Cement-chemical compounds

A unique key to substance data for cement-chemical compounds and to the work with cement-chemical calculations is available in the work o Babushkin, Matveyev Mchedlov-Petros an Thermodynamics of Silicates, Springer-Verlag, Berlin. 

There are four chemical compounds that are identified as being the active components of cements. 

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... 

The cement chemical notation refers to C = CaO, P = P2Os, H = H20 (see Figure 6.4). In addition to compounds occurring in the water-free binary Ca0-P205 phase diagram shown in Figure 6.4, the table lists also hydrated compounds. 

Chloroaluminates Chemical compounds formed in concrete when chlorides combine with the C A in the hardened cement paste. These chlorides are no longer available to cause corrosion. Sulphate resisting cements have a low C A content and are more prone to chloride induced corrosion than normal Portland cement based concretes. 

These are composed of fine mineral particles (bentonites, ground limestone, ground dolomite or lime, rock dust) and chemical compounds (e.g. sulpho-nates) in order to limit water migration when added as 1-2% of mass of Portland cement. Other effects are the workability improvement and decrease of mixing water by 5-10%. 

The normal mechanism of cement hydration is altered in the presence of wood extractives. There are many cases in which wood is used in contact with concrete. For example, cement is mixed in or molded with wooden forms, or cement and wood are mixed together for board production. Cement yields calcium ions when mixed with water and exhibits a pH of 13. The alkaline solution dissolves extractives from wood into the slurry. The resulting substances have a low solubility and they precipitate with CaO. CaO is one of the most important chemical compounds in concrete, and its loss retards the normal hydration reaction (35). These occurrences are dependent not only on the kinds of extractive, but also on whether the exposed wood surface is a transvers or a longitudinal section. 

Corrosion inhibitors are chemical compounds that, when added in adequate (preferably small) amounts to the concrete mixing water, can prevent corrosion of the rebars. These admixtures should not adversely affect the concrete properties (e.g. compressive strength) and the nature and microstructure of the cement paste. Inhibitors -well known in other areas of industry - have been studied since 1960 for reinforcing steel in concrete, mainly in relation to chloride-induced corrosion. A summary of information on steel corrosion inhibitors in concrete have been published (Page and Ngala, 1999 Elsener, 1998) and a state of the art report is to be published shortly by the European Federation of Corrosion (EFC, 1999). 

Pack cementation is the most widely used process for making diffusion aluminide coatings. Diffusion coatings are primarily aluminide coatings composed of aluminum and the base metal. A nickel-based superalloy forms a nickel-aluminide, which is a chemical compound with the formula NiAl. A cobalt-based superalloy forms a cobalt-aluminide, which is a chemical compoimd with the formula CoAl. It is common to incorporate platinum into the coating to improve the corrosion and oxidation resistance. This is called a platinum-aluminide coating. Diffusion chrome coatings are also available. 

Divide the following physical variables into intensive and extensive variables, respectively. Then show by examples how to convert the extensive variables into intensive variables. 1) The mass m (kg) of a system of substances. 2) The modulus of elasticity E (MPa) of a steel specimen. 3) The viscosity 77 (Pa s) of a given saline solution. 4) The electric charge Q (C) on a charged condenser plate. 5) The molar mass M (g/mol) of a chemical compound. 6) The cement content C (kg) in a given concrete specimen. 7) The elongation (m) of a loaded test steel rod. 8) The coefficient of thermal expansion a (K ) of pyrex glass. 

In addition to the four compounds discussed above, the final Portland cement may contain gypsum, alkali sulfates, magnesia, free lime and other components. These do not significantly affect the properties of the set cement, but they can influence rates of hydration, resistance to chemical attack and slurry properties. 

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