Burning, Slaking & Seiving of Lime

Mortars for re pointing should be softer than the masonry units and no harder than the historic mortar to prevent damage to the masonry units. Stresses caused by natural conditions should be relieved by the mortar rather than the by the masonry units. Stresses caused by mortars, which are too hard, can break the bond between the units and the mortar; they can also cause cracking and spalling of the units.

The process involves burning , slaking and sieving the lime.

BURNING OF LIMESTONE:

The process of burning involves the removal of carbon di oxide to form the oxide of calcium called ‘quick lime’.

CaCO3 --burning-->CaO (quick lime) + CO2

Type: SEMI-HYDRAULIC    72% of CaO & MgO

 

  Burnt Lime Stone

 
SLAKING OF QUICK LIME:

Slaking of lime is the process of hydration of quick lime.

CaO + H2O Ca(OH)2 (slaked lime)

The lime procured from the above sources is freshly burnt and delivered to the site in airtight bags.This lime is slaked on the site for 4 days.

 lime_m1.jpg (3048 bytes)  

  Lime Slaking
The method of slaking involves:
  • First fill the tank with water
  • Empty the bags into the tank. As this is an exothermic reaction giving out heat care needs to be taken by providing the worker with mask, gloves and gumboots.
  • At a time 5 bags are added and then stirred for some time, to see that all the limestone pieces start slaking.
  • About one inch of water is allowed to stand over the lime.
  • This is stirred twice a day for four days.
 

SIEVING OF SLAKED LIME:

The slaked lime is sieved into another storage tank, using a sieve of size1.18mm to remove any unslaked stones.

Lime itself, when mixed with water into a paste, is very creamy and plastic. It will remain workable and soft indefinitely, if stored in a sealed container. Lime (calcium hydroxide) hardens by carbonation viz, reaction with atmospheric carbon dioxide and not with water.

Ca(OH)2 + CO2 --->CaCO3 + H2O

   

  Sieving slaked lime
 

SIEVING OF SAND:

The sand acquired from the riverbed for the mortar is sieved with a sieve size 2.36mm.

A scoop of sand contains many small voids between the individual grains. A mortar that performs well fills all these small voids with binder (cement/lime combination or mix) in a balanced manner. Well-graded sand generally has a 30 per cent void ratio by volume. Thus, 30 per cent binder by volume generally should be used, unless the historic mortar had a different binder: aggregate ratio. This represents the 1:3 binder to sand ratios often seen in mortar specifications.

 

   Sieving sand

                                           Mortar          Lime Main Page