Lime is classified into quicklime and slaked lime. Quicklime's main component is calcium oxide (CaO), obtained by calcining limestone with a high calcium carbonate (CaCO₃) content in a well-ventilated lime kiln to above 900℃. It is hygroscopic and can be used as a desiccant, commonly used in my country to prevent items from becoming damp.
The process of quicklime (CaO) reacting with water to produce calcium hydroxide is called lime slaking or digestion. This reaction with water (releasing a large amount of heat) or absorption of moisture from the air produces slaked lime, also known as hydrated lime. Slaked lime dissolves 1.56 grams in one liter of water (at 20℃). Its saturated solution is called limewater, which is alkaline and absorbs carbon dioxide from the air to form calcium carbonate precipitate. The product of the reaction, calcium hydroxide, is called slaked lime or hydrated lime.
Lime slaking releases a large amount of heat and increases in volume by 1-2 times. Well-calcined lime with a high calcium oxide content slakes faster, releasing more heat and increasing in volume. Two common methods for slaking lime on construction sites are the slaked lime slurry method and the slaked lime powder method. In lime slurry formed after quicklime slaking, the lime particles form a calcium hydroxide colloidal structure with extremely fine particles (approximately 1 μm in diameter) and a large specific surface area (10–30 m²/g). A thick water film is adsorbed on its surface, allowing it to absorb a large amount of water, thus exhibiting strong water retention capacity. Adding it to cement mortar to form a mixed mortar significantly improves the mortar's workability.
Lime hardens through drying, crystallization, and carbonation. Due to the low carbon dioxide content in the air and the hardened calcium carbonate shell formed after carbonation preventing carbon dioxide penetration and water evaporation, hardening is slow, and the hardened strength is low. A 1:3 lime mortar has a compressive strength of only 0.2–0.5 MPa after 28 days. In humid environments, the water in the lime does not evaporate, and carbon dioxide cannot penetrate, halting hardening. Furthermore, calcium hydroxide is slightly soluble in water, causing hardened lime to dissolve and crumble upon contact with water. Therefore, lime is not suitable for use in environments with prolonged dampness or water immersion.
During the hardening process, lime evaporates a large amount of water, causing significant volume shrinkage and making it prone to drying shrinkage cracks. Therefore, lime should not be used alone; it is generally mixed with materials such as sand, paper pulp, and hemp fiber to reduce shrinkage, increase tensile strength, and conserve lime.
Lime has strong alkalinity and, at room temperature, can react with glassy activated silica or activated alumina to produce hydraulic products and form cement. Therefore, lime remains an important raw material in the building materials industry.
Lime slurry is generally produced by adding water to calcium oxide. Because calcium hydroxide has relatively low solubility, it often results in a suspension of calcium hydroxide (i.e., an aqueous solution containing undissolved calcium hydroxide). Lime slurry, on the other hand, is a turbid liquid obtained by diluting lime with water (approximately 2.5-3 times the mass of lime).
