Important cement-formers

The most important of the phosphate bonded cements are the zinc phosphate, dental silicate and magnesium ammonium phosphate cements. The first two are used in dentistry and the last as a building material. Copper(II) oxide forms a good cement, but it is of minor practical value. In addition, certain phosphate cements have been suggested for use as controlled release agents. The various phosphate cements are described in more detail in the remainder of this chapter. 

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The glass polyalkenoate cement, formerly known as the glass-ionomer cement, was invented by Wilson and Kent in 1969 (Wilson Kent, 1973) and is now well established as a material that has an important role in clinical dentistry. It has proved to have considerable development potential and has been subjected to continuous development, improvement and... 

Regeneration of S02 from gypsum has been done via the Krupp-OSW process, the most important facility formerly being the Fedmis plant in South Africa, which made about 300 tons per day H2S04 and a similar amount of cement. Apparently, because of the high cost of cement and sulfur in this remote location, the plant was not economically viable and shut... 

Cementation. Cementation is the precipitation of copper from copper leach solutions by replacement with iron. It was formerly the most commonly used method of recovering copper from leach solutions but has been replaced by solvent extraction—electro winning. The type of iron used ia cementation is important, and the most widely used material is detinned, light-gauge, shredded scrap iron. This operation can be performed by the scrap iron cone (Keimecott Precipitation Cone) or a vibrating cementation mill that combines high copper precipitation efficiency and reduced iron consumption (41). 

This phrase is understood as a process which maintains a population or community as an entity (Khailov, 1971). Particular metabolites act as signals , either attracting or repelling other fish, while others have trophic significance. The former have already been given close consideration in a number of books on behaviour and ethology, so need no further comment here. The latter captured the attention of researchers relatively recently. It was found that the uptake of metabolic products is an important factor that cements aquatic ecosystems, from plants to lower animals (protozoans, coelenterates, molluscs and some others). Metabolites that play a trophic role are inorganic... 

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). 

Chlorides in concrete and critical chloride levels. Chlorides in concrete exist in two basic forms, so-called free chlorides and boimd chlorides. The former are mobile chlorides dissolved in the pore solution, whereas the latter type represents relatively immobile chloride ions that interact (by chemical binding and/or adsorption) with the cement paste. At first glance, it may appear that only the free chlorides should be considered for corrosion reactions. However, Glass and Buenfeld have recently reviewed the role of both bound and free chlorides in corrosion processes in detail and have concluded that both types may be important. Boimd chloride may essentially buffer the chloride ion activity at a high value, and localized acidification at anodic sites may release some bound chloride. 

Out of all the unshaped refractory material subdivisions, refractory castables are the most important. These are produced as dense or insulating materials. Formerly, these were made as mixtures of aggregates and calcium aluminate cements. After continuous improvement, they are produced on a very high technical level today. Because of their varieties, they are further classified as follows. 

The family of the refractory castables is certainly the most important product group. Refractory castables can be either dense or insulating materials. The former relatively simple mixtures consisting of aggregates and calcimn aluminate cements have been continuously improved so that they are on a very high technical level today. This called for differentiation. The dense castables are classified according to EN 1402-1 as follows ... 

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