El interés del público

rences. Always pause when about to make an inter- pretation and ask “Is this observation characteristic (typical, common) for most of the sample? Does the observation represent a significant, but minor, portion of the sample, or merely a rare occurrence?” It has been said that one can find “almost anything” in clinker. Therefore, be constantly aware of a normal tendency to exaggerate the minutiae.

Now that phase analyses with electron micros- copy are available, one might foresee a routine, some- what automated microscopical basis for quality con- trol. Computer programs and equipment for automat- ing simultaneous microscopical and chemical analy- ses of polished sections do not appear to be beyond our reach and may, indeed, already be employed on a day-to-day basis in some plants. Application of the computerized scanning electron microscope and chemical analysis via energy dispersive x-ray was demonstrated by Diamond and Olek (1990) for a cement. The techniques were suggested to be appli- cable to supplementary cementitious materials, blended cements, and fly ash. However, the utility of computer-generated data, like all data, is only as good as the quality of the interpretations. Consequently, the microscopist should continue to serve in a role of objective verification, using the finely honed tech- niques of observation and inference to relate cause and effect. The value of x-ray diffraction can hardly be understated. XRD and microscopy combined are like two sides of the same coin.

Unfortunately, some of the observations and in- terpretations presented in Chapter 7 (“Microscopical Interpretation of Clinkers”) do not appear to be founded in systematic experimental design or statisti- cal analysis. Statistical measures to determine the degrees of correlation and association of the observa- tions, and their relationship to the various physical and chemical causal factors of the production process, are essential and urgently needed for several very important reasons:

1. To quantify microscopical data in order to re- move as much subjectivity as possible;

2. To strengthen the science of clinker and cement microscopy in general, that is, to fortify the foundation with statistical rigor; and

3. To enable the microscopist to communicate pre- cisely with others who are interested in quality control at the cement plant.

Little has been said in the present publication regarding these specific operational modifications because of the wide variety in types of kilns, and the large number of possible equipment changes that may be necessary to modify burning conditions. However, a few examples can be cited:

Of major importance in future work is the quanti- fication and statistical treatment of microscopical data from systematic investigations. Results of an exten- sive statistical study involving 754 clinker samples over a period of approximately two years to develop a database were described by McKenzie (1991). Spe- cific changes in the microscopy were related to equip- ment variations, failures, and other process fluctua- tions. For example, a drop in alite birefringence was related to inefficient coal mill grinding, resulting in a reduction of flame temperature. A partial blockage in a primary air duct, due to a snowman in the firing hood and cooler throat, led to an oxygen deficiency, reducing conditions, and a decrease in the burning zone temperature, indicated by lower birefringence of alite and high ash values. Cariou, Ranc, and Sorrentino (1988) utilized a statistical computer program to ana- lyze the microscopical and chemical data from more than 60 clinker samples representing many methods of pyroprocessing. As a result, after determination of the chemical composition of the raw material and definition of some of the burning-condition param- eters, a clinker mineralogy could be predicted and subsequently checked in actual production.

As a result of data from powder-mount study and polished-section examination, Prout (1985) recom- mended the following production process changes: location of burner pipe, feed rate, primary air, and the intergrinding of gypsum with kiln feed so as to pro- duce a molecular sulfur to alkali ratio of not less than 0.8. Cement quality greatly improved after the pro- posed changes were made, providing a clear-cut illus- tration of the successful application of the microscope to cement quality problems and, no less importantly, establishing support from operational personnel. For details concerning microscopical effects of changing burner pipe positions, the reader is referred to a paper by McKenzie (1989a).

Other recommended equipment changes related to the improvement of clinker quality can be found in papers by Hansen (1983) and Miller (1978). Jany and Love (1993) presented microscopical data correspond- ing to specific changes in flame profile, initial cooling, cooler bed depth, and I.D. fan, in a 158.6-meter, dry- process kiln.

Quality control of cement using microscopy finds daily use in Venezuela (Arbelaez, 1988) where statis- tical analysis of microscopical data from transmitted and reflected light, and other production variables, provides baseline parameters for problem solving and product improvement. Arbelaez described the following process changes: (1) use of the correct amount of compressed air in feed homogenization in the slurry tank to eliminate belite nests, (2) by modifying the

flame and position of the burner pipe, a shorter burn- ing zone and faster cooling rate resulted in smaller alite and better belite form, (3) maintenance of a uni- formly high heating value of the gas, combined with higher flow of primary air, to eliminate overburning but retain the high birefringence of the alite, (4) in- crease in bed depth in the grate cooler, increasing the secondary air temperature and the flame intensity, (5) a decrease in the lime saturation factor resulted in smaller alite and a minimum percentage of free lime, and (6) the feed retained on the 75-µm screen (No. 200-mesh) was corrected to 12%-14% for the wet plant and 20%-22% for the dry process.

The microscopical effects of improved clinker coolers, a new, innovative coal mill system, and con- trol of kiln feed blending and composition were de- scribed by Miller and Venable (1988). Changes in these items resulted in significant increases in 7- to 28- day concrete strengths. Specific details of equipment modifications were described along with the micro- scopical effects, illustrating several practical techniques to produce a clinker with relatively small alite, large belite, and a fine-grained matrix. Improvement of clinker appearance by cooler modification is described by Jany and Warmkessel (1987).

Recent work by Hamilton and Hamilton (1997) suggests that increasing kiln rotational speed (specific feed rate held constant) results in the following ben- efits: (a) shortening of the burning zone by 50%, (b) lower exit gas NO_x, (c) decrease in alite size, (d)

increase in quantity and quality of primary belite, (e) permits a quick quench of the clinker, reducing dust, (f) improves vaporization of alkali, (g) increases grind- ing rates by 15%, (h) increases kiln productivity, and (i) 90% to 100% return of kiln dust by insufflation.

Effects of common minor and trace elements de- rived from recycling waste materials in fuels and as raw materials for clinker production, as well as ce- ment hydration, are summarized by Uchikawa and Hanehara (1997). Crystal size and optical property variations in clinker phases (alite, belite, aluminates, and ferrite), and their hydraulic reactivities, are shown to be related to concentrations of sulphur, magne- sium, phosphorous, fluorine, chlorine, chromium, manganese, zinc, and many other elements. The ce- ment industry is based in crystal chemistry.

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. Micros- copy should be one of the first steps in that analysis.

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