3. Shrinking:
Fat lime has a greater tendency to shrink and crack as it dries. To prevent this, a large quantity of sand (2 to 3 times) must be mixed with it to prepare mortar.
4. Hardening or setting:
Fat lime is hydrated calcium oxide and sets by the absorption of CO2 from the air.
Ca (OH) 2 + CO2 ==> CaCO3 + H2O
Crystals of CaCO3 are formed and the water goes by evaporation. Thus fat lime hardens only where it comes in contact with air, as in plaster work.
In the interior of thick walls, it does not acquire strength as CO2 i.e. air cannot reach there. Mixing of sand (2 to 3 times) forms pores for access of CO2 and helps hardening.
5. Strength:
Crystals of CaCO3 formed by fat lime are not very strong. Fat lime, therefore, does not possess much strength and is used for plastering walls, while washing etc in exposed positions.
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