Refractory Materials: Scientific Connotation of Bulk Density, Porosity, Water Absorption and Air Permeability
Release time：2026-05-07
page views：
I. Core Definition and Essential Logic
II. Scientific Connotation, Principle and Engineering Significance of Each Indicator
1. Bulk Density (BD)
Scientific Connotation
Internal Mechanism
The higher the grade of aggregates, the more reasonable the gradation, and the more sufficient the sintering → the more compact the solid filling → higher bulk density;
Insufficient molding pressure, low firing temperature, and excessive introduction of pores in the formula → low bulk density.
Engineering Value
High bulk density: dense structure, high high-temperature creep strength, strong resistance to molten slag penetration, wear resistance and erosion resistance;
Low bulk density: more internal pores, good heat insulation performance, low thermal conductivity, but decreased mechanical strength and slag resistance.
2. Porosity (Divided into Apparent Porosity, Closed Porosity and Total Porosity)
Scientific Connotation
Apparent Porosity (Open Porosity): The percentage of the volume of open pores connected to the outside world in the total volume of the material;
Closed Porosity: The percentage of the volume of internal closed, non-connected and non-communicating pores with the outside world;
Total Porosity: The total proportion of open and closed pores.
Internal Mechanism
More apparent pores: pores are interconnected to form a network channel;
More closed pores: pores are isolated and closed, with no connected channels.
Engineering Value
High apparent porosity: molten slag and flue gas are easy to penetrate and erode along the pores, resulting in poor slag resistance and impermeability;
Reasonable closed pores: can prevent crack propagation, improve thermal shock stability, and reduce thermal conductivity;
Both refractory castables and shaped refractory bricks require strict control of apparent porosity to reduce slag erosion and penetration.
3. Water Absorption (W)
Scientific Connotation
Internal Mechanism
Core Logic
Higher water absorption → more open connected pores → looser material, worse impermeability and slag resistance;
Lower water absorption → fewer open pores, denser structure.
Engineering Value
Construction end: If the water absorption of castables and refractory bricks is too high, water absorption will dilute the binder and affect the setting and hardening strength;
Service end: High water absorption makes it easy to absorb moisture and be penetrated by flue gas/molten slag capillaries, accelerating high-temperature damage.
4. Air Permeability
Scientific Connotation
Internal Mechanism
Large pore size, straight connected channels and high porosity → high air permeability;
Small pore size, tortuous channels and few apparent pores → low air permeability.
Engineering Value
Excessively high air permeability: high-temperature flue gas, alkali vapor and molten slag vapor can easily penetrate the furnace lining, erode the inner lining and penetrate into the steel structure;
Excessively low air permeability: can block gas penetration, improve alkali corrosion resistance, flue gas penetration resistance and thermal insulation sealing performance;
Special air-permeable refractory products (permeable bricks) deliberately control high air permeability for ladle bottom argon blowing refining.
III. Internal Correlation Logic of the Four Indicators
Higher bulk density → lower total porosity → denser structure;
Higher apparent porosity → higher water absorption and higher air permeability;
Closed pores do not participate in water absorption or air permeability, but only affect bulk density and thermal shock stability;
All four are determined by raw material gradation, molding pressure, firing system and formula admixtures, and are the "four quantitative characterizations" of the microstructure of refractory materials.
