The high-temperature performance properties of refractory materials are measured at high temperatures, such as refractoriness, reburning line change, thermal shock resistance, slag resistance, refractoriness under load, and creep. These properties, to some extent, reflect the refractory material's performance during use. Understanding these properties is invaluable for selecting and using refractory materials.

1. Refractoriness

Refractory performance indicates a material's ability to withstand high temperatures. Raw materials such as mullite, kyanite, andalusite, and sillimanite require refractoriness testing to assess their purity. Impurities, particularly those with strong fluxing properties, can reduce refractoriness.

Refractoriness testing involves shaping the refractory raw material or material into a triangular pyramid. The pyramid has an upper base of 2 mm, a lower base of 8 mm, and a height of 30 mm. The pyramid is then placed on a cone tray with a high-temperature standard cone. The sample cone is heated at a specified heating rate and compared to the standard cone. The sample cone gradually softens due to the appearance of high-temperature liquid phase and bends toward the bottom under the action of its own gravity. The temperature when the top of the cone bends down and touches the cone plate is the refractoriness. The refractoriness is represented by the number of the standard cone that bends at the same time as the sample. Refractoriness is different from melting point and operating temperature: (1) Refractoriness and melting point have different meanings. Melting point is the equilibrium temperature of melting from solid to liquid for a single crystalline substance. Refractoriness is the melting range of multiple minerals. It is true that the melting point of crystalline minerals is high, and the temperature of the low-melting melt formed by mutual interaction is also high. (2) Refractoriness should not be misunderstood as the temperature at which the refractory material can be used under high temperature. Because it has completely softened to the point of losing mechanical strength. For example, alkaline refractory materials are damaged because the bonding part melts first at a lower temperature, and its damage is based on the softening of the bonding agent. This temperature is much lower than the refractoriness of the material. The operating temperature of general clay bricks is about 200~250℃ lower than the refractoriness. 2. Reburn Line Change
Reburn line change refers to the residual expansion or contraction of a sample after heating to a specified temperature, holding for a specified period, and cooling to room temperature.
Reburn line change is a key indicator for evaluating the quality of refractory products. Expansion or contraction of a refractory product after reburning indicates incomplete firing due to insufficient temperature, holding time, or uneven temperature distribution. If the reburn line change of a refractory product exceeds nationally specified values, it is likely that physical and chemical changes will continue during use in industrial kilns due to the high temperatures. This will cause the product to expand or contract, resulting in significant dimensional changes. This can cause cracks in the furnace bricks, compromise the integrity of the furnace lining, and in severe cases, damage the furnace structure. Therefore, standardized firing procedures must be implemented to ensure that the reburn line change of the product is within specified limits.
3. Thermal Shock Resistance
Refractory materials are the foundation of industrial kilns. They are inevitably affected by temperature, sometimes sudden temperature fluctuations, during use. Thermal shock resistance refers to the ability of refractory materials to withstand the thermal stresses caused by rapid temperature changes without breaking.
The test conditions for samples in my country are: 1100℃, water cooling; expressed in times.
There are many factors that affect the thermal shock resistance index of refractory materials, so there are also many expressions. From the above imperfect relationship (the above formula does not take into account the shape and size of the material, heating (cooling) conditions, local stress generated by sudden temperature changes, etc.), it can be seen that the following main factors affect the thermal shock resistance of the material:
(1) Low elastic modulus;
(2) Small linear expansion coefficient;
(3) High thermal conductivity of the main mineral of the refractory material;
(4) Appropriate binder composition and main structure.
In the refractory material production process, the basic ways to improve the thermal shock resistance of refractory products are:
(1) Changing the phase composition of the product to obtain low-expansion minerals, or superposition of low-expansion minerals, etc.;
(2) Properly selecting and controlling the production conditions to make the binder composition and main structure appropriate, such as forming microcracks, increasing the material particles of certain products, etc. Table 1 lists the expansion coefficient and other properties of several refractory materials.
CO2 Resistance
There are many types of industrial furnaces, and refractory materials are subject to various corrosive factors within these furnaces, including molten steel and iron, slag (acidic or alkaline), temperature fluctuations, thermal stress, and atmospheric changes (especially reducing atmospheres). Therefore, refractory materials must possess specific properties tailored to the operating environment of the industrial furnace, one of which is CO2 resistance.
CO2 resistance refers to the ability of refractory materials to resist cracking or disintegration in a CO2 atmosphere. When refractory products are exposed to a strong CO2 atmosphere at approximately 500°C (300-600°C), a chemical reaction (2CO = CO2 + C) occurs. The released free carbon deposits around the iron spots in the product, causing cracking or damage. During blast furnace smelting, cracking and structural deterioration in the 400-600°C section of the furnace shaft due to these factors are a major cause of blast furnace lining failure. Reducing the iron oxide content and porosity of refractory products can enhance their resistance to CO2 corrosion. 5. Softening Under Load
The refractory material's deformation under load at high temperatures is expressed as its softening under load. It measures the product's resistance to the combined effects of high temperature and load, and indicates the softening range within which the product exhibits significant plastic deformation. This index is often used to determine the maximum operating temperature of the refractory material.
The refractory material's softening under load can be used to determine the conditions under which the refractory material loses its compressive strength during use and to infer its internal microstructure.
6. High-Temperature Creep
High-Temperature Creep refers to the relationship between the deformation of a refractory material under constant high temperature and load and time. High-Temperature creep incorporates the refractory material's strength, temperature, and time at high temperatures, indicating the deformation rate of the refractory product at a specific temperature and over a specific period of time.
For commonly used refractory bricks, all of the above indicators need to be considered. In different high-temperature industrial furnaces, the operating environment and working conditions vary, so the key indicators for refractory bricks in different locations will vary depending on the specific situation. For example, for refractory brick products used in hot blast furnaces, in addition to considering the material content, the most important thing to consider is the thermal shock stability index.