Interstitial material clinker

Fig. 4. Photomiciogiaph of polished and etched sections of Poidand cement clinkers. The C A appears as dark interstitial material, the C AF as light...

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). 

All the effects described above indicate that rapid cooling is desirable the aluminate phase reacts more slowly with water when finely grained and intimately mixed with ferrite, making it easier to control the setting rate (S24), decrease in alite content either from reactions involving the interstitial material or from decomposition is avoided, a higher MgO content can be tolerated, and the clinker is easier to grind. 

Fig. 4.2 Reflected light micrograph of a polished and etched section of a Portland cement clinker, showing crystals of alite (dark, angular) and belite (less dark, rounded) embedded in a matrix of interstitial material, itself composed mainly of dendritic ferrite (light) and aluminate (dark). Courtesy Materials Science Department, British Cement Association.

There should be little free lime. What there is should occur as rounded grains, typically 10-20 pm in size, and associated with alite and interstitial material. Lime appears cream in sections etched with HF vapour. Its presence may be confirmed by a microchemical test using White s reagent (5 g of phenol in 5 ml of nitrobenzene + 2 drops of water) long, birefringent needles of calcium phenate are formed. The test also responds to CH. Alkali sulphates occur in the clinker pore structure they are etched black with HF vapour, and inhibit the etching of silicate phases with which they are in contact. 

The bulk composition affects the relative amounts of phases, the LSF, SR and AR affecting primarily the ratios of alite to belite, silicate phases to interstitial material and aluminate to ferrite phase, respectively. An LSF that is too high, either absolutely or in relation to the burning conditions, also gives rise to an excessive content of free lime. Belite may occur in such clinkers only as inclusions in alite. 

Fig. 4.3 Backscattered electron images of polished sections of (A) a Portland cement clinker and (B) grains of a Portland cement in a fresh paste. In both sections, alite is the predominant clinker phase. In (A), the relatively large, darker areas are of belite, and the interstitial material consists of dendritic ferrite (light) in a matrix of aluminate (dark) cracks and pores (black) are also visible. In (B), the belite forms well-defined regions, which are rounded, striated and darker than the alite the interstitial material, present, for example, in a vertical band left of centre within the larger grain, consists mainly of ferrite (light) and aluminate (dark). Scrivener and Pratt (S28).

Maki, I., "Nature of the Prismatic Dark Interstitial Material in Portland Cement Clinker," Cement and Concrete Research, Vol. 3, 1973, pp. 295-313. 

Except earlier mentioned elements others minor components do not form own phases, but solid solutions in principal clinker phases. The use of industrial by-products and untypical raw materials are introducing to clinker other foreign elements, which distribution in clinker phases was studied. In Table 2.14 the distribution of some foreign elements is presented [190]. As it results from these data vanadium is concentrated in alite, however, chromium and titanium principally in interstitial matter. 

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