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.
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