Fineness Of Cement By Blaine’s Air Permeability Method (IS-4031-Part-2)

The air permeability of a material is the measure of how easily air can pass through a material. Blaine's air permeability apparatus follows a means of drawing a definite quantity of air through a prepared bed of cement of definite porosity. The basic principle used in the air permeability test to determine the fineness of cement is based on the relationship between the fineness of cement particles and the rate at which air can pass through a compacted cement sample. 

Finer particles provide greater surface area, resulting in increased resistance to airflow. Therefore, by measuring the time it takes for a fixed volume of air to pass through a cement sample under standard conditions, we can infer the fineness of the cement.

This article explains in detail the test procedure followed for testing of fineness of cement using Blaine's Air Permeability Test  Apparatus as per (IS:4031 (Part 2)).

Principle of Air Permeability Test on Cement 

The principle behind air permeability method on cement is in "observing the time taken for a fixed quantity of air to flow through a compacted cement bed of specified dimension and porosity".

Under standard conditions, it is given that the:

Specific surface of cement is proportional to the √t

Here, 't' is the time taken for the given quantity of air to flow through the compacted cement bed. 

The number and size range of individual pores in the specified bed are determined by the cement particle size distribution which also determines the time for the specified air flow.

The method is comparative rather than absolute and therefore a reference sample of known specific surface is required for the calibration of the apparatus.

Equation to Determine the Specific Surface of Cement

In general, the fineness of cement is determined using the equation:

This equation for fineness can be simplified as: [Table-2]

S=K√t

Where:

S is the specific surface area of the cement sample in cm²/g,
K is the K value obtained from the test, and
t is the time taken for the air to pass through the cement sample under specified conditions. 

 

Materials Required

1. Blaine Permeability Apparatus
2. Mercury
3. Standard cement sample, whose specific surface is known = 3100cm2/gm
4. Permeability cell with a plunger
5. Perforated disc, Plain disc and filter paper
6. Stopwatch

Test Procedure

The test methodology followed here is initially determining the constant 'K' of the Blaine apparatus provided for the test. This is determined using the standard values of known cement values, which include the specific surface of the reference cement (So) and the density of the test sample. Once the constant is obtained, the specific surface of the unknown test sample is determined. 

The equation used to determine the specific surface is dependent on the temperature at which the test is conducted. 

In the Blaine apparatus, a low-viscosity and low-density liquid ( dibutyl phthalate or any light mineral oil ) is used to fill the tube till the ‘C’ mark. The markings on the manometer tube are marked A, B, and C as shown in the figure.

Fig.2. Blain Permeability Apparatus With Markings A,B and C in Manometer

1. Calculation of Bulk Volume of Cement (V)

Place a plain disc on the cell. Now, place filter paper to cover the plain disc. Now add mercury to the cell until full and level using a glass piece. Now weigh the amount of mercury in the cell by transferring it into another container. This weight is named Wa. 

Now place a perforated disc at the bottom of the cell, then filter paper. Take 2.8 grams of standard cement (whose specific surface So is known) and add it to the cell. Remove any air bubbles if any. Place a new filter paper on the top and add a plunger. Remove the plunger and add mercury until full and level using a glass plate. Now transfer the mercury in the cell to another dish, weigh it, and name as Wb.

Hence,

V Bulk Volume of the cement= (Wa- Wb)/ D

Wa = Grams of mercury required to fill the cell with no cement in the cell
Wb = Grams of mercury required to fill the portion of the cell not occupied by the prepared bed of cement in the cell 
D= density of the mercury at the temperature of the test (From Table 1, IS 4031 Part 2) selected based on temperature 

2. Determine the Weight of cement (W)

W = ρV (1-e)

 
V= Volume of cement obtained from before step

ρ = Density of Test Sample =3.15Mg/cm³ or 3.15g/cm³
e=porosity = 0.500

3. Determination of K Value

Put perforated disc and filter paper. Measure 'W' of standard cement and transfer to the cell.
Now use a lubricated plunger to expel out any excess air from the cell. 

Now place the cell on the top of the tube, the conical portion of the cell on the Blaine's Apparatus.

Open the stop cork and apply the pressure such that the liquid reaches at point A the top of the manometer with a suitable plug. And close the cork. (Fig.2)

Now remove the plunger from the cell, by rotating it slowly. Now remove the plug, and the liquid level starts to rise from A to B. When the level reaches ‘B’ start the stopwatch, until it reaches mark C. Record the time ‘ts’.

S = K.√t

K = So/√ts

Determine K, with So specific surface of standard sample.

4. Determination of Specific Surface of Unknown Cement Sample (So)

Measure W gms of unknown cement sample and fill the cell similar to the above procedure. Repeat the process in the Blaine apparatus. Record the time from B to C. and determine ‘t’.

S = K.√t

Calculation of Specific Surface of Cement as Per ASTM C 204-07

As per ASTM Standards, 

S = [Ss .  √T ]/ √Ts

T = Time measured from manometer for test sample

Ss is the specific surface of the reference cement in cm2/g

√Ts is the mean of three times measured on the reference cement

Result: The minimum specific surface for OPC should not be less than 2250 cm²/kg.

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