Chapter 3
Lime
Lime is one of the most important and largely used building materials. In fact, it used to be the main cementing material before the advent of Portland cement. Egyptians and Romans made remarkable application of this material for various constructional purposes. Even in India, various engineering structures such as big palaces, bridges, temples, forts and monuments were constructed with lime as a cementing material and some of these structures still exist in good condition. This is attributed to some of the unique properties of lime, such as its better workability, early stiffening, good strength and resistance to moisture and excellent adherence to masonry units. Although it has been largely replaced by cement in India and elsewhere, the material still stands comparable to cement in most important properties.
3.1 SOURCES OF LIME
Lime is not usually available in nature in free state. It is chiefly prepared by burning limestone. Depending on the percentage of calcium carbonate (CaCO3) in limestone, lime is classified into A, B and C types, which are used for masonry work, mortar and plaster and white washing, respectively. The type C variety is also known as pure lime or fat lime. Class A variety is only available in slaked form, while class B and C are available in slaked as well as unslaked forms. Lime is also obtained by burning kankar, shells of sea animals and boulders of limestone from beds of old rivers.
The lime that is obtained from the calcination of pure limestone is known as quicklime. It mainly consists of oxides of calcium and it is not crystalline. It shows great affinity to moisture.
3.2 CLASSIFICATION OF LIME
Lime has been conventionally classified into the following three types, namely
- Fat lime
- Hydraulic lime
- Poor lime
3.2.1 Fat lime
This lime is also known as high calcium lime, pure lime, rich lime or white lime. Fat lime is obtained from pure limestone, shell and coral. When it is left in air, it absorbs carbon dioxide (CO2) from air and gets transformed into calcium carbonate (CaCO3). It is popularly known as fat lime as it slakes vigorously and its volume gets increased to about 2-2½ times the volume of that of quick lime.
The following are the properties of fat lime:
- It hardens very slowly.
- It has a high degree of plasticity.
- Its colour is perfect white.
- It sets slowly in the presence of air.
- It slakes vigorously.
The following are the uses of fat lime:
- It is used in white washing and plastering of walls.
- With sand, it forms lime mortar, which sets in thin joints. Such mortar can be used for thin joints of brickwork and stonework.
3.2.2 Hydraulic lime
Hydraulic lime is different in composition from quick lime in that it contains a definite amount of clay, which gives it the hydraulic property, i.e., the capacity to set and harden even under water. Quick lime does not set under water. Hydraulic lime contains CaO between 70 and 80 per cent and clay about 15–30 per cent.
Hydraulic lime is generally manufactured from a limestone that is rich in clay, or by adding clay materials to the limestone during burning of limestone. This lime is further divided into feebly hydraulic, moderately hydraulic and eminently hydraulic lime, depending upon its hydraulicity. The comparison between these types of hydraulic limes is shown in Table 3.1.
3.2.3 Poor lime
This lime is also known as impure lime or lean lime. The following are the properties of poor lime.
- This lime contains more than 30 per cent clay.
- It slakes very slowly.
- It forms a thin paste with water.
- It does not dissolve in water though it is frequently changed.
- It hardens very slowly.
- It has poor binding properties.
- Its colour is muddy white.
- It can be used for inferior type of work where there is scarcity of good lime.
Table 3.1 Comparison Between the Types of Hydraulic Lime
3.3 CALCINATION OF LIME IN CLAMPS AND KILNS
3.3.1 Calcination
Lime is manufactured by the burning of limestone to bright red in suitable kilns or clamps. Theoretically, limestone dissociates when heated at 880°C into carbon dioxide and calcium oxide, which is also reversible.
CaCO3 ↔ CaO + CaO2
The burning or calcination of limestone can be carried out in one of the following ways:
- Clamps
- Kilns
- Intermittent kilns
- Continuous kilns
3.3.1.1 Clamp burning
It is a very common and quick method of obtaining small supplies of ordinary type of quick lime. No constructions have to be made. Simply the ground is levelled and cleaned and the limestone and fuel are stocked in alternative layers, if the fuel is wood. But if the fuel is coal or charcoal, the limestone and fuel are mixed together and placed in a heap form. Any type of burning material locally available is used. The whole heap is then covered with mud plaster and an attempt is made to preserve as much heat as possible. Small holes are left at the top of the plaster and also at the bottom. When the blue flame at the top disappears, it indicates the completion of the burning of lime. The clamp is then allowed to cool and the pieces of quick lime are hand-picked subsequently. Even though burning is quick and cheap for ordinary type of lime, it is not suitable for large supplies because of the following reasons:
- It proves to be uneconomical to manufacture lime on a large scale.
- The burning is not complete which results in poor quality lime containing unburnt limestone.
- Wastage is considerable, both in terms of heat generated and material produced.
- The quantity of fuel required is more and hence can only be practised where fuel is abundant (Figure 3.1).
3.3.1.2 Kiln burning
Most of the commercial lime is made by burning limestones in permanent structures called kilns. Kilns used in the manufacture of lime are of a number of variety and designs. A kiln may be of intermittent or continuous type. The kiln may be mixed feed type where the fuel and limestones are mixed up during burning. In separate feed type, the fuel is burnt separately and does not come into contact with the limestones.
Intermittent kilns
This is also known as batch type kiln. They are permanent structures of rectangular, oval or cylindrical shape. It may be made of bricks or stones. The interior of this kiln is lined with refractory bricks and does not break even at high temperatures. The kiln is provided with openings or flues for supply of air during burning. The roof may be covered or partly covered. An escape hole for gases is always provided at the top.
Figure 3.1 Clamp
In mixed feed type, the kiln is first loaded with a calculated amount of fuel and limestone, being deposited in alternative layers. The top of the kiln is covered with unburnt material. The kiln is ignited from the bottom for a required number of days till calcination is complete. It is allowed to cool and then unloaded. The next batch of fuel and limestone is charged to repeat the process (Figure 3.2).
In separate feed type, the fuel is not allowed to come in contact with the limestones. Bigger pieces are stocked in lower regions and smaller pieces above them, leaving open spaces for circulation of hot gases. Fuel is burnt in arch-type gates from where the hot gases rise and circulate between the limestones. When the limestones are sufficiently burnt, the kiln is cooled and unloaded. In this process the burning is more complete and mixing of ash and lime is avoided. There is a considerable wastage of lime in intermittent kilns as every operation includes loading, burning, cooling and unloading.
Figure 3.2 Intermittent flame kiln
Continuous kilns
These kilns are of such designs that from one end they are charged with raw materials and from the other end finished materials are taken out. Consequently, the kiln is not stopped for emptying or cooling operations. Naturally, the rate of production of lime is much higher.
In continuous flame kiln or mixed feed continuous kiln, the mixture of limestone and fuel is fed from the top. The widening of the middle portion is done so as to accommodate the hot gases of combustion. The bottom is covered by grating with holes. After burning, the lime is collected at the bottom and removed through the access shaft. The kiln is partly under the ground and partly above the ground. The inside surface of the kiln is covered with fire brick lining. The loading platform is provided at the top. As the level of material falls, the required amount of the mixture of fuel and limestone is fed from the top.
In continuous flare kiln or separate feed continuous kiln, the fuel is not allowed to come in contact with the limestone. The kiln consists of two sections. The upper section serves for the storage of limestones. The lower portion is provided with fire brick lining. Initially, a small quantity of fuel is mixed with limestone and ignited. The fuel is then fed through shafts around the lower and upper section of the kiln. The feeding of limestone is from the top. The removal of the calcinated material is done through a grating placed at the bottom of the kiln from where it can be removed (Figure 3.3).
There is a considerable saving in time and fuel, but the initial cost is high. Hence, this method can be adopted to manufacture lime on a large scale.
3.4 SLAKING OF LIME
The process of mixing water in quick lime is known as slaking of lime. It is an important operation in the preparation of lime at site for use in building construction. Improper slaking results in serious defects in mortars and plasters.
Figure 3.3 Continuous flare kiln
Two methods are commonly used for slaking of lime.
3.4.1 Tank slaking
In this method, two brick lined tanks are required. The first tank is about 45 cm deep and is made at a higher level, whereas the second tank is 60–75 cm deep and is made at a lower level, adjoining the first tank. Water is filled in the upper tank to about three-fourth its depth and quick lime is gradually added to it in small amounts. Water is constantly stirred during the process of addition of lime. This results in the formation of ‘milk of lime’, which is allowed to fall through a sieve into the lower tank.
After the lime-milk has been allowed to stand for the requisite time (2-3 days), it matures itself and forms lime putty, which is taken for use.
3.4.2 Platform slaking
The method provides dry slaking of lime which can be used as it is or may be converted to putty. A watertight masonry platform is built, over which the lime is spread in a 15 cm layer. Water is then sprayed over it using a hose pipe till lime disintegrates into a fine powder. During the water sprinkling process, the lime heap is turned over and over again. It is then left for 24 hours during which further slaking occurs.
The rate of hydration or slaking is greatly a function of the composition, physical state and degree of burning of lime. It is to be noted that over-burnt or under-burnt limestones do not slake easily. Hence, such undesirable pieces should be removed before slaking. It is also necessary to convert all lumps into powder or pulp form. It is observed that one part of fat quick lime is converted into about 1½ parts in paste form and about 2 parts in powder form.
The quantity of water required for hydrating 100 kg of lime is 32 litres, but practically even 100 litres of water may have to be added. This is generally determined by experience and depends on other factors like composition of quick lime, method of slaking and the form in which hydrated lime is required, i.e., either as putty or powder form.
3.5 COMPARISON BETWEEN FAT LIME AND HYDRAULIC LIME
3.6 USES OF LIME
Lime can be used for the following purposes:
- Lime mortar: Lime mortar has been extensively used in construction work from times immemorial. Lime mortar is used as a building medium in brick, stone and other masonry works as well as for plastering and pointing. The composition of the lime mortar for building work depends on:
- The type of masonry
- Situation/location of work
- Load which the masonry will have to take
- Condition of exposure to weather or soil conditions
- In case of hydraulic structures, weather conditions under water.
- Plastering: Plastering serves the following functions:
- To smoothen the surface of masonry
- To protect the masonry surface from weathering
- To cover unevenness of masonry
- To prepare surface for decorative treatment.
- Whitewashing: Whitewashing is applied on internal and external plastered surface as a decorative feature. Apart from decorative effect, the whitewashed outer surface reflects away the sun's rays and reduces the heating effect.
- Lime concrete: In situations where quick setting and high strength are not required, lime concrete serves as an economical substitute. Lime concrete can be used for foundation, terraced roofing, flooring, ditches for sullage water, etc.
- Lime sand bricks: It is a pearl gray brick like dry pressed burnt clay brick. This can be used for low-cost constructions and as a refractory material for lining open hearth furnaces.
- It is used as a chemical raw material in the purification of water and for sewage treatment.
- It is used for soil stabilization and for improving soil for agricultural purposes.
- It is used as a flux in the metallurgical industry.
- It is used for the production of glass.
REVIEW QUESTIONS
- Explain the importance of lime as a building material.
- What are the different sources of lime?
- Briefly discuss the classifications of lime.
- What is fat lime? What are the properties of fat lime?
- What is poor lime? What are the properties of poor lime?
- What is clacination of lime? How is clacination of lime carried out?
- Explain briefly
- Clamp burning of lime
- Kiln burning of lime
- Hydraulic lime
- Slaking of lime
- Uses of lime
- What is slaking of lime? What are the different methods used for slaking of lime?
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