Hydrated lime, chemically known as calcium hydroxide or slaked lime, is a versatile alkaline compound used across industrial, agricultural, and construction applications. Its utility stems from its chemical structure, allowing it to effectively neutralize acids, modify soil composition, and stabilize materials. The compound is widely adopted globally due to its cost-effectiveness.
The Chemical Identity of Hydrated Lime
Hydrated lime is a fine, white, powdery substance identified as calcium hydroxide. It is derived from limestone, which is first subjected to extreme heat (calcination) to produce quicklime (calcium oxide). The final product is created by mixing quicklime with a controlled amount of water in a process known as slaking.
This exothermic reaction releases heat as the calcium oxide converts into calcium hydroxide. The resulting compound is a strong alkali, with its solution having a high pH of around 12.5.
Role in Soil and Agriculture
In agriculture, hydrated lime acts as a liming agent, adjusting the pH of acidic soils to a range suitable for crop growth. The alkalinity of hydrated lime effectively neutralizes soil acidity, performed by hydroxide ions reacting with hydrogen ions in the soil solution.
Raising the soil pH unlocks essential nutrients, such as phosphorus, which tend to become chemically unavailable to plants in highly acidic conditions. This neutralization mitigates aluminum toxicity, a common condition that restricts root development and nutrient uptake. Hydrated lime also supplies calcium, an important nutrient for plant cell wall structure and overall health.
Farmers typically apply hydrated lime at rates ranging from 500 to 1,500 kilograms per hectare, depending on soil analysis. It is fast-acting and provides long-lasting benefits, often maintaining the desired soil pH for more than a single growing season. The resulting improvement in nutrient availability contributes directly to better crop yields and more vigorous plant growth.
Use in Construction and Material Stabilization
Hydrated lime is an established additive in construction, used for soil stabilization and enhancing masonry materials. For roadbeds and foundations, it is added to clay-rich subgrade soils to improve engineering properties. Calcium cations in the lime replace naturally occurring cations on clay particles, significantly reducing the soil’s plasticity and moisture-holding capacity.
This process, known as soil modification, quickly dries out wet clay, transforming it into a stable platform for construction. Over the long term, hydrated lime initiates a pozzolanic reaction with the silicates and aluminates present in the clay. This chemical reaction creates stable calcium silicate and aluminate hydrates, which are cementitious compounds that produce long-lasting strength gain in the stabilized layer.
In mortars and plasters, hydrated lime enhances the material’s workability and plasticity, making the mix smoother and easier to apply. It also improves the mixture’s water retention, necessary for proper curing and bonding with masonry units. During curing, the hydrated lime reacts slowly with carbon dioxide (carbonation), converting the calcium hydroxide back into calcium carbonate, contributing to the final strength and durability.
Functions in Water and Environmental Treatment
In water treatment facilities, hydrated lime adjusts pH and removes impurities from drinking water and wastewater. Its high alkalinity neutralizes acidic water, which helps prevent corrosion in distribution pipes and optimizes conditions for subsequent disinfection.
A common application is water softening, where lime is introduced to precipitate hardness minerals like calcium and magnesium. The addition of hydrated lime causes these dissolved minerals to form insoluble solids, which are easily removed through sedimentation or filtration. It also promotes flocculation, where small, suspended particles clump together, simplifying the removal of dirt, organic matter, and other contaminants to clarify the water.
Hydrated lime is also used for environmental air quality control, particularly in industrial flue gas treatment systems. In these scrubbers, the compound is injected to neutralize acidic pollutants, such as sulfur dioxide. The lime reacts with the sulfur dioxide, converting the harmful gas into a manageable solid byproduct, significantly reducing atmospheric emissions.