Hydraulic vs Non-Hydraulic Lime Mortar: What’s the Difference and Which Should You Use?

When it comes to restoring traditional buildings or working with natural stone, choosing the right mortar is essential. At Salthouse Stonemasonry, we often get asked about the difference between hydraulic and non-hydraulic lime mortar—and which is best for your project.

This blog breaks down the key differences, pros and cons, and where each type of lime mortar shines. Whether you’re a conservation specialist or a homeowner working on a listed building, understanding the lime mortar types available will help protect your property for generations to come.

What is Lime Mortar?

Lime mortar is a traditional building material used for thousands of years, made from a mix of lime, sand and water. It’s breathable, flexible, and ideal for buildings that need to manage moisture and movement—especially older or historic ones.

Lime mortars come in two main types:

• Hydraulic lime mortar

• Non-hydraulic lime mortar (also known as lime putty mortar)

Each behaves very differently depending on how it sets and performs over time.

What is Hydraulic Lime Mortar?

Hydraulic lime sets through a chemical reaction with water (hydraulic set), much like cement. It comes in different strengths: NHL 2, NHL 3.5, and NHL 5 (Natural Hydraulic Lime), with NHL 2 being the softest and NHL 5 the strongest.

Pros:

• Sets faster than non-hydraulic lime, even in damp or cold conditions

• Offers more strength—ideal for exposed or structural areas

• Available in a range of grades for different applications

• Can be used in wetter climates or seasons

Cons:

• Less breathable than non-hydraulic lime

• Less flexible—can be too rigid for softer historic materials

• Once mixed, has a limited working time

Common Uses:

• Chimneys, garden walls, retaining walls

• Exposed facades, especially in wetter areas

• Structural repairs where added strength is essential

What is Non-Hydraulic Lime Mortar (Lime Putty and Hot Lime)?

Non-hydraulic lime, or lime putty, sets by absorbing carbon dioxide from the air—a process called carbonation. It remains workable for longer and sets more slowly, which can be ideal for certain traditional applications.

Pros:

• Extremely breathable—ideal for solid walls without a cavity

• Softer and more flexible, reducing risk of cracking

• Remains workable for longer—good for intricate work

• Reversible and sympathetic to historic fabric

Cons:

• Slower setting time, especially in damp or cold conditions

• Not suitable for exposed or wet areas

• Requires more experience to apply well

Common Uses:

• Internal plastering and pointing

• Heritage conservation projects

• Softer masonry or stone, especially in protected environments

Which Lime Mortar Should You Use?

The right choice depends on your project’s specific needs:

• For listed buildings, soft stone, or internal repairs—non-hydraulic lime mortar is usually best.

• For external work, exposed locations, or structural elements—hydraulic lime mortar offers added durability.

Still unsure? At Salthouse Stonemasonry, we can advise on the best mortar type for your property, ensuring longevity, breathability, and compatibility with your building’s materials.

Final Thoughts

Choosing between hydraulic vs non-hydraulic lime mortar isn’t just about strength or setting time—it’s about what’s best for your building. Using the wrong type can lead to trapped moisture, cracked masonry, or long-term damage.

If you’re working on a restoration project or want expert advice on traditional building materials, get in touch with us today. We’re passionate about preserving historic structures the right way—with the right materials and skills.

Need help with your restoration project?

Contact us at Salthouse Stonemasonry—we’re always happy to chat lime, stone, and heritage!

email : info@salthousestonemasonry.co.uk

Telephone: 07428584294