Slaking

Part of Lime & Cement

Safely converting quicklime to slaked lime (calcium hydroxide) through controlled hydration.

Why This Matters

Quicklime straight from the kiln is one of the most reactive and dangerous substances you can produce with pre-industrial technology. It reacts violently with water, generating temperatures that can exceed 300°C, and the resulting calcium hydroxide is powerfully caustic. Slaking — the controlled addition of water to quicklime — is the critical transformation step that converts this hazardous raw material into the workable lime putty or dry hydrate used for mortar, plaster, and cement.

Get slaking wrong and you face two kinds of failure. The obvious one is physical danger: steam explosions, caustic burns, and fires from the intense heat. The less obvious failure is chemical: incomplete slaking produces mortar containing unreacted quicklime chunks that expand destructively when they eventually absorb moisture from rain or groundwater, cracking walls and popping out plaster.

Every civilization that built with lime mastered slaking. The techniques have been refined over thousands of years, and the principles are straightforward — but they demand respect, preparation, and protective equipment.

The Chemistry of Slaking

The slaking reaction is a hydration reaction:

CaO + H₂O → Ca(OH)₂ + 65 kJ/mol

This is strongly exothermic — each kilogram of quicklime releases enough heat to raise the temperature of 1 liter of water by approximately 280°C. In practice, the water boils.

What Happens Step by Step

1. Water contacts quicklime — Liquid water is absorbed into the porous structure of the calcined limestone
2. Exothermic reaction begins — Temperature rises rapidly, often within seconds
3. Steam generation — Excess water boils violently, producing dense clouds of caustic steam
4. Structural breakdown — The quicklime lumps crack and crumble as calcium hydroxide crystals grow inside them, expanding the material
5. Volume increase — Slaked lime occupies roughly 2-3 times the volume of the original quicklime
6. Cooling — As the reaction completes and excess water absorbs remaining heat, the mixture cools to a thick paste (lime putty) or dry powder (dry hydrate)

Factors Affecting Slaking

Factor	Effect
Quicklime quality	Well-burned, porous lime slakes fast and completely. Over-burned dense lime slakes slowly and may leave un-reacted cores.
Water temperature	Warm water accelerates the reaction. Cold water slows it but is safer.
Water quantity	Excess water produces putty. Limited water produces dry hydrate.
Particle size	Smaller pieces slake faster and more completely.
Age of quicklime	Fresh quicklime is most reactive. Old quicklime that has partially air-slaked is less vigorous but may contain carbonated (dead) material.

Safety Precautions

Slaking Is Dangerous

This is not optional safety theater. Lime burns are agonizing and slow to heal. Steam from slaking can blind you permanently. Take every precaution listed below.

Required Protective Equipment

• Eye protection — Sealed goggles, not open safety glasses. Caustic splash and steam must not reach your eyes.
• Heavy gloves — Leather gauntlets extending past the wrist. Rubber gloves melt from the heat.
• Long boots — Calf-height minimum. Splashed lime paste burns through cloth and skin.
• Full-coverage clothing — Long sleeves, collar up, trousers tucked into boots. No exposed skin.
• Face covering — Cloth mask or scarf over nose and mouth. Lime dust and steam irritate lungs severely.

Site Setup

1. Work outdoors or in a very well-ventilated area — never in enclosed spaces
2. Stand upwind — steam and spatters blow downwind
3. Clear the area — no children, animals, or bystanders within 5 meters
4. Keep clean water nearby — for immediately flushing any lime contact with skin or eyes
5. Keep vinegar available — neutralizes lime burns on skin (flush with water first, then apply vinegar)
6. Have a long-handled tool ready — wooden rake, hoe, or paddle for stirring from a safe distance
7. No metal containers — quicklime reacts with aluminum. Use wooden, stone, or steel vessels.

Slaking Methods

Method 1: Wet Slaking (Producing Lime Putty)

This is the traditional method for producing high-quality lime putty for mortar and plaster.

Equipment needed:

• Slaking pit or large wooden trough (minimum 200 liters capacity)
• Storage pit or containers for the finished putty
• Screen or sieve (6-10mm mesh)
• Long-handled hoe or rake
• Water supply (buckets or hose)

Procedure:

1. Fill the vessel half-full with water — Use clean water. The volume of water should be at least 3 times the volume of quicklime you plan to add.

2. Add quicklime gradually — Using a shovel or by hand (gloved), add quicklime in small batches — no more than 2-3 shovelfuls at a time. Allow each addition to begin reacting before adding more.

3. Stir continuously — Use the long-handled tool to agitate the mixture. This ensures all surfaces of the quicklime contact water and prevents lumps from forming an insulating crust.

4. Monitor the reaction — The mixture will boil violently, producing clouds of steam. Stand back during peak reaction. The temperature may exceed 100°C.

5. Continue adding quicklime — Add more as the reaction subsides from each batch. Total additions should bring the mixture to a thick, cream-like consistency.

6. Stir until reaction completes — The boiling will subside. Continue stirring for 15-20 minutes after the last visible reaction to ensure completeness.

7. Screen the slurry — While still liquid enough to pour, pass the mixture through a screen into the storage pit. This removes un-burned limestone, over-burned cores, and any foreign material.

8. Cover with water — Maintain at least 50mm of water over the putty surface at all times. This prevents carbonation (hardening) and begins the maturation process.

Method 2: Dry Slaking (Producing Hydrated Lime Powder)

Produces a dry powder rather than putty. Useful when storage space is limited or when you need to transport lime.

Procedure:

1. Spread quicklime in a thin layer (100-150mm) on a clean, hard surface — stone pavement, wooden platform, or metal sheet.

2. Sprinkle water — Add water gradually using a watering can or by flicking with a brush. Add approximately 30-35% of the quicklime’s weight in water. This is just enough for the hydration reaction without producing a paste.

3. Watch for reaction — The lumps will begin to crack, steam, and crumble. Add more water to any pieces that don’t react.

4. Turn the pile — Use a rake or hoe to turn the material, exposing all surfaces to water.

5. Continue until reaction completes — All lumps should break down to a fine, dry powder. If large cores remain, they are over-burned and should be removed.

6. Sieve — Pass through a fine mesh to produce uniform powder and remove any un-slaked material.

7. Store dry — Keep in sealed bags, barrels, or bins away from moisture. Dry hydrate will slowly carbonate if exposed to air, reducing its reactivity over weeks to months.

Choosing Between Methods

Wet slaking produces superior lime for mortar and plaster — the putty is smoother, more workable, and improves with age. Dry slaking is faster and the product is easier to store and transport. For construction, wet-slaked putty is almost always preferred.

Method 3: Hot Lime Mixing (In Situ Slaking)

Quicklime is added directly to damp sand, slaking in place within the mortar mix.

1. Measure damp sand onto a mixing surface — about 3 parts sand by volume
2. Add quicklime — 1 part by volume, broken to walnut-size pieces
3. Add water gradually — the quicklime will react within the sand pile
4. Mix vigorously as the reaction proceeds — use a hoe to chop and fold
5. The heat drives lime into sand particles — creating an intimate bond
6. Allow to cool before use, or use warm for cold-weather work

This method produces exceptionally strong mortar and was widely used historically. However, it requires more skill and carries higher burn risk.

Identifying Complete Slaking

Incomplete slaking is a serious problem. Un-slaked quicklime particles in mortar will eventually absorb moisture and expand, causing “lime pops” — cone-shaped defects that blow out of plaster and mortar joints.

Signs of Complete Slaking

Indicator	Complete	Incomplete
Texture (putty)	Smooth, no grit when rubbed between fingers	Grainy, gritty, with hard particles
Temperature	Cooled to ambient	Still warm hours after mixing
Lumps	None after screening	Hard cores remaining
Volume	Expanded 2-3x from original quicklime	Less expansion suggests unreacted material
Reactivity test	No reaction when a sample is placed in water	Fizzing or heating when a sample contacts water

Dealing with Over-Burned Quicklime

Over-burned lime (sintered at too-high temperature) has a glassy surface that resists water penetration. It slakes extremely slowly — sometimes taking weeks or months.

Solutions:

• Extended soaking — Submerge over-burned pieces in water for weeks. They will eventually slake but much more slowly.
• Crush before slaking — Breaking over-burned pieces to smaller sizes exposes more surface area.
• Hot water — Using near-boiling water accelerates the reaction on resistant material.
• Remove and discard — Screen out large, un-slaked cores. Using them risks lime pops in finished work.

Timing and Logistics

Freshness Matters

Quicklime begins absorbing moisture and CO₂ from the air immediately after leaving the kiln. Its reactivity decreases over time:

Age	Condition	Action
0-48 hours	Maximum reactivity, most dangerous	Slake as soon as safely cool enough to handle
2-7 days	Still highly reactive	Slake promptly — excellent results
1-4 weeks	Moderately reactive, some air-slaking	Still usable — remove any powdery surface material
1-3 months	Significantly reduced reactivity	Crush and soak longer; expect some waste
3+ months	Partially carbonated; poor quality	May not produce good lime; consider re-burning

Plan Ahead

Organize your workflow so that slaking happens within 48 hours of kiln firing. Have your slaking pit, water supply, and storage ready before you open the kiln.

Quantities and Ratios

Quicklime Input	Water Needed (wet slaking)	Approximate Putty Output
10 kg	30-40 liters	15-20 liters of putty
50 kg	150-200 liters	75-100 liters of putty
100 kg	300-400 liters	150-200 liters of putty

Note: Putty output varies significantly based on quicklime purity and how much excess water is used.

Historical Context

Ancient builders developed sophisticated slaking infrastructure:

• Rome: Dedicated slaking sheds with drainage channels and multiple pits for different ages of putty
• Medieval England: Lime putty was often specified to be aged 3+ years before use in cathedral construction
• India: Large-scale lime pits (chunam) were maintained for decades, producing putty of extraordinary quality
• Japan: Shikkui plaster used lime putty aged 5-10 years, polished to a mirror finish

These traditions persist because aged putty genuinely produces better results — the extended maturation time allows crystal refinement that cannot be rushed.

Troubleshooting

Problem	Likely Cause	Solution
Violent, uncontrolled boiling	Too much quicklime added at once	Add smaller batches; increase water volume
Putty is gritty	Under-burned limestone; over-burned cores	Screen through finer mesh; age longer
Putty won’t thicken	Too much water used	Allow to settle; decant excess water
Lime pops in finished mortar	Un-slaked particles	Screen more carefully; age putty longer; test for completeness
Low yield	Impure limestone; old quicklime	Source better limestone; slake sooner after burning
Putty hardens in storage	Water cover evaporated; air exposure	Always maintain water cover; seal containers