Effect of Alumina Micropowder on the Properties of High-Performance Aluminum-Magnesium Refractory Castables
Producing high-quality steel requires the use of high-performance refractory materials. Al₂O₃-MgO castables exhibit excellent thermal shock and erosion resistance, and extensive research has been conducted on these properties. These castables are widely used in ladles. Since castables are typically installed on-site, their workability is a critical factor. The workability of castables is primarily influenced by dispersants, particle size distribution, and the content of fine powders. To improve the workability of Al₂O₃-MgO castables, this study investigates the effects of dispersant type, alumina powder variety, and alumina powder blending on the flowability of ladle-grade Al₂O₃-MgO castables. Additionally, the study examines the influence of these factors on the apparent porosity, bulk density, room-temperature flexural strength, and room-temperature compressive strength of the castables after drying.
Part 1: Experiment
1.1 Raw Materials
The raw materials used in the experiment include: tabular corundum with particle sizes of 5–3 mm, 3–1 mm, ≤1 mm, and ≤0.074 mm (200 mesh); fused magnesia with a particle size of ≤1 mm; activated alumina micropowders PFR40, PBR, and calcined alumina micropowder AC34B5 from Alteo Aluminum Industry (Shanghai) Co., Ltd.; activated alumina micropowder CL and calcined alumina micropowder CT from a certain company; calcium aluminate cement (Secar71); and polycarboxylate water reducers WS, FS, and AD. The physicochemical property indices of the five types of alumina micropowders are shown in Table 1.
1.2 Sample Preparation
The basic formulation (by weight) of the sample is as follows: 69% tabular corundum granules (5–3 mm, 3–1 mm, ≤1 mm), 16% tabular corundum fine powder (≤0.074 mm), 3% fused magnesia (≤1 mm), 3% calcium aluminate cement (Secar71), 9% alumina micropowder, and 4% water (added externally). Based on this, two major types of test formulas were designed:
The alumina micropowder selected was either PFR40 or PBR, and then 0.4% (w) WS, 0.2% (w) FS, and 1% (w) AD were added, respectively. The corresponding samples were also labeled as WS, FS, and AD, respectively.
The alumina micropowders selected were PFR40-AC34B5 composite powder and CL-CT composite powder, with the composite ratio (mass ratio) gradually decreasing from 9/0 to 3/6. The admixture used was WS, at an addition level of 0.4% (w). The ingredients were mixed according to the designed formula, dry-mixed in a mixer for 1 minute, then water and dispersant were added, and stirring continued for 4 minutes. A portion of the mixed material was used to test the flow value, while the remaining portion was poured into a 160 mm × 40 mm × 40 mm mold, left at room temperature for 24 hours, demolded, and then dried at 110°C for 24 hours.
1.3 Performance Testing
The flow value of the castables for each formula was tested in accordance with GB/T 2419-2005. The apparent porosity and bulk density of the dried samples for each formula were tested in accordance with GB/T 2997-2000. The room-temperature flexural strength of the dried samples for each formula was tested in accordance with GB/T 3001-2007. The room-temperature compressive strength of the dried samples for each formula was tested in accordance with GB/T 5072.2-2004.
Part 2: Results and Discussion
2.1 Influence of Single Alumina Micropowder and Dispersant
The flow value of castables with added alumina micropowders PBR or PFR40, as a function of dispersant type and dosage, is shown in Figure 1. From Figure 1, it can be seen that for the castable with PBR, the fluidity with dispersant WS is significantly better than that with FS or AD. For the castable with PFR40, there is little difference in fluidity between dispersants WS and FS, both of which are better than that with AD. For the castable with the same dispersant, the fluidity of the castable with PFR40 is better than that with PBR.
The changes in apparent porosity and bulk density of castables with added alumina micropowders PBR or PFR40, as a function of dispersant type and dosage, are shown in Figure 2. From Figure 2, it can be observed that for the castables with added PBR or PFR40, those with dispersant FS exhibit the highest apparent porosity, followed by those with WS, and the lowest with AD. In terms of bulk density, the castables with dispersant WS have the highest value, followed by those with FS, and the lowest with AD. For castables with the same dispersant, the apparent porosity of the castable with PFR40 is lower than that of the sample with PBR, while the bulk density is higher. This may be because PFR40 has a smaller particle size and a larger specific surface area than PBR, which helps to better optimize the packing density of the castable.
The changes in room-temperature flexural strength and room-temperature compressive strength of castables with added alumina micropowders PBR or PFR40, as a function of dispersant type and dosage, are shown in Figure 3. As shown in the figure, for the castables with added PBR or PFR40, the room-temperature flexural strength and compressive strength exhibit a similar trend: the samples with the water reducer FS have the lowest strength, while the samples with water reducers WS or AD show little difference. For castables with the same dispersant, the room-temperature flexural strength and compressive strength of the castables with PFR40 are slightly higher than those of the samples with PBR. This can be explained by the relationship between strength and porosity.
2.2 Influence of Composite Alumina Micropowder
The influence of composite alumina micropowder type and its composite ratio on the flow value of castables is shown in Figure 4. As seen in the figure, for castables containing PFR40-AC34B5 composite powder or CL-CT composite powder, when their composite ratio (mass ratio) decreases from 9/0 to 5/4, the flow value of the castables changes little. However, when the composite ratio decreases from 5/4 to 3/6, the flow value of the castables gradually decreases. At the same composite ratio, the flow value of the castables with PFR40-AC34B5 composite powder is slightly higher than that of the castables with CL-CT composite powder.
The influence of the type and composite ratio of composite alumina micropowder on the apparent porosity and bulk density of dried samples is shown in Figure 5. As seen in the figure, for the castables containing PFR40-AC34B5 composite powder or CL-CT composite powder, as their composite ratio (mass ratio) decreases from 9/0 to 3/6, the apparent porosity of the samples gradually increases, while the bulk density gradually decreases. This may be because the particle sizes of calcined alumina AC34B5 and CT are larger than those of activated alumina PFR40 and CL (see Table 1), resulting in relatively poor packing efficiency. At the same composite ratio, there is little difference in the apparent porosity and bulk density between the castables with different types of composite alumina powders.
The influence of the type and composite ratio of composite alumina micropowder on the room-temperature flexural strength and compressive strength of dried samples is shown in Figure 6. As shown in the figure, as the composite ratio (mass ratio) decreases from 9/0 to 3/6, the room-temperature flexural strength and compressive strength of the castables change little, remaining approximately 8 MPa and 70 MPa, respectively.
Part 3: Conclusions
In the Al₂O₃-MgO castables with a single alumina micropowder (either PBR or PFR40) added, the castables with 0.4% (w) WS exhibit better fluidity and overall properties in the dried samples than those with 0.2% (w) FS or 1% (w) AD.
The flow value of the castables with PFR40-AC34B5 composite powder is slightly higher than that of the castables with CL-CT composite powder. As the composite ratio of PFR40 to AC34B5 or CL to CT decreases, both the flow value of the castables and the density of the dried samples show a decreasing trend, while the strength of the dried samples changes little. Given that the cost of activated alumina micropowder is relatively high, the cost of the composite alumina micropowder should also be considered when evaluating the performance of castables, in order to select an appropriate composite ratio.