Release Time：2018-03-26
Porosity refers to the percentage of the volume of pores contained in refractory bricks to the total volume of the product. According to experts from Henan Refractory Material Factory, the pores in refractory bricks can be divided into two types: pores enclosed in refractory bricks that are not connected to the outside world are called closed pores; The pores that are connected to the outside world and can fill the fluid are called open pores. In open pores, the pores that penetrate both sides of the product and allow fluid to pass through are also known as through pores.
Unlike refractory bricks used in blast furnaces, the ratio of the volume of open pores to the total volume of the product is called the open porosity, commonly known as the visible porosity, denoted as pa, and the ratio of the volume of closed pores to the total volume of the product is called the closed porosity, denoted as pc; The ratio of the total pore volume to the total volume of the product is called the true porosity, denoted as pt. The relationship between the three is: pt=pa+pc. Except for cast products, asphalt bonded or impregnated products, and partially insulated refractory products, the majority of pores in general refractory bricks are open pores. Due to the difficulty in directly measuring the volume of closed pores, the porosity index of products is usually represented by the apparent porosity. The apparent porosity of refractory bricks can be as low as close to zero or as high as 80%. The apparent porosity of general dense products is between 10% and 28%, and products with less than 15% porosity are called low porosity products. The true porosity of insulation materials is greater than 45%. Porosity is a fundamental technical indicator for most refractory materials. Its size almost affects all the properties of refractory products, especially strength, thermal conductivity, slag resistance, and thermal shock resistance. The increase in porosity leads to a decrease in strength, which is not only due to the decrease in solid cross-sectional area leading to an increase in actual stress, but more importantly, porosity is a defect in the material that can cause stress concentration and a decrease in strength. The influence of coarse pores is more significant.
Due to the extremely low thermal conductivity of air itself, an increase in porosity can significantly reduce thermal conductivity. The atmospheric porosity increases the penetration of slag into the product, or intensifies the oxidation of carbon in carbon containing refractory materials, significantly reducing the slag resistance of the material. Due to the complex influence of porosity on strength, fracture work, thermal conductivity, linear expansion coefficient, elastic modulus, etc., its impact on thermal shock resistance is also quite complex. The thermal shock resistance of most products is optimal when the porosity is within a certain range. Reducing porosity is beneficial for increasing the load softening temperature and high-temperature creep of refractory materials. The pores in refractory products are introduced by the raw materials themselves and formed during the molding, drying, and firing processes. It is distributed within particles, matrix, and between particles and matrix.
It is reported that different refractory materials have certain requirements for the size of porosity. The primary condition for producing dense products with low porosity is to choose raw materials with low porosity. According to the principle of the most compact packing, adopting a reasonable particle size distribution, using binders reasonably, fully and uniformly mixing, high-pressure forming, and appropriately increasing the firing temperature and prolonging the insulation time are important measures to reduce porosity in the production of dense refractory materials. In the production of dense castables, adding ultrafine powder and dispersants to the ingredients can form a tighter stack and obtain products with low porosity. In the production of thermal insulation refractory products that require high porosity, thermal insulation materials are used as raw materials, chemical foaming or the addition of combustible materials in the ingredients to generate and increase porosity.