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Porosity and porosity in refractory

In the refractory product, there are many pores of different sizes and shapes. The pores in refractory materials can be roughly divided into three categories:
(1) Open air hole, closed at one end and connected to the outside at the other end
(2) closed vents, closed products are not connected to the outside world
(3) through the pores, through the two sides of the refractory product, the fluid can pass
The presence of pores in the refractory material results in a decrease in its density and effective cross-section, which in turn causes changes in its mechanical properties and thermal properties. The performance of refractory materials subjected to thermal and heavy loads and resistance to thermal shock and slag corrosion during service is also greatly affected.
The through pores are easy to pass through the fluid, so that the aggressive fluid easily penetrates into the interior of the product, and the slag corrosion is intensified. The open pores enable the intrusion of fluid, but when the fluid invades, the gas in the pores is compressed, and the intrusion of the fluid is suppressed, so that the damage of the slag is smaller than that of the through pores. The closed pores are not invaded by external gas and liquid, and the damage of the slag is small. Sometimes it can also reduce the thermal conductivity and benefit from the thermal shock.
The pores which have a significant influence on the physical properties of the refractory material can be divided into: coarse pores and cracks of 1 mm or more; coarse pores of 25 μm or more and fine pores of 0.1 to 25 μm. The effect of different sized pores on the material properties is different.
The porosity is the percentage of the volume of gas pores contained in the refractory product to the total volume of the product. If the pores contain various pores, the ratio of the pore volume to the total volume of the material becomes the apparent porosity, which is also called the open porosity.
The apparent porosity is one of the important indicators for identifying the quality of refractories. In the process of using refractory bricks with high porosity, they are easily eroded by slag and easily absorb water during storage and transportation. Therefore, in the quality index of refractory bricks, the apparent porosity is generally specified. For example, the apparent porosity of ordinary clay bricks shall not exceed 20% according to the original metallurgical standard.