Slaking of lime is the process of adding water to quicklime (calcium oxide, CaO) to produce hydrated lime (calcium hydroxide, Ca(OH)2). This highly exothermic chemical reaction releases substantial heat and transforms a caustic powder or lump into a workable building material used in mortars, plasters, renders, and limewashes.
The chemical reaction
The reaction is straightforward: CaO + H2O produces Ca(OH)2 plus heat. When water contacts quicklime, temperatures can exceed 150 degrees Celsius, and the mixture may steam, boil, and spit. The quicklime lumps crack apart and dissolve into a fine paste or dry powder, depending on the amount of water added.
If just enough water is added to complete the reaction, the result is a dry powder called hydrated lime or slaked lime. If excess water is used, the product is a thick, creamy suspension called lime putty. The method chosen depends on the intended use and the desired working properties of the finished product.
Dry slaking versus wet slaking
Dry slaking uses a carefully measured quantity of water — roughly 32% of the weight of the quicklime — to produce a free-flowing powder. This is the method used in modern industrial lime production. The resulting hydrated lime powder is bagged and sold for construction, agriculture, and water treatment.
Wet slaking uses a large excess of water to produce lime putty. This traditional method, often performed in a slaking pit, yields a smooth paste that improves with age. Mature lime putty, stored under water for months or years, develops an exceptionally fine particle structure that enhances workability and final surface quality in plasters and renders.
Application
Slaked lime is one of the oldest and most widely used building materials. It forms the basis of lime mortar for bricklaying and stone masonry, lime plaster and render for interior and exterior wall finishes, and limewash for breathable, decorative coatings. In modern construction, hydrated lime is also added to cement mortars to improve workability, water retention, and flexibility.
Safety precautions are essential during slaking. Protective gloves, goggles, and face shields are required due to the intense heat and the caustic nature of both quicklime and wet lime.
Related terms
- Quicklime
- Lime mortar
- Lime slaking pit
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