Unlike modern cement renders, which seal walls and are rigid and impermeable, lime render acts as a genuine skin for the building. It accommodates micro-movements in the structure and actively regulates moisture.
The base binder is air lime (lime putty) or Natural Hydraulic Lime (NHL). The hardening of air lime does not occur through chemical hydration (as with cement) but through a slow process called carbonation. Calcium hydroxide absorbs the carbon dioxide (CO2) in the air, slowly returning to its original state: calcium carbonate (stone). During this process, the render can self-heal any micro-cracks.
To avoid detachment and shrinkage, application must be carried out wet-on-wet following decreasing grain sizes. It begins with the rough coat (rinzaffo) (coarse sand) to ensure mechanical adhesion to the substrate; it moves on to the browning coat (arriccio), which defines flatness; finally the setting coat (stabilitura or tonachino) is applied, the final millimetric layer that gives the velvety appearance typical of historic surfaces.
Standards
European and international references applicable.
Physical properties
Usage environment
Lime is mandatory in restoration of stone or solid-brick masonry: cement would cause salt disintegration and block evaporation of rising damp.