Determination of unreacted clinker phases

The experimental considerations applying to calcium silicate pastes (Sections 5.1 and 5.2) are equally relevant to cement pastes. Of the methods so far used in attempts to determine the degrees of reaction of the individual clinker phases as a function of time, QXDA (C39,D12,T34,P28) has proved much the most satisfactory. Procedures are essentially as for the analysis of a clinker or unreacted cement (Section 4.3.2), but it is necessary to take account of overlaps with peaks from the hydration products, and especially, with the C-S-H band at 0.27-0.31 nm. The water content of the sample must be known, so that the results can be referred to the weight of anhydrous material. If a sample of the unhydrated cement is available, and its quantitative phase composition has been determined, it may be used as the reference standard for the individual clinker phases in the paste. 

The precision and accuracy of the technique should not be overestimated. If a cement paste contains 50% of alite initially, an error of 3% in the determination of that phase leads to one of 6% in the percentage reaction if it contains 5% of aluminate phase initially, an error of I % leads to one of 20% in the percentage reaction. With existing techniques, errors much smaller than these are unlikely to be achieved for untreated pastes. The precision may be greatly increased for the aluminate and ferrite by first removing the silicates by chemical extraction methods (Section 4.3.3). Fig. 7.3 shows some results from the literature. The considerable differences 

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