Impact Of Aggregates On Lightweight High Alumina Castables

In recent years, researchers have studied a variety of lightweight aggregates, such as vermiculite, perlite, ceramic grains, refractory lightweight aggregates, refractory fibres, hollow balls, and so on. These lightweight aggregates used in the preparation of lightweight refractory castables each have their own scope of application.

Vermiculite and perlite can be used to produce castables with very low weight and excellent thermal insulation performance, but they are suitable only for use at lower temperatures. Ceramic castables not only operate at lower temperatures and have greater weight, but also exhibit higher strength. Refractory fibres can be used to produce castables with good thermal insulation properties, but they tend to have poor strength and are also limited to lower temperature applications.

Refractory lightweight aggregates can be used to manufacture castables suitable for a wide range of temperatures and offer high strength, but they generally have a higher bulk density and slightly poorer thermal insulation performance. Most natural lightweight aggregates, due to uncontrollable factors during their natural formation, tend to have structural defects, unstable composition, and high impurity content. These factors result in inconsistent properties across different parts of the material and limit their application temperature, which is generally not higher than 1200 °C.

In view of the drawbacks of natural lightweight aggregates, this study develops high-performance lightweight castables based on the use of synthetic lightweight aggregates. Synthetic lightweight aggregates offer high purity, uniform composition, and superior high-temperature performance. Examples include electrofused alumina hollow balls, complex-phase hollow spheres, and microporous mullite.

For the target temperature range of 1200–1500 °C, alumina hollow ball lightweight aggregates present certain limitations, such as excessive Al₂O₃ content and high cost. Therefore, the lightweight aggregates selected in this work are synthetic microporous mullite aggregates and complex-phase hollow spheres.

The physicochemical properties of the two lightweight aggregates are shown in Table 1.

01. Experimental Programme

1.1 Main Raw Materials

The raw materials used in this study include high-alumina bauxite clinker, bleaching beads, blue crystals, silica micropowder, alumina micropowder, pure calcium aluminate cement, raw bauxite powder, and rhodochrosite, in addition to synthesised lightweight mullite and complex-phase hollow spheres. The chemical compositions of the raw materials are shown in Table 2.

1.2. Experimental Formulations

In this part of the study on lightweight aggregate selection, lightweight mullite was gradually replaced with complex-phase hollow spheres, with the substitution ratio increasing from 0% to 40%, as shown in Table 3.

1.3. Sample Preparation Process

According to the proportions shown in Table 3, the various raw materials were accurately weighed. The materials were first dry-mixed in a mixer for 2 minutes, then water was added and mixed thoroughly. The mixture was cast into 40 mm × 40 mm × 160 mm strip samples using vibration casting. The samples were cured for 24 hours, followed by drying at 110 °C for 24 hours, and then heat-treated at 1350 °C for 3 hours before testing.

The bulk density, permanent linear change after heating, flexural strength at room temperature, compressive strength at room temperature, thermal conductivity, and other properties were tested in accordance with the relevant standards.

02 Results and Discussion

2.1. Effect of Aggregate Type on the Flexural Strength of Lightweight Castables

In lightweight castables, the main structural support is provided by the lightweight aggregates. Therefore, if the aggregate has low strength, it will inevitably lead to reduced overall strength of the castable.

Figure 1 shows the room-temperature strengths of samples Q1 and Q5 after heat treatments at different temperatures. CMOR refers to the room-temperature flexural strength, and CCS refers to the room-temperature compressive strength.

It can be seen that the strength of specimen Q5, which used complex-phase hollow spheres as the aggregate, was significantly higher than that of specimen Q1, which used lightweight mullite as the aggregate. This is because the strength of the complex-phase hollow sphere aggregate is considerably higher than that of the lightweight mullite aggregate. Therefore, to ensure that the lightweight castables possess sufficient strength at both room temperature and high temperatures, this study adopted a composite use of lightweight mullite and complex-phase hollow spheres as the aggregate.

2.2. Effect of the Proportion of Complex Hollow Spheres on the Bulk Density of Lightweight Castables

Figure 2 shows the change in bulk density when lightweight mullite aggregate is partially replaced with complex-phase hollow spheres. Since the bulk density of lightweight mullite aggregate is 0.80 g/cm³ and that of complex-phase hollow spheres is 0.60 g/cm³, the overall bulk density of the castables decreases as the proportion of complex-phase hollow spheres increases. When 40% of the lightweight mullite is replaced, the bulk density of the specimen after drying at 110 °C for 24 hours decreases from 1.86 g/cm³ to 1.68 g/cm³. This indicates that complex-phase hollow spheres have a significant effect in reducing the bulk density of the castables.

2.3. Effect of the Proportion of Complex-Phase Hollow Spheres on the Compressive Strength of Lightweight Castables at Room Temperature

Figure 3 shows the trend in compressive strength of the castables after incorporating different proportions of complex-phase hollow spheres. Since the strength of the complex-phase hollow sphere aggregate is significantly higher than that of the lightweight mullite aggregate, and because the complex-phase hollow spheres are denser and have lower water absorption, the required water content for the castables decreases. As a result, the compressive strength of the castables after drying and high-temperature firing tends to increase.

After heat treatment, the compressive strength increases from 25.2 MPa in Q1 to 52.6 MPa in Q4. This demonstrates that the addition of complex-phase hollow spheres has a significant positive effect on the room-temperature compressive strength of lightweight castables.

2.4. Influence of the Proportion of Complex-Phase Hollow Spheres on the Thermal Conductivity of Lightweight Castables

The thermal conductivity of the specimens was measured after heat treatment at 1350 °C for 3 hours to investigate the effect of different amounts of complex-phase hollow spheres on the thermal conductivity of the castables. As shown in Figure 4, the internal closed pores of the complex-phase hollow spheres provide good thermal insulation due to their hollow structure, resulting in a reduction of the castables' thermal conductivity after adding these spheres. The thermal conductivities of specimens Q1 and Q5 are 0.852 W/(m·K) and 0.682 W/(m·K), respectively. However, due to the relatively large pore diameter of the complex-phase hollow spheres, the reduction in thermal conductivity is limited by heat transfer mechanisms.

03 Conclusion

Replacing lightweight mullite aggregate with complex-phase hollow spheres reduces the water demand of the castables and significantly improves the room-temperature strength of lightweight high-alumina castables.

Complex-phase hollow spheres have good thermal insulation properties due to their hollow structure and internal closed pores. Therefore, adding these spheres reduces the thermal conductivity of the castables. However, because the pores inside the complex-phase hollow spheres are relatively large, the effect on lowering the material's thermal conductivity is limited.

The high-performance lightweight high-alumina castable developed using a composite of complex-phase hollow spheres and lightweight mullite microporous aggregates exhibits lower thermal conductivity, higher high-temperature strength, and excellent volume stability. It is suitable for use in various industrial furnaces operating at 1200–1500 °C. The castable achieves significant energy savings and offers considerable economic and social benefits.