Cement, Energy and Environment

·--- -- produces smaller alite crystals which are considered to be more reactive. In a slow burned clinker crystal growth by cannibalism is not usually found but in this example the large crystals appear to have formed from the amalgamation of several crystals by this process . The shape of the smaller alite crystals is also of interest. Those at the top right of Figure 2 are well-separated and euhdral. Those at the bottom left are of less well-formed shape and are tending to merge together. The former situation is typical of type M3 alite while the latter are more like the M1 alite variety. The difference has been described as being due to the impurities present in the clinker, primarily the balance between sulphate and magnesia. It was known that the raw feed which produced this cli nker contained some magnesia. It was know that the raw feed which produced this clinker contained some magnesia and was fired using a high-sulphur fuel. Having seen these differences within one field of view, the microscopist wou ld be investigating other possible indicators of poor homogenization of the raw feed components, leading to local preponderances of either so3 or magnesia relative to each other. The relative proportions of each of these types of alite and the different sized crystals will have implications for the potential quality of the clinker because finer, M1-type crystals are considered more reactive than coarser of type M3 crystals. Detail of belite On the left side of Figure 3 is a cluster of rounded crystals, generally brownish in colour but quite variegated and with a tendency to a lamellar appearance. These are belite crystals and Figure 3 shows the cluster to contain some light– coloured interstitial liquid phase although the crystals are quite tightly packed. In a coal-fired kiln this might be thought to be the result of coa l ash from insufficiently ground coal falling on the clinker bed and not being able to combine with free lime in the short journey through the hottest part of the burning zone. In a kiln fired by lower-ash fuels the cluster will have come from poorly-ground raw materials, particularly shales or sands with a high feldspar content. The lamellar structure is the result of a phase transition from a to a-prime on cooling. The origin and exact development of the lamellae and even their nature is a matter of debate still, but certainly they indicate that the crystal growth occurred at high temperature and the relatively small size of the crystals, averaging microns, indicates that the high temperature was experienced for a relatively short time because these a belite crystals grow rapidly once formed . Matrix Where the matrix is more abundant the fine grey and white crystals of tricalcium aluminate and tetracalcium alumino-ferrite, respectively are seen (Figure 4). The size of these crystals indicates a relatively fast cooling rate between the hottest part of the burning zone and the kiln exit. The relative quantities of each of the crystal types suggests a typical alumina-to– iron ratio for a normal Portland cement (ie not a white cement and not a sulphate resisting cement). Putting all of these observations together and even with no further knowledge of the cement plant or raw materials a certain number of suggestions 37