Types and Production of Lime

• Lime is derived from limestone or other calcium-rich rocks and is used as a binder in construction for mortar, plaster, and whitewash.
• Heating limestone (calcium carbonate, CaCO3) at high temperatures in a lime kiln causes calcination in lime production.
• Calcination drives off carbon dioxide (CO2), resulting in quicklime (calcium oxide, CaO).
• Quicklime is hydrated with water to produce slaked lime (calcium hydroxide, Ca(OH)2), also known as hydrated lime.
• Lime types are categorized by chemical composition and properties, including high-calcium, dolomitic, and hydraulic lime.

Types of Lime

• High-calcium lime contains over 90% calcium oxide (CaO), produced from limestone with low magnesium carbonate levels.
• High-calcium lime slakes rapidly and generates high heat during hydration.
• Dolomitic lime contains magnesium carbonate (MgCO3) in addition to calcium carbonate and is produced from dolomitic limestone.
• Dolomitic lime may be high-magnesium (more than 35% MgCO3) or low-magnesium (5-35% MgCO3).
• Dolomitic lime slakes more slowly and produces less heat than high-calcium lime.
• Hydraulic lime contains impurities like silica, alumina, and iron oxide that allow it to set and harden through a hydraulic reaction with water, similar to cement.
• Natural hydraulic lime (NHL) is derived from limestone that naturally contains these impurities, while artificial hydraulic lime has impurities added during manufacturing.
• NHL is classified into NHL 2, NHL 3.5, and NHL 5 based on compressive strength after 28 days.

Production Process

• The lime production process includes quarrying limestone, calcination, and hydration.
• Quarrying extracts limestone from the earth, which is then crushed and screened for the lime kiln.
• Calcination occurs in a lime kiln, heating limestone between 900°C and 1200°C to decompose calcium carbonate into calcium oxide (quicklime) and carbon dioxide.
• Calcination equation: CaCO3 (limestone) + Heat → CaO (quicklime) + CO2 (carbon dioxide).
• Hydration involves adding water to quicklime to produce slaked lime (calcium hydroxide).
• Hydration equation: CaO (quicklime) + H2O (water) → Ca(OH)2 (slaked lime) + Heat.
• Hydration generates significant heat and causes the quicklime to expand.

Uses in Construction

• Lime is used in mortar as a binder, providing workability, water retention, and autogenous healing.
• Lime mortar is more flexible and breathable than cement mortar, making it suitable for historic buildings and softer masonry units.
• In plaster, lime offers workability, adhesion, and a smooth finish.
• Lime plaster absorbs and releases moisture, regulating indoor humidity.
• Lime is used in whitewash as a protective coating for walls and ceilings, providing a bright, durable, breathable, and antiseptic finish that protects buildings from pests and decay.
• Lime is used in soil stabilization to improve clay soil properties, increasing strength and reducing plasticity.
• In road construction, lime modifies and stabilizes subgrade soils, providing a stable pavement base.

Advantages of Lime

• Breathability: Lime allows moisture to evaporate from walls, reducing dampness and decay risks.
• Flexibility: Lime is more flexible than cement, reducing cracking risks in masonry structures.
• Workability: Lime provides excellent workability in mortar and plaster, making it easier to apply and shape.
• Autogenous Healing: Lime mortar self-heals small cracks over time through calcium carbonate precipitation.
• Environmental Benefits: Lime production can be more energy-efficient than cement production; lime absorbs CO2 from the atmosphere during carbonation.

Disadvantages of Lime

• Slower Setting Time: Lime sets and hardens more slowly than cement, delaying construction.
• Lower Early Strength: Lime has lower early strength than cement, requiring longer curing times.
• Susceptibility to Acid Rain: Lime is susceptible to erosion from acid rain, which dissolves calcium carbonate.
• Availability: High-quality limestone availability for lime production can be limited in some regions.

Setting and Hardening

• Lime mortar and plaster set and harden through carbonation, where calcium hydroxide reacts with carbon dioxide to form calcium carbonate.
• Carbonation equation: Ca(OH)2 (slaked lime) + CO2 (carbon dioxide) → CaCO3 (calcium carbonate) + H2O (water).
• Carbonation is a slow process, taking months or years depending on temperature, humidity, and carbon dioxide availability.
• Hydraulic lime sets and hardens through both carbonation and hydraulic reactions.
• Hydraulic components in hydraulic lime react with water to form calcium silicate hydrates and calcium aluminate hydrates, contributing to its strength and durability.

Environmental Considerations

• Lime production can have environmental impacts, including greenhouse gas emissions and air pollution.
• Calcination of limestone releases significant carbon dioxide, a major greenhouse gas.
• Lime kilns can emit pollutants such as particulate matter, sulfur dioxide, and nitrogen oxides.
• Sustainable lime production practices include using energy-efficient kilns, capturing and storing carbon dioxide, and utilizing alternative fuels.
• Lime mortar and plaster can contribute to sustainable building practices by reducing the need for energy-intensive materials like cement and improving indoor air quality.
• Lime's ability to absorb CO2 during carbonation can help offset the emissions from its production.