Fineness Test :- The fineness of cement has an important bearing on the rate of hydration and hence on the rate of gain of strength and also on the rate of evolution of heat. Finer cement offers a greater surface area for hydration and hence the faster and greater the development of strength.
Fineness of cement is tested in two ways
(a) By sieving Test
(b) By determination of specific surface (total surface area of all the particles in one gram of cement) by air permeability apparatus.
a) Sieve Test: Weigh correctly 100 gms of cement and take it on a standard I.S sieve No. 9 (90m). Break down the air set lumps in the sample with fingers. Continuously sieve the sample giving circular and vertical motion for 15 minutes. Weigh the residue left on the sieve This weight is called “Fineness of cement”.
Fineness of cement > 10% for O.P.C.
Fineness of cement is also estimated in terms of specific surface area per unit weight.
Fineness of cement for O.P.C. = 2250 cm2/ gm.
(b.) Air permeability method.
This can be done by lea and Nurse permeability Apparatus. This apparatus can be used for measuring the specific surface of cement. The principle is based on the relation between the flow of air through the cement bed and the surface area of the particles comprising the cement bed. From this the surface area per unit weight of the body material can be related to the permeability of a bed of a given porosity. The cement bed in the permeability cell is I cm high and 2.5 cm in diameter. Knowing the density of content the weight required to make a cement bed of porosity of 0.475 can becalculate This quantity of cement is placed in the permeability cell inStandard manner. Slowly pass on air through the cement bed at a constant velocity Adjust the rate of air flow until the flowmeter shows a difference in level of 30-50 cm. Read the difference in level (h) of the manometer and the difference in level (of the flowmeter. Repeat these observations to ensure that steady conditions have been obtained as shown by a constant value of hy/h.
