Cementation, definition

Cement is a powder, which mixed with water forms plastic mass, easy to shape (paste), setting with time and hardening gradually with strength increase. In marty standards the more exact Portland cement definitions were introduced. It is necessary because the name of cement is given to other binding materials, for example to anhydrite or magnesium oxychloride (the last is called Sorel cement). 

Before proceeding further it is well to consider the term cement, for its definition can be the source of some confusion. Both the Oxford English Dictionary and Webster give two alternative definitions. One defines a cement as a paste, prepared by mixing a powder with water, that sets to a hard mass. In the other a cement is described as a bonding agent. These two definitions are quite different. The first leads to a classification of cements in terms of the setting process, while the second lays emphasis on a property. In this book the term cement follows the sense of the first of these definitions. 

The cement-forming reaction is a special case of an acid-base reaction so that concepts of acid, base and salt are central to the topic. In AB cement theory, we are concerned with the nature of the acid-base reaction and how the acidity and basicity of the reactants are affected by their constitution. Thus, it is appropriate at this stage to discuss the various definitions and theories available. 

The Arrhenius definition is not suitable for AB cements for several reasons. It cannot be applied to zinc oxide eugenol cements, for these are non-aqueous, nor to the metal oxychloride and oxysulphate cements, where the acid component is not a protonic acid. Indeed, the theory is, strictly speaking, not applicable at all to AB cements where the base is not a water-soluble hydroxide but either an insoluble oxide or a silicate. 

The theory of Bronsted (1923) and Lowry (1923a, b) is of more general applicability to AB cements. Their definition of an acid as a substance that gives up a proton differs little from that of Arrhenius. However, the same is not true of their definition of a base as a substance capable of accepting protons which is far wider than that of Arrhenius, which is limited to hydroxides yielding hydroxide ions in aqueous solution. These concepts of Bronsted and Lowry can be defined by the simple equation (Finston Rychtman, 1982) ... 

This concept covers most situations in the theory of AB cements. Cements based on aqueous solutions of phosphoric acid and poly(acrylic acid), and non-aqueous cements based on eugenol, alike fall within this definition. However, the theory does not, unfortunately, recognize salt formation as a criterion of an acid-base reaction, and the matrices of AB cements are conveniently described as salts. It is also uncertain whether it covers the metal oxide/metal halide or sulphate cements. Bare cations are not recognized as acids in the Bronsted-Lowry theory, but hydrated... 

The Lewis definition covers all AB cements, including the metal oxide/metal oxysalt systems, because the theory recognizes bare cations as aprotic acids. It is also particularly appropriate to the chelate cements, where it is more natural to regard the product of the reaction as a coordination complex rather than a salt. Its disadvantages are that the definition is really too broad and that despite this it accommodates protonic acids only with difficulty. 

It is better than the Lewis theory for describing acid-base cements, for it avoids the awkwardness that the Lewis definition has with protonic acids. However, as Day Selbin (1969) have observed, the generality of the theory is such that it includes nearly all chemical reactions, so that acid-base reactions could simply be termed chemical reactions . 

Also shown is how the topic of AB cements relates to these definitions. An ideal definition for a subject should be one that exactly fits it. It should cover all aspects of the subject while excluding all extraneous topics. Thus, a theory should be neither too restrictive nor too general. The Arrhenius and Germann-Cady-Elsey definitions do not relate to the subject at all as... 

Water as the solvent is essential for the acid-base setting reaction to occur. Indeed, as was shown in Chapter 2, our very understanding of the terms acid and base at least as established by the Bronsted-Lowry definition, requires that water be the medium of reaction. Water is needed so that the acids may dissociate, in principle to yield protons, thereby enabling the property of acidity to be manifested. The polarity of water enables the various metal ions to enter the liquid phase and thus react. The solubility and extent of hydration of the various species change as the reaction proceeds, and these changes contribute to the setting of the cement. 

Industrial furnaces are enclosed units that are integral parts of a manufacturing process and use thermal treatment to recover materials or energy from hazardous waste. These units may use hazardous waste as a fuel to heat raw materials to make a commodity (e.g., a cement kiln making cement) or the unit may recover materials from the actual hazardous waste (e.g., a lead smelter recovering lead values). The following 12 devices meet the definition of an industrial furnace12 ... 

Many dietary and environmental factors have been implicated as possible etiologic factors in the development of pancreatic cancer, but no definite causal relationships have been established. The strongest evidence points to cigarette smoking as a risk factor associated with pancreatic cancer (20-24). Occupational exposure to certain chemicals has also been linked to pancreatic carcinoma (25). Others in the high-risk group include stone miners, cement workers, gardeners, textile workers, and leather tanners (17,26). 

It is possible that a pretreatment step is required for organic ion-exchange materials before immobilization, although it is not definitely the case. The immobilization matrices currently used are cement, bitumen, and some polymers. 

In this technique, the material, say ethyl cellulose, is first plasticized and then a sleeve of a definite size and shape is extruded. This sleeve is applied to the grain by a special technique using ethyl cellulose cement which is prepared by dissolving ethyl cellulose in cellosolve-diethyl phthalate (1 1) mixture or ethyl lactate-butyl acetate (1 4) mixture. 

Carat, definition of 287, cassius, purple nt, J4, cementation, 362. 1 chemical examination of ores, 267. chlorides of, 203 ... 

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