The process of reacting quicklime (CaO) with water to form calcium hydroxide is called ripening or digestion of lime. The product produced by the reaction, calcium hydroxide, is called slaked lime or hydrated lime.
When the lime is ripened, a large amount of heat is released, and the volume is increased by 1 to 2.0 times. The lime with good calcination and high calcium oxide content is cooked faster, and the heat release and volume increase are also more. There are two methods for aging lime on the construction site: the slaked lime slurry method and the slaked lime powder method.
Depending on the amount of water added, the lime can be ripened into slaked lime or lime paste. The theoretical water requirement for lime ripening is 32% by weight of lime. In the quicklime, 60% to 80% of water is uniformly added to obtain a slaked lime powder having fine particles and uniform dispersion. If aged with an excess of water, a lime paste with a certain consistency will be obtained. Lime generally contains over-fired lime. Over-heated lime is slowly ripened. If the lime slurry is hardened and then matured, it will cause bulging and cracking due to expansion caused by ripening. In order to eliminate this hazard of over-fired lime, the lime should be "Chen Fu" for about 2 weeks after ripening.
The hardening of the lime slurry includes two simultaneous processes of dry crystallization and carbonization. The lime slurry is dried by evaporation or absorption of water, and capillary pressure is generated in the pore network of the slurry. The lime particles are made tighter to obtain strength. This strength is similar to the strength obtained by the loss of water from clay, which is not large and will be lost when exposed to water. At the same time, it loses water due to drying. The calcium hydroxide solution in the slurry is supersaturated, and crystals of calcium hydroxide are crystallized to produce strength; however, the number of crystals precipitated is small, and the strength growth is not large. In the atmosphere, calcium hydroxide reacts with carbon dioxide in the air to form calcium carbonate in the wet state, and releases water, which is carbonization.
The calcium carbonate crystals formed by carbonization cross each other or coexist with calcium hydroxide to form a tightly interwoven crystalline network, which further increases the strength of the hardened lime slurry. However, since the carbon dioxide content in the air is very low, the calcium carbonate layer formed on the surface is denser, which hinders further infiltration of carbon dioxide, and therefore, the carbonization process is very slow.
In the lime slurry formed after the lime is ripened, the lime particles form a colloidal structure of calcium hydroxide, the particles are extremely fine (having a particle diameter of about 1 μm), the specific surface area is large (up to 10 to 30 m 2 /g), and the surface is thickly adsorbed. The water film can absorb a large amount of water, so it has a strong ability to retain moisture, that is, good water retention. It is blended into cement mortar and blended into a mortar to significantly improve the workability of the mortar.
Lime is hardened by dry crystallization and carbonization. Because the carbon dioxide content in the air is low, and the hard shell formed by carbonization prevents carbon dioxide from penetrating into the interior, it also prevents the water from evaporating outward, so the hardening is slow and the strength after hardening is not High, 1:3 lime mortar 28 d compressive strength is only 0.2 ~ 0.5 MPa. When in a humid environment, the moisture in the lime does not evaporate, the carbon dioxide cannot penetrate, and the hardening will stop; plus the calcium hydroxide is slightly soluble in water, the hardened lime will dissolve and dissipate in contact with water. Therefore, lime should not be used in long-term wet and water-soaked environments.
During the hardening process of lime, a large amount of water is evaporated, causing a significant shrinkage of the volume, which is prone to dry shrinkage cracks. Therefore, lime should not be used alone, generally with sand, paper ribs, hemp knives and other materials to reduce shrinkage, increase tensile strength, and save lime.
Lime has a strong basicity, and reacts with glassy active silica or activated alumina at room temperature to form a hydraulic product, resulting in cementation. Therefore, lime is an important raw material in the building materials industry.
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