Ingredients: pure lime (CaCO3) as binder and sand as aggregate
Firing the limestone: Limestone is fired at approx. 800°C. CO2 is released from the limestone
and escapes as a gas. CaO remains. The limestone
pieces look unchanged after firing, but are much lighter.
The burnt CaO lime is stored as lump lime or ground as a powder.
Production of pure lime mortar
Lime (binder) and sand (aggregate) are mixed and applied as mortar. There are various methods of producing lime mortar:
Slaking the lime. If the burnt lime is slaked with water, calcium hydroxide Ca(OH)2 is formed. Heat is released and there is an increase in volume of approx. 2 to 2.5.
Setting / carbonation. The setting of the lime takes place when the calcium hydroxide Ca(OH)2 combines with the CO2 from the air to form pure limestone CaCO3. For this reaction to take place optimally, both CO2 must enter the mortar (via the air) and sufficient moisture must be present so that dissolved calcium hydroxide is present. It is therefore important for setting that the mortar does not dry out completely but is also not completely soaked. Regularly moistening the surface for around a month is ideal. The ambient temperature should not fall below 5°C. Hardening takes place from the surface towards the inside. This means that in its final state, the mortar has a hard surface layer that becomes increasingly softer towards the inside.

Advantages of lime mortar

• Strength is no greater than natural stone.
• Longer workability than cement mortar
• Vapor diffusion is not restricted.
• Stones can be easily separated from the mortar and can be reused
• Natural stone walls are built as structures without movement joints. Movements caused by temperature and moisture changes and shrinkage must be absorbed by the masonry. If the mortar has a higher strength than the natural stone, the stone will break as a weaker component. The mortar used should therefore not have a higher strength than the natural stone, otherwise damage to the natural stone can be expected.
• The mortar used should not hinder the evaporation of water from the masonry. Cement mortars are largely diffusion-proof. Water that has penetrated the masonry is prevented from evaporating by the cement mortar.
• The mortar used should enable future renovations. Lime mortar develops less adhesion to natural stone than cement mortar. When demolishing a lime mortar wall, the stones can be separated from the mortar without any problems.

Disadvantages of lime mortar

• Long and time-consuming post-treatment (covering, keeping moist) to ensure carbonation.
• When working with lime mortar, work must be carried out very cleanly. The stone surfaces must be free of impurities. The mortar work cannot be carried out under extreme weather conditions (rain, cold, wind and strong sun exposure). These restrictive conditions do not apply to dry masonry.