Crystallization of the sulphate phases

Much of the SO3 is present at the clinkering temperature in a separate liquid phase, immiscible with the main clinker liquid. The alkali cations are distributed between the two liquids and the alite and belite. During cooling, some redistribution of alkali cations and sulphate ions between the liquids may be expected to occur, the sulphate liquid finally solidifying below 900 C to yield alkali or potassium calcium sulphates. 

For most normal clinker compositions, the major sulphate phase will be aphthitalite with its maximum K/Na ratio of 3.0, with minor amounts of KS or C2KS3 or both only with unusually low values of K/Na in the clinker would one expect NS to be formed. It is only possible to estimate a maximum value for the content of anhydrite, based on the amount of SO3 that is not water soluble this will often be zero, as in the example given above. Anhydrite may, however, be determined by QXDA (Section 4.3.2). Rarely, the amount of water-soluble SO3 is insufficient to balance that of water-soluble alkalis this has been attributed to the presence of traces of alkali carbonates (P2), or of potassium oxide and potassium aluminate (KA) (F4). 

---