Porosity and pore size distribution affect these properties of refractory castables.

Porosity and pore size distribution affect these properties of refractory castables.

Influence on the Strength of Castables

After high-temperature heat treatment, the bond between the matrix and aggregate of a castable changes from hydration or coagulation provided by the binder to ceramic bonding formed by sintering. While ceramic phase materials are generally brittle and have high theoretical strength, their actual strength is much lower due to impurities, pores, and other defects. In fact, pores not only reduce the load-bearing area but also create stress concentration in the vicinity of the pores, thus weakening the material’s load-bearing capacity.

The pore size of refractory castables refers to the nominal diameter of the pores within the castable. It generally has an average or equivalent meaning and is characterized by maximum pore size, average pore size, and pore size distribution. Pore size distribution is another important aspect of the pore structure parameters of castables besides porosity. The strength of refractory castables is affected not only by porosity but also by factors such as the size and shape of the pores.

The Influence of Porosity on the Thermal Conductivity of Refractory Castables

The influence of porosity on the thermal conductivity of refractory castables is complex. When the porosity is low, the pore size is small, and they are uniformly dispersed in the castable medium, the pores can be considered as the dispersed phase in the continuous phase of the castable. Therefore, at relatively low temperatures, the thermal conductivity λ can still be calculated using the formula for the thermal conductivity of multiphase materials proposed by WDKingery.

Related literature also demonstrates that the thermal conductivity of high-alumina refractories decreases exponentially with increasing total porosity or apparent porosity. This is also the insulation principle of lightweight insulating castables, silicate fiber products, and hollow sphere lightweight ceramic products.

Influence of Thermal Expansion Coefficient on Refractory Castables

The thermal expansion of solid materials can essentially be attributed to the phenomenon that the average distance between particles in a lattice structure increases with increasing temperature. Since the forces between adjacent particles in lattice vibrations are nonlinear, the forces on either side of a particle’s equilibrium position are not symmetrical. The higher the temperature, the more pronounced this asymmetry becomes, leading to a greater increase in the average distance between adjacent particles, resulting in an increase in cell parameters and crystal expansion. Because refractory castables are ceramic-bonded after heat treatment, the theory of thermal expansion of solid materials also applies to them.

Many factors influence the coefficient of thermal expansion of materials, such as the material’s chemical and mineral composition, crystal structure and crystal form transformation, bond strength, micro-stress, external temperature, and the density of the internal structure. However, reports on its correlation with porosity are scarce.

Existing research indicates that the influence of porosity on the thermal expansion properties of materials largely depends on the distribution of pores within the material, and is not significantly related to the magnitude of the porosity itself.

The Influence of Slag Resistance on Refractory Castables

Slag resistance refers to the ability of refractory materials to resist the erosion and scouring of molten slag at high temperatures without being damaged. It is an important indicator for measuring the material’s resistance to chemical erosion and mechanical wear.

The erosion of castables by molten slag manifests in both surface dissolution and internal penetration. Slag penetration expands the reaction area and depth, causing qualitative changes in the composition and structure near the material surface, forming a highly soluble altered layer, leading to accelerated damage.

Therefore, when the castable material is the same, its matrix microstructure becomes crucial to its slag resistance. Yu Guocheng and Chen Zhaoyou pointed out that slag intrusion into refractory materials occurs through capillary channels, grain boundaries, liquid-phase channels formed by impurities within the material, and the crystal lattice. Among these, penetration along capillary channels is the most important. Open pores in refractory castables can be considered as capillaries, serving as channels for molten slag intrusion.

The higher the open porosity of the castable, the faster the molten slag intrusion rate; the intrusion ratio is approximately proportional to the porosity. In refractory castables, the matrix contains the majority of the total pores. Therefore, the matrix is more easily eroded than the aggregate, leading to aggregate exposure, increased reaction area, and eventual detachment and erosion, accelerating melting loss.

Furthermore, even with the same porosity, variations in pore size can alter the erosion rate. Maeda Eizo argues that slag penetration in basic refractories is governed by viscous flow in capillaries. According to the Hagen-Poiseuille fluid equation, pores larger than 1 μm in diameter will cause slag penetration. Therefore, an effective way to suppress slag penetration into the refractory matrix is ​​to maintain a fine pore size as much as possible. He Zhiyong et al., in their research on reducing slag penetration pathways in coatings, suggested selecting small-diameter organic fibers during coating production to improve the coating’s impermeability, and adjusting the particle size distribution of the coating without increasing its bulk density to minimize pore size.

Impact on the Anti-cracking Properties of Castables

For decades, the baking process of refractory castables has been a concern in industrial production. The main reason for sudden spalling of castables during baking is that free water boils at 100°C, generating pressurized gas that is not released in time. If the castable structure exhibits low permeability, the rate of steam generation is faster than its release from the pores; when the resulting pressure exceeds the ultimate strength provided by the binder, mechanical damage to the castable occurs. Therefore, permeability is a key parameter affecting the drying rate and cracking sensitivity of castables during heating.

The most successful method to improve the permeability of castables has been the addition of organic fibers such as polypropylene, polyglucan cellulose, and para-Aramine to the castable composition. These fibers create channels through which steam is released more quickly and efficiently. Since it is difficult to measure the permeability of castables during the heating and dehydration process, test data measured at room temperature are usually used as a reference. To obtain a more realistic water vapor dissipation mechanism, Japanese scholars conducted bursting and drying tests on alumina-based, bauxite-based, and clay-based refractory castables. They used an eddy current model to conduct a microscopic analysis of the water vapor dissipation mechanism of the castables and concluded that in actual castables, large pores are connected by small pores, as shown in the model below.

Conclusion

Refractory castables have undergone rapid changes in their materials, bonding systems, and construction methods, evolving from their initial use as repair materials for certain shaped furnace linings to their current widespread replacement of shaped products in various furnaces and kilns. However, these advancements are primarily reflected in technological progress. Research on the influence of matrix microstructure, especially pore structure parameters, on the mechanical and thermal properties of castables remains lagging, specifically in the following two aspects:

(1) Research on the pore structure characteristics after matrix microrefinement is insufficient, mostly limited to the porosity parameter, and quantitative characterization of other pore structure parameters has not yet been conducted;

(2) In-depth research on the influence of matrix pore structure on the physical properties of refractory castables is lacking, and quantitative research on the correlation between matrix pore structure parameters and the mechanical and thermal properties of castables has not yet been carried out.

Future work should focus on establishing the correlation between the pore structure of refractory castable matrix and its thermal and mechanical properties through quantitative characterization. This will allow for determining the sensitivity of different pore structure parameters to the material’s physical properties, leading to a more rational understanding of the role and significance of microstructural refinement in refractory castables. Furthermore, this work may provide a theoretical basis for the optimized design of refractory castable matrix structures, which is of great importance for promoting technological advancements in refractory castables.