The use of lime as a cementing material has been made since ancient times. The Egyptians and Romans made remarkable application of this material for various constructional purposes. Even in India, the various engineering structures such as big palaces, bridges, temples, forts, monuments, etc. were constructed with lime as a cementing material and some of these structures still exist in perfectly good condition.
At present, the cement has replaced lime to a great extent. But at places where lime is locally available and when there is acute shortage of cement, the lime certainly provides a cheap and a reliable alternative to the cement.
Sources of Lime:
The lime is not usually available in nature in free state.
It is procured by- burning one of the following materials:
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(i) Lime stones from the stone hills,
(ii) Boulders of lime stones from the beds of old rivers,
(iii) Kankar found below the ground, and
(iv) Shells of sea animals.
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It may be noted that white chalk is pure limestone and kankar is an impure limestone.
Constituents of Limestones:
The main source of getting lime is the lime stones obtained from nature and the properties of lime will depend on the composition of the limestone from which it is produced.
The constituents of such lime stones are as follows:
(1) Clay:
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This constituent is responsible for producing hydraulicity in lime. It also makes lime insoluble in water. If it is in excess, it arrests slaking. If it is in small quantity, it retards slaking. A proportion of 8 to 30 per cent is desirable for making a good lime.
(2) Soluble Silica:
It is essential to have silica and alumina present in chemical combination with limestone to develop hydraulicity. The silicates of calcium, magnesium and aluminium are responsible for hydraulicity. Such silicates are inert or inactive at low temperatures. But they become active and combine with lime at high temperatures.
(3) Magnesium Carbonate:
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The presence of this constituent allows lime to slake and set slowly, but it imparts more strength. Further, the production of heat and expansion are less. If the content of carbonate of magnesia is about 30 per cent, the hydraulicity is rendered to the lime even in absence of clay.
(4) Alkalies and Metallic Oxides:
When these are present in small amount upto about 5 per cent or so, they develop hydraulicity due to the formation of soluble silicates at low temperature.
(5) Sulphates:
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The presence of sulphates in small quantities accelerates the process of setting and reduces the slaking action.
(6) Iron:
If iron is present in small quantity, it develops a complex silicate at high temperature. But excess of iron is objectionable.
(7) Pyrites:
It is undesirable to have pyrites in the composition of lime stones. Such lime stones should therefore be rejected.
Classification of Limes:
The limes which are obtained by calcination of lime stones are broadly classified into the following three categories:
(1) Fat lime
(2) Hydraulic lime
(3) Poor lime.
(1) Fat Lime:
This lime is also known as the high calcium lime, pure lime, rich lime or white lime. It is popularly known as the fat lime as it slakes vigorously and its volume is increased to about 2 to 2½ times the volume that of quick lime.
It is prepared by calcining comparatively pure carbonate of lime which is composed of about 95 per cent of calcium oxide. The percentage of impurities in such limestone is less than 5 per cent.
Following are the properties of fat lime:
(i) It hardens very slowly.
(ii) It has a high degree of plasticity.
(iii) It is soluble in water which is changed frequently.
(iv) Its colour is perfectly white.
(v) It sets slowly in presence of air.
(vi) It slakes vigorously.
Following are the uses of fat lime:
(i) It is used in whitewashing and plastering walls.
(ii) With sand, it forms lime mortar which sets in thin joints. Such mortar can be used for thin joints of brickwork and stonework,
(iii) With surkhi, it forms lime mortar which possesses good setting and hydraulic properties. Such mortar can be used for thick masonry walls, foundations, etc. The surkhi is the powder obtained by grinding of the burnt bricks.
(2) Hydraulic Lime:
This lime is also known as the water lime as it sets under water. It contains clay and some amount of ferrous oxide.
Depending upon the percentage of clay, the hydraulic lime is divided into the following three types:
(i) Feebly hydraulic lime
(ii) Moderately hydraulic lime
(iii) Eminently hydraulic lime.
Following facts should be noted:
(i) The increase in percentage of clay makes the slaking difficult and increases the hydraulic property.
(ii) With about 30 per cent of clay, the hydraulic lime resembles natural cement.
(iii) The hydraulic lime can set under water and in thick walls where there is no free circulation of air.
(iv) The colour of hydraulic lime is not perfectly white. It therefore appears less sanitary than the fat lime.
(v) It forms a thin paste with water. It does not dissolve in water though it is frequently changed.
(vi) If hydraulic lime is to be used for plaster work, it is to be ground in fine powder and then it is mixed with sand. The mortar thus prepared is kept as heap for one week or so and it is then ground again. Such mortar can then be used for plaster work.
Table 5-1 shows the points of comparison between these types of hydraulic limes.
(3) Poor Lime:
This lime is also known as the impure lime or lean lime. It contains more than 30 per cent of clay. It slakes very slowly. It forms a thin paste with water. It does not dissolve in water though it is frequently changed. It sets or hardens very slowly. It has poor binding properties and its colour is muddy white.
This lime makes a very poor mortar. Such mortar can be used for inferior type of work or at places where good lime is not available.
I.S. Classifications of Lime:
The building limes according to BIS:712-1984 (Third revision) are classified under six categories, namely, Class A, Class B, Class C, Class D, Class E and Class F.
(1) Class A:
Class A lime is the eminently hydraulic lime which is used for structural purposes and it is to be supplied in the hydrated form only. Its minimum compressive strength with lime sand mortar of proportion (1:3) by weight at the end of 14 days and 28 days should be respectively 1.75 N/mm2 and 2.80 N/mm2.
(2) Class B:
Class B lime is the semi-hydraulic lime which is used for mortars for masonry work and it can be supplied either as quick lime or as hydrated lime. Its minimum compressive strength with lime sand mortar of proportion (1:3) by weight at the end of 14 days and 28 days should be respectively 1.25 N/mm2 and 1.75 N/mm2.
(3) Class C:
Class C lime is the fat lime which is used mainly for finishing coat in plastering, whitewashing and with suitable admixture such as surkhi or any other pozzolanic material to produce artificial hydraulic mortars. It is to be supplied in hydrated or quick form.
(4) Class D:
Class D lime is the magnesium or dolomitic lime which is used for finishing coat in plastering, whitewashing, etc. It is to be supplied in hydrated or quick form.
(5) Class E:
Class E lime is the kankar lime which is used for the masonry mortars and it is to be supplied in the hydrated form only.
(6) Class F:
Class F is the siliceous dolomitic lime which is used for undercoat and finishing coat of plaster. It is to be supplied in hydrated or quick form.
It may be noted that the applications of various categories of limes are suggestive only.
The chemical and physical requirements to be compiled by the lime of each category are mentioned in the above BIS with methods of testing as per BIS:6932-1973.
Manufacture of Natural Hydraulic Lime:
Following three distinct operations are involved in the manufacture of natural hydraulic lime:
(1) Collection of kankar
(2) Calcination of kankar
(3) Slaking and grinding of burnt lime.
(1) Collection of Kankar:
The kankar is an impure limestone and it is used for manufacturing natural hydraulic lime. The kankar is available in two forms, namely, nodules and block.
The nodules are found either on surface of ground or slightly below ground level.
The nodules of kankar are easy to collect and kankar in nodular form is considered as superior material for manufacturing the natural hydraulic lime for the following reasons:
(i) It can withstand heat and rain without disintegration.
(ii) It contains higher percentage of clay and hence it possesses better hydraulic properties.
The blocks of kankar are found from the underground strata below or on or near the banks of rivers or streams. The thickness of blocks is usually 50 mm to 300 mm.
The nodules or blocks of kankar are quarried with the help of pick-axes and crowbars. Such kankar is then cleaned of mud or earth and converted into suitable sizes.
(2) Calcination of Kankar:
The calcination or burning of kankar to bright red heat is done either in clamps or kilns as in case of manufacture of fat lime.
(3) Slaking and Grinding of Burnt Lime:
The slaking of hydraulic lime occurs very slowly. Hence the quick lime is first ground dry before water is added for slaking.
The grinding of quick lime can be carried out in one of the following ways:
(i) With hand with the help of wooden beaters, or
(ii) With the help of mills working with bullocks or steam power, or
(iii) With the help of special machines.
Following points of differences in case of slaking of fat lime and hydraulic lime should be noted:
(i) In case of fat lime, the required quantity of water for slaking is added at a time. In case of hydraulic lime, the water is added gradually to cause thorough slaking.
(ii) One part of fat quick lime, when slaked, is converted into about 1½ parts in paste form and 2 parts in powder form. One part of hydraulic quick lime, when slaked, is converted into about 1 part in paste form and 1½ parts in powder form.
(iii) The quantity of water required to slake fat lime is more than that required for hydraulic lime.
(iv) The time taken by fat lime to slake is about 3 to 4 hours and that by hydraulic lime is about 12 to 48 hours.
Manufacture of Artificial Hydraulic Lime:
If natural raw material is not suitable for the manufacture of hydraulic lime, it is possible to prepare the hydraulic lime artificially. In fact, the fat lime may be converted into hydraulic lime by addition of clay in required proportion.
Following are the two methods of preparing the artificial hydraulic lime:
(1) Conversion of soft limestone
(2) Conversion of hard limestone.
(1) Conversion of Soft Limestone:
The available limestone, if of soft quality such as chalk, is ground and converted into powder form. It is mixed with the required proportion of clay. It is then burnt in a kiln and slaking is carried out as in case of manufacture of natural hydraulic lime.
(2) Conversion of Hard Limestone:
The available limestone, if of hard quality, is first burnt and slaked. To this slaked lime, the required proportion of clay is added to obtain raw material for good variety of hydraulic lime.
This mixture is converted into balls of suitable size and after drying, these balls are burnt in kiln. The slaking is carried out as in case of manufacture of natural hydraulic lime. As this lime is produced after burning twice in kiln, it is also known as the twice-kilned lime.
Precautions to be Taken in Handling Lime:
Following precautions are to be taken while handling lime to avoid accidents:
(1) Contact with Water:
The quick lime should not be allowed to come in contact with water before slaking.
(2) Facilities for Workers:
The lime dust causes irritation and hence the workers handling lime should be provided with goggles and respirators. The lime also causes skin burns, especially if the skin is moist. It is therefore advisable to provide rubber gloves, gum boots and skin protective cream to the workers likely to get skin burns.
(3) Fire Hazard:
The quick lime gives out immense heat while slaking and hence all suitable measures should be taken to avoid chances of any fire hazard.
(4) Instructions to Workers:
After handling lime, the workers should be instructed to wash the exposed portions of the body with abundant fresh water. Similarly the workers handling milk of lime, which is hot, should be advised to oil their skin daily to avoid skin burns.
Uses of Lime:
The lime is an important engineering material and its uses can be enumerated as follows:
(i) It is used as chemical raw material in the purification of water and for sewage treatment.
(ii) It is used as a flux in the metallurgical industry.
(iii) It is used as a matrix for concrete and mortar.
(iv) It is used as a refractory material for lining open-hearth furnaces.
(v) It is used in the production of glass.
(vi) It is used for making mortar for masonry work.
(vii) It is used for plastering of walls and ceilings.
(viii) It is used for the production of artificial stone, lime-sand brick, foam- silicate products, etc.
(ix) It is used for the soil stabilization and for improving soil for agricultural purposes.
(x) It is used for whitewashing and for serving as a base coat for distemper.
(xi) When it is mixed with Portland cement, the mortar attains valuable properties and such a mortar can be used in place of the costly cement mortar.
Tests Conducted for Limestones:
The lime stones are tested to determine the quality of lime which can be obtained from them. For determining exactly the suitability of limestone, the detailed chemical tests are carried out in a laboratory.
But for general information, the following four practical tests are made:
(1) Physical properties
(2) Heat test
(3) Acid test
(4) Ball test.
(1) Physical Properties:
The pure limestone is indicated by white colour. The hydraulic lime stones are indicated by bluish grey, brown or some dark colour. The taste of hydraulic lime stones is clayey and they give out earthy smell. The presence of glistering particles on the surface of lime stones indicates the presence of free sand. The presence of lumps gives an indication of quick lime or un-burnt limestone.
(2) Heat Test:
A piece of dry limestone is weighed and it is heated in an open fire for few hours. The sample is weighed again and loss of weight indicates the amount of carbon dioxide. From this data, the amount of calcium carbonate in limestone is worked out.
(3) Acid Test:
A teaspoon of powdered lime is taken in a test tube and dilute hydrochloric acid is poured in it. The contents are now stirred and the test tube with its contents is then kept standing in its stand for 24 hours.
If content of calcium carbonate is high, there will be vigorous effervescence and less formation of residue. Such an action will indicate pure limestone. On the other hand, if content of calcium carbonate is less, there will be less effervescence and more formation of residue. Such an action will indicate impure or hydraulic limestone.
If a thick gel or semi-solid material is formed which does not flow even when the test tube with its contents is turned upside down, it indicates lime of class A. If the gel is not quite thick and tends to flow when the test tube is tilted, it indicates lime of class B. If however there is no gel formation, it indicates lime of class C.
(4) Ball Test:
In this test, the balls of about 40 mm size of stiff lime formed by adding enough water are made and they are left undisturbed for six hours. The balls are then placed in a basin of water. If there are signs of slow expansion and slow disintegration within minutes after placing in water, it indicates class C lime. If, on the other hand, there are very little or no expansion and numerous cracks, it indicates class B lime.