LIME

The use of lime as a cementing material has been made since ancient times. The Egyptians & Romans made use of this material for various constructional purposes. Even in India, big palaces, bridges, temples, forts, monuments, etc. were constructed with lime. At present, places where lime is locally available & when there is acute shortage of cement, lime

certainly provides a cheap & reliable alternative to cement.

Definitions

Calcination: o The heating of limestone to redness in contact with air is known as ‘Calcination’.

Hydraulicity:o Property of lime by which it sets or hardens in damp places, water or thick masonry

walls with no circulation of air. Lime:

o Due to calcinations, the moisture & carbon-dioxide are removed from it & the remaining product is known as ‘Lime’.

o Chemical reaction – CaCo3 = CaO + Co2

Quick lime: o Lime obtained by the calcinations of pure limestone is known as ‘Quick Lime’ or ‘Caustic

lime’. o It is amorphous (i.e) not crystalline & has no affinity for carbonic acid, but has affinity for

moisture. o The quick lime as it comes out from kilns is known as the ‘lump lime’.

Setting: o The process of hardening of lime after being converted into paste form is known as

‘setting’. o It is different from drying because incase of drying, water evaporates from lime & no

setting action takes place. Slaked lime:

o Product obtained by slaking of quick lime is known as ‘Slaked lime’ or ‘hydrate of lime’.o It is in the form of white powder & should always be used as fresh as possible because it

has a tendency to absorb carbonic acid from atmosphere & thus gets converted into carbonate of lime.

o Such slaked lime becomes useless as it loses its setting properties.o Therefore, it should not be kept in a damp place.o Chemical reaction – CaO + H20 ----slaking---- Ca (OH)2 + heat.o The theoretical amount of water required for lime slaking is about 32%, but in practice,

the amount of water required is about 2 to 3 times more because of lime composition, degree of burning, method of slaking & evaporation of water.

o A thin pourable suspension of slaked lime in water is known as ‘Milk of lime’. Slaking:

1

LIME

o Process in which, when water is added to quick lime, a chemical reaction takes place & the quick lime cracks, swells & falls into a powder which is the Calcium hydrate known as ‘Hydrated lime’.

Sources of lime

Lime is not usually available in nature in free state. It is produced by burning one of the following materials;

o Lime stones from the stone hillso Boulders of lime stones from the beds of old riverso Kankar (impure limestone) found below the ground &o Shells of sea animals.

Constituents of limestone

The properties of lime depend on the composition of the limestone from which it is produced. The constituents of lime stones are as follows; Clay:

o It is responsible for producing hydraulicity in lime. o If excess it arrests slaking & if less, it retards slaking. o 8% to 30% is desirable for making a good lime. Clay also makes lime insoluble in water.

Soluble Silica:o It is essential to develop hydraulicity in lime. o The silicates of calcium, magnesium & aluminum are responsible for hydraulicity. o They are inert or inactive at low temperatures & become active & combine with lime at

high temperatures. Magnesium Carbonate:

o This constituent allows lime to slake & set slowly, but imparts more strength.o 30% of carbonate of magnesia renders hydraulicity to lime, even in the absence of clay.

Alkalies & Metallic oxides:o These when present about 5% or so, develop hydraulicity.

Sulphates: o Its presence in small quantities, accelerates the process of setting & reduces slaking

action. Iron:

o If present in small quantity, it develops a complex silicate at high temperature, but excess is objectionable.

Pyrites: o Undesirable to have pyrites in lime stones. Such lime stones should be rejected.

Classification of lime

Lime obtained by calcinations of limestone is classified as;o Fat lime

2

LIME

o Hydraulic limeo Poor lime

Fat lime

This lime is also known as High Calcium lime, Pure lime, Rich lime or White lime. It is popularly known as Fat lime. It slakes vigorously & its volume increases about 2 to 2 ½ times the volume of quick lime. It is prepared by calcining pure lime composed of 95% of calcium oxide. Impurities in such limestone are less than 5%.

Properties

Hardens very slowly High degree of plasticity Soluble in water Colour is perfectly white Sets slowly in presence of air, and Slakes vigorously.

Uses

White washing & plastering walls With sand, it forms lime mortar which is used for brickwork & stonework. With surkhi, it forms lime mortar used for thick masonry walls, foundations, etc. (surkhi: powder obtained by grinding of burnt brick).

Hydraulic lime

This lime is also known as ‘Water lime’ as it sets under water. It contains clay & some amount of ferrous oxide & depending upon the percentage of clay,

hydraulic lime is divided as;o Feebly hydraulic limeo Moderately hydraulic limeo Eminently hydraulic lime

Properties

Increase in percentage of clay makes slaking difficult & increases the hydraulic property. With 30% of clay, hydraulic lime resembles natural cement. Can set underwater & in thick walls with no free circulation of air. Colour is not perfect white. Forms a thin paste with water & does not dissolve in it.

Uses3

LIME

Used for plaster works Hydraulic lime is ground to a fine powder & then mixed with sand & kept aside for 1 week. It is grounded again & then used for plastering work.

Poor lime

It is also known as ‘Impure lime’ or ‘Lean lime’.

Properties

Contains more than 30% of clay & slakes very slowly. Forms a thin paste with water but does not dissolve in it. Sets or hardens very slowly & has poor binding properties. Colour is muddy white.

Uses

It makes a very poor mortar, that can be used for inferior type of work or places where good lime is not available.

Building lime classification

Class – Ao It is eminently hydraulic lime used for structural purposes & is supplied in the hydrated

form only. Class – B

o It is the semi – hydraulic lime used for masonry work & is supplied as ‘Quick lime’ or as ‘Hydrated lime’.

Class – Co It is fat lime used for plastering, white washing & supplied in hydrated or quick form.

Class – Do It is dolomitic lime used similar as class – C lime.

Class – Eo It is kankar lime used for masonry works & supplied as hydrated lime.

Class –Fo It is siliceous dolomitic lime used for plastering & supplied as quick & hydrated lime.

Manufacture of lime

3 distinct operations are involved in the manufacture of lime;o Collection of limestoneo Calcinations of limestoneo Slaking of limestone.

Collection of limestone

4

LIME

The lime stones with 5% impurities are collected at site of work. It is desirable to use pure carbonate of lime in the manufacturing process of fat lime.

Calcination of limestone

The calcination’s or burning of lime stones can be achieved either in kilns or clamps. Clamps are temporary structures whereas Kilns are permanent structures that may be

intermittent type or continuous type. The fuel required for calcinations consist of charcoal, coal, firewood or coal ashes & initial firing

is achieved with few chips of dry wood or cow-dung cakes.

Clamps

The ground is leveled & cleaned. The lime stones & fuel (incase of wood) are placed in alternate layers, whereas, if fuel (is coal or

charcoal), it is mixed with lime stones & placed in a heap form. The sloping sides are covered with mud plaster to preserve heat as much as possible. It is then fires from bottom & a small opening is provided at top for draught. When the blue flame at top disappears, it indicates the completion of the process. The clamp is then allowed to cool down & pieces of quick lime are handpicked. It is adopted to manufacture lime on a small scale.

Disadvantages

Uneconomical to manufacture lime on a large scale. Loss of heat is considerable as mud plaster cracks by heat inside & allows heat to escape. Quality of lime produced is not good, Quality of fuel required is more.

Intermittent Kilns

These are of various patterns & sizes & shapes depending on the practice. 2 important types are

o Intermittent Flame Kiln & o Intermittent Flare Kiln.

Intermittent Flame Kiln

Alternate layers of limestone & fuel are arranged in kiln. Horizontal & vertical flues are suitably formed & top of kiln is covered with unburnt material. The kiln is ignited from the bottom & lime stones are allowed to burn for 3 days or so. The kiln is then cooled & unloaded. The process is the repeated.

Intermittent Flare Kiln5

LIME

In this type, a rough arch of selected big pieces of lime stones are formed & smaller pieces of lime stones are packed over this arch.

The fuel is placed below the arch & thus is not allowed to come into contact with limestone.

When fuel is ignited, only the flame comes into contact with limestone. When the lime stones are burnt, the kiln is cooled & unloaded. The process is then repeated. This type of kiln is easy to manage & produces lime of better quality as lime stones are not

mixed with fuel & the finished product does not contain ashes.

Continuous Kiln

There are various types, sizes of such kilns depending on practice in the locality. The 2 important types of Continuous Kilns are;

o Continuous Flame Kilno Continuous Flare Kiln

Continuous Flame Kiln

It is in the form of a cylinder with diameters of 1.8m, 2.3m & 1.4m of top, middle & bottom portion resp.

Widening of middle portion is done to accommodate hot gases of combustion. The mixture of lime stones & fuel are fed from the top & the bottom is covered by grating. The Kiln is partially above the ground & partly below ground. A loading platform is provided at the top. The inner surface of the kiln is covered with fire-brick lining & to facilitate cleaning of the grating,

a rake hole is provided. After burning, the lime is collected sat the bottom & is removed through access shaft. As the level of material inside the kiln falls, the required quantity of mixture of limestone & fuel is

fed from top. A roof may be provided at top to protect the kiln.

Continuous Flare Kin

This kiln consists of 2 sections – upper & lower. The upper section serves as storage of limestone & the lower portion is provided with fire-brick

lining. A small quantity of fuel is mixed with limestone & ignited. Fuel is then fed through shafts around the upper & lower sections of the kiln. Lime stones are fed from the top & the calcined material removal is done through a grating

placed at the bottom of kiln. A roof is provided at top to protect the kiln. There is considerable saving of time & fuel in this kiln, as the fuel does not come in contact with

lime stones. Initial cost is high & thus these are adopted to manufacture lime on a large scale.

6

LIME

Facts to be remembered during the process

Dark red colour indicates completion of burning process & presence of Co2. Burnt limestone should be withdrawn from kiln as soon as Co2 is driven off & colour changes to

a brilliant white. Over burning or under burning should be avoided during burning of stones. Ideal temperature is

800°C & for several hours. Heating should be gradual as sudden heating results in blowing of stones to pieces. Imperfectly calcined lime does not slake with water & is referred to as ‘Dead-burnt’ lime. Lime stones should be broken to suitable sizes before they are burnt. Fat lime stones – 200mm to 250mm Hydraulic lime stones – 75mm to 100mm. Quantity of fuel for burning should be carefully proportional.

Slaking of burnt lime

The objects of slaking are;o To ensure soundness (i.e.) bring about volumetric stability.o To ensure plasticity

The common methods employed for slaking are;o Air slakingo Basket slakingo Platform slakingo Tank slaking

Air Slaking

Quick lime obtained is exposed to atmosphere for slaking This is known as ‘Natural slaking’ or ‘Air slaking’ & is a very slow process. Other 2 methods of slaking are;

Slaking to paste

In this method, quick lime is spread evenly of 150mm depth in a wooden or masonry basin. Water in sufficient quantity is then poured over the layer till the quick lime submerges. Excess water retards slaking & less water results in unsatisfactory slaking. The basin is covered with wooden planks to preserve heat & ensure proper slaking. Stirring is not required & slaking is completed in 10 minutes.

Slaking to powder

In this method, the quick lime is slaked to powder form by 2 ways;

7

LIME

I method: (Basket slaking)

Quick lime is broken to pieces of sizes 50mm. It is then immersed in water for a considerable time in a basket. The basket with lime is then removed from water & the lime pieces are thrown on a wooden or

masonry platform in a heap form. Quick lime crumbles & falls as powder.

II method: (Platform slaking)

Quick lime is spread in layer of 150mm depth on a wooden or masonry platform. Water is sprinkled above this layer from a water can or vessel with perforated nozzle. Quick lime thus swells & crumbles into powder from. This method is generally adopted to slake quick lime obtained from the shells.

Tank slaking

2 brick-lined tanks are constructed, one generally at the ground level 45cm deep & the other tank is made adjacent to the first one, but at a lower level (below ground) usually 60cm to 75cm deep.

The first tank (at ground level) is filled half with water & sufficient quicklime is added to fill the tank to half the level of water.

It may be noted that lime should always be added to water & not water to lime. The lime is then stirred with no part of lime to be exposed above water. As lime slakes, the temperature increases & water begins to boil. Thus additions of lime & water

are made in small quantities with constant stirring so as to maintain the required temperature (85°C - 98°C).

The operation is continued until the required quantity of lime or whole of lime has been slaked. The lime suspension (milk of lime) in the state of suspension is then allowed to pass through a

sieve & flow into the second tank at lower level. The particles of lime settle down & water gets partly absorbed, partly evaporated & remaining

surplus water is removed. The putty, prior to use, is allowed to mature in the tank for 2 to 3 days & by doing so, the

complete slaking is ensured & the workability of the putty improves.

Manufacture of Natural Hydraulic Lime

3 distinct operations are involved in the manufacture of natural hydraulic lime.o Collection of kankaro Calcinations of kankar, ando Slaking & grinding of burnt lime.

Collection of Kankar

Kankar is an impure limestone & it is used for manufacturing natural hydraulic lime. It is available in 2 forms, nodular & blocks.

8

LIME

Nodules are found either on surface of ground or slightly below ground level. They are easy to collect & are superior material for manufacturing natural hydraulic lime,

because It can withstand heat & rain without disintegration It contains higher % of clay & thus better hydraulic properties. The blocks of kankar are found form the underground strata below or near river banks or

streams. Thicknesses of blocks are usually 50mm to 300mm. Nodules or blocks of kankar are quarried with pick-axes or crowbars. It is the cleaned of mud or earth & converted to suitable sizes.

Calcinations of Kankar (burning)

Calcinations of kankar to bright red heat are done either in clamps or kilns as manufacture of fat lime.

Slaking & Grinding of burnt lime

The slaking of hydraulic lime occurs very slowly. Thus quick lime is ground dry before water is added for slaking. Grinding of quick lime can be done;

o By hand with the help of wooden beaters, oro By mills working with bullocks or steam power, or o By special machines

Differences between slaking of fat lime & hydraulic lime

Fat Lime Hydraulic Lime

Required quantity of water for slaking is added at time

Required quantity of water for slaking is gradually added through slaking

1 part of Fat lime when slaked is converted into 1 ½ parts in paste form & 2 parts in powder form

1 part of Hydraulic lime when slaked is converted into 1 part in paste form & 1 ½ parts in powder form.

Quantity of water required for slaking is more

Quantity of water for slaking is less

Time taken for slaking is about 3 to 4 hours.

Time taken for slaking is about 12 to 48 hours.

Manufacture of Artificial Hydraulic Lime

9

LIME

Artificial hydraulic lime can be prepared when natural raw material is not suitable for the manufacture of hydraulic lime.

Fat lime may be converted into hydraulic lime by addition of clay in required proportion. 2 methods of preparing artificial hydraulic lime are;

o Conversion of soft limestoneo Conversion of hard limestone

Conversion of Soft Limestone

Limestone of soft quality, such as chalk, is ground & converted into powder form. It is then mixed with required proportion of clay & burnt in a kiln & slaking is carried out as in the

manufacture of natural hydraulic lime.

Conversion of Hard Limestone

Limestone of hard quality is first burnt & slaked. To this slaked lime, required portion of clay is added. This mixture is converted into balls of suitable sizes & after drying, these balls are burnt in kiln. Slaking is then done as in the manufacture of natural hydraulic lime. As this lime is produced after burning twice in kiln, it is also known as ‘Twice-kilned lime’.

Precautions to be taken in handling lime

Following precautions are to be taken while handling lime to avoid accidents; Contact with water: quick lime should not be allowed to come in contact with water before

slaking. Facilities for workers:

o Workers should be provided with googols & respirators as lime dust causes irritation.o They should also be provided with rubber gloves, gum boots & skin protective cream as

lime causes skin burns, esp. when skin is moist. Fire hazard: As quick lime gives out immense heat while slaking, suitable measures should be

taken for any fire hazard. Instructions to workers:

o Workers handling lime – wash exposed parts of their body with abundant fresh water.o Workers handling milk of lime which is hot – oil their skin daily to avoid burns.

Uses of lime: used as;

Chemical raw material in the purification of water & for sewage treatment. Flux in the metallurgical industry Matrix for concrete & mortar. Refractory material for lining open-hearth furnaces. Production of glass Making mortar for masonry work Plastering of walls & ceilings

10

LIME

Production of artificial stone, lime – sand brick, foam – silicate products, etc. Soil stabilization & improving soil for agricultural purposes. White washing & as a base coat for distemper. When mixed with Portland cement, can be used in place with costly cement mortar.

Tests for lime stones

Lime stones are tested to determine the quality of lime by detailed chemical tests in a laboratory.

The following practical test are made for general information; Physical properties Heat test Acid test Ball test Visual inspection Soundness test Workability test Transverse strength test Compressive strength test Impurity test Plasticity test

Physical properties

Pure limestone is indicated by white colour Hydraulic limestone id indicated by bluish grey, brown or some dark colour, tastes clayey &

gives out earthy smell. Presence of lumps gives an indication of quick lime or un-burnt limestone.

Heat test

A piece of dry limestone is weighed & heated in an open fire for a few hours & a sample is weighed again.

The loss of weight indicates the amount of calcium carbonate in limestone is worked out.

Acid Test

A teaspoon of powdered lime is taken in a test tube & dilute hydrochloric acid is poured in it, stirred & kept standing with its contents for 24 hours.

If there is vigorous effervescence & less residue formation, calcium carbonate is high – indicates pure limestone.

If there is less effervescence & more residue formation, calcium carbonate is less – indicates impure or hydraulic limestone.

If a thick gel is formed – indicates class – A of lime

11

LIME

If gel is not thick & tends to flow – indicates class – B of lime. If no gel is formed – indicated class – C of lime.

Ball Test

Stiff lime balls of about 40mm size are made by adding enough water & left undisturbed for 6 hours.

The balls are then placed in a basin of water. If there is slow expansion & disintegration within minutes after placing in water – it indicates

class – C lime If there is little or no expansion & numerous cracks – it indicates class – B lime.

Visual inspection

A sample of lime is examined for its colour & lumps.o White colour – fat or pure lime.o Lumps of lime – quick lime or un-burnt lime.

Chemical Analysis

The analysis determines the cementation & hydraulic properties of lime. Cementation value of lime = 2.8A+1.1B+0.7C

D + 14 E

Where, A = silica oxide content (SiO2) B = aluminium oxide content (Al2o3) C = ferric oxide content (Fe2o3) D = calcium oxide content (CaO) E = magnesium oxide content (MgO)

Soundness test

The test is done to find the quality, (i.e.) the unsoundness or disintegration property of lime using the Le-chatelier apparatus.

To test hydrated lime; Cement, hydrated lime & sand (1:3:12) are mixed in the cylinder of the Le-chatelier apparatus &

is covered with a glass sheet & left for an hour. The distance between the indicator pointers is measured. The apparatus is then kept in damp air for 48 hours & is then subjected to steam for 3 hours. The sample is cooled to room temperature & the distance between the pointers is measured

again. The difference in two measurements should not be more than 10mm. To determine the soundness of fat lime; Pats are prepared by mixing 70g of hydrated lime, 10g of POP & 70ml of water. The pats are subjected to steam & then tested for disintegration, popping ^ piping & if any of

these occur, the lime is considered to be unsound.12

LIME

This test is also known as ‘Popping & Piping test’.

Workability test

A handful of mortar is thrown on the surface on which it is to be used & the area covered by the mortar & its quantity is recorded along with its sticking quality.

This data indicates the workability (if sticks well – workability is good) of the lime mortar & is a very crude field test performed with actual mortar.

Transverse Strength test

25 x 25 x 100mm specimens are cured for 28 days at 90% HUMIDITY. They are immersed in water for 30 minutes, taken out & placed on two parallel rollers, 80mm

apart & load is applied uniformly starting from 0 increasing at a rate of 150 N/minute through a 3rd roller of same size, at a midway point between the two, till the specimen breaks.

Modulus of rupture of test specimen, m = 3Ws = 0.0768w 2 bd2

Where, m = modulus of rupture of the specimen in N/mm2

w = breaking load in N s = spacing between the rollers in mm

b, d = the width & depth (each 25mm) of the specimen in mm. Minimum value should be 1.05 N/mm2 for class – A lime, and

0.7 N/mm2 for class – B lime.

Compressive Strength test

12 cubes of 50mm size are made from standard lime –sand mortar & are kept undisturbed for 72 hours at room temperature (27˚C ± 2˚C)

They are then taken out of the mould & kept in open air for 4 days. 6 of the 12 cubes are cured for 7 days & tested in a compressive testing machine & the balance

6 cubes are cured for 21 days & tested. The load is steadily & uniformly applied, from 0 increasing at the rate of 150 N/minute & the

crushing load is divided by the area of the cube denotes the compressive strength of mortar.

Impurity test

A known weight of lime is mixed with water in a beaker & the solution is decanted. The residue is dried well in hot sun for 8 hours & then weighed. If residue is less than 10% - lime is good

10% - 20% - lime is fairabove 20% - lime is poor

Plasticity test

The lime is mixed with water to a thick paste & left overnight. It is then spread on a blotting paper with a knife to test its plasticity.

13

LIME

Good lime is plastic in nature.

Lime putty

More popularly known as Calcium hydroxide, it is used as a base material for a number of different applications in building construction.

It is prepared by mixing lime chalk with required quantity of water & heated to a high temperature that helps the mixture to thicken.

Once the desired consistency is reached, lime putty is allowed to settle & mature over several methods.

In order to keep the product from drying out, a thin layer of water is applied over the top level of the putty & the container is sealed.

It can be used in a thick composition for plastering & grouts & also as the base for a plastering technique.

Storing lime

Lime reacts to the moisture present in the atmosphere & that from the ground. Hence, it should be stored with utmost care & so following precautions should be followed

properly;o Should be stored in properly insulated (against moisture) container & off the ground.o When delivered as hydrated lime – kept dry, stored under cover & off the ground.o When delivered as quick lime – should be used as soon as possible positively within a

week.o Lime putty stored without any deterioration for many weeks & actually improves by

keeping. (Maximum storage – 14 days).o (Deterioration is caused due to the action of the atmosphere moisture on un-slaked

lime).o In case of semi-hydraulic lime – putty must not be stored for more than 3 days of its

preparation.o In case of hydrated eminently hydraulic lime – coarse stuff & putty should be used within

12 hours.

14

LIME

15