Slaked lime is Calcium Hydroxide (Ca(OH)2). It is also known as hydrated lime, builders’ lime, or “cal.” This substance is manufactured by adding water to calcium oxide (quicklime) in an exothermic process called “slaking.” The resulting fine, white powder is a versatile and historically significant material in construction, environmental management, and agriculture.
As a Key Component in Building Materials
Slaked lime functions as a binder in traditional construction, where it is incorporated into lime mortar, plaster, and stucco to provide structural cohesion and a breathable finish. The hardening process involves a slow chemical reaction known as carbonation. During carbonation, the calcium hydroxide slowly absorbs carbon dioxide (CO2) from the atmosphere, gradually reverting to a stable form of calcium carbonate.
The slow setting process provides lime-based materials with a self-healing capability, as the material can re-absorb atmospheric CO2 over time to repair micro-cracks. Although it sets much slower than modern Portland cement, slaked lime significantly improves the plasticity and workability of fresh masonry mixes. This allows masons to achieve smoother finishes and better adhesion, making it a preferred component in restoration work and specific architectural applications.
Improved workability stems from the fine particle size and shape of the calcium hydroxide, which allows the mixture to flow smoothly under troweling. This physical characteristic reduces the amount of water needed in the mix while still maintaining a cohesive paste. This ultimately yields a more durable and less brittle final product compared to some early cement-only applications.
Treating Water and Neutralizing Acids
Slaked lime functions as a cost-effective alkali, used for adjusting the pH of liquid and gas streams in industrial and municipal settings. This basic property is utilized in municipal water purification systems. A primary application is softening hard water by precipitating calcium and magnesium ions out of the solution, which subsequently reduces scaling in pipes and appliances.
The compound also aids in flocculation, where its addition causes fine, suspended solids and impurities to clump together. These larger clumps, or flocs, can then be easily filtered or settled out of the water, greatly improving clarity and potability. In industrial settings, slaked lime is regularly used to neutralize acidic contaminants in wastewater before discharge.
This neutralization process is chemically straightforward, as the base (Ca(OH)2) reacts with acids to produce harmless salts and water. Another industrial use is in flue gas desulfurization systems, where it is injected into smokestacks to scrub sulfur dioxide (SO2) emissions. This air pollution control method effectively captures and neutralizes acidic gases generated by burning fossil fuels.
Slaked Lime in Soil and Agriculture
Slaked lime is used in agriculture to modify soil chemistry, primarily by reducing soil acidity and raising its pH level. Many crops struggle to efficiently absorb necessary nutrients, such as phosphorus and nitrogen, when the soil becomes overly acidic. Applying calcium hydroxide corrects this imbalance, optimizing the environment for nutrient uptake and beneficial microbial activity.
Beyond pH adjustment, slaked lime serves as a fungicide and insecticide. It is sometimes dusted onto crops or around livestock areas to deter pests and control fungal growth. However, farmers must carefully distinguish this material from dolomitic lime, which is used specifically when the soil also requires the addition of magnesium alongside calcium.
Safe Handling and Practical Considerations
Given its alkaline nature, handling slaked lime requires strict adherence to safety protocols to prevent caustic irritation. Users must wear protective gear, including chemical-resistant gloves, sealed eye protection, and a dust mask. Direct contact with skin, eyes, or inhalation of the fine powder can cause burns or severe respiratory distress.
Proper storage is necessary to maintain its efficacy. The material should be kept in a tightly sealed container in a cool, dry place, preventing exposure to atmospheric carbon dioxide or moisture. The versatility of slaked lime across construction, environmental protection, and agriculture underscores its continued importance.