Strätlingite and calcium hemicarboaluminate hydrate in belite-calcium sulphoaluminate cement

Belite-calcium sulphoaluminate (BCSA) cement is a promising low-CO₂ alternative to ordinary Portland cement. With high water cement ratio, phase compositions of the BCSA pastes at different curing ages were investigated via X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Large amounts of water accelerated the hydration reaction significantly and substantial ettringite (C₆AŠ₃ · H₃₂, AFt) was generated in the early stages. With the carbonation of AFt in the later stages, lots of strätlingite (C₂ASH₈) and calcium hemicarboaluminate hydrate (C₄AČ_0.5H₁₂, H_c) formed after curing age of 28 days. The microstructure of C₂ASH₈ and H_c in the BCSA pastes were observed and confirmed via a scanning electron microscope (SEM) and an energy dispersive X-ray spectrometer (EDS). The results indicated that C₂ASH₈ was thick hexagonal plate product and H_c was thin sheet-like product. C2ASH8 and H_c would stack together respectively in the later stages of hydration. In addition, spherical aluminum hydroxide (AH₃) and rod-like AFt were also observed.