What is lime?
Lime is calcium oxide obtained by the calcination of limestone, kankar, and other calcareous substances at over 900°C. It was one of the most common construction materials before the advent of Portland cement. Wherever locally available, it is still a very popular and cheap construction material. It is used for plastering the interior surface of walls and preparing concrete for the foundation and flooring of ordinary buildings.
#Raw Material for Lime
Calcium carbonate (It is obtained by the calcinations of lime stone), Kankar. Lime is obtained by burning limestone at a temperature of about 800°C.
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Types of lime
a) Fat lime
Fat lime consists of mainly calcium oxide (CaO) which reacts with carbon dioxide (CO2) in the air to form calcium carbonate after hardening.
b) Hydraulic lime
It is a lime containing small quantities of silica and alumina and/or iron oxide which are in chemical combination with the calcium oxide content. It differs from fat lime in the process of hardening in which it chemically combines with water to form calcium silicate and aluminate hydrate. Hydraulic lime hardens in combination with water while fat lime in combination with air.
What are the classifications of limes?
# Classification of Lime
- Based on Source
i) Stone lime (from limestone)- Uses: Mortar. Flooring & terracing.
ii) Kankar lime (from kankar)- Uses: Mortar (substructure)
iii) Shell lime (from shells of sea animals)- Uses: Lime punning; whitewash & color wash.
2. Based on Purity
- Fat, rich, pure, high calcium or white lime:-
Impurity not 5%, volume- 2 to 3 times after slaking, Uses: Plaster, whitewashing but not for mortar due to poor strength & slow hardening. When MgO exceeds one-tenth of the CaO & MgO in the lime then it is called magnesium lime and when MgO exceeds one-quarter of the CaO and MgO, the lime is called dolomite lime. It doesn’t set underwater.
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Fat lime is obtained by burning limestone and hydraulic lime is obtained by burning Kankar.
ii) Poor or lean lime:- Impurity>5%, Uses: Poor mortar & plaster. It doesn’t set underwater.
iii) Hydraulic lime:- It gets underwater unlike fat and poor limes which can set only by absorbing CO, from the atmosphere. It contains 5-30% (silica & alumina) and iron oxide which are in chemical combination with the calcium oxide. It is of three types:
1. Feebly hydraulic lime- Silica & alumina <10%, slaking time 5 to 15 min, setting time-21 days
2. Moderately hydraulic lime: Silica & alumina- 10-20%, slaking time- Ito 2 hrs, setting time- 7 days
3. Eminently hydraulic lime Silica & alumina- 20-30%, slaking time 2 to 5 hr, setting time- 2 to 48 hours, Uses: Plaster, mortar, etc.
3. Based on the purpose
- Class A (Eminently hydraulic lime):-
CaO (60-70%), Clay (20-30%), Setting time- 2 to 48 hr, Uses:- Mortar & concrete.
- Class B (Semi hydraulic lime)- CaO (70%), Clay (15% ).
Setting time-7days, Uses:-Mortar, flooring & concrete
- Class C (Fat lime):- CaO (93%), Clay (5%), volume-2 to 3 times after slaking, Uses:-Plaster & whitewashing. It can be used as a mortar only after mixing with pozzolana.
iv) Class D (Magnesium lime):- CaO & MgO (85%), Uses Plaster & whitewashing.
v) Class E (Kankar lime):- CaO (20%), MgO (5%), Uses: Mortar, Plaster & White washing.
ISI Classification of lime
a) Class A: It is an eminently hydraulic lime normally used for structural purposes. It is usually supplied as hydrated lime.
b) Class B: Semi-hydraulic lime contains both hydraulic lime and fat lime. It is used for mortar in masonry work. It is supplied both as hydrated or quick lime.
c) Class C: It is predominantly fat lime used for finishing coat in plastering, whitewashing, etc., and with suitable admixtures such as Surkhi or any other pozzolanic material to produce artificial hydraulic lime. It is supplied both as quick lime and hydrated lime.
d) Class D: It is lime containing substantial proportions of magnesium oxide and is similar to fat lime. Its important uses are for finishing coats in plastering, whitewashing, etc.
e) Class E: It is Kankar lime generally used for masonry mortars and is supplied generally as hydrated lime.
Terminologies in context to lime
a) Quick lime: The calcined material, a major part of which is calcium oxide in natural association with a relatively small amount of magnesium oxide and which is capable of slaking with water, is called quick lime. Quick lime (CaO) is a lime obtained after the calculations of Jime stone, It is capable of slaking with water.
b) Fat lime: The lime which has high calcium oxide content and depends solely on the absorption of carbon dioxide for setting and hardening, is called fat lime.
c) Hydraulic lime: Lime containing small quantities of silica and alumina and/or iron oxide which are in chemical combination with some of the calcium oxide content, giving a putty or mortar which has the property of setting and hardening under water, is called hydraulic lime. i.e. When quick lime is sprinkled with water, the fine powder obtained is called hydrated lime.
d) Hydrated lime: A dry powder obtained by treating quick lime with enough water to satisfy its chemical affinity for water under the condition of its hydration, is called hydrated lime.
e) Lump lime: It is the quick lime as it comes from the kiln in the form of lumps.
f) Milk lime:- It is a thin pourable solution of slaked lime with water.
g) Slaking: The process of adding water to quick lime to form calcium hydroxide is called slaking.
h) Lime putty:- It is prepared by stirring hydrated lime in water so as to get thick creamy consistency and allowing it to stand and mature for a period of about 16 hours in the case of nonhydraulic lime before using.
i)Coarse stuff: –
When the required quantity of sand is mixed with hydrated lime then mix water & follow the same process as lime putty is called coarse stuff.
#Tests on lime
a) Visual inspection:- Colour & lumps
b) Hydraulic acid test:- Carbonate content of lime (10ml of 50% HCI)
c) Soundness test:- Quality of lime, Le-chatelier apparatus
d) Workability test:- To know the workmanship
e) Impurity test:
If <10% Good
If 10-20% Fair
If >20% Poor
f) Ball test:- This test is performed to know the expansion and disintegration of ball of lime after 6 hours.
Test of the freshness of lime
When the original limestone has a relatively low level of CaCO, and there are other components containing magnesia, silica, and alumina, or if the limestone was poorly burnt os that the lime contained some original limestone, the fineness test is done to remove the non-lime components.
In the test, the lime is passed through a fine mess sieve of 0.18 mm and a good quality lime hydrate should not have more than 1% retained on it. The principle behind the test is that hydrated lime is very much finer than the impurities present and can pass through the fine sieve.
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