Alite Size

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). 

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). 

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Portiand cement clinker structures (18,19) vary considerably with composition, particle size of raw materials, and burning conditions, resulting in variations of clinker porosity, crystallite sizes and forms, and aggregations of crystallites. Alite sizes range up to about 80 p.m or even larger, most being 15—40 )J.m. 

Long (LIO) noted that alite size is indirectly affected by both bulk composition and fineness of the raw feed. Increases in LSF, SR or AR all make a clinker harder to burn, which tends to increase the size, though with increased SR or AR, the effect is counteracted by decreases in the quantity and mobility of the liquid, both of which tend to decrease the size. The... 

Alite size was related to bulk clinker SO3 in the following linear equation ... 

For example, assume the following data alite size (AS) 50 im (slow heating rate), alite birefringence (AB)... 

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. 

Mor and Perez (1994) presented a critical evaluation of Ono s method (powder mount only) using laboratory heating stages, concluding that the alite size and alite birefringence did not correlate. Correlation was said to be much better with regard to belite size and color. The correspondence between laboratory and industrial kiln microscopy was seriously questioned and differences in environmental conditions, mainly atmospheric composition, were alleged to be responsible. 

Lee (1983) stated that kiln variables such as feed granulometry, feed composition, fuel characteristics, and alkali sulfur circulation are more or less fixed in a normal kiln. Therefore, alite size reflects the heating rate, as Ono has maintained in many papers. 

A partial least-square regression analysis was applied by Svinning and Bremseth (1993) on alite crystal size and various process parameters, showing that alite size accounted for only approximately 40% of the total variance and that an increase in the >60-pm crystals occurred with decreasing secondary air temperature measured in the cooler. Statistical analyses of this type, in the present writer s opinion, are greatly needed in our industry to decipher relative importance of the many kinds of measurements. 

Increase in silicate abundance, decrease in liquid, higher temperature required for oombination of feed ingredients, deorease in alite size Relatively higher silica ratio (Long, 1982b)... 

Increasing alite size, densification of clinker Relatively lower alumina ratio (Long, 1982a)... 

Alite size ranges by factor of two at distance of 100-200 pm Insufficient diffusion due to low temperature as a result of too brief burning (Tsuboi and Ogawa, 1972)... 

Alite size decreasing, alite abundance increasing Increasing burning temperature from 1500°C to 1700°C (Suzukawa, Kono, and Fukunaga, 1964)... 

Alite size variation Absorbed nests or prior nonhomogeneities (Gille and others, 1965)... 

Wide variation in alite size, small alite Nonhomogeneous raw mix locally high lime saturation factor (Fundal, 1980)... 

Alite size is small, dominance of Mj variety Rapid crystallization during 10 minute retention at 1600°C (Maki and Goto, 1982)... 

Increase in alite size and idiomorphism Increasing TiO content in clinker (Kndfel, 1977)... 

Increase in alite size and idiomorphism Lowered viscosity of melt resulting from increase in clinker sulfate and degree of sulfatization (sulfate to alkali ratio) (Strunge, Knofel, and Dreizier, 1985)... 

Decrease in alite size Rapid burning, resulting in growth rate of crystals slower than formation of nuclei (Butt, Timashev, and Starke, 1973)... 

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)... 

Photograph 7-25 Experimental laboratory burn with raw mix containing marl instead of quartz as a silica source. Resulting belite was well scattered in clinker and, to a minor extent, as nests. 1000°C for 30 min, 1425°C for 10 min. Very rapid temperature change. Average alite size = 20 pm. Clinker courtesy of Joe Garcia, Capitol Cement, San Antonio, Texas. (S A6645)... 

Increase in free lime and belite percentages decrease in alite and interstitial percentages decrease in alite size Correlated with increase in porosity (Akatsu, Monna, and Maeda, 1965)... 

A relatively "hard-to-burn" feed from the western U.S., with a very impure dolomitic limestone, has 6.7% >125 pm and an acetic-acid insoluble residue of 9.5% >45 pm, the latter comprised of quartz, feldspar, medium to finely crystalline igneous and metamorphic rock fragments, and an abundance of ferro-magnesian minerals (mainly amphiboles and pyroxenes). = 1.6% and = 4.5%. Belite nests (many with tightly packed crystals), solitary belite, and periclase are abundant in this fine- to medium-crystalline clinker. Nevertheless, a high-compressive strength mortar (44.8 to 48.3 MPa) is made, mainly because of the small alite size and the well-scattered solitary belite. 

Recent work by Hamilton and Hamilton (1997) suggests that increasing kiln rotational speed (specific feed rate held constant) results in the following benefits (a) shortening of the burning zone by 50%, (b) lower exit gas NO, (c) decrease in alite size, (d)... 

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