Belite in clinkers

Also belites in clinkers are, as a rule, p phase containing different minor elements. However, on the XRD pattern some peaks are broadening and some shifted a little. The composition of these belites is unbalanced. Ca + is usually substituted by Mg +, K+, Na+, Cr3+, Mn and [SiOJ by [AlOJ -, [FeOJ lPOJ - and [SOJ - [129]. 

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. 

Thanks to numerons measnrements with electron microprobe there are tnaity chemical analysis of belites in clinkers (Table 2.12). Midgley [162] gives also the following content of foreign elements inbehtes from industrial clinkers (mass %) ... 

Beta dicalcium silicate is metastable below 670°C and monoclinic, with an n ofl.717,an n ofl.722,and of 1.736 and parallel to b birefringence is 0.019. The (h-) 2 Vis 64° to 69°, with the axial plane (010). The beta form exhibits very close polysynthetic twinning (Chromy, 1970). It comprises most of belite in clinker. Cleavage is poor on (100) and (010). Colors range from colorless to yellow or brown. Density is 3.28 Mg/m. Boikova (1980) reported a typical yellow color and spotty surface (not striated). 

Relatively large belite in clinker fines, small belite in nodules Comparatively long time in burning zone for clinker fines (Samuel, Rao, and Chopra, 1984)... 

Sbirasaka, T. Nimura, T. Kanaya, M. and Ucbida, S., "Influence of Burning Condition and Kind of Raw Materials on Formation of Cluster Belite in Clinker," Reviews, 47tb General Meeting, Cement Association of Japan, 1993, pp. 28-33. 

Studies on synthetic belites suggest the possible occurrence of modulated structures, i.e. ones in which compositional or structural variations occur with a statistical periodicity that is not a rational multiple of that of the basic structure (H5). Such effects might occur in clinker belites. 

Table 1.2 includes an average composition for belites in ordinary production clinkers, based on the results of many studies by X-ray microanalysis (T2,K1.G3,B2,G4,S1,H3,H4) or chemical analysis of separated material (Yl). As with alite (Section 1.2.3), most of the results from different laboratories are in relatively close agreement. Belites in white cement clinkers have also been analysed (B3). Pure C2S, for comparison, contains 34.9% of Si02 and 65.1% of CaO. 

Clinker belites typically contain 4-6% of impurity oxides, of which the chief ones are usually AI2O3 and Fe203. Correlations have been reported to exist between the MgO contents of belite and clinker (Kl), the Fe20, contents of belite and clinker (Kl), and the total impurity content of the belite and the MgO content of the clinker (B4). Early reports mention a compound KC23S,2, but the K2O content of 3.5% corresponding to this formula is well above those found in clinker belites, and recent electron optical work indicates that the limit of K2O substitution is about 1.2% (C1). 

Regourd et al. (R3) synthesized several belites of P-C2S type with compositions similar to those found in clinkers and determined their cell parameters. With increased substitution, the crystallinity tended to decrease, causing broadening of XRD powder peaks. Two of their results are given in Table 1.5. 

Alite may be expected to form in clinkers as the R polymorph, and belite as u l, ot n or a, according to the temperature. Chromy (C9) and Hofmanner (H21) discussed DTA curves of raw meals (Fig. 3.4). The main exotherm is due to belite formation, and the main endotherm to melting. A small... 

As it was aforementioned clinker contains phase p and seldom a [160,161]. The synthetic preparations obtained by Forest [83], of the formula Ca2M Si, 04 2 with oxygen vacancies, are similar to belite. It turned out [100] that a -C2S contains more K2O and less AI2O3. K O in clinker is principally in belite, albeit there are exceptions from this prineiple. 

XRD belite identifieation in clinker is difficult, because its peaks show coincidences with C3S and usually its eontent is too low for 120 peak, 20=31° (CukTa) to be analyzed. For elear separation of this peak should be at least 12% of P-C2S- Regourd [103] affirmed also the presenee of a -C2S in clinker on the basis of 111 peak. 

Fig. 2.63 Change of MgO content in clinker phases alite ( ), belite (o), interstitial matter (x) in relation to MgO amount in clinker. (After [183])...

According to Taylor [218] Bogue s formitlae give to low values in the case of alite and tricalcium aluminate, too high for belite and relatively correct for ferrite phases. More close to reahty results can be obtained with modified by Taylor [218] Bogue s formulae, which take into account the presence of minor components solid solutions in clinker phases. Taylor simplifying assumed that in clinker phases the quantity of minor components are on the level given in Table 2.17. 

In the last two decades the unquestionable superiority in clinker phase composition has gained XRD. The inner standard is usually used, frequently mtile or comn-dnm. The last one has advantage that it does not coincidence with clinker phases peaks in the range of 20 till 60°. Taylor [218] states that this method is not useful for belite determination, because the coincidence of its peaks with alite, which cause that the weak ones must be used. 

Geis and Knofel (1986), in a laboratory-produced belite-rich cement (56% belite), state that alpha belite may be visible as light-gray reflecting lamellae with the alpha prime polymorph in clinkers with Na contents of approximately 1.5%. Asimilar occurrence, but less distinct, was observed in clinkers with KjO of... 

Total MgO content in clinker is normally maintained at less than 2.0% to avoid expansion in the autoclave test. Belite containing periclase (at an early stage of crystallization) indicates coarse grains of MgO-bearing silicates, such as diopside, pyroxene, and hornblende in the raw feed. Dendritic periclase occurs in slowly cooled clinkers from large kilns (200 t/hr) and is hardly observed in clinkers from small kilns (20 t/ hr), according to Ono (1995). 

Note If MgO in clinker is higher than 1.8%, birefringence of alite in the table should be increased by 0.001. If MgO is less than 1.2%, birefringence is decreased by 0.001. Belite crystals with abundant dotlike impurities indicate slow cooling. Ono s numerical designations of 4, 3, 2, and 1 were placed in the table by the present writer. 

Increase in abundance of alite and belite Increase in TiO content in clinker (Knofel, 1977)... 

Photograph 7-21 Blue, subhedral alite crystals with very narrow belite fringe (almost the only belite in the clinker) and matrix of gray aluminate and dull ferrite. Average alite size is approximately 26 pm. Coal-fired, dry-process kiln, 550 tons/day. (S A6641)... 

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