Alite crystallization

Several workers have discussed the kinetics of the clinker-forming reactions (J5,B31,T1,C11,C15,C16,G26). Because the material is not uniform, different factors may dominate at different times and in different regions (Tl) but the principal one appears to be diffusion of Ca through the liquid present between the alite crystals in the layers coating the lime clusters (Cl 6). 

Alite crystallizing from the melt has very differentiated morphology. Frequently of very disturbed constitution. Maki [42] distinguishes two periods of alite crystals growth quick and in condition close to equilibrium. The crystals formed during the quick growth period contains the inclusions of liquid phase. They are usually in the core of crystals. In the similar conditions the inclusions of small crystals of C2S and CaO as well as air bubbles can be formed. The latter, according to Maki, as... 

Different 38 phases, in the case of low content of belite in clinker, is most convenient to identify using light microscopy. Among others, Metzger [161] applying this method found the presence of phase a. The striations of belite crystals always observed in industrial ehnkers due to polysynthetic twinning, are distinctly differentiate this phase from alite crystals and are formed in the polymorphic transformation process. 

The subject of clinker grindability also has microscopical aspects and the most complete literature survey, to date, is that of Hills (1995) who enumerated most of the prevailing agreed-upon relations (such as decreasing alite crystal size increasing grindability). Other variables on which the interpretations were not as clear cut (such as percent liquid phase) were also listed. 

Photograph 3-2 Blue coloration on alite with an otherwise uniform tan color on alite crystals. Possible explanations include differences in crystallographic orientation, chemical composition, structural state (for example, monoclinic versus triclinic), or perhaps combinations of these. (S A6612)... 

Therefore, any cross section of typical clinker displays (1) the more or less loosely tied framework of alite crystals, (2) belite that occurs as single crystals and as concentrations, and (3) a matrix of aluminate and ferrite formed as the molten liquid cools and crystallizes. Microscopical observations clearly suggest aluminate (CjA) crystallizes after the ferrite, the latter forming a prismatic crystal mesh, the holes of which are partially filled with aluminate. Ferrite can be seen within alumi nate and, extremely rarely, vice versa. The matrix com monly contains secondary belite and shows effects of reaction with alite. Voids remain in areas not filled by the liquid, forming sites for crystallization of alkali sulfates on the cavity walls. Thus, the typical clinker is a somewhat porous mass of interlocking crystals, a truly glassless crystalline mosaic. Recent studies of the sequence of crystal development in the production of Portland cement clinker can be found in papers by Imlach and Hofmanner (1974), Moore (1976), Ono (1981, 1995), Chromy (1974, 1982), and Maki (1982, 1995). 

Alite crystal chemistry was discussed in a paper by Ono (1974) in which he described changes in the atomic structure of alite in response to such variables as solid solution, exsolution, thermal vibration, states of disorder, inversion, and partial decomposition. 

Burning too near the discharge end of the kiln, where the temperature change is 1400°C to 1000°C, also produces small alite, and the clinker is usually poorly burned. Alite sizes of less than 15 pm in a 1000 tons-per-day kiln can be indicative of poor burning a 20-pm alite size is typical of poor burning in a 4000 tons-per-day kiln (Ono, 1980c). A well burned clinker (f-CaO < 0.6%) does not have alite crystals under 20 pm (Ono, 1995). 

Insert the Senarmont compensator, properly oriented so that the darkness of the background results in cross-polarized light. Place a green filter in the path of the incoming light. The green filter facilitates observation of the alite crystal compensation point determined in the next step. 

Select a bright alite crystal (one that exhibits the maximum interference color in cross polarized light) and rotate the microscope stage so that the crystal is at its extinction position (dark). 

Insert the gypsum accessory plate (wavelength = 530 nanometers). The interference color of the alite crystal will be first order red (530 nanom eters), as seen on the Chart. 

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