Microstructure and Microchemistry of Calcium Sulfoaluminate Cement
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RODUCTION Calcium sulfoaluminate, 4CaO - 3Ae 20 3 S03 0 or "C4A3 S"', has long been known to have cementitious properties. Recent experience in China shows that by controlling the clinker mineralogy, a range of dimensionally-stable, cementitious matrices can be formulated. These may replace Portland cement in many constructions or find application in special cements. However, relatively little is known about the mineral composition and microstructure of sulfoaluminate clinkers and of the resulting cement hydration products. Muzhen, et al [1] have presented SEM fractographs of burnt clinkers and claimed that the CaO content of"C4A 3S' was variable on "a relatively large scale". This paper summarises several years of work undertaken in our laboratories on this interesting and novel class of structural materials. Studies of hydration properties will be reported elsewhere [2].
CLINKER COMPOSITION AND MICROSTRUCTURE Much of the clinker is open-structured and friable, perhaps as a result of the relatively low firing temperatures which are reported; ca 1200'-1 300'C. Despite local differences in texture, coherence, colour and apparent bulk density, the clinker is chemically quite uniform on a gram scale: generally within +0.5 - 0.6% with respect to its major constituents. Table I presents an averaged analysis by XRF on a dry weight basis: loss on ignition was variable but generally low, in the range 0.08 - 0,3%. With respect to the minor constituents of Portland cement, sulfoaluminate clinker is very low in alkali but relatively high in TiO 2. The microstructure of the clinker is shown in Figs. 1-4. Figure 1, at low magnification shows the lace-like open-textured structure of the more friable clinker lumps.
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Mat. Res. Soc. Symp. Proc. Vol. 370 01995 Materials Research Society
Fig. 1. Backscattered electron micrograph
Fig. 2. B.e.m. of clinker polished
(B.e.m.) of clinker polished section showing lace-like, open textured clinker structures.
section showing pores surrounded by bettersintered clinker. The cluster of dark grains (right center) is periclase, MgO.
Fig. 3. B.e.m. of clinker polished section. A nest of beliteferrite core with a little sulfoaluminate, ca 30pm,is surrounded by a more sulfoaluminate-rich zone. Note ferrite (bright) and occasional periclase (dark).
Fig. 4. B.e.m. of clinker polished section, showing region of exaggerated grain growth of belite. Partial local melting may have occurred in these zones.
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Table I. CHEMISTRY OF SULFOALUMINATE CLINKER
This porous structure undoubtedly contributes to the ease of grinding sulfoaluminate clinker and its
K20
wt % 4.6 1.5 36.5 2.7 0.02 2.5 42.0 0.08 0.07
subsequent reactivity during hydration. Figures 2 and 3 show the microstructure of rather better sintered regions of clinker. Backscattered electron images at higher magnifications, coupled with analytical microscopy, disclose some of the mineralogical features. The well shaped, equant gray crystals are calcium sulfoaluminate. These are embedded in a matrix consisting mainly of two phases: b
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