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1Ye'elimite is the main phase in calcium sulfoaluminate cements and also a key phase in sulfobelite 2 cements. However, its hydration mechanism is not well understood. Here we reported new data on 3 the hydration behaviour of ye'elimite using synchrotron and laboratory powder diffraction coupled 4 to the Rietveld methodology. Both internal and external standard methodologies have been used to 5 determine the overall amorphous contents. We have addressed the standard variables: water-to-6 ye'elimite ratio and additional sulfate sources of different solubility. Moreover, we report a deep 7 study of the role of the polymorphism of pure ye'elimites. The hydration behaviour of orthorhombic 8 stoichiometric and pseudo-cubic solid-solution ye'elimites is discussed. In the absence of additional 9 sulfate sources, stoichiometric-ye'elimite reacts slower than solid-solution-ye'elimite, and AFm-10 type phases are the main hydrated crystalline phases, as expected. Moreover, solid-solution-11 ye'elimite produces higher amounts of ettringite than stoichiometric-ye'elimite. However, in the 12 presence of additional sulfates, stoichiometric-ye'elimite reacts faster than solid-solution-ye'elimite. 13 14 15

Belite calcium sulfoaluminate (BCSA) cements are low-CO 2 building materials. However, their hydration behavior and its effect on mechanical properties have still to be clarified. Here, we report a full multitechnique study of the hydration behavior up to 120 days of nonactivated and activated BCSA laboratory-prepared clinkers, with βor α H -belite as main phase, respectively. The effects of the amount of gypsum added were also studied. The hydration and crystallization processes are reported and discussed in detail. Finally, shrinkage/expansion data are also given. The optimum amount of gypsum was close to 10 wt %. Our study has demonstrated that β-belite reacts at a higher pace than α′ H -belite, irrespective of the gypsum content. The hydration mechanism of belite determines the development of the mechanical strengths. These are much higher for activated BCSA cement, ∼65 MPa at 120 days, against ∼20 MPa for nonactivated BCSA cement, with the latter having larger amounts of stratlingite.

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Belite–ye'elimite–ferrite (BYF) cements are a sustainable alternative to ordinary Portland cement in attempts to reduce carbon dioxide emissions. The aim of this research was to understand the influence of the sulfate source on the hydration of two laboratory-prepared BYF cements. One of the studied clinkers contained β-belite and orthorhombic ye'elimite (non-active) and the other contained α′H-belite and pseudo-cubic ye'elimite (activated with borax during clinkering). The pastes were mainly characterised through Rietveld quantitative phase analysis of powder patterns, thermal analysis and scanning electron microscopy. The active mortars developed higher compressive strengths than the non-active mortars, regardless of the sulfate source. The highest values of compressive strength for active mortars (w/c = 0·55) were 40 ± 1 MPa (28 d) with anhydrite and 68 ± 1 MPa (120 d) with gypsum.