Slacked lime, a versatile chemical compound, is the common name for what is scientifically known as calcium hydroxide. This white, powdery substance is also frequently referred to as hydrated lime or builder’s lime. The substance is integral to a wide range of fields, from construction and agriculture to environmental purification. Its widespread use is a direct result of its unique chemical properties, which derive from the specific process used to create it.
The Chemical Identity and Slaking Process
Slacked lime is chemically identified as calcium hydroxide, with the formula Ca(OH)2. This compound is produced in a manufacturing process known as “slaking” or hydration. The process begins with quicklime, or calcium oxide (CaO), the precursor material.
Water is introduced to the quicklime, initiating an intense chemical transformation. This reaction is highly exothermic, meaning it releases a significant amount of heat. The chemical equation for this transformation is CaO + H2O \(\rightarrow\) Ca(OH)2 + Heat.
The addition of water converts the calcium oxide into calcium hydroxide. This hydration process yields a fine, dry powder when water is precisely measured, or a paste known as lime putty when a larger amount is used. The term “slacked” refers directly to this manufacturing step, signifying that the quicklime has reacted with water.
Distinguishing Slacked Lime from Quicklime and Limestone
The entire family of materials is derived from limestone, which is primarily calcium carbonate (CaCO3). Limestone is the raw material, a stable sedimentary rock that is not chemically reactive with water.
Quicklime, or calcium oxide (CaO), is the second stage. It is produced by heating limestone to temperatures above 900°C in a process called calcination. This heating drives off carbon dioxide (CO2), leaving the highly reactive quicklime.
Slacked lime (Ca(OH)2) is the third stage, created when water is added to quicklime. These three forms—limestone, quicklime, and slacked lime—represent chemically distinct stages of the same base mineral. Each form has unique properties suitable for different industrial uses.
Key Uses Across Different Industries
Slacked lime is widely used in the construction industry, where its plasticity and binding properties are valued. It is a traditional component in mortar, plaster, and stucco, improving the mixture’s workability. As the slacked lime slowly reacts with atmospheric carbon dioxide, it reverts to calcium carbonate, which hardens and gives the structure strength.
In agriculture, the compound is primarily used for soil amendment in a practice called liming. Since slacked lime is strongly alkaline, it is applied to neutralize acidic soils, raising the soil’s pH to an optimal level for crop growth. This adjustment increases the availability of essential plant nutrients.
The environmental sector relies on slacked lime for various water and waste treatment processes. It is used to purify drinking water by acting as a flocculant, helping to remove impurities and suspended particles. Its alkaline nature makes it an effective agent for neutralizing acidic industrial wastewater and flue gases, controlling pollution before discharge.
Handling and Safety Precautions
Slacked lime remains a strongly alkaline substance that requires careful handling. It has a high pH, around 12.4, which means it can cause irritation upon contact with bodily tissues. Direct contact with the powder or its solutions can irritate the skin, eyes, and lungs.
When working with the material, appropriate personal protective equipment is necessary to minimize exposure:
- Safety glasses with eye shields.
- Chemical-resistant gloves.
- Long-sleeve clothing to protect the skin.
- A dust mask or respirator if handling the dry powder.
Slacked lime should be stored in a manner that protects it from moisture and air to maintain quality and prevent unwanted reactions. Storage areas should be dry and well-ventilated, ideally using purpose-designed containers like silos for bulk storage. If skin or eye contact occurs, the affected area must be flushed immediately with large amounts of water, and medical attention sought for eye exposure.