Lime mortar is an essential material in construction and the application of finishes. Its versatility, breathability, and strength make it an ideal choice for a variety of projects, from traditional masonry to stone repointing for themed applications. If you're a professional looking to master the technique of preparing lime mortar, this guide will walk you through it step by step: exact proportions, tips to avoid mistakes, and its specific application for repointing.
Preparation of lime mortar
1. Add water to the cement mixer
Begin by pouring the required amount of water into the cement mixer. The precise amount will depend on the type of lime and sand used, but a water-to-dry ratio of 40-50% of the total amount of dry materials is generally recommended.
2. Add the sand
Gradually add 3 parts sand for every 1 part NHL 3,5 hydraulic lime. The particle size of the sand also influences the final texture of the mortar, so it is important to choose the right one for each project.
3. Mix the hydraulic lime
Add the NHL 3,5 hydraulic lime to the water and sand mixture. Make sure to mix well until you obtain a smooth, lump-free texture.
Tips for a perfect lime mortar
- Use quality materials: The quality of the lime and sand will determine the strength, flexibility, and workability of the mortar. Opt for products from recognized brands with quality certification.
- Respect the proportions: The ideal ratio of water, sand, and lime is crucial for obtaining mortar with the desired properties. Adjust the quantities according to the manufacturer's recommendations or your experience.
- Mix correctly: Proper mixing is key to avoiding lumps and ensuring the mortar is homogeneous. Use a concrete mixer or an electric mixer for optimal results.
- Let the mixture rest: After mixing, let the mortar rest for a few minutes so that the particles hydrate and acquire the appropriate plasticity.
Lime mortar for jointing stone in theming
One of the most popular applications of lime mortar is stone repointing . Compared to pure cement mortar, lime provides greater breathability and flexibility to the joint, reducing the risk of cracking due to thermal movement in natural or artificial stone facades. To further enhance this flexibility and adhesion, EST Gel Cal is often added to the mix— see the complete recipe with proportions for making repointing mortar for stone and brick.
If your grouting project requires applying the mixture to narrow or hard-to-reach joints, the piping bag technique makes the job much easier compared to the traditional trowel —see more on mortar for grouting stone with a piping bag—.
Lime mortar vs cement mortar: which one to choose
The choice between lime mortar and cement mortar is common, and the answer depends on the substrate and its intended use. Cement mortar offers greater mechanical strength and sets faster, making it preferable for structural elements or surfaces highly exposed to wear. Lime mortar, on the other hand, is more breathable and flexible, making it more suitable for older stone or brickwork, where overly rigid cement mortar can cause cracking by failing to accommodate the substrate's minor movements due to changes in temperature and humidity.
In theming, the choice also depends on the technique: for coatings with a molded thixotropic mortar texture, such as those worked with silicone molds, ESTonetex TXT thixotropic mortar is still used —see how to mix ESTonetex TXT thixotropic mortar— , while lime mortar is specifically reserved for joints and applications that require breathability, such as the repointing of natural stone.
Curing and protection after application
Lime mortar sets more slowly than cement mortar, as its hardening depends in part on carbonation with CO2 from the air. For the first few days, it's best to keep the surface slightly damp, protecting it from direct sunlight and strong winds, which would dry the surface too quickly and prevent even, deep curing. Outdoors, avoid applying lime mortar if frost is expected within the next 48-72 hours, as the water in the unset mix can freeze and damage the mortar's internal structure.
Other applications of lime mortar
- Brickwork: Lime mortar is used to join bricks, stones, and other building materials.
- Plastering and rendering: It is ideal for creating smooth and decorative surfaces on walls and ceilings.
- Restoration: It is an ideal material for the restoration of historic buildings and monuments, due to its compatibility with old factories.
- Theming: In addition to grouting, it is used in finishes that require breathability, such as coatings on substrates that must allow moisture to escape.
Common mistakes when preparing lime mortar
- Not respecting the 3:1 ratio of sand to limeToo much lime makes the mortar more fragile; too much sand reduces its adhesion and breathability.
- Add all the water at once.This makes it difficult to control the final consistency. It is preferable to add the water gradually while mixing.
- Do not let the mixture sit.Applying the mortar immediately after mixing it, without allowing the particles to hydrate, results in a worse plasticity than the mortar can offer.
- Use non-hydraulic lime for exteriorsFor applications exposed to the elements, NHL 3,5 hydraulic lime offers better resistance than an air lime without a hydraulic component.
- Do not add EST Gel Cal to grout jointsIn joints exposed to thermal movement, omitting this flexibility additive increases the risk of microcracks over time.
Tools needed to prepare lime mortar
- Concrete mixer or electric mixer: essential to achieve a homogeneous mixture without lumps, especially in large quantities.
- dosing bucket: to accurately measure the proportion of sand, lime and water in each batch, avoiding variations between batches.
- Trowel and float: for application on surfaces or in jointing using traditional techniques.
- Pastry bag or grout gun: an alternative to the trowel when working in narrow stone joints, with greater precision and less material waste.
- Gloves and protective glassesLime is an alkaline material that can irritate the skin and eyes during handling when fresh.
FAQ
What is the correct proportion of lime mortar?
The usual ratio is 3 parts sand to 1 part NHL 3,5 hydraulic lime, with 40-50% water on the total dry materials.
Is lime mortar suitable for jointing stone?
Yes, and it is one of its most common applications in theming: lime provides breathability and flexibility to the joint, reducing the risk of cracking compared to a pure cement mortar.
What is the difference between hydraulic lime and air lime?
Hydraulic lime (such as NHL 3,5) also sets in the presence of moisture and offers better resistance outdoors, while air lime depends on contact with CO2 in the air to set and is less suitable for applications exposed to the elements.
How long should the mixture be left to rest?
A few minutes after mixing are enough for the particles to hydrate and for the mortar to acquire the appropriate plasticity before applying it.
What additive is recommended for lime-based grouting joints?
EST Gel Cal, which provides flexibility and additional adhesion to the mixture, reducing the risk of microcracks in joints subjected to thermal movement.
Can lime mortar be used for the restoration of old buildings?
Yes, it is one of the most suitable materials for restoration due to its compatibility with historical structures and its breathability, similar to the original mortars of the time.
What happens if the proportion of sand and lime is not respected?
An excess of lime makes the mortar more brittle, while an excess of sand reduces its adhesion and breathability, affecting the durability of the finish.