Belite crystallization

Belite crystals have been observed that show only one set of striations. whose behaviour in transmitted light between crossed polars shows them to be twin lamellae. Such crystals are typically irregular in shape they have been called Type II belites, and are rare in modern clinkers. The striations arise from the aY to p transformation. Such crystals can form either by... 

At 1200-1300°C, high-temperature light microscopy showed movement of the silica cores and CS crystals the amorphous material was softening. Where the belite shell had burst, clusters of belite crystals with a central pore usually replaced the original quartz grain, but where it had not, these clusters only formed at a higher temperature and were more compact, with very little interstitial material. Results at higher temperatures are described in Section 3.4.4. 

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. 

Belite crystals with an internal microstructure of lamellae arranged in a concentric hexagonal pattern are rare. 

Type I belite crystals according to Insley (1936) were generally considered characteristic of alpha dicalcium... 

Type A—belite crystals with typical multidirectional lamellae (Photograph 7-14),... 

TypeF belites with a ring ofsmaller satellite belite crystals formed by lamellar extension and detachment (Maki s remelted variety, 1994). 

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. 

Photograph 6-4 Clear to faint yellow, round belite crystals in powder mount (44- to 75-pm fraction). Fast to moderately fast cooling. Coal- and coke-fired, dry-process kiln, 45 MPa. (S A6619)... 

Photograph 6-5 Amber belite crystals in 44- to 75-pm fraction of crushed clinker. Low maximum temperature, long burning time, slow heating rate, moderately slow to slow cooling. Coal-fired, wet-process kiln. (S A6620)... 

This parameter is the average of the longest diameters of approximately 10 crystals. Typical belite crystals are seen in Photographs 6-3 through 6-5. In selecting the crystals for measurement one should look for crystals representing the most commonly occurring crystal size. Belite size, in the writer s opinion, is best determined with polished, etched sections or thin sections. 

Belite—crystal size, optics, morphology and tex ture, recrystallization, gelappster, exclusion of impurity ... 

Large belite nests were shown to be indicative of poor grindability, even though the same clinker may have relatively high porosity. Clusters of belite crystals remained undivided even at the final grinding stage. 

Tiny belite crystals formed as a surficial decomposition of the alite during slow cooling have always been clear in the writer s observations secondary belite (formed out of the matrix during cooling) is also clear, perhaps because these crystals have little to exsolve. Research is needed on this point. Amoeboid crystals are rarely colored dendritic crystals have never been observed to be colored in the writer s experience. Ono s belite color interpretation seems most applicable to properly formed crystals. 

Colorless belite, according to Lee (1983), can occur as very small inclusions in alite and contains very little impurity. Large-crystal, ringlike belite nests (from coarse alkali feldspar) are also colorless because of iron-oxide deficiency. These occurrences of belite, therefore, do not reflect the cooling rate. Consequently, Ono (1978) recommends color observation of roughly 20 pm belite crystals in order to judge the cooling rate. 

Detailed microscopical characteristics of clinkers from vertical shaft kilns, compared to laboratory burns, are presented by Ahluwalia and Raina (1992). Alite and belite crystal sizes in plant clinkers averaged approximately 21 and 19 pm, respectively, possibly accounting for high mortar strengths (44.8 MPa at 28 days). 

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)... 

Photograph 7-12 Streak of belite crystals in sulfate-resisting cement clinker, suggesting nonuniformity in feed. Gas-fired, dry-process kiln, 770 tons/day. (S A6632)... 

Very large alite and belite crystals (averaging or exceeding 100 pm), cubic C3A, K SO, and CaSO crystals Burning with fuel oil containing 2.5 to 3.5% sulfur (Hawthorne, Richey, and Demoulian, 1981)... 

---