Mineral and Chemical Admixtures

At this point, it is important to distinguish admixtures from additives, the latter being materials that are added during the manufacture of a cement. Additives include gypsum as a grinding aid and set controllers that function by inhibiting the hydration of calcium aluminate (see Section 5.2.4.2). 

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Finally, microscopical examination alone may not provide sufficient answers to the questions of clinker microstructure or a cement s inferior performance. Cement particle size distribution, variations in crystal chemistry, mineral and chemical admixtures, as well as the effectiveness of the set-controlling material (normally gypsum or similar minerals), may have stronger effects on cement hydration than the clinker production problems inferred by routine microscopy. Some clinker and cement problems, however, are simple and easily solved others require the analysis of a tangled set of multiple causes and effects. Microscopy should be one of the first steps in that analysis. 

Hobbs, D.W. (1989) Effect of mineral and chemical admixtures on alkali aggregate reaction, in Proceedings 8th International Conference on AAR, Kyoto, pp. 173-186. 

Civjan, S. A., LaFave, J. M., Lovett, D., Sund, D. J., and Try-bulski, "Performance Evaluation and Economic Analysis of Combinations of Durability Enhancing Admixtures (Mineral and Chemical) in Structural Concrete for the Northeast U.S.A., NETCR36 Project No. 97-2, The New England Transportation Consortium, February 2003. 

For special high strength appHcations, ie, up to 69 MPa (10,000 psi), special formulations of Pordand cement concretes have been developed. These ate based on the use of chemical and mineral admixtures. The typical mineral admixtures ate fumed siUca and other po22olanics. The chemical admixtures ate generally chemicals termed supetplastici2ets that allow very low water to cement ratios, ie, between 0.4 and 0.25, and reduce the amount of water needed to provide plasticity or dow to the concrete. PubHc works appHcations take just under 32% of the total Pordand cement market streets and highways represent 68% of this usage, and water and waste account for 23%. 

The relative reactivity of the different mineral phases of cement with water is usually given as C A>C S>C S>C AF. Aluminate phases and their hydration products therefore play an important role in the early hydration process. Because of the high reactivity of calcium aluminate, the aluminate hydration reaction is carried out in the presence of sulfate ions. The latter provide control of the reaction rate through the formation of mixed aluminum sulfate products (ettringite and monosulfoaluminate) Calcium sulfate which is added to the cement clinker hence controls the properties of the aluminate hydration products. Sulfates thus play a crucial role in cement hydration and the influence of chemical admixtures on any process where sulfates are involved may be expected to be significant [127],... 

Their physical properties are listed in Table 4. FA and SF were also used as mineral admixtures. The physical properties of each admixture are listed in Table 5 and 6. For the chemical admixtures, polycarboxylate-based superplasticizer and negative ion type AE agent were used. 

Mineral additions mnst be distinguished from chemical admixtures, which are added to the binder only in small amounts (usually below 1%), and which influence the properties of the fresh mix or the hardened paste, but do not alter snbstantially the product of the hydration reaction. 

Wood consists of 40-50% cellulose, 20-30% hemi-celluloses, 21-23% lignin, 0.2-0.5% protein, and 1-5% mineral matter, which is left behind as ash when the wood is burned. Besides its straightforward use as timber, much wood is pulped for use in papermaking. Lignin is extracted in the process by either sulfonation or chlorination. Sulfonation produces lignosulfonates, which are widely used as dispersing agents, for example as chemical admixtures for concrete to disperse the cement particles (see the section on Cement and Concrete Admixtures, below). 

Billions of years were required for the formation of the earth s crust with its minerals and ores—a process bearing witness to many whims of nature which, to be more exact, reflect the laws of geochemistry. Some elements were less fortunate they did not succeed in forming their own minerals, that is, those in which they would be the principal or, at least, a noticeable component. They exist only as admixtures to all sorts of minerals consisting of other elements. They seem to be widely dispersed in the earth s crust and are called trace elements. Only in the rarest cases do they form their own minerals and if the scientists were lucky to come across them, the new element immediately became the target of chemical analysis. As we shall see later, this was the case of germanium extracted from argyrodite, a uniquely rare mineral. 

Zirconium is extracted mainly from the siUcate mineral, zircon (ZrSi04), which is found as a beach sand, usually in association with other minerals of commercial value, in Australia, India, Madagascar, Malaya, Ceylon, parts of Africa and the United States. The mineral is obtained in a relatively pure condition by ore dressing methods, except that it is usually associated with about 2 per cent of the isomorphous, and chemically very similar, hafnium silicate. The sihcate has been used directly, in admixture with charcoal, for chlorination to zirconium tetrachloride. The reaction can be represented as,... 

The large group of cements and concretes covers many traditionally used materials that have for some 20 years been the subject of rapid development and innovation. Also, their variety has been extended, mainly due to use of various polymer additions and chemical and mineral admixtures. In this chapter the main kinds of cements and concretes are introduced briefly. 

Concrete, made from cement, aggregates, chemical admixtures, mineral admixtures, and water, comprises in quantity the largest of all synthesized materials. The active constituent of concrete is cement paste and the performance of concrete is largely determined by the nature of the cement paste. Admixtures are chemicals that are added to concrete for obtaining some beneficial effects such as better workability, strength, durability, acceleration, retardation, air entrainment, water reduction, plasticity, etc. Mineral admixtures, such as blast furnace slag, fly ash, silica fume, and others, are also incorporated into concrete to improve its quality. 

The dissolution of zinc in a mineral acid is much faster when the zinc contains an admixture of copper. This is because the surface of the metal contains copper crystallites at which hydrogen evolution occurs with a much lower overpotential than at zinc (see Fig. 5.54C). The mixed potential is shifted to a more positive value, E mix, and the corrosion current increases. In this case the cathodic and anodic processes occur on separate surfaces. This phenomenon is termed corrosion of a chemically heterogeneous surface. In the solution an electric current flows between the cathodic and anodic domains which represent short-circuited electrodes of a galvanic cell. A. de la Rive assumed this to be the only kind of corrosion, calling these systems local cells. 

Modem concretes often incorporate a mixture of chemical and mineral admixtures, each of which may interact with the various constituents of cements and influence cement hydration reactions. The admixture-cement interactions may in fact be viewed as the reaction between two complex chemical systems - the multicomponent, multiphasic inorganic materials in the cement and the organic compounds of multicomponent admixture systems. For example, lignosulfonate water-reducers are intrinsically complex mixtures of chemical compounds derived from the chemical degradation of lignin, while synthetic admixtures such as superplasticizers contain species with a broad distribution of molecular weights, reaction products, or other chemicals added for a specific purpose [125]. The performance of an admixture in concrete is highly dependent on many... 

McCarthy, G. J., Solem, J. K., Manz, O. E. Hassett, D. J. 1990. Use of a database of chemical, mineralogical and physical properties of North American fly ash to study the nature of fly ash and its utilization as a mineral admixture in concrete. Materials Research Society Symposium Proceedings, 178, 3-34. 

Chemical composition of the mineral admixture - coal burning fly ash is presented in Table 5. Because of the lower content of Si02 and high content of CaO this fly ash was not so suitable for the cement production. It also has the increased loss on ignition, yet complying with the Yugoslav standard JUS B.C1.018. It also could be seen that the MgO, alkalies, and S03 content were in the limits of the mentioned Yugoslav standard. 

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