Characterisation of ye’elimite by scanning electron microscopy, electron backscatter diffraction and energy-dispersive X-ray spectroscopy

Ye’elimite containing sodium and iron (Ca_4−xNa_2xAl_5.6Fe_0.4SO₁₆ with x = 0.1) was synthesised using reagent-grade chemicals. Characterisation of the sample was performed using X-ray diffraction (XRD) and Rietveld analysis in combination with analytical scanning electron microscopy. A combination of energy-dispersive X-ray spectroscopy (EDX) and electron backscatter diffraction (EBSD) was applied to characterise phases. A full documentation of how to obtain the EBSD pattern of ye’elimite is given. The obtained EBSD pattern quality of the ye’elimite was good and allowed indexing. Indexation using the Hough transformation and measuring angles between planes showed mostly a best fit with the orthorhombic structure of ye’elimite. This finding deviated from the XRD analysis and is discussed using results from EDX and dynamic pattern matching, a novel method for indexing EBSD patterns. The results of the EBSD–EDX mapping data revealed the mean chemical composition and crystal size of ye’elimite. The crystal size (average maximum Feret diameter of 2.6 µm) was significantly smaller than the size of alite crystals in Portland cement clinker. Therefore, EBSD mapping data has to be recorded at a small step size (<0.2 µm). Phase identification by EBSD–EDX analysis revealed that, besides the target ye’elimite phase, ferrite, sodium sulfate and a monocalcium aluminate phase (CaAl₂O₄) were also present.