Lime|Types|Properties|Classification|Test of lime

1.Fat lime:

This lime is known as fat lime because it increases 2 to 2.5 times in volume when slaked. This lime is obtained by burning comparatively pure lime stone. This lime is also sometimes known as pure lime, rich

lime, white lime or high calcium lime. The limestone from which this lime is obtained should contain about 95% calcium oxide and about 5% other material in the form of impurities after burning. When limestone containing mostly calcium carbonate is burnt in atmosphere, carbonate dioxide is driven out, leaving back calcium oxide (CaO), known as

quick lime. Fat lime is obtained by slaking quicklime.

2.Hydraulic lime:

This lime has the property of setting under water. It is obtained by burning limestone. Containing a lot of clay, and other substances which develop . Hydraulicity of this lime, depend upon the

amount of clay and type of clay present in it. Silica, alumina and or iron oxide are present in a chemical combination with calcium oxide (CaO). Depending upon the amount, of clay (Silica and alumina)

present hydraulic limes may be further divided into following three categories.

I.Feebly hydraulic lime: It contains silica, alumina and/or iron oxide less than 15%. The usual percentage of these constituent

varies between 5% to 10% on slacking, it increases in volume by small amounts. It slakes slowly. This lime may be used in the form of mortar with sand in not very important work.

II.Moderately hydraulic lime: The lime contains 15 to 25% silica and alumina. It slakes very slowly and increases by a very small amount on slaking. This lime provides a very good mortar with sand. It is mostly used as mortar in good types of masonry works.

III.Eminently hydraulic lime: This lime is of even better quality than moderately hydraulic lime. It contains 25 to 30% clayey (silica and alumina) ingredients. It resembles very much to Portland cement in chemical composition. Slaking of this lime is hardly noticeable. Its initial setting starts after 2 hours and the final setting within 48 hours. This lime is mostly used for structural purposes and for the works carried out under coat or in damp situations.

Properties

●Its hardening action is slow.

●Its setting action, being dependent upon atmospheric air, is also slow.

●Slakes vigorously with hissing sound and a lot of heat is generated during slaking.

●It swells 2 to 2.5 times of quick lime after slaking.

●It has a very high degree of plasticity.

Soluble in water.

●It is milky white in its purest form.

Properties of hydraulic lime

●Increased percentage of clay renders lime more hydraulic and makes slaking more difficult.

●With 30% clay content, its chemical composition resembles

more or less that of cement.

●It can be set under water.

●It can be set under situations where free air cannot reach. This is because setting action of this line does not depend upon

atmospheric air.

●This lime is not perfectly white but forms a thin plastic paste with water.

●It does not dissolve in water but forms a thin plastic paste with water.

Classification of lime based on purpose:

Class-A Lime: It is predominantly hydraulic lime. It is normally supplied as hydrated lime and is commonly used for structural works.

Class-B Lime: It contains both hydraulic lime and fat lime. It is supplied as hydrated lime or as quick lime. It is used for making mortar for masonry works.

Class-C Lime: It is predominantly fat lime, supplied both as quick lime and fat lime. It is used for finishing coats in plastering and for white washing.

Class-D Lime: This lime contains a large quantity of magnesium oxide and is similar to fat lime. This is also commonly used for white washing and for finishing coats in plastering.

Class-E Lime: It is an impure limestone, known as kankar. It is available in modular and block form. It is supplied as hydrated lime. It is commonly used for masonry mortar

Setting of Lime

Calcination: The process of heating limestone at very high temperature about 800°C, so that it decomposed and volatiles materials such as CO₂ escaped out and giving quicklime CaO as final

Thel product is known as calcination. It is also called the burning process of limestone to get quick lime either in lumps or powder form. Calcination is an exothermic reaction, which is shown below

Setting and Hardening of Lime

When lime in paste form is exposed to the atmosphere, it gradually hardens. This phenomenon of hardening of lime in exposed conditions is known as setting of lime. The hardening of lime paste or

Mortar is caused by the absorption of carbon dioxide from the air which, in the presence of excess water, unites with calcium hydroxide to form calcium carbonate. Hydraulicity and setting properties of fat lime can be improved by the addition of  and grinding the mixture in a mortar-mill. An addition of 10 to 15 percent of cement to a fat lime mortar also improves its quality considerably.

The hardening of hydraulic lime does not depend on the absorption of air; the setting of hydraulic lime and cement is facilitated by the presence of water. The setting action of hydraulic lime is much quicker than that of fat lime. Only eminently hydraulic lime is suitable for underwater works but it should not be immersed within 48 hours.

Tests on Lime

The following practical tests are made on limestones to determine their suitability:

Physical Test

Pure limestone is white in color. Hydraulic limestones are bluish gray, brown or have dark colors. The hydraulic lime gives out an earthy smell. They have a clayey taste. The presence of

lumps give indication of quick lime and unburnt lime stones.

Heat Test

A piece of dry stone weighing W, is heated in an open fire for a few hours. If weight of sample after cooling is Ws the loss of weight is W₂ - W₁. The loss of weight indicates the amount of carbon dioxide. From this the amount of calcium carbonate in limestone can be worked out.

Chemical Test

A teaspoon full of lime is placed in a test tube and dilute hydrochloric acid is poured in it. The

content is stirred and the test tube is kept in the stand for 24 hours. Vigorous effervescence and less residue indicates pure limestone. If effervescence is less and residue is more it indicates impure limestone. If thick gel is formed and after test tube is held upside down it is possible to identify class of lime as indicated below:

Class A lime, if gel does not flow.

Class B lime, if gel tends to flow down.

Class Clime, if there is no gel formation.

Ball Test

This test is conducted to identify whether the lime belongs to class C or to class B. By adding sufficient water about 40 mm size lime balls are made and they are left undisturbed for six hours Then the balls are placed in a basin of water. If within minutes slow expansion and slow

disintegration starts when it indicates class C lime. If there is little or no expansion, but only cracks appear it belongs to class B lime.