Mineralogical phase analysis of alkali and sulfate bearing belite rich laboratory clinkers

The activation of laboratory belite clinkers has been carried out by adding variable amounts of alkaline salts (K2CO3, Na2CO3), and/or SO3 as gypsum in the raw materials but keeping almost constant the main elements ratios, Ca/Si/Al/Fe. Quantitative phase analyses by the Rietveld method using high resolution synchrotron and strictly monochromatic CuKα1 laboratory X-ray powder diffraction data has been performed. Quantitative phase analysis results have been compared to validate the protocol using laboratory X-ray data. The agreement in the results is noteworthy, which indicates that good quantitative phase analyses can be obtained from laboratory X-ray powder data. Qualitative studies have confirmed that 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. Quantitative studies gave the phase assemblage for ten different laboratory belite clinkers. As an example, an active belite clinker with 1.0 wt.% of K2O and 1.0 wt.% of Na2O (amounts added to the raw mixtures) contains 8.5(3) wt.% of β-C2S, 21.2(3) wt.% of α'H-C2S, 24.1(2) wt.% of α-C2S, 18.9(3) wt.% of total C3S, 17.3(2) wt.% of C3A and 10.0(2) wt.% of C4AF. A belite clinker with 0.8 wt.% SO3 (nominal loading) contains 60.7(1) wt.% of β-C2S, 6.7(2) wt.% of α′H-C2S, 12.3(7) wt.% of C3S, 9.1(2) wt.% of C3A and 11.2(2) wt.% of C4AF. Overall, quantitative phase analyses have shown that alkaline oxides stabilize α′H-C2S and α-C2S, sulfur stabilizes β-C2S, with a large unit cell volume, and the joint presence of alkaline oxides and sulfur promotes mainly the stabilization of the α′H-C2S polymorph.