Hydraulic lime sets under water through chemical reaction, whereas non-hydraulic lime sets only by absorbing carbon dioxide from the atmosphere and cannot set underwater. This fundamental difference in curing mechanism makes hydraulic lime suitable for masonry repair work where traditional lime compatibility is required.
All lime production begins with calcining a calcium-rich feedstock such as limestone, chalk or seashells. Heating this material in a kiln produces calcium oxide, known as quicklime. Quicklime is then combined with water in a process called slaking to produce usable limes.
Lime is a traditional binding material used in mortars, plasters, renders and limewashes and is valued for its flexibility, permeability, and compatibility with historic structures. Not all lime however is created equal.
The European classification scheme divides natural hydraulic lime into three grades — NHL 2, NHL 3.5 and NHL 5. Each grade serves different applications in building conservation and construction.
Non-hydraulic lime, also termed fat lime or air lime, cures exclusively through absorbing carbon dioxide from the atmosphere. Hydraulic lime, by contrast, sets through chemical reaction in the presence of water, independent of atmospheric CO₂.
For those maintaining listed and historic properties, selecting appropriate lime is integral to preserving the structural and aesthetic integrity of their buildings. Lime selection requires assessment of substrate conditions, exposure severity, and original mortar composition.
