Tuesday, June 26, 2018

TESTING OF CEMENT-FINENESS TEST

A Vision To Clear Solutions.


FINENESS TEST
Finer the cement, greater the surface area for hydration and hence the strength attained is faster. Disadvantage of fine grinding is, it is susceptible to air set and early deterioration. The size of particle ranges from 100microns to 1.5microns (avg ~ 10microns). Fineness of cement can be measured using 1. Sieve test and 2. Air permeability method
1. Sieve Test
100 gms of cement is taken for this test. Any air set lumps found in the sample is broken down. The weighed cement sample is sieved through sieve no 9 (i.e., sieve size of 90microns) for 15minutes. The weight retained should not exceed 10% for ordinary cement and 5% for rapid hardening cement.
2. Air Permeability Test
The principle is based on the relation between the flow of air and the surface area of the cement particles. The surface area per unit weight of the material can be related to permeability. Lea and Nurse permeability apparatus is used for this test.
A cement bed of 1 cm high and 25cm dia, having a porosity of 0.475 is prepared. Knowing the density of the cement, the weight of cement required can be calculated. The cement bed is made in the permeability cell in a standard manner. Air is passed through the cement bed and the air flow is adjusted until the flow meter  shows a difference of 30-40cm. The corresponding readings in the manometer (h1) and flow meter (h2) are noted. The experiment is repeated until a steady value of (h1/h2) is obtained. Then the surface area is calculates as
Sw = k*√(h1/h2) cm^2/gm
k = 14/D*(1-ξ) * √((ξ^3 * A)/(C*L))
where,
Sw – surface area
D – Density of cement
ξ – porosity = 0.475
A – area of cement bed
C – Flow meter constant
L – Length of cement bed
Surface area shall not be less than 2250 cm2/gm for ordinary cement, 3250 cm2/gm for rapid hardening cement and 3200 cm2/gm for low heat cement.

No comments:

Post a Comment