What Properties Of Mullite-Bauxite Castables Can Be Enhanced By Adding Cordierite?

Materials containing cordierite not only possess excellent high-temperature resistance but also exhibit superior thermal shock resistance. In addition, cordierite-based kiln furniture and cordierite honeycomb ceramics are highly favored in high-tech fields such as electronic packaging materials, bioceramics, foam ceramics, printed circuit boards, and low-temperature thermal radiation materials. Consequently, research on cordierite materials has garnered unprecedented attention, and the study of artificially synthesized cordierite has become a hotspot in the field of ceramic materials.

Vibrated castables with mullite and bauxite as the matrix demonstrate strong explosion resistance during initial heating and require a small amount of mixing water. They exhibit high mechanical strength over a wide temperature range and are suitable for lining parts that demand excellent spalling resistance and wear resistance. Adding a portion of cordierite to mullite-bauxite castables can improve their thermal shock resistance.

This paper investigates the effects of adding different amounts of cordierite on the properties of mullite-bauxite castables, including bulk density, linear shrinkage, flexural strength, compressive strength, thermal expansion coefficient, and thermal shock resistance. The aim is to enhance the thermal shock resistance of mullite-bauxite castables.

01 Experiment

1. Raw Materials and Experimental Scheme

The main raw materials of this experiment are mullite, bauxite, cordierite, silica fume, and calcium aluminate cement. The main chemical compositions of the raw materials used are shown in Table 1.

The ingredients were prepared according to the formulations in Table 2. Specifically, aggregates and powders were added to a mixing tank and stirred uniformly. Water was then added, and the mixture was mixed for 3 minutes to prepare samples measuring 160 mm × 40 mm × 40 mm. After drying at 110°C, the samples were calcined at 1000°C, 1300°C, and 1500°C for 3 hours each. The bulk density, linear shrinkage, flexural strength, and compressive strength of the samples after different heat treatments were tested. Samples measuring Φ20 mm × 100 mm were prepared to test the thermal expansion coefficient of the material. To test the thermal shock resistance, samples measuring 160 mm × 40 mm × 40 mm were dried at 110°C and then calcined at 1300°C for 3 hours.

Performance Testing

The bulk density, linear shrinkage, flexural strength, and compressive strength of the fired samples were tested in accordance with the following standards: YB/T5200-1993 (Test Method for Apparent Porosity and Bulk Density of Dense Refractory Castables), YB/T5203-1993 (Test Method for Linear Change Rate of Dense Refractory Castables), and YB/T5201-1993 (Test Method for Room Temperature Flexural Strength and Compressive Strength of Dense Refractory Castables). The thermal expansion coefficient of the samples was determined using GB/T7320.1-2000 (Test Method for Thermal Expansion of Refractories - Push Rod Method).

The shrinkage of the samples was measured using a vernier caliper, and the linear shrinkage and bulk density were calculated from this measurement. The flexural strength was tested using a Japanese-made CT-1000 flexural testing machine, while the compressive strength was measured with a Japanese-made MS-20-S1 compressive testing machine. The thermal expansion coefficient was determined using an RPZ-03 high-temperature dilatometer, and the thermal shock resistance was evaluated using an RZ-2A high-temperature thermal shock resistance test furnace.

Experimental Process:

The electric furnace was heated to 1200±10°C and held for 30 minutes. The samples were then quickly placed into the furnace and insulated at 1200°C for 15 minutes to ensure uniform heating from the surface to the core. Subsequently, the samples were removed and rapidly cooled in circulating water at room temperature. After cooling for 3 minutes, the samples were immediately removed and placed in air until they reached room temperature. This cycle was repeated 5 times. The residual flexural strength of the samples was measured, and the percentage of strength loss was calculated.

02. Results and Discussion

1. Effects of Cordierite Addition Amount and Heat Treatment Temperature on the Linear Shrinkage Rate of the Material

Figure 1 shows the linear shrinkage rates of samples JA1, JA2, JA3, JA4, and JA5 with different cordierite contents after heat treatment at various temperatures. As shown in Figure 1, after drying at 110°C, the linear shrinkage rate of the samples changes slightly with increasing cordierite content. After heat treatment at 1000°C and 1300°C, the linear shrinkage rate of the samples gradually decreases as the cordierite content increases. This is because the theoretical composition of cordierite is w(Al₂O₃) = 35% and w(SiO₂) = 51%, but the cordierite used in this experiment has w(Al₂O₃) = 29.99% and w(SiO₂) = 46.31%, resulting in some excess SiO₂. As the cordierite content increases, the content of this free SiO₂ also gradually increases. At high temperatures, the excess silica transforms into cristobalite, which undergoes an expansion process. This partially offsets the shrinkage of the material, leading to a gradual decrease in the linear shrinkage rate of the samples as the cordierite content increases.

After heat treatment at 1500°C, as the cordierite content increases, the samples gradually transition from shrinkage to expansion, with the expansion rate increasing steadily. This is because cordierite melts and decomposes into mullite and magnesia glass at 1435°C, a process that is accompanied by expansion. Therefore, after heat treatment at 1500°C, as the cordierite content increases, the samples gradually shift from shrinkage to expansion, and the expansion rate continues to rise.

In practical applications, products containing cordierite are typically used at temperatures not exceeding 1400°C. This is because cordierite decomposes above this temperature, resulting in the loss of many of its excellent properties, such as thermal shock stability and a low thermal expansion coefficient.

2. Effects of Cordierite Addition Amount and Heat Treatment Temperature on the Bulk Density of the Material

Figure 2 shows the bulk densities of samples JA1, JA2, JA3, JA4, and JA5 with different cordierite contents after heat treatment at various temperatures. As shown in Figure 2, after drying at 110°C and heat treatment at 1000°C, 1300°C, and 1500°C, the bulk density of the samples consistently decreases with increasing cordierite content and also decreases with the rise in heat treatment temperature.

The reason for this is that after drying at 110°C, the hydration product of cement is C₃AH₆ (tricalcium aluminate hexahydrate). As the heat treatment temperature continues to increase, the hydration product C₃AH₆ gradually transforms into C₁₂A₇ (dodecacalcium heptaaluminate). During this transformation, crystal water is gradually released, leading to a continuous decrease in the mass of the samples, while the volume changes only slightly. Therefore, the bulk density of the samples steadily decreases as the heat treatment temperature increases.

At low, medium, and high temperatures, as the cordierite content increases, the content of excess silica also increases correspondingly, triggering a series of crystalline phase transformations in silica, resulting in volume expansion. This volume expansion leads to a looser internal structure and an increase in the number of pores in the samples. Additionally, as shown in Figure 1, with the increase in cordierite content, the linear shrinkage rate of the samples gradually decreases. This also leads to a decrease in the density of the material due to the addition of cordierite. Therefore, the bulk density of the samples decreases as the cordierite content increases.

3. Effects of Cordierite Addition Amount and Heat Treatment Temperature on the Flexural Strength and Compressive Strength of the Material

Figures 3 and 4 show the flexural strength and compressive strength of samples JA1, JA2, JA3, JA4, and JA5 with different cordierite contents after heat treatment at various temperatures. As shown in Figures 3 and 4, after drying at 110°C, the flexural strength and compressive strength of the material exhibit the same trend: both first increase and then decrease with increasing cordierite content, with the maximum values for both flexural strength and compressive strength occurring when the mass fraction of cordierite (w(cordierite)) is 5%.

As shown in Figure 3, after heat treatment at 1000°C and 1300°C, the flexural strength of the samples decreases with increasing cordierite content. After heat treatment at 1500°C, there is little change in the flexural strength among the samples with different cordierite contents, but all are lower than that of the sample without cordierite.

From Figure 4, it can be observed that after heat treatment at 1000°C, 1300°C, and 1500°C, the compressive strength of the samples first increases and then decreases with increasing cordierite content, with the maximum compressive strength occurring when w(cordierite) = 5%.

It can be concluded that when the addition of cordierite is 5% by mass, it increases both the compressive strength and the flexural strength after drying. However, the introduction of excessive cordierite reduces both the flexural strength and compressive strength of the material after heat treatment at various temperatures.

4. Effect of Cordierite Addition Amount on the Thermal Expansion Coefficient of the Material

As the cordierite content increases, the thermal expansion coefficient of the samples increases slightly. Theoretically, the introduction of cordierite should reduce the thermal expansion coefficient of the material. However, as mentioned earlier, the chemical composition of the cordierite used in this experiment deviates from the theoretical composition, resulting in an excess of silica. This excess silica leads to a series of quartz crystalline phase transformations during the heating process, causing volume expansion (see Table 3), which in turn increases the thermal expansion coefficient of the material.

Meanwhile, it can be observed that there is little difference in the thermal expansion coefficients between samples JA3 and JA5, with cordierite mass fractions of 10% (w(cordierite) = 10%) and 20% (w(cordierite) = 20%), respectively.

5. Effect of Cordierite Addition Amount on the Thermal Shock Resistance of the Material

A comparison of the flexural strength of sintered samples before and after thermal shock is shown in Table 4. As shown in Table 4, with the increase in cordierite content, the strength retention rate of the material gradually increases. As observed in Figure 2, the bulk density of the samples consistently decreases with increasing cordierite content, indicating a gradual increase in porosity. Although excessively high porosity adversely affects the strength of the material, it is precisely this high porosity that enables the material to exhibit excellent resistance to thermal shock damage, resulting in a higher strength retention rate as porosity increases.

In addition, as shown in Figure 5, the addition of cordierite does not reduce the thermal expansion coefficient of the material, but instead increases it slightly. However, the introduction of cordierite reduces the bulk density of the material and increases its porosity, which compensates for the reduction in strength by improving the material's thermal shock resistance. Therefore, adding cordierite to mullite-bauxite castables remains an effective way to produce materials with excellent thermal shock resistance.

03 Conclusions

(1) After heat treatment at 1000°C and 1300°C, the linear shrinkage rate of mullite-bauxite castables decreases as the cordierite content increases.

(2) The bulk density of mullite-bauxite castables decreases with the increase in cordierite content.

(3) After heat treatment at 1000°C and 1300°C, the flexural strength of mullite-bauxite castables decreases as the cordierite content increases.

(4) When the mass fraction of cordierite (w(cordierite)) is 5%, the compressive strength of mullite-bauxite castables reaches its maximum value after heat treatment at various temperatures.

(5) As the cordierite content increases, the thermal shock resistance of mullite-bauxite castables improves. The strength retention rate of the material gradually increases, and the lower bulk density contributes to a higher strength retention rate after thermal shock.