Evaluation of old mortars
Introduction
Lime mortars are those "Mortar (construction)" mortars that are made with lime, sand and water.
The lime used can be aerial or hydraulic, with the difference of carbonating in contact with air (aerial) or setting in water (hydraulic).
In Spain, the type of lime is regulated in the standards "Standard (technology)") UNE UNE-EN 459-1:2011 (Lime for construction. Part 1: Definitions, specifications and conformity criteria), UNE-EN 459-2:2011 (Part 2: Test methods) and UNE-EN 459-3:2012 (Part 3: Conformity evaluation).
The air lime
Aerial lime in traditional construction
Lime was already known in the 6th millennium BC. C. as a construction material for mortars and coatings. In Çatal Hüyük, walls plastered with lime mortar and frescoed have been found. Later, thanks to archaeological research it has been proven that it was used in Ancient Egypt, the Assyrian Empire, Classical Greece or the Roman Empire; Also, outside the Mediterranean, it was used by the Mayans, the Incas and the Aztecs in America and since the first Chinese dynasties or the first Indian dynasties").
It is very important to distinguish aerial lime from hydraulic lime"), since the latter contains many silicates and has a different behavior, especially as a construction material.
Aerial lime has very suitable properties since its mortars have the particularity of being permeable to water vapor (although not to rain) that allows the walls to breathe, which gives the walls hygroscopic qualities to regulate the humidity of the environments.
Once the lime is used, it begins to crystallize and carbonate, from the surface inwards, preserving a moist core that is what gives it its properties and elasticity, thanks to which it has better mechanical behavior than Portland Cement, both for exterior and interior renders, as well as for mortars and other uses.
After hundreds of years, the slaked lime, after being completely carbonated, returns to its original composition in the limestone quarries (calcium carbonate, CaCO).
Freshly slaked lime does not adhere chemically to surfaces, but mechanically. This makes it necessary to chip smooth surfaces for proper adhesion of the render and render layers.