Lime, a material used across various industries, is a chemical compound distinct from the citrus fruit. Its journey from a naturally occurring rock to diverse chemical forms involves specific processes and transformations. Understanding what lime is made of begins with its primary raw material and follows its evolution through controlled chemical reactions.
The Primary Raw Material
The starting point for producing lime is limestone, a sedimentary rock composed primarily of calcium carbonate (CaCO3). This abundant mineral is found globally, forming over millions of years through both biological and nonbiological processes. Much of the limestone on Earth originates from the accumulation of calcium-rich shells, coral, and other marine debris from ancient organisms that settled on ocean floors.
Over geological timescales, these organic and inorganic remains are compressed and hardened, transforming into the rock known as limestone. While calcium carbonate is the main component, limestone can also contain other minerals such as magnesium carbonate, clay, or silica, influencing its purity and characteristics. Quarrying operations extract this rock from the Earth, preparing it for the next stage of its transformation into various lime products.
The Transformation Process
The conversion of limestone into lime occurs through a high-temperature process called calcination, also known as lime-burning. This industrial process involves heating calcium carbonate (CaCO3) to elevated temperatures, typically around 900°C (1,652°F) or higher, but below its melting point.
During calcination, a chemical decomposition reaction takes place where calcium carbonate breaks down into two main products. The solid product is calcium oxide (CaO), commonly known as quicklime, and carbon dioxide (CO2) gas is released. The balanced chemical equation for this transformation is CaCO3 (s) → CaO (s) + CO2 (g). This process is endothermic, meaning it requires a significant input of heat to drive the reaction. The quicklime produced is the immediate result of this thermal decomposition, forming a porous layer as carbon dioxide escapes.
Different Forms and Their Composition
Following calcination, quicklime (calcium oxide, CaO) emerges as the initial product. This white solid is highly reactive, particularly with water. Quicklime is also referred to as burnt lime or unslaked lime due to its direct production from the heating process. Its chemical formula is CaO.
When quicklime is reacted with water, a process known as slaking, it transforms into hydrated lime. This reaction is exothermic, releasing a significant amount of heat. Hydrated lime, also called calcium hydroxide or slaked lime, has the chemical formula Ca(OH)2. It is typically a fine, white powder.
A third form, dolomitic lime, is derived from dolomitic limestone, which naturally contains magnesium carbonate (MgCO3) in addition to calcium carbonate (CaCO3). When dolomitic limestone undergoes calcination, it yields a product that contains both calcium oxide (CaO) and magnesium oxide (MgO), known as dolomitic quicklime. If this dolomitic quicklime is subsequently slaked with water, it forms dolomitic hydrated lime, composed of calcium hydroxide (Ca(OH)2) and magnesium hydroxide (Mg(OH)2). The presence of magnesium distinguishes its chemical makeup from high-calcium lime forms.