Portland Cement Clinker Burning

Raw mix, composed of limestone, marl and small addition of iron corrective component is transformed, as the effect of several complicated reactions, in clinker containing from 55 to 65 % C3S, 15-25% C2S, 8-12% CjAand 8-12% C2(A, F). 

It is convenient to follow the advancement of clinkering process by determining the free calcium oxide in the raw mix, called frequently free lime . The content of this component on the beginning is increasing quickly, as a result of calcium carbonate decomposition, and then is gradually decreasing (Fig. 2.1), as the new compounds are formed with silica, alumina and iron. 

In the process formation two stages can be distinguish at the range of lower temperature to about 1300 °C, when the reactions proceed, as a rule, in the sohd state, in the presence of very low quantities of the liquid phase, and at higher temperature, at which it is already about 25 % of the melt. It is assumed that the dicalcium sihcate and calcium aluminate and ferrite are formed principally as the result of reactions in the sohd state. However, tricalcium silicate is formed by ciystalhzation from the liquid phase. 

The kinetic of this reaction was studied by Jander [3 ], which assumed that its rate is governing by the diffusion of one substrate (CaO) through the layer of the product formed to the surface of second substrate (SiOj). Under different simplifying assumptions, using first Pick s law, Jander derived the formula 

The layer thickness can be easily replaced by degree of transformation  

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ASTM C845 Type E-I (K) expansive cement manufactured ia the United States usually depends on aluminate and sulfate phases that result ia more ettriagite formation duriag hydration than ia normal Portland cements. Type K contains an anhydrous calcium sulfoaluminate, C A SI. This cement can be made either by iategraHy burning to produce the desired phase composition, or by intergrinding a special component with ordinary Portland cement clinkers and calcium sulfate. 

The burning temperature for production of Portland cement clinker can be decreased by about 150°C through the use of fluxes, but opinions have differed as to the energy saving thereby obtainable, Klemm and Skalny (K52), who reviewed the subject, estimated it at 630kJkg" . Christensen and Johansen (C56) considered that this figure, while possibly realistic for an inefficient, wet process kiln, was unlikely to be so for a modern, precalciner-preheater kiln, in which heat recovery is efficient. They considered a value of lOSkJkg" more realistic. 

Burning temperature for Portland cement clinkers ca. I450°C... 

Composition of ferrite phase in clinker depends in great degree on the A/F ratio and also on the burning conditions. The compositions ate ranging from CgAp2 through C AF to CgAjF. Many authors found that in Portland cement clinkers of common composition ferrite phase has the composition close to brownmillerite [171, 180]. Ferrite phase has always the orthorhombic synunetiy. 

Long, G.R., "The Effect of Burning Environment on the Microscopic Characteristics of Portland Cement Clinker," Proceedings of the Fourth International Conference on Cement Microscopy, International Cement Microscopy Association, Las Vegas, Nevada, 1982b, pp. 128-140. 

Suzukawa, Y. Kono, H. and Fukimaga, K., "High-Temperature Burning of Portland Cement Clinker," Reviews, 18th General Meeting of the Cement Association of Japan, 1964, pp. 96-99. 

Fig.11 Portland cement clinker under-burned (porous) micrograph obtained with reflected light free lime black pocket belite light-coloured textured areas alite dark textured areas pores (here filled with resin) grey areas with grinding scratches...

The result of the burning process is Portland cement clinker, consisting of the clinker phases. 

In the manufacture of Portland cement, many otherwise-waste materials can be used either as a substitute for the traditional raw material, or as a secondary fuel (e.g., used tires) [334,1577]. In particular, drilling wastes can be introduced in the clinker burning process [878]. For both waste disposal and cement manufacturers, a mutual benefit will emerge. The cement manufacturing companies reduce their demand for traditional raw materials and save the limited capacity of landfills and other waste-treatment industries. 

Two types of materials are necessary for the production of portland cement one rich in calcium (calcareous), such as limestone or chalk, and one rich in silica (argillaceous) such as clay. These raw materials are finely ground, mixed, and heated (burned) in a rotary kiln to form cement clinker. 

Liquid or pulverized solid fuels are blown into the kiln through a nozzle with primary air. Additional secondary air is drawn into the kiln through the clinker cooler. The flame in the rotary kiln must meet several requirements. The clinker must be correctly burned, so as to minimize its content of free lime, with the least expenditure of fuel. The ash from a solid fuel must be uniformly absorbed by the clinker. For normal Portland cements, the conditions must be sufficiently oxidizing that the iron is present as Fe however, for white cements, mildly reducing conditions may be preferable. Proper flame control also extends the life of the refractory lining of the kiln. Computer-aided or fully automated control of kiln operating conditions is increasingly used. 

Portland cement is an aluminosilicate powder which sets to a solid mass on treatment with water. It is usually manufactured by grinding limestone and clay to a fine powder, mixing with water to form a slurry, and burning the mixture, with a flame of gas, oil, or coal dust, in a long rotary kiln. At the hot end of the kiln, where the temperature is about 1500 C, the aluminosilicate mixture is sintered together into small round marbles, called clinker. The clinker is ground to a fine powder in a ball mill (a rotating cylindrical mill filled with steel balls), to produce the final product. Over 100,000,000 barrels of cement per year is made in the United States. 

S. Brown deserves special recognition for his observational skills and interpretive acumen. Brown worked for Lone Star Research Laboratory in Hudson, New York, in the 1930s and in 1940 joined the research staff at the Portland Cement Association, where he spent approximately 25 years in cement and concrete investigations. Most of his scientific efforts were dedicated to the microscopical interpretation of clinker burning, cement hydration, and concrete deterioration. An unpublished report (Brown, 1936) contains the following interesting observations ... 

Brown s most widely known published work is his Microscopical Study of Clinkers (1948) in which 21 different lots of clinkers were microscopically studied at the Portland Cement Association laboratories in order to correlate mineral composition with what Brown termed the degrees of burning in the cement kiln and concrete durability. Although Brown s description and interpretation of what he termed the glass and dark prismatic phase may be questionable in the light of recent research, his work in clinker and concrete microscopy was seminal. Brown contributed significantly to the discussion of clinker phases in a book by Insley and Frechette (1955). 

Ono s method and theory of kiln control were introduced to the Western world by Mau (1975). In that same year Ono conducted a seminar for North American cement-company personnel in Hawaii, where he taught the details of his theories and method of kiln control with powder-mount microscopy. The dissemination of Ono s technique to the Western world was largely due to this seminar. Since that time Ono s theories and method of clinker interpretation have been subjects of research in laboratories of many North American cement companies, the Portland Cement Association, and in Europe. Mau (1979) reported on the routine application of the Ono technique in Hawaii and strongly supported Ono s Method and theories, stressing their use to control burning temperature. 

As an economical and rapid method to control the quality of portland cement, the value of routine clinker microscopy should be an inescapable conclusion from the numerous observations and interpretations given on previous pages. Quality control of clinker without microscopy of raw feed, in the writer s opinion, is less than adequate. Profound cause-effect relationships exist between the raw feed/particle size distribution, energy required for grinding and burning, clinker quality, and cement performance. Visually appreciating the characteristics of raw feed via microscopical examination gives additional comprehension to quality control. 

It has been reported (Albats and Shein, 1997) that the strength of Portland cement with an elevated belite content may be increased by using a very high heating rate in the clinker burning process (as high as 20 °C/min) and by simultaneously increasing the... 

Table 2 Chemical transformations in the thermal treatment of Portland cement raw meal (principal reactions in clinker burning)...

It can reasonably be presumed that the chemical reaction pattern, the actual phase content and the strength of portland cement are at least loosely interassociated. For one thing, the new phases formed in the burning process (clinker phases) are dependent on the chemical character of the raw material. Furthermore, the strength-determining hydration products (hydration phases) are formed by reaction with water from the clinker phases. 

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