Structural modifications of the aluminate phase in clinkers

Production clinkers have been found to contain cubic or orthorhombic forms of the aluminate phase, alone or in combination. The monoclinic modification has not been observed. The orthorhombic modification is also known as the prismatic, dark interstitial material, and is sometimes pseudo-tetragonal. It can arise only if sufficient alkali is available, but its formation appears to be favoured also by rapid cooling and by bulk compositions potentially able to yield a relatively high proportion of aluminate phase (M12). 

The orthorhombic phase is recognizable in the light microscope or SEM by its occurrence as characteristic, lath-shaped crystals, which are often twinned (Ml 2). An XRD powder pattern of material from which the silicate phases had been removed showed a splitting of the strong peak at 33.3 20 peak into a strong singlet at approximately 33.2 and a weaker, close doublet at 32.9 -33.0° (Fig. 1.7B). The unit cell parameters were a = 1.0874 nm, h = 

The aluminate phase in clinkers can also be characterized by its composition, determined by X-ray microanalysis this is discussed in the next section. 

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