Cement, Energy and Environment

quickly the clinker was cooled by secondary air after leaving the burning zon.e and before falling into the cooler and even about how the cement made from this clinker is likely to behave in service. The photomicrograph in Figure 1 shows a portion of a clinker nodule sliced and polished as described above and etched with a mixture of alcohol and nitric acid. If submerged in this liquid for the appropriate amount of time (up to about 30 seconds) the alite crystals in the clinker turn blue and the belite crystals turn brown when viewed in reflected light under the microscope. From the relative sizes and shapes of the crystals , their positions in relation to each other, any inclusions within the crystals, the nature of the interstitial material, even from the shapes of the pores formed within the clinker and filled with polishing debris during preparation, we can put together the story of the clinker's origins and future as well as some indications of the degree of efficiency in how the kiln is being burned. Detail of alite The blue, angular crystals in Figure 2 are of alite, tricalcium silicate. These are generally described as angular, hexagnol crystals, commonly approximately 30-35 microns across in section. They comprise typically between 50 and 70 per cent of the cement clinker, sometimes more or less than this. The reaction of these crystals with water is crucial to the development of strength when mortar or concrete are mixed and placed. The size, shape and composition of alite crystals is dependent on the chemistry of the raw mix, the fineness of the kiln feed, the maximum temperature experienced by the clinker during its formation in the cement kiln and the length of time at high temperature. The crystals in the picture are very varied in size. The examples in the top right corner of detail (Figure 2) are not much over 25 microns in length. They are angular in shape (euhedral) and separated in the matrix, formed by the crystallization of the liquid phase of the clinker. The larger crystals in the centre of Figure 2 appear to be agglomerations of smaller crystals, each of them larger than the first set of crystals at about 35-40 microns in length, with overall length of the combined crystals approaching 80 microns. The shape of the crystals is less clearly hexagonal and can be described as subhedral. These large composite crystals contain large numbers of rounded inclusions of brown belite crystals. As the smaller alite crystals coalesced to form the larger crystals some belite became included as lime was unable to reach the belite and convert it to alite. Although there are a small number of inclusions of white liquid phase, trapped with in the alite as the smaller crystals coalesced, these are rare and the local shortage of flux contributed to the inability to convert the belite inclusions to alite. At the bottom of Figure 2 the alite crystals are finer again, but poor in shape with rounded corners. These crystals have partly coalesced and included portions of belite and liquid phase. The larger crystals with belite inclusions are typical of those formed from conversion of belite clusters due to the presence of coarse silica grains in the raw feed. These form from coarse belite crystals frequently surrounding a central pore and if several such clusters are present this would be evidence that the siliceous component of the kiln feed was insufficiently finely ground. The smaller alite crystals close to the coarser examples are also typical of this situation because the excessive growth of the alite is a local phenomenon brought about by the pre-growth of coarse belite, alite crystals formed by other means are not similarly affected. If the size were due to excessive overburning as a result of poor heat distribution in the cement kiln all the alite crystals would be expected to be large, which is generally ascribed to slow burning in a long flame. Quick burning in a short flame 36 .....