Hydrated lime, also known as slaked lime, is a high-utility chemical compound used across numerous global industries. This white, powdery substance is manufactured by adding water to quicklime, or calcium oxide, in a process called slaking. The resulting product is characterized by its strong alkalinity and highly reactive nature, making it an effective neutralizer and chemical reagent.
Hydrated Lime in Construction and Infrastructure
The compound plays a central function in civil engineering, primarily by enhancing the stability and durability of construction materials and groundworks. A major use is in soil stabilization, where lime is mixed into fine-grained clay soils to create a stable base for foundations, roads, and embankments. The calcium ions in the lime displace the cations on the surface of the clay particles, which reduces the soil’s plasticity and allows for immediate drying of wet construction sites.
This initial modification is followed by a long-term strengthening process known as the pozzolanic reaction. The hydrated lime reacts with silica and alumina naturally present in the clay to form cementitious compounds. These materials are similar to the binding agents in concrete and provide permanent strength gain. This significantly increases the soil’s load-bearing capacity and resistance to moisture fluctuations.
In asphalt pavement, hydrated lime functions as a superior anti-stripping agent, improving the adhesion between the asphalt binder and the aggregate stone. This chemical interaction enhances the pavement’s resistance to moisture damage, preventing the asphalt from separating or “stripping” from the aggregate. The addition of lime also stiffens the asphalt mixture, which helps reduce rutting and cracking, thereby extending the service life of roadways.
Traditional masonry also relies on slaked lime, where it is a component in mortar and plaster. Lime-based mortars possess a unique ability to absorb and release moisture, allowing the wall assembly to “breathe” and preventing water from being trapped inside the structure. When cracks form, atmospheric carbon dioxide reacts with the lime to create calcium carbonate, a process that enables the mortar to self-heal minor imperfections over time.
Essential Roles in Water and Wastewater Treatment
Hydrated lime is an established chemical agent in the purification of both drinking water and industrial wastewater streams. One principal application is in water softening, accomplished through a method known as the Clark process. Here, the lime raises the water’s pH, which causes the precipitation of hardness minerals like calcium and magnesium bicarbonates as insoluble solids that can be easily removed.
The compound is also utilized for general pH adjustment and coagulation, especially in acidic water sources. By raising the water’s alkalinity, hydrated lime promotes the precipitation of dissolved heavy metals, such as iron, manganese, and lead, which form solid hydroxides at higher pH levels. These solids aggregate into larger, visible particles called floc, which are then physically separated from the water column.
In municipal wastewater treatment, lime is an effective agent for stabilizing sewage sludge before disposal or reuse. The high alkalinity of the lime raises the sludge pH to levels that reduce pathogens and harmful bacteria, making the material safer to handle. It also improves the sludge’s dewatering characteristics, reducing its volume and improving treatment efficiency.
Applications in Farming and Soil Management
In agriculture, hydrated lime is primarily used as a soil amendment to optimize growing conditions for crops. Many agricultural soils naturally become acidic over time, which inhibits a plant’s ability to absorb necessary nutrients. The alkalinity of the lime neutralizes this acidity, raising the soil’s pH to a more favorable range, typically between 6.0 and 7.0.
This adjustment in pH unlocks nutrients, particularly phosphorus and potassium, making them readily available for uptake by plant roots. The application also supplies the essential nutrient calcium, which is necessary for healthy cell wall development and overall plant structure. In some farm settings, the alkaline environment created by the lime can help suppress the growth of certain acid-loving soil-borne pathogens and fungi.
Use in Chemical and Industrial Processes
Hydrated lime’s strong alkaline nature makes it a valuable reagent in a wide array of industrial chemical manufacturing operations. It serves as a base for neutralizing acidic byproducts and as a source of calcium in the production of various calcium salts, plastics, and lubricants. In the pulp and paper industry, it is a key component in the Kraft process, where it is used in the causticizing step to regenerate the cooking chemicals required for wood pulping.
One of its most environmentally significant uses is in air pollution control, specifically in Flue Gas Desulfurization (FGD) systems. Industries, such as power plants and cement kilns, inject dry hydrated lime powder or a lime slurry into their exhaust stacks to neutralize acidic gases. The lime reacts chemically with sulfur dioxide and hydrogen chloride, capturing these pollutants and converting them into stable solid byproducts.