Alite clusters

Alite cluster, small crystals (10 pm) plus minor f-CaO or belite In same cluster Marly grain near cement composition (Fundal, 1980)... 

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). 

Ono (1995) characterized the alite in raw (poorly) burned clinker in relation to the occurrence of the alite (1) next to free lime and (2) next to a belite nest. Where alite is next to a free lime cluster, the alite is small, isolated and dispersed, separated from free lime, con tains a large amount of melt, is strongly etched, and is heterogeneous and zoned. Alite next to a belite nest is relatively large, connected and welded, touching the belite, contains a small amount of melt, is weakly etched, and is homogeneous and flat (not zoned). 

Poorly burned clinker frequently contains many small belite clusters. If the raw mix contains coarse quartz grains, belite clusters appear even in well-burned clinker, and the alite size has a wide range. 

Small, poorly formed phases very high porosity uneven phase distribution alite surrounded by large amounts of liquid phase belite in clusters Flushing or sintering temperature is too low (Hofmanner, 1973)... 

Photograph 7-7 Variation in alite crystal sizes clusters of small crystals (14 pm), noncluster crystals (38 pm). Round belite crystals in nest in upper center. High maximum temperature, long burning time, moderately slow heating rate, quickly cooled, 42 MPa. Gas-fired, dry-process kiln, 770 tons/day. (S A6627)... 

Large alite in cluster Derived from belite nest from quartz grains near critical size (44 pm) or highertemperature burning (Fundal, 1980)... 

Subhedral, pseudohexagonal, lath shaped alite and belite clusters (subround), some hexagonal belite Coal ash absorption (Rao and others, 1993)... 

Figure 8.55 SEM-BSE image of an alite grain with Mg-rich and Al-rich clusters.

Low elemental concentrations, below 1 at.%, are difficult to quantify by EDS analysis. The standardless quantification methods used in STEM may mean that low concentrations cannot be as accurately measured as by SEM, as illustrated in the following example. The alite studied contained aluminium and magnesium and Al-rich and Mg-rich clusters and can be identified as indicated by the BSE image of a polished section in Figure 8.55. 

XRE gives the total amount without differentiating between the amounts in exsolved regions and in solid solution in alite. The microscopy techniques permit the selection of regions without clusters. With SEM techniques, because of the large interaction volume, subsurface clusters not visible from the surface may be included, but the relatively large distance between exsolved regions makes this unlikely. The STEM technique allows a localised analysis of the composition, since the interaction volume is small because of the small thickness of the sample. Table 8.1 compares the results obtained with the different techniques to the calculated amount of each element from the synthesis recipe. 

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