The ratio of the volume of pores in a material to its total volume can be categorized into true porosity, closed porosity, and apparent porosity. In the refractory materials community in China, porosity generally refers to apparent porosity. Pores in refractory materials can be broadly classified into three types: closed pores, open pores, and through pores. Typically, these three types of pores are combined into two categories: open pores (including through pores) and closed pores. Apparent porosity refers to the ratio of the volume density of all open pores in the material to the total volume of the refractory material (the sum of the volumes of solids, open pores, and closed pores), which is the mass per unit volume of the material, expressed in g/cm3 or kg/m3. Porosity is a fundamental technical parameter for most refractory materials, and it affects almost all properties of refractory products, especially strength, thermal conductivity, resistance to erosion, and thermal shock resistance. Generally, as porosity increases, strength decreases, thermal conductivity decreases, and resistance to erosion decreases. Refractory castable is a type of unshaped refractory material made by mixing granular and powdered refractory materials in a certain composition, adding a certain amount of binder and water, and forming it through stirring and vibration casting. It is widely used in high-temperature industries such as metallurgy, glass, cement, petrochemicals, and energy. During the forming and heat treatment processes of refractory castable, pores are inevitably generated in the dehydration and sintering stages. Statistics show that in dense refractory castable, the matrix accounts for 25% of the total volume of the castable, and pores account for about 10% of the total volume of the matrix. Therefore, pores are an important part of the microstructure of the matrix. Pore structure parameters include a wide range of content, such as porosity, pore shape and distribution, pore size and pore size distribution, and pore volume, all of which largely determine the mechanical and thermal properties of refractory castable. In this paper, the research progress on the effects of three pore structure parameters—porosity, pore size, and pore size distribution—on the strength, thermal conductivity, thermal expansion coefficient, slag resistance, and spalling resistance of refractory castable is mainly introduced.

The thermal expansion of solid materials can essentially be attributed to the phenomenon that the average distance between particles in the lattice structure increases with rising temperature. Since the forces between adjacent particles in lattice vibrations are nonlinear, the forces acting on particles are asymmetric on either side of their equilibrium positions. The higher the temperature, the more pronounced the asymmetric force situation becomes, and the greater the increase in the average distance between adjacent particles, leading to an expansion of the unit cell parameters and the crystal as a whole. As refractory castables form a ceramic bond after heat treatment, the theory of thermal expansion of solid materials is equally applicable to them. There are many factors that influence the thermal expansion coefficient of materials, such as the chemical and mineralogical composition of the material itself, crystal structure and phase transformations, bond strength, microstress, external temperature, and the compactness of the internal structure. However, there are relatively few reports on the correlation between thermal expansion coefficient and porosity. Existing research indicates that the effect of porosity on the thermal expansion performance of materials largely depends on the distribution state of pores within the material, rather than the magnitude of porosity.

Slag resistance refers to the ability of refractory materials to withstand the erosion and scouring of molten slag at high temperatures without being destroyed. It is an important indicator for measuring the resistance of materials to chemical erosion and mechanical wear. The erosion of castable by slag is manifested in the dissolution of the surface and the penetration into the interior of the material. The penetration of slag into the castable expands the reaction area and depth, causing a qualitative change in the composition and structure near the surface of the material, forming a highly soluble altered layer, which accelerates damage. Therefore, under the same material conditions, the matrix microstructure of the castable is the key to its slag resistance. The pathways for slag penetration into refractory materials include capillary channels, grain boundaries, and the liquid phase channel network and lattice formed by impurities inside the material, among which penetration along capillary channels is the most important. Open pores in refractory castables can be regarded as capillaries, which are channels for slag penetration. The higher the open porosity of the castable, the faster the slag penetration rate, and the penetration ratio is approximately proportional to the porosity. In refractory castables, the majority of pores are in the matrix, so the matrix is more susceptible to erosion than the aggregate, causing the aggregate to be exposed, increasing the reaction area, and gradually detaching and being scoured away, accelerating melting loss. At the same time, even if the porosity of the castable is the same, but the size of the pores is different, the erosion rate will also change. In basic refractory materials, slag penetration is dominated by viscous flow in capillaries. According to the Hagen-Poiseuille fluid formula, pores with a diameter of more than 1 μm can cause slag penetration. Therefore, an effective means to suppress slag penetration in the matrix of refractory materials is to maintain a fine pore size level as much as possible.

For decades, the baking process of refractory castables has been a matter of concern in industrial production. The main reason for sudden spalling of castable during baking is that free water boils at 100°C, generating pressurized gas that is not promptly released. If the structure of the castable exhibits low permeability, the rate of steam generation is faster than the rate at which it is released through the pores. When the pressure formed exceeds the ultimate strength provided by the binder, mechanical damage to the castable occurs. It is evident that permeability is a major parameter affecting the drying speed of castable and its sensitivity to cracking during heating. The most successful method to increase the permeability of castable has always been to add organic fibers such as polypropylene, polyglucosan cellulose, and aramid to the castable composition. The channels formed by the burning of fibers provide a faster and shorter pathway for steam release. Since it is difficult to determine the permeability of castable during the dehydration process, experimental data measured at room temperature are usually used as a reference. To obtain a more realistic mechanism of water vapor escape, Japanese scholars conducted spalling and drying tests on alumina-based, bauxite-based, and clay-based refractory castables. Using an eddy current model, they conducted a microscopic analysis of the water vapor escape mechanism in castable and concluded that in actual castable, large pores are connected by small pores.

Since its inception, refractory castable has evolved from being used only as a repair material for certain shaped furnace linings to now largely replacing shaped products and being directly used in various kilns and furnaces. Its material, bonding system, and construction methods have undergone rapid changes. However, these advancements are mainly reflected in the progress of its process technology. Research on the impact of matrix microstructure, especially pore structure parameters, on the mechanical and thermal properties of castable is still lagging behind, specifically in the following two aspects: (1) Insufficient research on the pore structure characteristics after matrix microfine processing, mostly limited to the parameter of porosity, without conducting quantitative characterization research on other pore structure parameters; (2) Lack of in-depth research on the influence of matrix pore structure on the physical properties of refractory castable, without conducting quantitative research on the correlation between matrix pore structure parameters and the mechanical and thermal properties of castable. Future work is necessary to establish the correlation between the thermal and mechanical properties of castable and its matrix pore structure through quantitative characterization of the matrix pore structure of refractory castable. This will determine the sensitivity of different pore structure parameters to the physical properties of the material, providing a more rational understanding of the role and significance of matrix microstructure microfine processing in refractory castable. On the other hand, it may also offer a theoretical basis for the optimization design of the matrix structure of refractory castable, which is of great significance for promoting the technological progress of refractory castable.