Building Materials, Third Edition

w——2

2w—

2g

™Es†2v 217

VQ gveƒƒspsge„syx

According to the percentage of calcium oxide and clayey impurities in it, lime can be classified
as lean, hydraulic and pure lime. Since magnesium oxide slakes slowly, an increase in its
percentage decreases rate of hydration and so is with clayey impurities as well.

v—2 2€ 2v X It consists of CaO + MgO 80 to 85% with MgO less than 5% and clayey
impurities of about more than 7 per cent in the form of silica, alumina and iron oxide. It sets on
absorbing CO 2 from atmosphere.

g——™

™

1. Slaking requires more time and so it hydrates slowly. Its expansion is less than that of fat
   lime.


2. It makes thin paste with water.


3. Setting and hardening is very slow.


4. The colour varies from yellow to grey.

…  X It gives poor and inferior mortar and is recommended for less important structure.

r—™2v X It is a product obtained by moderate burning (900°-1100°C) of raw limestone
which contains small proportions of clay (silica and alumina) 5-30 per cent and iron oxide in
chemical combination with the calcium oxide content (CaO + MgO 70-80% with MgO less than
5%). In slaking considerable care is required to provide just sufficient water and no excess,
since an excess would cause the lime to harden. Depending on the percentage of clay present
these are classified further as, feebly, moderately and eminently hydraulic limes. It sets under
water.

p˜2r—™2v has less than 5-10 per cent of silica and alumina and slakes slowly, after
few minutes (5 to 15). The setting time is twenty one days. It is used in damp places and for less
important structures.

w—
2r—™2v has 10-20 per cent of impurities, slakes sluggishly after 1-2 hours.
The setting time is seven days. It is used in damp places.

i 
2r—™2v has clayey impurities 20-30 per cent and slakes with difficulty. Its
initial setting time is 2 hours and final setting time is 48 hours. It is used in damp places and for
all structural purposes.

€D2‚™2 2p—2v X It is soft lime (CaO + MgO more than 85% with MgO less than 4%)
obtained by the calcination of nearly pure limestone, marble, white chalk, oolitic limestone and
calcareous tufa. Also known as white washing lime should not have impurities of clay and
stones, more than 5 per cent. Fat lime is nearly pure calcium oxide and when it is hydrated with
the required amount of water the solid lumps fall to a soft fine powder of Ca(OH) 2 and the high
heat of hydration produces a cloud of steam. It sets on absorbing CO 2 , from atmosphere.

g——™

™

1. Slaking is vigorous and the volume becomes 2-3 times.


2. It sets slowly in contact with air, and hence is not suitable for thick walls or in wet climate.