Apparent porosity is a key physical property of high alumina bricks that significantly impacts their performance in various high-temperature industrial applications. It refers to the percentage of open pores connected to the surface of the brick that are capable of absorbing water or gas. Understanding apparent porosity helps to select the ideal high alumina brick for specific applications, such as high-heat, high-corrosion, or insulation zones in industries like steelmaking, cement, and glass production.
At Highland Refractory, we control the apparent porosity of our high alumina bricks (48%-80% Al₂O₃) within the range of 16%-28%, optimizing raw material gradation and sintering processes to meet diverse industrial needs.
Apparent porosity (denoted as “Pa”) is the ratio of the volume of open pores to the total volume of a high alumina brick, expressed as a percentage. These open pores are connected to the brick’s surface and play a critical role in the material’s interaction with the environment. Closed pores, which are not connected to the surface, do not impact this interaction and are not included in the apparent porosity calculation.
Apparent porosity is measured according to ASTM C20 (American standard) or ISO 5017 (international standard). The measurement steps are as follows:
Weigh the dry brick (m1).
Immerse the brick in boiling water to saturate the pores (m2).
Weigh the saturated brick in water (m3).
Use the formula to calculate apparent porosity:
Highland Refractory uses ASTM C20 to test every batch of high alumina bricks for apparent porosity, providing test reports to customers upon request.
The apparent porosity of high alumina bricks directly impacts several key performance factors, including thermal insulation, slag resistance, and mechanical strength. Below, we explore how apparent porosity influences these properties.
Higher apparent porosity (22%-28%) results in lower thermal conductivity, making the brick an ideal choice for insulation in low-heat zones. For example, Highland Refractory’s high alumina bricks with Pa=24%-26% exhibit a thermal conductivity of ≤0.9W/(m·K) at 800°C. Conversely, lower apparent porosity (16%-20%) increases thermal conductivity (1.1-1.3W/(m·K)), making it more suitable for high-heat environments such as steel blast furnaces.
Lower apparent porosity (16%-20%) means fewer open pores for slag penetration, leading to better slag resistance. For instance, Highland Refractory’s high alumina bricks for steel Electric Arc Furnaces (EAF) have a Pa=17%-19%, resulting in a slag erosion rate ≤0.18mm/h. In contrast, bricks with higher porosity (22%+) trap more slag, increasing the risk of accelerated corrosion in high-slag environments.
Bricks with lower apparent porosity (16%-20%) are denser and have higher compressive strength, making them ideal for high-impact zones. For example, Highland Refractory’s high alumina bricks for charging ports have Pa=18%-20%, with compressive strength ≥65MPa. Bricks with higher porosity (22%+) may have lower strength (≥45MPa) but offer better thermal shock resistance, which is beneficial in environments with frequent temperature fluctuations.
Ideal for high-heat, high-slag, and high-impact scenarios, such as steel blast furnace linings, EAF, and cement kiln burning zones. Highland Grade I high alumina bricks (Al₂O₃ ≥75%) have a Pa=17%-19%, making them well-suited for these demanding applications.
Suitable for medium-heat, balanced scenarios, such as ceramic kiln chambers and glass furnace regenerators. Highland Grade II high alumina bricks (Al₂O₃ 65%-75%) have a Pa=22%-24%, providing a balance of thermal insulation and mechanical strength.
Ideal for low-heat, insulation scenarios, such as cement kiln preheaters and industrial flues. Highland Grade III high alumina bricks (Al₂O₃ 48%-65%) have a Pa=26%-28%, providing excellent insulation properties while maintaining low thermal conductivity.
According to industry standards like ASTM C155, high alumina bricks should have apparent porosity ≤30%. Highland Refractory ensures that all our high alumina bricks meet this requirement, with tighter control over apparent porosity (≤28%).
Apparent porosity is a critical factor that influences the performance of high alumina bricks. It directly impacts the material’s thermal insulation, slag resistance, and mechanical strength. Lower porosity is ideal for high-heat and high-slag environments, while higher porosity is better for insulation.
Highland Refractory offers high-quality high alumina bricks with controlled apparent porosity ranging from 16%-28%, optimized to meet the needs of various industrial applications. Our expertise in raw material selection and sintering processes ensures that our products deliver superior performance in every scenario.
To get Highland’s high alumina brick “Apparent Porosity – Performance – Scenario” chart, or to request a porosity test report for our products, contact our brand specialist directly.
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