Cement Quicklime, Slaked Lime

Calcinating limestone (composed of calcium carbonate) removes its volatile component (carbon dioxide) and results in the formation of quicklime (composed of calcium oxide) (see Textbox 33). 

This process, usually carried out in a kiln and at a temperature well above 600°C, seems to have been practiced as early as the Stone Age (Gourdin and Kingery 1975). Quicklime is a basic component of calcareous cement. Before the cement can be prepared, however, it is essential to slake (disintegrate and break up) the quicklime by the addition of water water reacts with quicklime to form slaked lime, composed of calcium hydroxide  

When the excess water (that is, the amount of water in excess of that required to bring about the formation of slaked lime) evaporates, the 

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Lime (CaCOj is a common material used for making a hard and water-resistant cement. It is found in many parts of the earth as a component of limestone. It can be chemically altered by heating to produce stronger binding agents and it can be rather easily ground to a fine powder. Lime burned at 900°C turns into quicklime (CaCOj), which can be combined with water in the absence of carbon dioxide to produce slaked lime (CaCOH) ). This wet mass sets slowly with the loss of water to produce interwoven crystals of CaCOH) and then combines with carbon dioxide in the air to produce lime (Fig. 3.12). 

The thermal decomposition of limestone - which was reported by Cato in 184 bc -at about 900 °C produces calcium oxide (lime, technically called quicklime ) (CaCOj CO2 + CaO). This is an important basic step in glass and ceramics production, and quicklime in combination with clay is also a cheap essential raw material for the cement industry. For the estimated worldwide production of cement in 1994, the consumption of limestone was about 1420x10 (metric) tons. Quicklime reacts with water to calcium hydroxide Ca(OH)2. Hydrated lime is a dry calcium hydroxide powder, while slaked lime is an aqueous suspension of calcium hydroxide particles in water. Both forms are the cheapest industrial alkaline chemical and are frequently used together with limestone as a neutralizer for acids, for example in flue gas desulfurization (see below) (Oates 1998). The annual global production of lime and lime compounds is estimated to exceed 300x10 tons, with highest amounts in China, followed by the USA, the former Soviet Union, Germany, Japan, Mexico and Brazil (Oates 2002). 

Its natural and prepared compounds are widely used. Quicklime (CaO), which is made by heating limestone that is changed into slaked lime by carefully adding water, is a great base of chemical refinery with countless uses. Mixed with sand, it hardens as mortar and plaster by taking up carbon dioxide from the air. Calcium from limestone is an important element in Portland cement. 

Already in ancient times it was known that the reaction between calcium oxide, also called lime or quicklime, and water could yield a binder in building construction. The Etrascans, for instance, added water to lime to form calcium hydroxide, or slaked lime, which they mixed with sand and stone into what today would be called a primitive concrete. The Romans discovered a way to improve cemeut making by burning a mixture of volcanic ash, which essentially consisted of silica, and lime. The many impressive constructions that have lasted to om days testify to the durability of their cement. 

Limestone is used as a building material and is a component of Portland cement. By heating it forms quicklime (CaO), which is changed to slaked lime [Ca(OH)2l hy the addition of water. This is a cheap base in the chemical industry with countless uses, e.g., as mortar and plaster. 

Active metals are highly reactive metals. Oxides of active metals react with water to produce metal hydroxides. Calcium oxide, CaO, also known as lime or quicklime, is manufactured in large quantities. The addition of water to lime to produce Ca(OH)2, which is also known as slaked lime, is a crucial step in the setting of cement. 

Some carbonates are important industrial chemicals. Calcium carbonate occurs naturally in several forms, including limestone, and is used in the production of quicklime, calcium oxide CaO, slaked (or hydrated) lime, calcium hydroxide Ca(OH)2 and cement. 

Following Black, Anderson held that pure limestone was made up of calcareous earth united with fixed air. In his words, calcareous earth was a general term denoting all those substances that consist of the matter which lime may be made, in whatever state it may be found - whether alone - or mixed with other substances, that prevent it from being reduced to powder after calcination. 33 For reasons of simplicity, he often used lime synonymously with calcareous earth. 34 Lime, as he explained, existed in three states. First, mild calcareous earth was limestone in its pre-calcined state. Second, Caustic calcareous earth was exactly synonymous with quicklime, in its strict and philosophical acceptation. 35 Finally, Effete calcareous earth was the hardened cement that formed from the quicklime which meant that it was a post-calcination form of limestone. As Anderson explained it Lime is no sooner slaked, than it immediately begins to absorb its air, and return to its former mild state or, in other words, it becomes effete, in which state it possesses the same chemical qualities, in every respect, as limestone. 36... 

Lime was often used in combination with natural cement. The Ume used by the Corps of Engineers in the 19th century was almost invariably quicklime, packed in sealed barrels in a lump condition. This material would be slaked on site, as the economic penalty for shipping lime as a paste or putty with excess water would have been out of the question. The primary objective of slaking is described as lump reduction, allowing the Ume to be distributed uniformly in the mortar and to achieve a smoother, mote workable consistency. 

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