Mortar Mixing

Part of Brick Making

Preparing lime and mud mortars for bricklaying.

Why This Matters

Mortar is what turns a pile of bricks into a structure. It fills the gaps between bricks, distributes loads evenly across the wall, compensates for dimensional variations in hand-made bricks, and seals the wall against wind and water. Without mortar, bricks are just a dry stack that can be pushed over by hand. With the right mortar, those same bricks become a wall that stands for centuries.

The mortar must be weaker than the bricks it joins. This sounds counterintuitive, but it is critical: when a wall settles, expands, or contracts (as all walls do with temperature and moisture changes), the movement must be absorbed by the mortar joints, not by cracking the bricks. Cracked mortar can be repointed. Cracked bricks cannot be repaired.

In a rebuilding scenario, you will likely progress through several mortar types: starting with simple mud mortar for temporary shelters, advancing to lime mortar for permanent buildings, and potentially producing hydraulic lime or even crude cement for specialized applications. Each type has its place, and understanding their properties helps you choose correctly.

Mud Mortar

The simplest mortar — a mixture of clay-rich soil and water, sometimes with fibrous reinforcement.

Preparation

1. Select your soil. Good mortar clay is the same material you would use for adobe bricks — a mixture of clay and sand. Too much clay and the mortar shrinks and cracks as it dries. Too much sand and it crumbles. A ratio of roughly 30% clay to 70% sand and silt is ideal.

2. Test the soil. Take a handful of damp soil and roll it into a ball. Drop it from waist height onto a flat surface. If it shatters, it is too sandy. If it flattens like a pancake, it is too clayey. If it deforms but mostly holds its shape, the ratio is about right.

3. Mix with water. Add water gradually and mix by hand, foot, or with a hoe until the mortar reaches a thick, creamy consistency — like peanut butter. It should spread easily with a trowel but hold its shape when piled.

4. Add fiber reinforcement (optional but recommended). Chop straw, dried grass, or animal hair into 20-50mm lengths and mix in at roughly 5-10% by volume. This dramatically reduces cracking as the mortar dries.

5. Let it slake. If time allows, mix the mortar and let it sit covered for 24-48 hours before use. This allows water to fully penetrate the clay particles, improving workability and adhesion.

Properties

Property	Mud Mortar
Strength	Low — adequate for single-story, sheltered walls
Water resistance	Poor — dissolves in sustained rain
Flexibility	Good — absorbs movement well
Set time	Days to weeks (air-drying only)
Cost	Free — uses local soil
Skill required	Minimal

Best Uses

• Interior walls and partitions
• Adobe construction (mud brick with mud mortar)
• Temporary shelters
• Kiln walls where the mortar will be fired in place
• Any sheltered construction where rain does not reach the joints

Water Vulnerability

Mud mortar must be protected from rain. Walls using mud mortar need deep eaves (overhangs of at least 300mm), raised foundations to prevent splash-back, and regular re-pointing of weathered joints. Without these protections, mud mortar walls degrade rapidly in wet climates.

Lime Mortar

The standard mortar for permanent construction throughout most of human history. Lime mortar is strong, weather-resistant, slightly flexible, and self-healing (it slowly absorbs CO2 and re-hardens over time).

Making Quicklime

Lime mortar starts with limestone (calcium carbonate, CaCO3). You must first convert it to quicklime (calcium oxide, CaO) by burning it in a kiln.

1. Source limestone. It is one of the most common rocks on Earth — chalk, marble, seashells, and coral are all forms of calcium carbonate. Crush or break it into fist-sized pieces.

2. Build a lime kiln. A simple updraft kiln works — stack limestone over a fire grate, with fuel below and a chimney effect to draw heat through the stone. The kiln must reach 900-1000°C and hold that temperature for 24-48 hours.

3. Burn the limestone. Maintain a bright orange fire for at least 24 hours. The stone loses roughly 44% of its weight as CO2 gas escapes. Properly burned quicklime is white, lightweight, and crumbles easily.

4. Test for completion. Drop a small piece of burned limestone into water. If it fizzes, heats up, and breaks apart within minutes, it has been properly converted to quicklime. If it just sits there, it needs more burning.

Quicklime Safety

Quicklime is extremely caustic. It reacts violently with water, generating intense heat (enough to boil water and cause severe burns). Always add quicklime to water, never water to quicklime. Wear eye protection and keep skin covered. Store quicklime dry and away from moisture.

Slaking Quicklime

Slaking converts quicklime to hydrated lime (calcium hydroxide, Ca(OH)2), which is the actual binder in lime mortar.

1. Pit slaking (traditional method). Dig a pit or use a large container. Fill it half-full with water. Slowly add quicklime in small amounts, stirring constantly. The mixture will boil and steam violently. Keep adding quicklime until the reaction subsides and you have a thick, creamy paste — this is “lime putty.”

2. Allow the putty to mature. Cover the lime putty with a thin layer of water to prevent it from drying out and leave it for at least 2 weeks, preferably 2-3 months. Longer maturation produces smoother, more workable mortar. Historically, lime putty was stored for years before use.

3. Alternative: dry slaking. Spread quicklime on a flat surface and spray lightly with water. It will crack and crumble into a dry white powder — hydrated lime. This is faster but produces a less workable mortar than aged putty.

Mixing Lime Mortar

The standard ratio is 1 part lime putty to 2-3 parts sand by volume.

1. Select your sand. Sharp, angular sand (crushed rock, not river-rounded) gives the strongest mortar because angular grains interlock better. Avoid beach sand — salt contamination causes efflorescence (white staining). The sand should be clean and free of organic matter.

2. Measure and combine. For general bricklaying, use 1:3 (lime:sand) for a workable, moderately strong mortar. For load-bearing or exposed walls, use 1:2.5 for higher strength. For pointing or plastering, 1:2 gives a smooth, dense finish.

3. Mix thoroughly. Combine lime putty and sand on a clean surface or in a mixing trough. Turn the pile with a shovel, chop through it with a hoe, and turn again. The mortar should be uniform in color with no streaks of pure lime or clumps of sand.

4. Adjust consistency. Add water sparingly if the mortar is too stiff. It should hold a shape when troweled but spread easily under the weight of a brick. Too wet and it squishes out of joints; too dry and it will not bond to the brick surface.

Mix Ratio (Lime:Sand)	Strength	Best Use
1:2	High	Pointing, renders, chimney tops
1:2.5	Medium-High	Load-bearing walls, foundations
1:3	Medium	General bricklaying, most walls
1:3.5	Low-Medium	Interior walls, lightweight partitions

Properties

Property	Lime Mortar
Strength	Moderate — appropriate for most construction
Water resistance	Good when carbonated; slow initial cure
Flexibility	Excellent — accommodates movement without cracking
Set time	Initial set: 1-3 days. Full carbonation: months to years
Self-healing	Yes — absorbs CO2 and re-hardens over time
Breathability	High — allows moisture to pass through, preventing damp

Carbonation and Curing

Lime mortar does not “dry” — it carbonates. The calcium hydroxide reacts with CO2 from the air to form calcium carbonate (basically turning back into limestone). This process takes months to years, depending on joint thickness, humidity, and CO2 exposure.

• Keep freshly laid brickwork damp for the first week to prevent the mortar from drying too fast and cracking before it can carbonate
• Protect from freezing for at least 4 weeks — frost destroys uncarbonated lime mortar
• The wall will gain strength continuously for years as carbonation progresses

Hydraulic Lime Mortar

Hydraulic lime sets by reacting with water (like cement) in addition to carbonating. It is stronger and more water-resistant than non-hydraulic lime, and it sets faster — even underwater.

Sources of Hydraulic Lime

Hydraulic lime comes from burning impure limestone that contains clay minerals (silica and alumina). The clay content causes the lime to develop hydraulic properties:

• Feebly hydraulic: Limestone with 5-12% clay content. Mild water resistance.
• Moderately hydraulic: 12-18% clay. Good water resistance, faster set.
• Eminently hydraulic: 18-25% clay. Sets hard even underwater. Approaches cement.

If your local limestone is impure (gray rather than white, feels gritty), it may naturally produce hydraulic lime when burned.

Alternative: Pozzolanic Additives

You can convert ordinary lime mortar into hydraulic mortar by adding pozzolanic materials — substances rich in reactive silica that combine with lime in the presence of water:

Pozzolan	Source	Mixing Ratio
Crushed fired brick (brick dust)	Your own rejected bricks	Replace 1/3 of the sand with fine brick dust
Volcanic ash / tuff	Volcanic regions	Replace 1/3 to 1/2 of sand
Calcined clay (metakaolin)	Fire clay to 600-800°C, then crush	Replace 1/4 of sand
Wood ash (high-silica)	Burning rice husks, bamboo, straw	Replace 1/5 of sand

The most practical pozzolan for a rebuilding community is crushed brick dust. You already have a supply of broken and rejected bricks from your kiln. Grind them to a fine powder (the finer the better — aim for flour-like consistency), and substitute it for part of the sand in your lime mortar mix.

Roman Concrete

The Romans built structures that have lasted 2,000 years using lime mortar with volcanic ash (pozzolana) as the pozzolan. You can achieve similar results with brick dust. A mix of 1 part lime putty, 1 part brick dust, and 2 parts sand produces a mortar that sets hard, resists water, and gains strength over decades.

Mixing Tips and Troubleshooting

Common Problems

Problem	Cause	Solution
Mortar cracks as it dries	Too much clay (mud mortar) or too much lime	Add more sand; add fiber reinforcement
Mortar crumbles and falls out	Too much sand, too little binder	Increase lime or clay proportion
Mortar does not stick to bricks	Bricks too dry — absorbing water from mortar	Dampen bricks before laying (dip in water briefly)
Mortar squishes out of joints	Mix too wet	Add sand to stiffen; let mortar rest and stiffen
White stains on wall (efflorescence)	Salt in sand or bricks	Use clean sand; rinse bricks if salt-contaminated
Mortar freezes before setting	Laid in cold weather	Do not lay mortar below 5°C; protect with tarps/straw

Batch Sizing

Mix only as much mortar as you can use in 2-3 hours. Lime mortar begins to stiffen as it starts to carbonate, and adding water to re-temper it weakens the final product. Mud mortar is more forgiving and can be re-wetted, but even so, mix in manageable batches to maintain consistent quality.

Mortar Board

Always mix and hold mortar on a clean, flat surface — a board, a slab of stone, or a sheet of metal. Never mix on bare ground where dirt, pebbles, and organic matter can contaminate the mortar. A dedicated mortar board also makes it easy to re-chop the mortar (turn it and cut through it repeatedly) to maintain workability.

Mortar Joint Types

The shape of the finished joint affects water shedding, appearance, and mortar durability:

Joint Profile	Description	Water Resistance	Best Use
Concave (bucket handle)	Curved inward, smoothed with a rod	Excellent	Exterior walls — the standard
Weathered (struck)	Angled to shed water downward	Very good	Exposed walls in wet climates
Flush	Cut flat with the wall face	Good	Interior walls, walls to be plastered
Raked	Recessed inward from the face	Poor	Decorative only, sheltered walls
V-joint	Pressed to a V-shape	Good	Decorative, hides slight irregularities

For any exterior wall, always use concave or weathered joints. Raked joints look attractive but trap water and accelerate mortar deterioration.