Methods of physical tests for hydraulic cement, Part 2: Determination of fineness by specific surface by Blaine air permeability method
1999 Edition

The Blaine air permeability method estimates cement fineness by measuring the specific surface area through air flow resistance. Cement powder is compacted into a permeable bed, and air is forced through it. The resistance encountered correlates with particle size; finer particles restrict airflow more, resulting in longer flow times. By measuring the time required for air to pass through the bed under a known pressure difference, the specific surface area (cm²/g) is calculated using Blaine's formula, enabling a rapid and reliable fineness determination.

The apparatus constant (K) is derived by testing a standard reference cement with known specific surface area. Using measured parameters such as cement density, bed volume, manometer readings, and fluid displacement, K is calculated via a prescribed formula and averaged over multiple trials. This constant links the physical properties of the standard cement and the apparatus readings, calibrating the equipment to provide accurate fineness measurements. Maintaining an accurate K value through periodic recalibration is essential to ensure consistency and reliability in test results.

The test environment must be maintained at a temperature of 27 ± 2°C, with a relative humidity not exceeding 65%, to ensure consistent results. Alternatively, a temperature of 20 ± 2°C may be used if mutually agreed upon. All materials involved in testing and calibration should be conditioned to laboratory temperature and protected against moisture absorption. These controlled conditions account for the effects of temperature on air viscosity and mercury density, which influence the permeability measurements.

Porosity, defined as the fraction of air volume within the compacted cement bed, directly affects the particle packing and air permeability. A standard porosity value of 0.500 (i.e., 50% air by volume) is targeted to ensure uniform test conditions. The mass of cement required to achieve this porosity depends on cement density and bed volume. Deviations from the standard porosity, especially with very fine or special cements, necessitate experimentally determining the actual porosity and adjusting the cement mass accordingly. Since porosity influences the air flow resistance, accurate control and measurement are crucial for precise fineness determination.

Calibration involves determining the apparatus constant using a secondary reference cement with known fineness, ensuring airtight manometer connections, and precise adjustment of manometer fluid levels. Recalibration is mandated after every 1,000 tests, upon changing the manometer fluid, filter paper, or manometer tube, and whenever systematic discrepancies in reference cement readings are observed. Leakage tests must also be conducted to confirm system integrity. Adhering to these procedures preserves the apparatus's accuracy and repeatability, thus ensuring reliable cement fineness measurements.