Several active belite laboratory clinkers with a theoretical composition of 60 wt% of C2S have been synthesized. The activation process has been carried out by adding variable amounts of alkaline salts, K2CO3, or Na2CO3 to the raw materials. In addition to an ordinary Portland clinker and a nonactive belite clinker, four active belite clinkers have been prepared with 1.0 and 2.0 wt% of K2O or 0.5 and 1.5 wt% of Na2O. Phase‐enriched residues, silicate, and aluminate fractions have also been prepared in order to identify the most suitable structural description for each phase. High‐resolution synchrotron X‐ray powder diffraction patterns have been collected for the residues and for the laboratory clinkers. Quantitative phase analyses were performed by the Rietveld method using the synchrotron patterns. The reference belite laboratory clinker has a large amount of β‐C2S but the addition of alkaline salts to raw mixtures promotes the stabilization at room temperature of the highest temperature polymorphs: α′H‐C2S and α‐C2S. As an example, the active belite clinker with 2.0 wt% nominal content of K2O contains 4.5(2) wt% of β‐C2S, 19.2(1) wt% of α′H‐C2S, 35.9(2) wt% of α‐C2S, 17.6(1) wt% of C3S, 13.2(1) wt% of total C3A, and 9.6(1) wt% of C4AF. The role of K2O and Na2O, which stabilize different polymorphs, will be discussed. The partial volatilization of the alkaline metals will also be addressed. Finally, a calorimetric study has been undertaken in order to correlate the phase assemblage determined in the powder diffraction study with the hydration behavior.

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