In industries such as refractory materials, ceramics, and powder metallurgy, the apparent porosity (open porosity) and bulk density are key physical parameters used to evaluate material compactness, strength, thermal shock resistance, and corrosion resistance. Accurate determination of these two properties is of great significance for material research and development, quality control, and process optimization.

At present, one of the most classical and widely used testing methods is the vacuum method (also known as the immersion method) based on the Archimedes principle. This article will systematically introduce its testing principle, operating procedure, applicable standards, and technical advantages.

I. Testing Principle

The core principle of the Archimedes displacement method (vacuum method) is based on Archimedes’ law of buoyancy, which states that a body immersed in a static fluid experiences an upward buoyant force equal to the weight of the fluid displaced by the body.

In the testing of porous materials, a vacuum environment is used to ensure that the liquid fully penetrates all open pores within the material. The mass of the specimen is then measured under three conditions: dry state, saturated state, and suspended (immersed) state, and the following formulas are used for calculation:

Apparent porosity: The percentage of open pore volume to the total volume of a material, reflecting the amount of pores in the material that are connected to the outside environment.

Bulk density: The ratio of the dried mass of a material to its total volume (solid volume + open pore volume + closed pore volume), reflecting the macroscopic density of the material.

II. Testing Procedure (Standard Steps)

The following procedure conforms to national standard GB/T 2997 and general standards such as ISO 5017:

1. Drying and Weighing

Dry the specimen to constant mass and record its dry weight m₁

2. Vacuum Impregnation

Place the specimen into a vacuum chamber and evacuate to a residual pressure of less than 2.5 kPa, maintaining thonis cdition for a specified period (typically about 30 minutes). Then introduce the immersion liquid (such as distilled water, kerosene, or other inert liquids that do not react with the material), and continue vacuuming to ensure the liquid fully penetrates and fills all open pores within the specimen.

3. Saturated Weight Measurement

Remove the saturated specimen and gently wipe off excess surface liquid (without drawing liquid from the pores). Weigh the specimen in air to obtain the saturated mass (m₂).

4. Suspended Weight Measurement

Suspend the saturated specimen in the immersion liquid using a basket, and measure its apparent submerged weight (m₃) while immersed in the liquid.

5. Calculation

Apparent Porosity (P)

P = (m₂ − m₁) / (m₂ − m₃) × 100%

Bulk Density (ρb)

ρb = m₁ / (m₂ − m₃) × ρliquid

Where:

ρliquid = density of the immersion liquid at the test temperature

III. Why Use the Vacuum Method?

Conventional immersion methods often fail to allow the liquid to fully penetrate the tiny pores, leading to lower-than-expected measurement results. A vacuum environment effectively removes air from the pores, ensuring the liquid fully fills all open pores, resulting in high-precision, highly repeatable measurement results.

This method is particularly suitable for:

Porous materials with high apparent porosity (> 5%)

Sintered products with small pore sizes or complex pore structures

Solid materials that are insensitive to immersion (do not dissolve, expand, or react)

IV. Applicable Materials and Industries

Industry Fields & Typical Materials

1. Refractory Materials

Clay bricks, high-alumina bricks, magnesia-carbon bricks, castables, insulating bricks

2. Advanced Ceramics

Alumina (Al₂O₃), zirconia (ZrO₂), silicon carbide (SiC), silicon nitride (Si₃N₄) ceramics

3. Building Materials

Permeable bricks, aerated concrete, natural stone, ceramsite (expanded clay aggregate)

4. Powder Metallurgy

Sintered metal filters, oil-impregnated bearings

5. Abrasives & Grinding Tools

Ceramic bonded grinding wheels, oil stones

V. Equipment Selection Recommendations

A qualified apparent porosity/bulk density analyzer should possess the following features:

✅ High-precision electronic balance (accuracy 0.01 g or higher)

✅ Large-capacity vacuum chamber and stable vacuum pump (ultimate vacuum < 2 kPa)

✅ Automatic liquid replenishment and level control function

✅ Dedicated rack and basket conforming to standards (GB/T 2997, ASTM C20, ISO 5017)

✅ Automatic data acquisition and calculation system to reduce human error

VI. Limitations of the Method

It should be noted that the Archimedes vacuum drainage method has the following limitations:

1. Only open porosity can be measured: Closed porosity or total porosity cannot be obtained.

2. Not suitable for powders or ultrafine particles: Powders cannot be formed into block samples, and they may be suspended or lost during the immersion process.

3. Samples must not react with the immersion liquid: If the material hydrates, dissolves, or swells upon contact with water, it must be replaced with an alternative liquid such as kerosene or alcohol.

If it is necessary to determine the closed-porosity, total porosity, or true density, a true density analyzer using the gas expansion method (helium displacement method) should be used.

VII. Conclusion

The Archimedes displacement method (vacuum method), with its clear principle, standardized operation, and reliable results, has become the benchmark method for determining the apparent porosity and bulk density of porous block materials.The corresponding Apparent Porosity and Bulk Density Testing Instrument is an indispensable basic testing device for quality control laboratories in the refractory, ceramic, and building materials industries.

By strictly following standardized procedures and integrating modern automatic weighing systems and vacuum control technology, this method can significantly improve testing efficiency and data accuracy, providing strong and reliable data support for material performance evaluation and process optimization.