Air Permeability Method:
The fineness of cement is represented by specific surface, i.e. total surface area in cm2 per gram or m2 per kilogram of cement and is measured by Lea and Nurse apparatus or by wagner turbidimeter..
The Lea and Nurse apparatus shown in Fig. 28 essentially consists of a permeability test cell-where cement is placed and air pressure is applied, flowmeter-to determine the quantity of air passing per second through its capillary tube per unit difference of pressure, and manometer-to measure the air pressure.
To determine the fineness, a cement sample of 20 mm height is placed on a perforated plate (40 micron perforations) and air pressure is applied. The manometer is connected to the top of the permeability cell and the air is turned on. The lower end of the permeability cell is then slowly connected to the other end of the manometer. The rate of flow is so adjusted that the flowmeter shows a pressure difference (h2) of 30-50 cm. The reading (h 1) in the manometer is recorded. The process is repeated till the ratio h1/h2 is constant. The specific surface is given by the expression
The minimum specific surface for various cements should be as specified in Table 6.
Table 6 Minimum Specific Surfaces of Cements