Lime for restoration: air lime and hydraulic lime
How to choose the right binder for the task
Plaster is fast and practical but in older and protected buildings you often work with lime because that is the material the original building was built with. The joint European standard DS/EN 459-1 uses the term "building lime" as an umbrella term for both air lime and hydraulic lime and defines the requirements the products must meet.
§Lime - the classic
Lime, chemically calcium hydroxide, is produced by adding water to burnt lime. It sets by absorbing CO2 from the air in a process called carbonation, which in practice restores the limestone that the lime originally came from. Setting is slow, and lime is therefore soft and flexible compared to modern cement-based products — which makes it suitable for old masonry that needs to be able to absorb and release moisture.
§Hydraulic lime - when it needs to go faster or withstand moisture
Hydraulic lime contains both air lime and hydraulic components and therefore hardens in two ways: both by carbonation and by reaction with water (hydration). The standard divides natural hydraulic lime into three strength classes - NHL2, NHL3.5 and NHL5 - where the number roughly indicates the compressive strength. The higher the number the harder and more moisture resistant the lime becomes but at the same time it becomes less forgiving towards old soft material.
§When do you choose what
| Situation | Recommended lime type | Why |
|---|---|---|
| Internal stucco decoration in dry room | Lime | Soft, flexible and true to historical technique |
| Facade with moderate weather exposure | NHL2 or NHL3.5 | Better moisture resistance without becoming too hard |
| Plinth or highly moisture-exposed structure | NHL5 | Higher strength and faster hardening upon water contact |
§Lime in practice
Historically, lime mortar for ceilings was often reinforced with animal hair, which was mixed into the mass to give it cohesion and counter cracking — a technique you can still encounter when examining older stuck ceilings during restoration. Today there are suppliers who produce both traditional air lime and NHL products specifically targeted at restoration tasks, and you should always check the product's data sheet for classification according to DS/EN 459-1 before ordering.
“The right choice of lime is not the strongest — it's the one that fits best what you're working on.”