What Are the Factors Affecting the High-Temperature Performance of Alkali-Resistant Refractory Castables for Cement Kilns?
The temperatures in the preheater system, precalciner, ascending flue, tertiary air duct, and other parts of a cement kiln range from 800 ℃ to 1200 ℃. Due to the enrichment of alkalis in the kiln gas, the materials used in these areas are susceptible to alkali corrosion, which tends to form expansive minerals such as leucite (KAlSiO₄) and kaliophilite (KAlSi₂O₆) within the bricks. This process causes the material structure to become loose and cracked, seriously affecting kiln operation. In areas where shaped bricks are not applicable, high-performance alkali-resistant castables are usually adopted as the kiln lining.
Alkali-resistant castables are hydraulic castables with alkali resistance. They are formulated using aluminosilicate materials as raw materials, aluminate cement as the binder, and appropriate admixtures. When alkali-resistant castables come into contact with alkali vapor under high-temperature conditions, a dense glaze layer can form on the surface, preventing further corrosion and thereby extending the service life of the kiln lining.
Cement and silica fume are the main raw materials of refractory castables, and their addition levels have a significant influence on both the room-temperature and high-temperature performance of the materials. In this study, architectural ceramics were used as the primary raw material to systematically investigate the effects of cement and silica fume additions on the properties of alkali-resistant castables. In addition, the microstructure of the samples was analyzed.
01 Experiment
1.1 Experimental Raw Materials
The architectural ceramics used in the experiment employed a five-grade particle size distribution, namely 8–5 mm, 5–3 mm, 3–1 mm, 1–0 mm, and fine powder with a particle size of less than 200 mesh. Other raw materials included silica fume, alumina powder, calcium aluminate cement, and a water reducer. The chemical compositions of the main raw materials are shown in Table 1.
1.2 Experimental Methods and Performance Testing
The mass ratio of aggregate to matrix was maintained at 64:36, while the mix proportion of the matrix was adjusted. The required raw materials were accurately weighed according to the mix proportions, placed into a mixer, and blended thoroughly. An appropriate amount of water was then added, and the mixture was stirred for 3 minutes before being poured into a three-compartment mold measuring 40 mm × 40 mm × 160 mm and formed by vibration.
After curing at room temperature for 72 hours, the samples were dried at 110 ℃ for 24 hours. A portion of the dried samples was then fired at 1100 ℃ with a holding time of 3 hours.
02 Results and Discussion
2.1 Effects of Calcium Aluminate Cement Dosage on Properties of Castables
The effects of calcium aluminate cement dosage on the room-temperature properties of samples after heat treatment at 110 ℃ for 24 h and at 1100 ℃ for 3 h are shown in Figures 1–3. As shown in the figures, the room-temperature strength of samples treated at 110 ℃ for 24 h increases with increasing cement dosage. In contrast, the room-temperature strength of samples subjected to heat treatment at 1100 ℃ for 3 h first increases and then decreases as the cement dosage rises. Meanwhile, the permanent linear change upon heating of samples treated at 1100 ℃ for 3 h shows increasing shrinkage with higher cement content.
In the formulation design, the increase in calcium aluminate cement dosage was achieved by reducing the content of 200-mesh fine powder of architectural ceramics. With a fixed dosage of silica fume, changes in sample strength were mainly attributed to variations in cement dosage. The higher cement content produced more hydration products, which acted as a binder and simultaneously filled the pores in the samples, improving compactness and strength.
After heat treatment at 1100 ℃ for 3 h, some hydration products formed by calcium aluminate cement at room temperature were destroyed due to the removal of structural water, losing their bonding effect. When the cement dosage was no more than 8%, the increase in room-temperature strength was mainly due to the smaller particle size and higher reactivity of the cement compared with architectural ceramic fine powder, with most of the strength derived from sintering.
Under the experimental conditions, when the cement dosage exceeded 8%, CaO in the cement reacted with Al₂O₃ and SiO₂ in the matrix to form low-melting eutectics. At high temperatures, a large amount of liquid phase was produced, and the cement colloid shrank due to water loss, generating cracks that reduced strength. Therefore, the formation of low-melting eutectics is the main reason for the increase in permanent linear change upon heating of samples after treatment at 1100 ℃ with increasing cement dosage.
In addition, the surface of the architectural ceramic raw materials contains a certain amount of glaze components. These glaze components have a relatively low melting temperature and are more likely to form a liquid phase during the sintering process. On one hand, the formation of the liquid phase promotes structural densification, enhancing sealing and alkali resistance. On the other hand, the liquid phase primarily forms on the surface of aggregate particles and within the matrix, improving the bonding performance between the matrix and aggregates. While this strengthens the integration of aggregates and matrix in the castables, it also reduces room-temperature toughness and increases brittleness. In particular, for castables with such an inhomogeneous internal structure, their strength becomes highly sensitive to defects and cracks. Considering these factors, the optimal dosage of calcium aluminate cement is around 7%–8%.
2.2 Effects of Silica Fume Dosage on Properties of Castables
With the calcium aluminate cement dosage fixed at 7.5%, the effects of silica fume dosage on the room-temperature properties of samples after heat treatment at 110 ℃ for 24 h and at 1100 ℃ for 3 h are shown in Figures 4–6.
As shown in the figures, with increasing silica fume dosage, the flexural and compressive strengths of the samples after heat treatment at different temperatures both improve, although the trend is not pronounced. At the same time, the linear shrinkage rate of the samples also increases.
Silica fume is an ultrafine powder formed by the rapid gas-phase reaction and condensation of SiO₂ and Si gases generated during the smelting of ferrosilicon alloys and industrial silicon in the presence of oxygen. It has high surface activity and, after hydration, can exert a bonding effect similar to silica sol, thereby providing a certain degree of strength and improving the strength of samples after heat treatment at various temperatures.
Silica fume can effectively fill the gaps between aggregates and the matrix, reduce the water demand of the samples, and improve the compactness of the alkali-resistant castables. With its filling and sintering-promoting properties, silica fume, when used together with appropriate dispersants, can endow castables with good rheological properties using only a small amount of water, while enhancing their high-temperature strength.
In the experimental scheme, the increase in silica fume dosage was achieved by reducing the content of 200-mesh fine powder of architectural ceramics. Since the reactivity of silica fume at high temperatures is much higher than that of architectural ceramic fine powder, increasing the silica fume dosage leads to a rise in the post-firing shrinkage rate of the samples.
An insufficient dosage of silica fume cannot effectively improve the strength of the alkali-resistant castables, whereas an excessively high dosage tends to cause cracking, impairing the high-temperature volume stability of the castables. Based on a comprehensive analysis, the optimal silica fume dosage is determined to be 5%–6%.
2.3 Microstructure Analysis
The cross-sectional microstructure of the sample prepared with 7.5% calcium aluminate cement and 5% silica fume, after heat treatment at 1100 ℃ for 3 h, is shown in Figure 7.
It can be seen from the figure that a certain amount of micropores is still present in the sample, while the bond between aggregates and matrix is strong. This structure mainly produces transgranular fractures across particles, which is beneficial for improving strength. The aggregate particles are primarily flaky in shape, which is related to the source, forming method, and crushing process of the architectural ceramics used as raw materials; however, the flaky shape is not conducive to improving the fluidity of the castables. The silica fume introduced into the matrix reacts with alumina powder to form columnar or acicular mullite, which has high bonding strength between grains and is interspersed within or fills the gaps of the skeleton structure. The network structure formed during mullite development plays a significant role in strengthening and toughening. A certain amount of glass phase also exists in the matrix. This glass phase improves the compactness and alkali resistance of the castables and originates both from the glaze components in the architectural ceramics and from the formation of low-melting eutectics in the matrix.
03 Conclusions
(1)The change in room-temperature strength of the samples after heat treatment at 110 ℃ for 24 h and at 1100 ℃ for 3 h is related to the dosage of calcium aluminate cement. Increasing the cement dosage leads to more low-melting eutectics in the samples at high temperatures and simultaneously increases the permanent linear change rate. Under the experimental conditions, the optimal calcium aluminate cement dosage is around 7%–8%.
(2)The addition of silica fume reduces water consumption and improves the compactness and strength of the castables. Under the experimental conditions, the optimal silica fume dosage is determined to be 5%–6%.
(3)After heat treatment at 1100 ℃ for 3 h, the aggregates and matrix of the sample are firmly bonded, and dual reactions of mullitization and low-melting eutectic formation occur in the matrix.