What does the compressive strength of refractory castables represent? How is it measured?
Normal Temperature Compressive Strength
Normal temperature compressive strength refers to the ultimate pressure that a specimen can withstand per unit area at room temperature after the completion of curing. In other words, it is the ultimate stress at which the specimen fails when subjected to a compressive load at room temperature.
The magnitude of this strength depends on the type and amount of the binding agent and its additives. It is also influenced by the purity of the raw materials, the mixing ratio, the amount of liquid used for mixing, the construction method, and the curing conditions.
The compressive strength test is conducted by applying compressive stress to the specimen at a specified loading rate using a special pressurizing device until the specimen fails. The loading surface measures 40 mm × 40 mm.
For the room temperature compressive strength test, each group of specimens shall consist of not fewer than % specimens. Each prismatic specimen should be tested for compressive strength after the bending test. The calculation formula is as follows:
The strength results are rounded to one decimal place. Table 1 shows the compressive strength of the refractory castables at room temperature.
Compressive Strength After Drying and Burning
The drying compressive strength refers to the ultimate pressure that a specimen can withstand per unit area at the end of the curing period after drying. This index serves as a basis for kiln design and as a reference standard for evaluating material performance. It should be noted that tests of unshaped refractory materials are generally conducted after the specimens have been dried.
The post-burning compressive strength refers to the ultimate pressure per unit area that a specimen can withstand after being heated to a specified temperature, held at that temperature, and then allowed to cool naturally to room temperature in the furnace. This index can be used to macroscopically analyze changes in the mineral composition and microstructure of the material, and to predict certain performance characteristics.
For example, refractory castables bonded with CA-50 cement exhibit low strength at medium temperatures, making furnace linings prone to cracking or spalling. In contrast, clay-bonded refractory castables maintain their strength as the temperature increases, meaning that their mid-temperature strength is almost unchanged and spalling is less likely to occur.
It should be noted that when specimens are fired at temperatures above 1300 °C, their post-burning strength becomes similar to the room-temperature strength of shaped refractory products.
Table 2 shows the drying and post-burning compressive strengths of refractory castables. From the table, it can be seen that the mid-temperature strength of low-cement, ultra-low-cement, and cement-free refractory castables does not decrease. Their high-temperature post-burning compressive strength, which is more than twice that of conventional refractory castables, reaches approximately 100 MPa.
High-Temperature Compressive Strength
High-temperature compressive strength refers to the ultimate stress at which a specimen fails under a compressive load at a specified high temperature. It serves as a basis for evaluating the abrasion resistance, slag resistance, and spalling resistance of unshaped refractory materials, as well as for selecting appropriate materials and determining their application areas.
Figure 1 shows a schematic diagram of the high-temperature compressive strength testing apparatus. The specimen is a square prism with a length of 70 mm.
1 – Nonmetallic upper and lower compression bars
2 – Furnace lining
3 – Specimen
4 – Heat generator
6 – Insulation layer
7 – Heat-resistant steel gasket plate
When the heating temperature is below 800 °C, amorphous refractory specimens are in the elastic deformation stage, and their high-temperature compressive strength is generally similar to, or slightly lower than, the post-burning compressive strength. When the temperature exceeds 800 °C, the specimens enter the plastic deformation stage, so the high-temperature compressive strength gradually decreases as the temperature increases. At 1400 °C, it typically ranges from 1 to 10 MPa.
Table 3 shows the high-temperature compressive strength of refractory castables. From the table, it can be seen that clay-bonded refractory castables exhibit higher high-temperature compressive strength than other types of castables, and their medium-temperature strength is also superior.
