Lime Processing

Part of Lime & Cement

Quicklime fresh from the kiln is dangerously reactive and difficult to use directly. Processing it into slaked lime and lime putty transforms it into the workable materials that build walls, purify water, and enable dozens of essential chemical processes.

From Quicklime to Usable Forms

Quicklime (calcium oxide, CaO) is the raw product of limestone calcination. It is caustic, generates extreme heat on contact with water, and crumbles unpredictably. Before it can be used in mortar, plaster, or any application, it must be “slaked” — reacted with water in a controlled manner.

The slaking reaction:

CaO + H2O → Ca(OH)2 + heat

This reaction is highly exothermic, generating temperatures up to 300°C at the reaction surface. One kilogram of quicklime releases enough heat to boil nearly half a liter of water. This is both the danger and the utility of quicklime.

Slaking Methods

Dry Slaking (Sprinkling)

The traditional method for producing hydrated lime powder.

1. Spread quicklime lumps on a clean, dry surface (stone slab or packed earth)
2. Sprinkle water gradually — roughly 30-35% of the quicklime weight (300-350 ml per kg)
3. The lime heats, steams, cracks, and expands
4. Continue sprinkling until all lumps have broken down to a fine white powder
5. Screen through a mesh to remove overburned or unslaked pieces

Result: Dry hydrated lime powder — Ca(OH)2. Good for agricultural liming, water treatment, and quick mortar mixing.

Stand upwind during slaking. The reaction produces intense steam mixed with caustic lime particles. Wear eye protection and cover your mouth and nose. A single particle of quicklime in the eye causes severe burns.

Wet Slaking (Drowning)

Produces lime putty — the superior form for high-quality mortar and plaster.

1. Fill a container (wooden barrel, stone trough, or lined pit) one-third full with water
2. Add quicklime lumps gradually — roughly equal weight to the water
3. Stir continuously with a wooden paddle or hoe
4. The mixture boils violently — add more water as needed to keep the lime submerged
5. Continue stirring until all lumps have dissolved into a smooth, cream-like paste
6. Allow to settle, then screen through a fine mesh (1-2 mm) to remove grit
7. Store the resulting putty under a thin layer of water in a sealed container

Result: Lime putty — a smooth, plastic paste of Ca(OH)2 suspended in water. The finest material for plastering, pointing, and limewash.

Slaking Pit (Large Scale)

For settlement-scale production:

1. Dig a pit roughly 2 m x 2 m x 1 m deep, lined with clay or planks
2. Fill half with water
3. Dump quicklime into the water from wheelbarrows (stand clear of the splash zone)
4. Stir with long-handled tools
5. Allow to settle for several days
6. Drain excess water
7. Transfer putty to storage pits or barrels

Large-scale slaking is best done by two or more people — one adding lime, one stirring, and one standing by with additional water. The reaction is vigorous and unpredictable. Never lean over a slaking pit.

Lime Putty Aging

Lime putty improves dramatically with age. Fresh putty works adequately, but aged putty (matured for months or years) produces noticeably better mortar and plaster.

What Happens During Aging

• Particle size decreases — individual Ca(OH)2 crystals become finer and more uniform
• Plasticity increases — the putty becomes creamier and easier to work
• Workability improves — mortar made from aged putty spreads more smoothly and bonds better to stone
• Water retention improves — aged putty holds water longer, giving the mason more working time

Aging Protocol

Age	Quality	Best Use
Fresh (0-1 week)	Adequate	Emergency repairs, rough work
1-3 months	Good	Standard mortar, garden walls
3-12 months	Very good	Fine masonry, interior plaster
1-3 years	Excellent	Heritage-grade work, finish plaster
3+ years	Superior	Best possible mortar and plaster

Storage Requirements

• Keep putty covered with at least 2 cm of standing water at all times
• Use sealed containers — wooden barrels, clay vessels, or plastic-lined pits
• Water prevents CO2 from contacting the putty and converting it back to CaCO3 (carbonation)
• Putty stored correctly is effectively immortal — Roman lime putty has been found still viable after centuries

Hydrated Lime Powder

For applications where dry material is needed, hydrated lime powder (dry-slaked lime) is more convenient than putty.

Producing Fine Powder

1. Dry-slake quicklime as described above
2. Screen through the finest mesh available — ideally 0.5 mm or finer
3. Store in airtight containers to prevent carbonation
4. Use within weeks for best reactivity

Powder vs. Putty Comparison

Property	Hydrated Lime Powder	Lime Putty
Storage life	Weeks to months (in airtight containers)	Years to decades (under water)
Mortar quality	Good	Superior
Ease of transport	Easy (dry, lightweight)	Difficult (heavy, wet)
Mixing convenience	Just add water on site	Ready to use
Plasticity	Moderate	Excellent
Cost of production	Lower (less water, simpler)	Higher (needs storage vessels)

Testing Lime Quality

Reactivity Test

Drop a small piece of quicklime into a cup of water:

• Vigorous reaction (hissing, steaming, disintegrates within minutes) — high-quality, well-burned lime
• Slow reaction (gentle warming, takes 30+ minutes) — underburned or dead-burned
• No reaction — not lime, or completely carbonated (reverted to CaCO3)

Slaking Residue Test

After slaking, check what remains on the screen:

• Less than 5% residue — excellent burn quality
• 5-15% residue — acceptable; residue is either overburned cores or impurities
• More than 15% residue — poor burn; reprocess the residue in the next kiln firing

pH Test

Properly slaked lime dissolved in water should produce a pH of 12-12.5. Test with plant pH indicators (red cabbage juice turns green-yellow in lime water). If the pH is below 11, the lime is either weak or contaminated.

Safety Protocols

Lime in all forms is a strong alkali that causes chemical burns.

Personal Protection

• Eyes — the most critical. Lime dust or splashes cause severe, potentially blinding burns. Wear goggles or at minimum tight-fitting eye shields
• Skin — lime dries and cracks skin at minimum, causes full burns with prolonged contact. Wear gloves and long sleeves
• Lungs — lime dust is a respiratory irritant. Cover mouth and nose in dusty conditions
• Feet — wear boots when working around slaking pits. Stepping in hot slaking lime causes deep burns

First Aid

• Skin contact — brush off dry lime first, then wash with large quantities of water. Do not rub.
• Eye contact — flush with clean water for at least 30 minutes continuously. This is a medical emergency.
• Ingestion — drink large quantities of water or milk. Do not induce vomiting.

Keep a large container of clean water within arm's reach whenever handling quicklime or fresh slaked lime. Reaction time matters — a lime burn worsens every second it remains in contact.

Applications of Processed Lime

Building Applications

• Lime mortar — lime putty + sand (1:2 to 1:3 ratio)
• Lime plaster — lime putty + fine sand + hair or fiber (1:2:handful)
• Limewash — lime putty thinned with water to paint consistency (antimicrobial coating)
• Lime concrete — lime + sand + aggregate

Chemical Applications

• Water purification — a small amount of slaked lime added to turbid water raises pH, kills bacteria, and causes suspended particles to settle
• Hide tanning — lime bath removes hair and swells collagen
• Causticizing lye — lime + soda ash produces sodium hydroxide
• Nixtamalization — lime water processes corn to release niacin and improve nutrition
• Soil amendment — corrects acidic soil pH (apply 1-2 tonnes per hectare)

Common Mistakes

1. Adding water too fast during dry slaking — causes violent spattering. Sprinkle gradually and stand back.
2. Not screening slaked lime — overburned particles in mortar create “lime pops” — small explosions that leave craters in plaster as they hydrate weeks later.
3. Letting lime putty dry out — exposed to air, putty carbonates on the surface and becomes unusable. Always keep under water.
4. Using fresh putty for fine plaster — it shrinks and cracks more than aged putty. Age for at least 3 months for any finish work.
5. Ignoring eye protection — lime is the most common cause of chemical eye burns in construction, historically and today. Goggles are not optional.

Summary

Lime Processing — At a Glance

• Slaking converts quicklime (CaO) to slaked lime Ca(OH)2 — an extremely exothermic reaction (up to 300°C)
• Dry slaking produces hydrated lime powder for transport and quick mixing
• Wet slaking produces lime putty — superior for mortar and plaster, improves with age
• Lime putty stored under water is effectively immortal; powder degrades in weeks without airtight storage
• Screen all slaked lime to remove overburned particles that cause “lime pops” in plaster
• Test quality by reactivity (water reaction speed), residue percentage, and pH
• Always protect eyes first — lime burns are the most common and most serious chemical eye injury
• Processed lime serves building (mortar, plaster), chemistry (lye, water treatment), and agriculture (soil pH correction)