Mortar Mixing

Part of Lime & Cement

Mortar is the glue that holds masonry together — the difference between a pile of stones and a permanent structure. Getting the mix right determines whether your walls stand for generations or crumble within years.

What Mortar Does

Mortar fills the gaps between masonry units (stone, brick, or block), distributing loads evenly and preventing water from penetrating the wall. Good mortar does three things:

1. Bonds — adheres to the masonry surfaces through chemical and mechanical grip
2. Cushions — accommodates minor movement from thermal expansion, settling, and loading without cracking
3. Seals — prevents wind and water penetration through the wall

Contrary to instinct, mortar should generally be softer than the masonry units it joins. Hard mortar forces stress onto the stones, which crack. Soft mortar absorbs stress and can be repointed (replaced) without damaging the masonry.

Lime Mortar Basics

The Simple Formula

Lime mortar consists of just three ingredients:

Lime putty + Sand + Water

The standard ratio by volume is:

Lime : Sand	Use Case
1 : 2	Rich mortar — for pointing, thin joints, interior work
1 : 2.5	General purpose — standard for most masonry
1 : 3	Lean mortar — for thick joints, rough stone walls
1 : 4	Very lean — for rubble fill and backing

When in doubt, use 1:3. A slightly lean mix is more forgiving than a rich one — excess lime causes shrinkage cracking during curing.

Sand Selection

Sand quality has more impact on mortar performance than most people realize.

Good mortar sand should be:

• Sharp (angular grains, not rounded) — angular grains interlock and bond better
• Well-graded — a mix of coarse (2 mm), medium (1 mm), and fine (0.25 mm) particles. This fills voids and produces a dense mortar
• Clean — free of clay, silt, organic matter, and salt
• Local — for aesthetic reasons, local sand gives mortar a color that harmonizes with local stone

Sand washing test: Fill a jar one-third with sand, add water, shake vigorously, and let settle. Sand settles in seconds; clay and silt remain suspended as a cloudy layer on top. If this cloudy layer exceeds 5% of the total volume, wash the sand before use.

Sand Type	Source	Suitability
Pit sand	Quarries, hillside deposits	Excellent — naturally angular
River sand	Stream beds	Good — somewhat rounded
Beach sand	Coastal	Poor — rounded, salt-contaminated
Crusite	Ground-up brick or pottery	Excellent additive (pozzolanic)
Desert sand	Dunes	Poor — too fine and uniform

Never use beach sand without thorough freshwater washing. Salt in mortar absorbs moisture, causes efflorescence (white surface deposits), and accelerates deterioration. Even after washing, beach sand is inferior to pit sand.

Mixing Procedure

Using Lime Putty

1. Measure sand into a mixing area (a flat board, stone slab, or shallow wooden trough)
2. Form a crater in the center of the sand pile
3. Add lime putty to the crater
4. Fold sand over the putty from the edges using a shovel or hoe
5. Chop and fold repeatedly — this is called “beating” the mortar
6. Add small amounts of water as needed to reach the desired consistency
7. Continue beating for at least 10-15 minutes — thorough mixing is critical

Using Hydrated Lime Powder

1. Mix dry hydrated lime powder and sand together first
2. Form a crater and add water gradually
3. Mix thoroughly to eliminate dry pockets
4. Let the mixture “soak” for 10-15 minutes (allows lime to hydrate fully)
5. Remix and adjust consistency

Consistency Guide

The correct consistency depends on the application:

Application	Consistency	Test
Bedding mortar (laying courses)	Stiff but workable	Holds shape on trowel, slides off with a flick
Pointing mortar (filling joints)	Slightly stiffer	Holds shape when pressed into joint
Plastering (base coat)	Creamy	Sticks to vertical surface without slumping
Plastering (finish coat)	Smooth cream	Spreads easily with float, no lumps
Limewash	Thin liquid	Like heavy cream, brushable

Mortar should never be so wet that it flows like soup — this is the single most common mixing error. Excess water weakens the cured mortar by creating voids as the water evaporates. Add water sparingly and err on the dry side.

Hydraulic Lime Mortar

Standard lime mortar (non-hydraulic) sets by absorbing CO2 from the air — a slow process called carbonation. This means it does not set in wet conditions or underwater. Hydraulic lime sets by reacting with water, making it suitable for foundations, bridges, cisterns, and damp environments.

Natural Hydraulic Lime (NHL)

Some limestones contain clay minerals. When burned and slaked, the silica and alumina from the clay react with the lime to form compounds that set hydraulically.

Classification:

Grade	Strength	Set Time	Use
NHL 2 (feebly hydraulic)	Low	Slow (weeks)	Interior walls, soft stone
NHL 3.5 (moderately hydraulic)	Medium	Days to weeks	General exterior work
NHL 5 (eminently hydraulic)	High	Days	Foundations, water contact

Making Hydraulic Lime

If your limestone naturally contains 10-20% clay, simply burning and slaking it produces natural hydraulic lime. Test by checking how quickly a slaked lime mortar hardens — if it begins to set within days rather than weeks, you have a hydraulic lime.

Pozzolanic Additives

If your limestone is pure (non-hydraulic), you can add pozzolanic materials to create a hydraulic mortar:

• Crushed brick or pottery (crushed fine) — the historical Roman additive
• Volcanic ash — if available, the original pozzolan (named after Pozzuoli, Italy)
• Burnt clay — fire ordinary clay at high temperature, then grind fine
• Wood ash — mild pozzolanic effect

Pozzolanic mortar recipe:

• 1 part lime putty
• 1 part pozzolan (crushed brick or volcanic ash)
• 2 parts sand

Roman concrete — one of the most durable building materials ever created — was simply lime mortar with volcanic ash pozzolan and stone aggregate. Structures built with this material have survived 2,000+ years, some still intact underwater. The recipe is straightforward and reproducible.

Fiber Reinforcement

Adding fibers to mortar and plaster reduces cracking and improves toughness.

Suitable Fibers

Fiber	Source	Length	Best For
Animal hair	Goat, horse, cow	2-5 cm	Plaster coats
Plant fiber	Hemp, sisal, straw	3-8 cm	Structural mortar
Chopped straw	Any cereal crop	2-4 cm	Base plaster coats

Add fiber at roughly 1-2% by volume of the total mix. Mix in during the final beating stage to distribute evenly.

Working With Lime Mortar

Application Rules

1. Dampen masonry before applying — dry stone sucks water from the mortar too quickly, preventing proper setting. Spritz with water until the surface is damp but not dripping.
2. Work in layers — for thick mortar beds or plaster, apply in layers of 10-15 mm maximum. Let each layer stiffen before adding the next.
3. Press firmly — mortar bonds better when pressed into contact with the masonry. Use a pointing trowel or your thumb.
4. Protect from rain — fresh lime mortar washed by rain before it sets will fail. Cover new work with boards, sacking, or tarps.
5. Protect from frost — lime mortar that freezes before setting is destroyed. Do not lay mortar if temperatures will drop below 5°C within 48 hours.
6. Mist-cure — in hot or dry weather, spray completed work with a fine mist 2-3 times daily for the first week. This slows drying and improves carbonation.

Setting and Curing

Lime mortar sets in two phases:

1. Initial set (hours to days) — water evaporation makes the mortar rigid
2. Carbonation (weeks to years) — Ca(OH)2 absorbs CO2 from air and converts back to CaCO3, gaining full strength

Non-hydraulic lime mortar reaches roughly:

• 50% strength at 1 month
• 75% strength at 3 months
• Full strength at 1-2 years

This slow curing is actually an advantage — the mortar remains flexible longer, accommodating settlement in new structures.

Common Mistakes

1. Mortar stronger than masonry — hard mortar transfers stress to the stone, causing stone faces to crack and spall. Mortar should always be the sacrificial element.
2. Too much water in the mix — creates a weak, porous mortar full of voids. Mix stiff and add water only as needed.
3. Using unwashed sand — clay in sand prevents lime from bonding to sand grains, drastically weakening the mortar.
4. Not dampening masonry — dry stone sucks water from mortar, causing it to set too fast with poor adhesion. Always wet the wall.
5. Working in frost or heavy rain — either condition ruins fresh mortar. Check the weather forecast (or your own sky-reading skills) before starting masonry work.

Summary

Mortar Mixing — At a Glance

• Standard lime mortar: 1 part lime putty to 2.5-3 parts sharp, well-graded sand
• Sand quality matters enormously — use angular, clean, well-graded pit or river sand
• Beat the mortar thoroughly for 10-15 minutes to ensure complete mixing
• Non-hydraulic lime sets by carbonation (weeks-years); hydraulic lime sets by water reaction (days)
• Add pozzolans (crushed brick, volcanic ash) to make non-hydraulic lime set in wet conditions
• Mortar should be softer than the masonry it joins — always
• Dampen stone before applying mortar, protect from rain and frost, mist-cure in hot weather
• Full strength develops over 1-2 years — slow curing is a feature, not a bug