Slaked lime is made by adding water to quicklime (calcium oxide), triggering a chemical reaction that produces calcium hydroxide. The process is straightforward in concept but demands respect: quicklime reacts violently with water, releasing enough heat to cause serious burns. Whether you need a dry powder for construction or a wet putty for traditional plastering, the core method is the same, with the amount of water determining the final product.
What Happens When Quicklime Meets Water
The reaction is simple: one part calcium oxide combines with one part water to produce one part calcium hydroxide. In practice, this means roughly 56 grams of quicklime reacts with 18 grams of water. The reaction generates significant heat, enough to boil the water and send steam shooting upward. This is why the process is called “slaking,” an old English word meaning to quench or cool.
The heat release is not a minor detail. Quicklime can reach temperatures well above 100°C (212°F) during slaking, which is why accidental contact between quicklime and even small amounts of moisture, including sweat on your skin, can cause thermal burns on top of chemical burns.
Dry Powder vs. Lime Putty
The amount of water you add determines which form of slaked lime you get. These two products behave quite differently and suit different purposes.
Dry hydrated lime is made by adding a carefully controlled amount of water, just enough to complete the chemical reaction without leaving excess moisture. The quicklime crumbles and expands into a fine, dry white powder. Industrial hydrators do this precisely, but the principle applies at any scale: use only as much water as the quicklime can absorb. The result stores easily in sealed bags and is the form most commonly sold in hardware stores.
Lime putty (lime slurry) is made by adding a large excess of water, far more than the reaction requires. The quicklime breaks down into a thick, creamy suspension. This is the traditional form used in heritage plastering, lime mortar, and fine finishing work. Lime putty improves with age. Research on lime putty maturation has confirmed what Roman builders codified into law over two thousand years ago: putty aged for at least 36 months produces noticeably better mortar than younger material. Putties aged less than 12 months show incomplete mineral development, while those aged longer approach their full potential. If you’re making lime putty for building, patience pays off.
Step-by-Step: Slaking Quicklime
Before you begin, gather your protective equipment. You need chemical splash goggles (not just safety glasses), a face shield, heavy rubber or neoprene gloves, long sleeves, long pants, and closed-toe boots. Do not wear contact lenses. Work outdoors or in a very well-ventilated area, because the reaction produces steam and can release fine caustic dust.
For Dry Hydrated Lime
Place your quicklime in a large, heat-resistant metal container. Slowly sprinkle water over the quicklime using a watering can or spray nozzle, adding small amounts at a time. The lime will begin to crack, hiss, and steam. Stir carefully with a long-handled metal tool to ensure even wetting. Stop adding water once the quicklime has fully broken down into a dry, powdery consistency. The goal is complete reaction with no leftover lumps of unslaked quicklime and no pooling water. Let the powder cool completely before storing it in an airtight container, since calcium hydroxide absorbs carbon dioxide from the air and gradually reverts to calcium carbonate (chalk), losing its reactive properties.
For Lime Putty
Place quicklime in a large metal drum or pit. Add water generously, roughly three to four times the volume of the quicklime. The mixture will boil and churn violently for several minutes. Stand back during the initial reaction and add water from a safe distance. Once the vigorous phase subsides, stir thoroughly to break up any remaining lumps. The result should be a thick, smooth slurry. Cover the putty with a few inches of standing water to prevent it from drying out and reacting with air. For basic use, the putty is workable within a few days. For high-quality lime mortar, store it submerged in water for 12 months or longer.
Choosing the Right Quicklime
The quality of your slaked lime depends entirely on your starting material. Quicklime is produced by heating limestone (calcium carbonate) in a kiln to around 900°C (1,650°F), which drives off carbon dioxide and leaves calcium oxide behind. You can buy quicklime from agricultural suppliers, building material stores, or chemical suppliers.
Purity matters. For general construction, limestone with at least 85% calcium carbonate produces acceptable quicklime. For more demanding applications, aim higher. Food-grade calcium hydroxide must contain a minimum of 95% calcium hydroxide, with strict limits on contaminants: arsenic below 3 parts per million, lead below 2 parts per million, and fluoride below 0.05%. If you plan to use slaked lime for food preparation (such as nixtamalization for tortillas, or pickling), buy certified food-grade calcium hydroxide rather than attempting to produce it yourself. The contaminant risks from unknown limestone sources are not worth taking.
Common Uses and Which Form to Choose
- Garden soil amendment: Dry hydrated lime works well. It raises soil pH and adds calcium. Spread and mix into soil, then water in.
- Traditional plastering and lime mortar: Lime putty is the preferred form, ideally aged for at least a year. It produces a more workable, cohesive mortar than dry hydrate mixed on site.
- Water treatment: Dry hydrated lime dissolves more predictably and is easier to dose by weight.
- Cooking (nixtamalization, betel leaf preparation): Use only purchased food-grade calcium hydroxide. Do not slake your own quicklime for culinary purposes unless you can verify the purity of both the quicklime and the water source.
Storage and Shelf Life
Dry hydrated lime absorbs moisture and carbon dioxide from the air, gradually turning back into chalk. Store it in tightly sealed plastic bags or airtight containers, away from humidity. Properly sealed, it lasts for months. Once the bag is opened, use it within a few weeks or reseal it thoroughly.
Lime putty, by contrast, actually improves over time when stored under water. Keep it in a sealed bucket with a layer of water on top, and it can remain usable for years. The longer it sits, the finer and more workable the particles become. Some specialist suppliers sell putty that has been aged for three years or more, specifically because of the quality benefits that extended maturation provides.