🧪 Cement Fineness by Blaine’s Air Permeability Method

🔍 Purpose

The Blaine’s Air Permeability Method is used to determine the specific surface area (fineness) of cement particles. The test measures the air flow resistance through a compacted cement bed, which indicates how fine the cement is ground. Finer cement leads to faster hydration, higher early strength, and better performance in construction.

📘 Reference Standard

• IS 4031 (Part 2):1999 – Methods of Physical Tests for Hydraulic Cement – Determination of Fineness by Air Permeability Method
• ASTM C204 – Standard Test Method for Fineness of Hydraulic Cement by Air Permeability Apparatus

⚙️ Apparatus Required

• Blaine air permeability apparatus
• Vacuum pump or aspirator
• Non-absorbent filter paper discs
• Mercury manometer (for pressure measurement)
• Stopwatch
• Weighing balance (accuracy ±0.001 g)
• Cement sample and standard reference cement

🧫 Principle

The test is based on the relationship between the air flow rate through a compacted cement bed and the surface area of the particles. Finer cement has more surface area, causing greater resistance to air flow. The time required for a fixed quantity of air to pass through the cement bed is measured and used to calculate fineness.

📐 Formula

The specific surface area (S) of the test sample is calculated using the formula:

S = S_s × √(t / t_s) × (η / η_s) × (ρ_s / ρ)

• S = Specific surface area of test sample (cm²/g)
• S_s = Specific surface area of standard cement (cm²/g)
• t = Time for air to pass through the sample (s)
• t_s = Time for standard sample (s)
• η, η_s = Viscosities of air at test and standard conditions
• ρ, ρ_s = Densities of cement (g/cm³)

🧪 Procedure

1. Determine the porosity (packing density) required to achieve the standard air permeability rate.
2. Weigh a suitable quantity of cement (about 2–3 g) and fill it into the permeability cell.
3. Compact the cement using a plunger to achieve uniform packing.
4. Place filter papers at the top and bottom of the cement bed.
5. Connect the cell to the Blaine apparatus.
6. Pass air through the sample and measure the time taken (t) for the manometer reading to fall between two marks.
7. Repeat the test with a standard reference cement having a known specific surface area (S_s).
8. Calculate the fineness of the test sample using the formula.

📊 Example Calculation

• Specific surface area of standard cement, S_s = 2800 cm²/g
• Time for standard sample, t_s = 90 s
• Time for test sample, t = 110 s
• Assume ρ and η are same for both samples.

S = 2800 × √(110 / 90) = 2800 × 1.105 = 3094 cm²/g

✅ Result: Specific surface area = 3094 cm²/g

📏 Permissible Limits (as per IS 4031)

• Ordinary Portland Cement (OPC): 225 – 325 m²/kg
• Rapid Hardening Cement: ≥ 325 m²/kg
• Pozzolanic Cement: 300 – 400 m²/kg

📋 Significance

• Indicates the rate of hydration and strength gain.
• Helps control grinding operations in cement production.
• Ensures uniform quality and performance in concrete.

⚠️ Precautions

• Ensure air pressure and temperature remain constant during the test.
• Use clean, dry apparatus and fresh cement sample.
• Repeat test at least twice and take the average value.

📘 Conclusion

The Blaine’s Air Permeability Method provides a reliable and accurate measure of cement fineness. Finer cement improves early strength and durability, but excessive fineness increases water demand and shrinkage. Maintaining the specific surface area within the recommended limits ensures high-quality and consistent cement performance.