Preparing NHL Mortar — Mixing, Dosage & Sands

Correct preparation is essential to obtain the workability, strength, vapour permeability and durability expected from a Natural Hydraulic Lime mortar. This practical field guide covers mixing, dosage and reworking, and sand selection for Saint-Astier® NHL mortars.

01 Mixing NHL mortar

The order in which materials are introduced, and the time spent mixing, directly determine the quality and workability of the finished mortar.

Small mixers

Add half of the required sand to the drum.
Add all of the lime.
Mix until the colour is uniform, approximately 2 minutes.
Add the remaining sand and mix again for 1 to 2 minutes.
Add water slowly while mixing until the required plasticity is achieved.
Allow approximately 5 to 8 minutes of total mixing time.

Larger mixers

Introduce equal parts of the required sand.
Add an equal part of lime.
Add more sand in equal parts.
Dry mix for a few minutes until homogeneous.
Continue mixing while slowly adding water.
Mix until the mortar becomes workable, generally 5 to 8 minutes.

Expert tip: NHL mortar should be closer to a dough-like texture than a slurry. Longer mixing improves workability; adding excess water reduces performance and may increase shrinkage.

Technical note — Using lime putty with NHL mortar

A small amount of lime putty may be added to improve immediate workability and reduce mixing time. Do not exceed 10% of the NHL binder weight.

Always incorporate lime putty before adding water.
Do not add lime putty late in the mix, as this may produce an excessively wet mortar.
Do not use lime putty with NHL 2.
Remember that lime putty can reduce the final mortar strength.

02 Dosage & reworking

Use standard buckets, always levelled. Mortar ratios should be agreed with the supplier or specifier according to the masonry, exposure and expected performance.

Mortar class	Strength	Recommended binder	Best suited to
Class II	2 N/mm²	NHL 2	Sensitive historic masonry and softer substrates.
Class III	3.5 N/mm²	NHL 3.5	General pointing, bedding and many traditional masonry applications.
Class IV	≥ 5 N/mm²	NHL 5	Exposed situations, denser masonry and stronger mortar requirements.

24h

Reworking: Saint-Astier® NHL mortars can be reworked for up to 24 hours after initial preparation. Keep the mortar covered overnight and add as little additional water as possible when re-mixing.

03 All about sands for NHL mortars

Sand selection is one of the most important variables in lime mortar design. The wrong sand can affect strength, vapour permeability, workability and durability.

CleanlinessSand should be free from clay and silt. Very fine particles may require wet sieve analysis, as dry sieving may not reveal clay coatings.
Well-graded structureA good mortar sand should contain several particle-size grades. Mono-granular sands reduce workability and compromise the void structure.
Vapour permeabilityThe void structure of the sand directly influences vapour permeability. Sharp, well-graded sands help maintain the breathable nature of lime mortars.
Colour and finesMortar colour is mainly determined by the fines in the sand. Avoid sands coloured by clay, as they can alter the effective binder ratio.

Important: Improve workability through longer mixing time, not by adding more water. Fine sands usually require more water and may reduce strength, increase shrinkage and encourage lime leaching.

Application	Maximum particle size	Fines content
General masonry / bedding	Approximately 1/3 of joint height	About 20% of mass
Render — stipple, base and rough coats	3 mm down to 0.075 mm	Maximum 15%
Render — smooth finishing coat	2.36 mm down to 0.075 mm	Maximum 25%
Fine / ashlar work	Fine, well-graded sand	Maximum 25%
Interior smooth plastering	Fine, well-graded sand	Maximum 25%

This guide is provided for general information only. Specifications should always be project-specific and issued by the relevant specifier. Site conditions, substrate type, exposure, sand grading and workmanship must be assessed before application.

Continue to Part 2 — Consumption, Protection & FAQ →