Calcium hydroxide (\(\text{Ca}(\text{OH})_2\)) is an inorganic compound produced by mixing calcium oxide (quicklime) with water in a process called slaking. It is commonly known as slaked lime, hydrated lime, or caustic lime. This substance appears as a white powder or colorless crystal and is characterized by its strong basicity, possessing a high pH of approximately 12.4 in a saturated solution known as limewater.
The compound is sparingly soluble in water, with its solubility decreasing as temperature increases. Due to its low cost, low toxicity, and ability to neutralize acids and precipitate various ions, calcium hydroxide is utilized across a vast range of industrial and specialized applications, including construction, environmental control, and food preparation.
Construction and Infrastructure
Calcium hydroxide holds an important role in the construction industry, serving as a fundamental component in traditional building materials. When mixed with sand and water, it forms lime-based mortars and plasters used in masonry and finishing works. Its inclusion enhances the workability and plasticity of these mixtures, making them easier to apply and reducing the likelihood of shrinkage cracks after they are set.
The hardening process, known as carbonation, is a chemical reaction where calcium hydroxide slowly reacts with carbon dioxide absorbed from the air. This reaction results in the formation of calcium carbonate, the same substance that makes up natural limestone. This transformation provides structural strength and contributes to the durability and longevity of the constructed elements.
Hydrated lime is also integral to geotechnical engineering for soil stabilization, particularly for roads and foundations. When added to clay-rich soils, the calcium ions react with the clay minerals, improving the soil’s bearing capacity and reducing its plasticity. This chemical modification stabilizes the subgrade, providing a stronger base for infrastructure projects.
Water Purification and Environmental Control
In environmental management, calcium hydroxide is widely used to treat and purify water for both municipal and industrial purposes. One of its main functions is in water softening, where it is a component of the lime-soda process. The addition of limewater precipitates hardness-causing calcium and magnesium ions by converting them into nearly insoluble solids like calcium carbonate and magnesium hydroxide.
The compound’s strong basicity makes it an effective neutralizer for acidic wastewater generated by industrial processes. By raising the pH of the water, it helps prevent corrosion in pipes and facilitates the removal of heavy metals, which become less soluble in alkaline conditions. This neutralization is an important step in sewage treatment to ensure effluent water complies with environmental discharge regulations.
Calcium hydroxide also functions as a flocculant, helping suspended particles clump together so they can be easily removed. It forms a charged solid that attracts and binds to smaller impurities, creating larger, heavier clusters called flocs. These flocs then settle out of the water column through sedimentation, resulting in a clearer product.
Food Processing and Preparation
Calcium hydroxide is safely incorporated into various food processes, designated as the food additive E526. It is used as an acidity regulator and a firming agent, helping to maintain the desired texture and shelf life of products. Its most culturally and nutritionally significant use is in the preparation of corn through nixtamalization.
In nixtamalization, corn kernels are cooked and soaked in an alkaline solution made with calcium hydroxide. This softens the outer hull and makes the corn easier to grind into masa for tortillas. This process also causes a chemical change that significantly increases the bioavailability of niacin (vitamin \(\text{B}_3\)), which helps prevent the deficiency disease known as pellagra. Furthermore, the compound is utilized in the sugar industry to purify sugar cane and sugar beet extracts by precipitating impurities.
As a firming agent, it is applied to fruits and vegetables during pickling or canning to help them retain a desirable crispness. The calcium ions interact with pectin in the cell walls of the produce, forming stable structures that resist softening during preservation.
Dental and Endodontic Procedures
In clinical dentistry, calcium hydroxide is a standard material used in endodontic procedures, leveraging its unique biological and antimicrobial characteristics. When mixed into a paste, it has a high pH of approximately 12.5 to 12.8. This extreme alkalinity is the source of its strong antimicrobial action, as the hydroxyl ions (\(\text{OH}^-\)) damage the cell membranes and denature the proteins of bacteria within the root canal system.
It is frequently used as an inter-appointment intracanal medicament, placed inside the tooth’s root canal between visits to disinfect the space before the final filling. The sustained release of hydroxyl ions helps to sterilize the dentine and the surrounding tissues.
This basic compound is also applied directly to the exposed dental pulp in a procedure called pulp capping. Its application stimulates the underlying pulp tissue to form a protective layer of reparative dentin, often referred to as a dentin bridge. The high pH encourages the formation of new, hard tissue, which ultimately helps to preserve the vitality of the tooth.
Agricultural and Soil pH Management
In agriculture, calcium hydroxide is a fast-acting liming material used to manage soil acidity and promote better crop growth. Many soils naturally become acidic due to factors like rainfall and the use of nitrogen fertilizers. The compound’s high alkalinity is used to rapidly neutralize the acid, effectively raising the soil’s pH.
This adjustment improves the availability of essential plant nutrients, which are often locked up and unavailable to roots in highly acidic conditions. By increasing the soil pH, calcium hydroxide promotes a more favorable environment for nutrient uptake and also supports beneficial soil microbes. Although it provides a rapid change in soil pH, it is less stable and may revert to calcium carbonate over time through reaction with soil carbon dioxide.
Calcium hydroxide is also a component in the preparation of Bordeaux mixture, a traditional fungicide and bactericide used to protect fruit trees and vegetables from diseases. In this mixture, the slaked lime is combined with copper sulfate and water. The lime component neutralizes the acidity of the copper sulfate, preventing the copper from damaging the plant foliage, while also helping the mixture adhere to the surfaces of the leaves and branches.