There are several naturally available civil engineering materials or materials directly produced from natural materials. Among them are binding materials, which play a major role. Lime, cement and bitumen are important for civil engineers.
Binding materials may be of air binding, hydraulic binding and autoclave binding.
Air binding materials, also known as air-setting binding substances, pass into a stone state in air, gain strength and then retain their mechanical properties. Lime, cement, gypsum, etc., are examples of air binding materials.
Hydraulic binding materials, also known as hydraulic-setting substances, pass into a stone stage in air and as well as in water, gain strength and then retain their mechanical properties. Hydraulic lime, cement, etc., are examples of hydraulic binding materials.
Autoclave binding materials, also known as autoclave-setting substances, set only when treated in autoclaves with saturated steam under pressure of 8–12 atmosphere and at a temperature between 170 and 200ºC. Lime–silica, sand, Portland cement, etc., are examples of autoclave binding materials.
Lime is one of the oldest cementing materials as discussed in this chapter.
5.2 CONSTITUENTS OF LIMESTONE
Lime is not freely available in nature but is obtained by burning naturally available materials such as:
- Limestone found in limestone hills
- Limestone boulders found in the bed of old rivers
- Kankar found below ground and
- Shells of sea animals.
Lime is more or less pure calcium carbonate. The main sources of obtaining lime are the stones which possess clay, soluble silica, magnesium carbonate, alkalis and metallic oxides, sulphates, iron compounds and carbonaceous matter.
It is the material which gives the property of producing hydraulicity in lime. In small quantity it retards slaking and is also capable of arresting slaking when it is in excess. It gives the property of insolubility for lime in water. In order to get the best quality lime it is recommended to have 8–30% of clay.
5.2.2 Soluble Silica
In order to develop hydraulicity in lime it is necessary to have silica and alumina during chemical combination with limestone. Hydraulicity in lime is caused due to silicates of calcium, magnesium and aluminium. These silicates are inert at low temperatures and they become active to combine with lime at a high temperature.
5.2.3 Magnesium Carbonate
Hydraulicity is rendered to lime when the magnesium carbonate content is about 30% and this can happen even with the absence of clay. But limes containing large proportions of magnesium carbonate are liable to crack. It has the important property of increasing the setting process but reducing slaking.
5.2.4 Alkalis and Metallic Oxides
These materials convert as soluble salts at a low temperature and thereby cause hydraulicity when present in small quantities (5%).
The presence of sulphates in small quantities tend to accelerate the setting and reduces the slaking action.
The presence of iron compounds lowers the temperature of calcination of limestone. But excess of iron is objectionable.
It is undesirable to have pyrite in limestone as it is harmful and liable to produce poor-quality lime.
5.3 PROPERTIES OF LIME
Lime is a binding material used in several ancient civil engineering works. A good quality lime should possess the following properties:
- It is a flexible material and is easily workable.
- When used as a mortar it provides strength to the masonry.
- It possesses good plasticity.
- It is highly resistant to moisture and is used for pointing works.
- It solidifies with less time and hardens.
- An excellent binding material which adheres to brick or stone masonry units perfectly.
- As it is less shrinkable during drying it is highly durable when used in mortar.
5.4 CLASSIFICATION OF LIME AND THEIR USES
5.4.1 General Classification
In general, classification of lime is based on the calcination of limestone. They are
- Fat lime,
- Hydraulic lime and
- Poor lime
1. Fat Lime
It is the lime which has high calcium oxide content. This can set and become hard only in the presence of carbon dioxide. This is white in colour.
This lime is also called high-calcium lime, pure lime, rich lime or white lime. It is mostly called fat lime as it slakes vigorously. By this process, the volume is increased two-fold.
It is obtained by calcining pure carbonate of lime, which contains about 95% of calcium oxide. The percentage of impurities will be less than 5%.
It has the following specific properties:
- Very slowly hardening
- High degree of plasticity and
- Soluble in water.
The uses of fat lime are as follows:
- It is used as lime mortar (lime and sand) for plastering and pointing works.
- It is used for white washing of plastered surfaces.
- Lime surkhi mortar is used for thick masonry walls, foundations, etc., where surkhi is the powder obtained by grinding burnt bricks.
2. Hydraulic Lime
It is the lime containing small quantities of silica, alumina and iron oxide, which are jointly in chemical composition with calcium oxide. Hydraulic lime can set and become hard even in the absence of CO2 and can set even under water. This is also known as water lime.
Based on the percentage of clay, lime may be classified as (i) Feebly hydraulic lime, (ii) Moderately hydraulic lime and (iii) Eminently hydraulic lime. The increase in clay content makes slaking difficult and increases the hydraulic properties.
The uses of hydraulic lime are as follows:
- It can set under water and in thick walls where there is no free circulation of air.
- As it contains about 30% of clay, it resembles that of cement and can be used for major civil engineering works.
- It forms a thin paste with water.
3. Poor Lime
This lime is also known as impure lime or lean lime. More than 30% of clay present in poor lime makes the lime to slack slowly.
It sets and hardens very slowly and has very poor binding properties. It can form a thin paste with water. Because the lime is of inferior quality, it is used for inferior types of works.
5.4.2 Indian Standard Classification
Limes are classified into six categories as per Bureau of Indian Standards (IS:712–1984). They are explained below.
1. Class A: Eminently Hydraulic Lime
It is available in hydrated form. The compressive strength of lime sand mortar (1:3 by weight) should have 1.75 N/mm2 and 2.8 N/mm2 at 14 and 28 days, respectively. With such a strength they can be used for structural works such as arches, domes, etc.
2. Class B: Semi-hydraulic Lime
This is available as quick lime (lime obtained by calcining pure limestone) or as hydrated lime (dry power obtained by treating quick lime with water). The compressive strength of lime and mortar (1:3 by weight) should have 1.25 N/mm2 and 1.75 N/mm2 at 14 days and 28 days, respectively. This mortar is used for masonry work.
3. Class C: Fat Lime
This should be available in quick or hydrated form. It can be used as the finishing coat in plastering, white washing, etc. It is also used for masonry mortar with the addition of pozzolanic material (it is volcanic dust containing 80% clay with lime magnesia and iron oxide in varying proportions).
4. Class D: Magnesium or Dolomite Lime
This is also available in hydrated or quick form. It is used as the finishing coat in plastering and white washing.
5. Class E: Kankar Lime
The lime is produced by burning lime nodules (found in soils like black cotton soil containing silica) in the hydrated form. It can be used for masonry mortar.
6. Class F: Siliceous Dolomite Lime
It is available in quick or hydrated form. It is generally used as an undercoat and finishing coat of plaster.
5.5 MANUFACTURE OF LIME
Manufacturing of lime comprises three stages, viz.:
- Collection of raw materials
- Burning of limestone and
- Slaking of burnt lime
5.5.1 Collection of Raw Materials
For manufacturing fat lime, the percentage of impurities in limestone should not exceed 5%. Kankar lime may be used for manufacturing hydraulic lime.
Limestones required for the manufacture of specific lime may be collected and slacked in sufficient quantity near the manufacturing site.
Fuels used for calcining limestone are wood, coal, coke, coal gas, charcoal, cinder, oil, etc.
5.5.2 Burning of Limestone
Limestones may be burnt in clamps or kilns. For kiln burning, an intermittent kiln or a continuous kiln may be used.
1. Clamp Burning
When the burnt lime required is small, this method may work out to be more economical. Further, this method can be adoptable in places where limestone and the fuel are available.
The clamp comprises of alternate layers of limestone and fuel. The fuel to be adopted in this case is wood. If the fuel is other than wood, viz., coal or charcoal, the fuel is mixed with the limestone and the clamp is formed. The clamp is of 3.6 m max height with sloped sides. The clamp is covered with mud plaster to prevent the escape of heat (Fig. 5.1).
Clamp burning has several disadvantages, viz., wastage of fuel, is uneconomical, is not a continuous process, can not be produced during the rainy season, etc.
Figure 5.1 Clamp burning
2. Intermittent Kiln Burning
Various kinds of intermittent kilns are in practice, the most prevalent among them being the intermittent flame kiln and the intermittent flare kiln.
The intermittent flame kiln is similar to the clamp kiln. Here the limestone and fuel are arranged in alternate layers between two vertical walls. In some cases vertical flue is also provided. The top of the kiln is covered with unburnt material. The kiln is operated for three days. Then the kiln is cooled and unloaded and the process is repeated.
The intermittent flare kiln is one in which fuel is not allowed to come into contact with limestone. Big pieces of limestone are chosen, and a rough arch is formed over which small pieces of limestone are packed (Fig. 5.2).
Figure 5.2 Intermittent flare kiln
The fuel is placed below the arch. When it is ignited, only the flame comes into contact with the limestone. After the limestone is adequately burnt, the kiln is cooled and unloaded. This process is then repeated.
This type of kiln is used only for the manufacture of lime on a moderate scale. The main drawback is the wastage of time for every operation.
3. Continuous Kiln Burning
This type of kiln is used when large quantities of lime is needed. In the conventional type, alternate layers of limestone and coal or wood are used. Feeding is done from the top, ignition is done from below, and the lime is collected at the bottom. Such a kiln does not produce high-purity lime as it gets mixed with ash of fuel (Fig. 5.3).
Figure 5.3 Continuous lime kiln
High-purity lime is prepared by using continuous separate feed kilns. The arrangement is shown in Fig. 5.3. Here the limestone is fed from the top and two or four fireplaces are provided by the side of the kiln near the bottom. Coal or wood is used as the fuel, and hot gases of combustion move up the shaft of the kiln. The use of wood as a fuel has an added advantage in that long flames are produced, and the temperature of limestone is not raised too much.
5.5.3 Slaking of Burnt Lime
From quicklime, slaked lime and lime putty are prepared as discussed below:
1. Slaked Lime
Quicklime is heaped on a masonry or wooden platform. Then water is gradually sprinkled over it till the lime is slaked and reduced to a powder form. During the sprinkling of water, the heap is turned over and over again. This process is continued till no more water can be added than that required for the lime to convert into a powder form. The slaked lime is then sieved through an I.S sieve of mesh size 3.35 mm, and the residue if any is rejected. The end product is slaked lime.
2. Lime Putty
Sometimes before using lime in mortar, quicklime is converted into a putty form. For this purpose two large tanks are made, one 50 cm deep and the other 80 cm deep. The first tank is constructed as a pucca masonry at a higher level. The lower level tank is constructed at the ground level. The lower tank is made of dry brick masonry with joints filled with sand.
The upper tank is filled to half of its depth with water. Quicklime is added gradually till it fills half the depth of water. Only lime should be added to water and not otherwise. It is then stirred taking care to ensure that lime at no time is exposed to the air. For about 5 minutes the mixture is stirred till the boiling stops and it thickens. More water is added, if needed, and then the products are allowed to flow to the lower tank.
The mixture is then allowed to stand in the lower tank for 72 hours. As the lower tank is made of dry brick masonry with sand joints, water in the slurry is partly absorbed by the ground and partly through the brick joints. Thus, excess water is removed, and lime putty is obtained as a paste. It can be stored for about two weeks if adequately protected from drying.
5.6 PRECAUTIONS IN HANDLING LIME
Lime easily affects the skin and also is a health hazard if breathed in. The moisture on the body reacts with lime resulting in skin diseases. This may be avoided by coating the skin with oil.
Slaking of lime is an exothermic reaction, i.e., it produces a large amount of heat. Necessary precautions should be taken by workers who are involved in the use of lime. It is customary to use goggles, respirants, gloves and boots as protection.
5.7 STORAGE OF LIME
Quicklime should be kept in air-tight vessels or in large heaps under cover; otherwise, it should be slaked soon after delivery or run into putty to prevent deterioration.
Slaked lime can be stored safely for 3–4 months if packed in gunny bags lined with polythene, craft paper, etc. The storing should be done in weather-proof sheds, which are perfectly dry. As per specifications, the lime should be slaked not less than 1 week and not more than 4 weeks before its use.
Hydrated lime can be safely stored for considerable time without deterioration provided the bags are kept in a dry place.
5.8 LABORATORY TESTING OF LIMESTONES
Physical and chemical tests are conducted on limestone to determine their suitability.
5.8.1 Physical Tests
Physical tests are based on the texture, appearance, colour and odour of limestone. Limestone showing amorphous textures are suitable for making lime.
Chalk and fat lime deposits are white or whitish-brown or grey in colour. Magnesium limestone is distinguished from its heaviness and hardness.
Limestone with white, bluish-grey, yellow or brown colours are suitable for producing hydraulic lime. These limestones show a compact texture with an irregular dull fracture. A freshly fractured surface of such limestone has an earthy smell and is clayey in taste.
5.8.2 Chemical Tests
1. Heat Test
A piece of dry limestone is weighed and heated on an open fire for a few hours. The limestone disintegrates emitting CO2. Due to the removal of CO2 from the sample, it loses weight. The sample is weighed again. The loss of weight indicates the amount of CO2. From this, the percentage of calcium carbonate present in the limestone is determined.
2. Acid Test
The purpose of this test to classify the limestone and to determine the percentage content of calcium carbonate.
A teaspoon of powdered lime is placed in a test tube, and dilute hydrochloric acid is added. The contents are stirred with a glass rod and set aside for 24 hours.
Abundant effervescence indicates a high percentage of calcium carbonate. On the other hand, the formation of residue signifies that the limestone is impure or hydraulic. The absence of formation of gel signifies fat lime.
The formation of a thick gel shows primarily the presence of hydraulic lime, and gel of moderate thickness signifies feebly hydraulic lime.
3. Slaking Test
Quick lime in combination with water hydrates, and the process is known as slaking. A lot of heat is generated during slaking, which indicates that the percentage of calcium content is high in the limestone. In the case of hydraulic lime, the slaking action is very slow.
The Bureau of Indian Standards (IS: 6932–1973) specifies 10 laboratory tests for lime (Methods of Test for Building Lime – Parts 1 to 11). The reader may refer to the codes for details.
5.9 FIELD TESTING OF BUILDING LIME
The Bureau of Indian Standards (IS: 1624–1974) provides a number of field tests for building lime, which can be easily conducted in the field. They are discussed below.
5.9.1 Visual Examination
Class C lime is identified by its pure white colour.
5.9.2 Hydrochloric Acid Test
Hydrochloric acid of 50% strength is added to powdered lime kept in a test tube till effervescence ceases. Generally, a teaspoonful of powdered lime of about 10 cc is required. It is kept for 24 hours. The bubbling reaction indicates the presence of lime. The volume of insoluble residue signifies unwanted inert material.
Class A lime shows the formation of good thick gel after 24 hours of letting the mixture stand. Class B lime shows the formation of gel, whereas no gel is formed in the case of Class C lime.
5.9.3 Ball Test
Sufficient water is added to lime and a ball of the size of an egg is made. It is stored for 6 hours and then placed in a basin of water.
If it shows expansion and disintegration in a few minutes, it signifies Class C lime. If the expansion is less with a number of cracks, it can be categorised as Class B lime. Class A lime will not show any adverse effects.
5.9.4 Impurity Test
A known weight of lime is mixed with water in a beaker and the solution is decanted. The residue is dried in the sun for 8 hours, cooled and weighed. Based on the percentage of residue, the quality of lime is decided. Residue of less than 10% indicates that the lime is good, 10–20% indicates fair and above 20% indicates poor.
5.9.5 Plasticity Test
A small quantity of lime is mixed with water and left overnight. The so-formed material is spread like butter on a blotting paper with a knife to test its plasticity. Good lime indicates high plasticity.
5.9.6 Workability Test
An 1:3 lime mortar is made with adequate water. The mortar is thrown on a brick wall by a trovel. The sticking quality is examined. If it sticks well, then its workability is good.
5.10 POZZOLANIC MATERIALS
Pozzolanic materials are siliceous and aluminous materials, which do not possess any cementitious properties but form cementitious compounds by reacting with lime in the presence of water. It is volcanic ash containing about 80% clay.
The use of pozzolanic materials provides the following improvements:
- Improves workability
- Lowers the heat of hydration
- Assists in early setting and hardening of the mortar
- Reduces shrinkage
- Improves hydraulic properties and resistance to chemical attacks.
Naturally available pozzolanic materials are clay and shale, diatomaceous earth, volcanic tuffs and pumicites. Artificially available pozzolanic materials are fly ash, blast furnace slag, silica fume, rice husk ash, metakaoline and surkhi.
- Binding materials may be of the following kinds: air binding, hydraulic binding and autoclave binding.
- Lime is not freely available in nature but obtained by burning one of the naturally available materials such as (i) limestone found in limestone hills, (ii) limestone boulders found in the bed of old rivers, (iii) kankar found below ground and (iv) shells of sea animals. Lime is more or less pure calcium carbonate.
- The main sources of obtaining lime are the stones which possess clay, soluble silica, magnesium carbonate. alkalis and metallic oxides, sulphates, iron compounds and carbonaceous matter.
- The general classification of lime based on the calcination of limestone is (i) fat lime, (ii) hydraulic lime and (iii) poor lime.
- Fat lime has high calcium oxide content. This can set and become hard only in the presence of carbon-di-oxide. This is white in colour.
- Hydraulic lime is lime containing small quantities of silica, alumina and iron oxide, which are jointly in chemical composition with calcium oxide. Hydraulic lime can set and become hard even in the absence of CO2 and can set even under water. This is also known as water lime.
- Poor lime is also known as impure lime or lean lime. More than 30% of clay present in poor lime makes the lime to slake slowly. It sets and hardness very slowly and has very poor binding properties. It can form a thin paste with water.
- The Bureau of Indian Standards classifies lime into six categories as (i) Eminently Hydraulic Lime (Class A), (ii) Semi-hydraulic lime (Class B), (iii) Fat lime (Class C), Magnesium or Dolomite lime (Class D), Kankar lime (Class F) and siliceous Dolomite lime (Class F).
- The manufacture of lime comprises three stages, viz., collection of raw materials, burning of limestone, and slaking of burnt lime.
- Burning of limestone comprises clamp burning, intermittent kiln burning and continuous kiln burning.
- Laboratory tests of limestone are of two kinds: physical tests and chemical tests. Chemical tests are the Heat Test, Acid Test and Slaking Test.
- Field tests of building lime are Visual Examination, Hydrochloric Acid Test, Ball Test, Impurity Test, Plasticity Test and Workability Test.
- Briefly explain the constituents of limestone.
- List the properties of lime.
- What is meant by hydraulic lime? How can it be obtained?
- Compare fat lime with quick lime.
- Give the classification and properties of different types of lime.
- How is lime classified by Indian Standards?
- Describe the process of lime burning in India.
- Explain the types of intermittent kilns.
- Provide a sketch of the continuous kiln, and explain its working.
- How do you proceed to get (a) lime putty, (b) quick lime and (c) slaked lime?
- Explain chemical tests conducted on limestone.
- What are the field testing of building lime?