Cement process chemistry

Understanding of the chemistry of autoclave processes is due primarily to the work of Kalousek and co-workers (K32,K59-K62). Above about I. SO C. for the time scales of a few hours that are used in practice, two features of cement hydration chemistry are added to those relevant at lower temperatures. Firstly, the hydration products tend to crystallize in the absence of reactive silica, C-S-H tends to be replaced by a structurally unrelated, crystalline phase, a-CjS hydrate. Secondly, the range of siliceous materials having effective pozzolanic properties is widened, and includes quartz and various other crystalline minerals, if sufficiently finely ground. 

There are over 280 million scrap tires produced annually in the United States. Of these over 100 million are used as fuel and most of these are burnt to supplement fuel use in cement and other rotary kiln operations. The cement process is particularly convenient for tire combustion because the reinforced steel wire in the tire tread can be a source of iron for the cement chemistry. Kilns burning tires must comply with the EPA s boiler and industrial furnace act and hence are heavily regulated as a pollution source. Table 6.3 gives a typical average composition of tires supplied by the Rubber Manufacturers Association of America. 

The solvation (hydration) and desolvation of ions is important to the gelation process in AB cement chemistry. The large dipole moment of ion-pairs causes them to interact with polar molecules, including those of the solvent. This interaction can be appreciable. Much depends on whether the solvent molecule or molecules can intrude themselves between the two ions of the ion-pair. Thus, hydration states can affect the magnitude of the interaction. The process leading to separation of ions by solvent molecules was perceived by Winstein et al. (1954) and Grunwald (1954). 

It is interesting that this cement has been known for over 100 years and yet certain features of its chemistry remain obscure. The exact nature of the matrix is still a matter for conjecture. It is known that the principal phase is amorphous, as a result of the presence of aluminium in the liquid. It is also known that after a lapse of time, crystallites sometimes form on the surface of the cement. A cement gel may be likened to a glass and this process of crystallization could be likened to the devitrification of a glass. Therefore, it is reasonable to suppose that the gel matrix is a zinc aluminophosphate and that entry of aluminium into the zinc phosphate matrix causes disorder and prevents crystallization. It is not so easy to accept the alternative explanation that there are two amorphous phases, one of aluminium phosphate and the other of zinc phosphate. This is because it is difficult to see how aluminium could act in this case to prevent zinc phosphate from crystallizing. 

Zhuravlev, V. F., Volfson, S. L. Sheveleva, B. I. (1950). The processes that take place in the roasting of zinc-phosphate dental cement. Journal of Applied Chemistry (USSR), 23, 121-8. 

The sensitivity of Magnetic Resonance (MR) to the local concentration, molecular dynamics and molecular environment of these nuclei make it well suited for the study of deterioration processes in concrete materials. Hydrogen (water), lithium, sodium, chlorine and potassium are all MR sensitive nuclei and play an important role in cement chemistry. The ability of MRI to spatially resolve and non-destructively examine test samples as a function of treatment or exposure has the potential to provide new insight to better understand deterioration mechanisms and mass transport properties of concrete materials. 

See also Iron entries hydration, 5 477-478 in Portland cement, 5 467 in Portland cement clinker, 5 473t classification of, 11 55-58 crystal chemistry of, 11 59-71 defined, 11 55 energy losses in, 11 64-66 physical properties of, 11 59-71 processing of, 11 71-75 properties of spinel and M-type,... 

Surface and colloid chemistry principles impact many aspects of our daily lives, ranging from the cleaners and cosmetics we use to combustion engines and cement. Exploring the range of this field of study, Surface and Colloid Chemistry provides a detailed analysis of its principles and applications and demonstrates how they relate to natural phenomena and industrial processes. 

Fresh concrete and cement mortars—which are identical from a chemical point of view—are relatively strongly alkaline (pH approximately 12.5). It later falls, however, due to the binding of carbon dioxide from the air. Depending on the special chemistry of the cement mortar, this process proceeds very slowly in the depth of the material. According to the composition of the cement mortar, this may last from a few months to many decades, until the pH value of such a mortar or concrete becomes neutral, even in the deepest layers.396"398 This chemical behavior explains the entire secret of the stability of reinforced concrete, which prevents the embedded steel from rusting further in the environment within the concrete, which remains alkaline for lengthy periods of time.399... 

The rest of the chapter deals with the hydration chemistry of Portland and composite cements at temperatures outside the range 15-25 C, including that of autoclave processes, and with specialized uses of cements in casing oil wells and in making very high strength materials. 

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