Calcium Hydroxide

Calcium hydroxide, commonly known as slaked lime or hydrated lime, is an inorganic compound with the chemical formula Ca(OH)₂. It is a white, odourless crystalline powder or colourless crystal that plays a vital role in numerous industrial, environmental, agricultural, and laboratory applications. As a strong base, calcium hydroxide is extensively used in water treatment, construction, food processing, and medicine. It has a profound historical significance, having been employed for centuries in building materials, particularly lime mortar and plaster.

Chemical Structure and Properties

Calcium hydroxide is a metal hydroxide derived from calcium oxide (quicklime) by the addition of water. The compound consists of one calcium ion (Ca²⁺) bonded to two hydroxide ions (OH⁻). It is a sparingly soluble base, producing an alkaline solution known as limewater when dissolved in water.
Chemical characteristics:

• Chemical formula: Ca(OH)₂
• Molar mass: 74.09 g/mol
• Appearance: White powder or colourless crystals
• Density: 2.21 g/cm³
• Melting point: 580°C (decomposes to calcium oxide and water vapour)
• Solubility in water: 1.73 g/L at 20°C
• pH of saturated solution: Approximately 12.4
• Nature: Strong base, weakly soluble in water, reacts readily with acids to form salts.

Calcium hydroxide’s limited solubility decreases with increasing temperature, an unusual property among salts, as heating causes the formation of insoluble calcium oxide. Its aqueous solution, limewater, readily reacts with carbon dioxide to form calcium carbonate (CaCO₃), a reaction used in qualitative detection of CO₂:
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
This reaction forms the scientific basis for its use in air and gas analysis, and for assessing carbonation in construction materials.

Preparation and Manufacture

Calcium hydroxide is industrially prepared by the controlled hydration of quicklime (CaO), a process known as slaking:
CaO + H₂O → Ca(OH)₂ + heat
This exothermic reaction releases a significant amount of heat, causing the mixture to expand and form a dry, powdery solid. The process can be carried out as either dry slaking, producing a fine powder, or wet slaking, yielding a slurry known as milk of lime.
Raw materials for this process are obtained from limestone, chalk, or other calcium carbonate-rich sources, which are calcined at temperatures around 900°C–1000°C to produce quicklime. The quality of the final calcium hydroxide product depends on the purity of the lime and control of hydration conditions.

Historical Background

Calcium hydroxide has been in use since ancient times. The Egyptians, Greeks, and Romans utilised slaked lime for construction and sanitation. Roman concrete, one of the most durable ancient materials, was made from lime mixed with volcanic ash and water. During the Middle Ages, limewash and mortar were essential components in architecture.
In the 18th and 19th centuries, calcium hydroxide gained scientific recognition as chemistry advanced, leading to its wide-scale industrial and agricultural application. Today, it remains one of the most important alkaline compounds in global chemical industries.

Physical and Chemical Behaviour

Calcium hydroxide behaves as a strong base in water, partially dissociating to release hydroxide ions (OH⁻):
Ca(OH)₂ ⇌ Ca²⁺ + 2OH⁻
This confers high alkalinity to its aqueous solutions, enabling it to neutralise acids effectively. It also reacts with various gases and salts:

• With carbon dioxide to form calcium carbonate.
• With chlorine to produce calcium hypochlorite, an active bleaching agent.
• With ammonium salts to release ammonia gas.

Due to its reactivity with acids, calcium hydroxide forms salts such as calcium nitrate, calcium chloride, and calcium sulphate. It also participates in precipitation reactions, a property widely exploited in water and wastewater treatment.

Applications in the Construction Industry

Calcium hydroxide is a cornerstone material in the construction and civil engineering sectors.
Key applications include:

• Mortar and plaster preparation: When mixed with sand and water, it forms lime mortar, which hardens by absorbing atmospheric CO₂ to form calcium carbonate.
• Cement and concrete manufacture: Serves as an intermediate in cement hydration reactions, influencing setting time and strength development.
• Limewash: Used for coating walls due to its whiteness, antibacterial nature, and ability to reflect light.
• Soil stabilisation: Added to clayey soils to improve load-bearing capacity in road and foundation engineering.

Its ability to bind carbon dioxide also contributes to carbon capture in buildings, as the curing of lime-based mortars naturally removes CO₂ from the atmosphere.

Role in Water and Wastewater Treatment

One of the most significant applications of calcium hydroxide is in water purification and wastewater treatment. It serves as both a coagulant and a pH adjuster.

• Water softening: Reacts with bicarbonates of calcium and magnesium to precipitate them as insoluble carbonates, reducing water hardness.
• Neutralisation: Used to correct the acidity of industrial effluents and mine drainage.
• Flocculation and coagulation: Facilitates the removal of suspended solids and heavy metals by forming flocs that can be filtered out.
• Disinfection: In conjunction with chlorine, calcium hydroxide produces calcium hypochlorite, a powerful disinfectant used in drinking water and sanitation systems.

The use of lime in water treatment not only ensures safe potable water but also contributes to environmental protection by reducing toxic metal concentrations.

Agricultural and Environmental Uses

Calcium hydroxide plays an integral role in agriculture as a soil conditioner and pesticidal agent.
Agricultural applications:

• Soil pH regulation: Neutralises acidic soils, enhancing nutrient availability and microbial activity.
• Liming agent: Prevents soil acidification from fertilisers, improving crop productivity.
• Pest control: In combination with sulphur, it forms Bordeaux mixture, an effective fungicide for vineyards and fruit trees.

In environmental management, calcium hydroxide is used to treat acidic industrial wastes, flue gases, and sludge stabilisation. It removes sulphur dioxide (SO₂) from emissions via flue gas desulphurisation (FGD) processes:
SO₂ + Ca(OH)₂ → CaSO₃ + H₂O
This reaction reduces air pollution and contributes to compliance with environmental regulations.

Food and Pharmaceutical Applications

Calcium hydroxide is widely used in the food and pharmaceutical industries due to its recognised safety profile and functional properties.
Food industry uses:

• pH control: Acts as an acidity regulator in beverages and food processing.
• Calcium fortification: Serves as a source of dietary calcium in fortified foods.
• Corn processing (nixtamalisation): Used in traditional preparation of maize for tortillas, enhancing nutritional value and flavour.
• Sugar refining: Clarifies sugarcane juice by precipitating impurities as calcium salts.
• Preservative: Helps in pickling and in the firming of fruits and vegetables during processing.

Pharmaceutical uses:

• Antacid: Neutralises stomach acid in mild formulations.
• Dental applications: Used in root canal treatments and as a pulp capping agent due to its antibacterial and remineralising properties.
• Excipient: Functions as an inactive filler in tablet manufacture.

Calcium hydroxide is included in pharmacopeias worldwide, approved as a safe and effective compound in medical formulations.

Industrial Chemical Uses

In industry, calcium hydroxide serves as a base reagent, neutraliser, and intermediate in numerous chemical processes.
Examples include:

• Paper industry: Used in the causticising process during Kraft pulp production to regenerate sodium hydroxide from sodium carbonate.
• Chemical synthesis: Precursor for producing calcium salts, such as calcium stearate (lubricant), calcium hypochlorite (bleach), and calcium soap (stabiliser).
• Leather tanning: Helps remove hair and flesh from hides before tanning.
• Petroleum refining: Used for neutralising acidic residues and removing sulphur compounds from fuels.

Its wide applicability stems from its basicity, reactivity, and relatively low cost compared with other alkalis such as sodium hydroxide.

Safety, Toxicity, and Handling

Although calcium hydroxide is not toxic when used appropriately, it is corrosive and must be handled with care. Contact with skin or eyes may cause irritation or burns, and inhalation of dust may lead to respiratory discomfort.
Safety measures:

• Use protective gloves, goggles, and masks during handling.
• Avoid prolonged exposure to moist air to prevent carbonation.
• Store in airtight containers away from acids and moisture.

In controlled doses, calcium hydroxide is considered safe in food and medical products, as it is a naturally occurring compound in many environmental systems.

Analytical and Laboratory Uses

In laboratory practice, calcium hydroxide serves as a standard reagent for neutralisation reactions and pH control. Limewater is often used for detecting carbon dioxide, a classical experiment in chemistry education. It also serves as a source of calcium ions in precipitation studies and as a reagent in qualitative inorganic analysis.

Environmental and Sustainability Aspects

From a sustainability perspective, calcium hydroxide is an eco-friendly material. Its production and use are part of a natural cycle involving limestone and carbon dioxide:
CaCO₃ → CaO → Ca(OH)₂ → CaCO₃
This reversible cycle exemplifies material circularity, where calcium compounds continually interconvert with minimal ecological impact. The use of calcium hydroxide in carbon sequestration, wastewater treatment, and air pollution control further enhances its environmental importance.
Efforts are underway to develop carbon-neutral lime production technologies, capturing CO₂ released during calcination for reuse, thus supporting global decarbonisation strategies.

Economic and Commercial Importance

Calcium hydroxide holds substantial economic value due to its widespread applications. Major producers include countries with abundant limestone resources such as China, the United States, and India. It is a critical component of the construction, water treatment, paper, and chemical industries.
Global demand continues to rise, driven by infrastructure development, environmental regulations, and sustainable agriculture. The versatility and affordability of calcium hydroxide ensure its enduring relevance across sectors.