Lime

Burning Lime.

Limestone is burnt to make it useful for mortar purposes, its still done the same way today.
Shells were even burnt to make lime in places where there was no ready supply of limestone.

Limestone unburnt is calcium carbonate (CaCo3).
Burning pure limestone in air to redness produces calcium oxide(CaO), or Quick Lime as the carbon dioxide (CO2) is liberated, this process is called calcining.

Quick Lime or Caustic Lime

When water is added to Quick Lime it absorbs it for a considerable time without showing moisture, and at the same time gives out heat and vapour.
This process is called slaking or hydration.

Lime is Hydrated when it has taken up the maximum amount of water without appearing moist.
It bulks when hydrated to at least twice that of when it was just Quick Lime.

Hydrated Lime.

Hydrated lime is generally sold as builder lime in bags of 20Kgs

The use of hydrated lime in modern mortar shouldnt be overlooked, it makes the mortar more user friendly by making it more plastic and fatty, helping it to stay on the trowel rather than just sliding off.

Without lime the mortar is liable to be crumbly and thin as you try to use it. It also stops the mortar drying out too quickly, especially on hot days.

Another benefit of lime is its ability over time, to heal any small cracks (autogenous healing) that appear in the mortar joints.

Lime Mortar.

Harder to use than cement based mortar.

Lime mortar is sand and lime, course sand seems to have be the preferred sand as most lime mortar brickwork I have seen has course sand.

1 lime, 2 sand was the preferred mix.

Cement was never added to lime mortar, the setting properties of the lime made the mortar go off.

Cement based mortars should never be used to repair brickwork that is lime mortar based, the cement mortar will rapidly deteriorate lime mortar brickwork.
It will also make dampness a problem in the brickwork as the cement based mortar will trap moisture behind it.

Healing Properties of Lime.

Lime has the ability to self heal small cracks even in cement based mortars, known as autogenous healing.