•Lime is produced from the calcination of its ore calcium carbonate.
•Calcination is the process in which, calcium carbonate is heated upto redness resulting in loss of moisture & CO2 from it and leading to the formation of lime.
CaCO3 ⇌ CaO+CO2
• Lime which is preduced from relatively pure lime stone is termed as quick lime.
• Quick lime has very high affinity for water hence reacts with it vigorously, resulting in the formation of slake lime or hydrated lime.
Slaking of lime:
CaO + H2O → Ca(OH)2 (Hydrates (slaked) lime)
• Slaking is the process in which quick lime reacts with water swells, cracks and falls out as calcium hydroxide (exothermic reaction) Slake lime should be used as fresh as possible as it has very high affinity for Co2 hence, reacts with it & results in the formation of precipitate of calcium carbonate.
On the basis of percentage purity lime may be further divided into three :
(1) Fat Lime / White / Pure / Rich / High Calcium Lime :-
• It is the type of lime that is being obtained from relatively pure lime stone (% purity is approx. 90 to 95 % or clay content is approx 5 to 10 %). This lime slakes vigorously and its volume is increase to about 2 to 25 times the volume of original lime.
• This lime sets very slowly and posse’s high plasticity hence is generally not used in the works where strength is required.
• This lime posse’s pure white colour hence, is generally used for finishing works, like plastering & white washing lime also referred as white lime.
(2) Hydraulic lime / water lime :- It is the type of lime that is being obtained from relatively impure lime stone (percentage purity is in the range of 190 to 10 % any clay content in the range of 10 to 30 %)
• This lime is capable of setting under water or in damped conditions where there is no free circulation of air, this property of lime is referred as Hydrolicity.
• This lime takes comparatively lesser time for its setting and posses higher strength hence, is generally used for the works where is required.
Eg :- Masonry work. The colour of this lime is not perfectly white hence it is generally not used for finishing works.
(3) Poor lime / Impure lime / Lean lime :- It is the type of lime that is produced from the lime stone having high percentage of impurity in it (having clay content greater than 30 %, percentage purity less than 70%). This lime does not slakes, sets very slowly, posses poor binding property and muddy white colour hence, is generally used for inferior quality works like brick works around the foundation.
Constituents of Lime :-
(1) Clay :- It imparts hydrolicity in lime and makes it insoluble in water. If it is in excess it retards slaking and if it is in deficiency it arrests slaking. For good lime clay content should be in the range of 8 to 30 %.
(2) Soluble silicates :- Silicates in the form of calcium, magnesium & aluminum also impart hydrolicity to the lime.
(3) Magnesium carbonate :- Presence of magnesium carbonate in lime allows it to slake(increase in volume) and set slowly.
Functions / uses of lime :-
(1) It is used for finishing works like plastering & white washing.
(2) Lime is used in masonry work.
(3) Lime is used for the stablisation of soil.
(4) Lime is used for the manufacturing of the glass.
(5) It is used as a flux in metallurgical operations.