃. When the temperature reaches 1500℃, the thermal expansion rate rises again to 0.5%.
The performance characteristics of low cement castables are high strength, and the mid-temperature strength does not decrease but rather increases significantly, for various reasons. The dehydration of calcium aluminate hydrate is carried out slowly and continuously over a wide temperature range, with minimal disruption to the crystal structure.
The molded body of refractory castables has a heterogeneous microstructure with a small number of pores. The size and number of these pores directly affect the performance of the castable. The pore size distribution of two castables was determined using mercury intrusion porosimetry. The pore size of the low-cement refractory castable was no larger than 100 nm (1000 Å), accounting for 70.7%~73.2%, which is 2~3 times higher than that of the CA-50 cement refractory castable. The total porosity of the two castables after drying at 110℃ was basically similar. After firing at 800℃, the porosity of the low cement castable increased by about 6%, while that of the CA-50 cement castable increased by nearly 39%. This indicates that the low-cement refractory castable has a lower total porosity, more numerous and uniformly distributed small capillaries, resulting in characteristics such as low porosity, good pore structure, poor permeability, and strong resistance to slag erosion. This is also one reason for the high strength of this castable.
The apparent porosity of low-cement refractory castables is also relatively low compared to that of CA-50 cement refractory castables. After 800℃, with increasing heating temperature, the apparent porosity of low-cement castables increases only slightly, reaching a maximum of only 20.4%. In contrast, the apparent porosity of CA-50 cement castables increases significantly, reaching a maximum of 26.4%. Due to the low apparent porosity and dense microstructure of low-cement refractory castables, they exhibit high strength at medium and high temperatures.
Low-cement refractory castables have high strength, good wear resistance, low apparent porosity, and fewer low-melting-point substances, thus exhibiting strong resistance to slag erosion. For example, in a crucible method slag erosion test using converter steel slag as the medium, the heating temperature was 1450℃, and the holding time was 36 hours, measuring the erosion amount. Castables with 5% and 7% cement content were low cement castables, while those with 15% cement content were CA-50 cement refractory castables. A 2% CA-50 cement-based clay-bonded refractory castable was used as an accelerator. Under the same material and process conditions, the slag erosion resistance of the low-cement refractory castable was significantly improved compared to the CA-50 cement refractory castable, even surpassing that of the clay-bonded castable. Furthermore, the lower the cement content, the better the slag erosion resistance. Higher cement grades resulted in better slag erosion resistance in the low-cement castable. Specifically, the slag erosion resistance of the castable prepared with CA-70 cement was better than that prepared with CA-50 cement, with an approximately 25% reduction in erosion. The slag erosion resistance of the low cement castable was tested using the rotary method and compared with that of the clay-bonded castable. The medium was blast furnace slag, the heating temperature was 1400℃, the rotation speed was 6 r/min, and the rest of the operation followed standard procedures. The results showed that the slag erosion depth of the former was 1.25 mm, while that of the latter was 2.65 mm. This indicates that low-cement refractory castables exhibit significantly better slag erosion resistance than clay-bonded refractory castables.
The thermal shock resistance of low-cement refractory castables, measured by the number of water-cooling cycles after heating to 1100℃, is the same as that of CA-50 cement- and clay-bonded refractory castables, both exceeding 15 cycles. Due to its low porosity and dense structure, its thermal conductivity is slightly higher than that of CA-50 cement- and clay-bonded refractory castables, typically ranging from 1.05 to 1.35 W/(m·K).
Low-Cement Mullite and Corundum Castables
Low-cement mullite and corundum refractory castables use sintered or electrofused mullite, brown corundum, bauxite-based corundum, and white corundum as refractory aggregates. Sometimes, high-grade bauxite clinker powder is also used, while ultrafine powder is mixed with α-Al₂O₃ and silica fume. Aluminate cement is used as a binder, and dispersants are added. This material is used in steel rolling furnace linings, water-cooled pipe wrapping, burner bricks, slag weirs in tundishes, tapping troughs in medium and small blast furnaces, and electric furnace covers, among other thermal equipment, with good performance.
Low-Cement SiC Refractory Castables
Low-cement SiC refractory castables are characterized by a low coefficient of linear expansion, high thermal conductivity, high strength, and good wear resistance. They have been applied in thermal equipment such as power generation boilers, non-ferrous metallurgical furnaces, and incinerators, with good results.
Low-cement SiC refractory castable uses silicon carbide with SiC content greater than 97% as refractory aggregate and powder, with added SiO2 ultrafine powder and metallic silicon antioxidant. CA-70 cement is used as the binder, and a polyphosphate water-reducing agent is added. The main properties of this material are: SiC content 85%; at 110℃, the bulk density, compressive strength, and flexural strength are 2.5 g/cm³, 45 MPa, and 9 MPa, respectively; after firing at 1000℃, the linear shrinkage, compressive strength, and flexural strength are -0.2%, 107 MPa, and 24 MPa, respectively; after firing at 1450℃, the linear shrinkage, compressive strength, and flexural strength are +0.3%, 130 MPa, and 54 MPa, respectively; and the thermal conductivity at 400℃ is 12.2 W/(m·K).
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