In situ powder diffraction study of belite sulfoaluminate clinkering

Belite sulfoaluminate (BSA) cements have been proposed as environmentally friendly building materials, as their production may release up to 35% less CO₂ into the atmosphere when compared with ordinary Portland cement fabrication. However, their formation mechanism has not been studied in detail so far. Here, an in situ high‐temperature high‐resolution synchrotron X‐ray powder diffraction study is reported. Two types of BSA clinkers have been characterized, both containing 50–60 wt% C₂S and 20–30 wt% C₄A₃ as main phases. One type is iron‐rich and a second type (with different phase assemblage) is aluminium‐rich. Furthermore, the C₂S phase reacts slowly with water, thus activation of this compound is desirable in order to enhance the mechanical strength development of the resulting cements. To do so, iron‐rich BSA clinkers have been doped with minor amounts of B₂O₃ and Na₂O to promote stabilization of α‐forms of C₂S, which are more reactive with water. The decarbonated raw materials were loaded into Pt tubes and heated to between 973 K and 1673 K, and patterns were collected using a high‐energy synchrotron beam of wavelength λ = 0.30 Å. The thermal stability of Klein's salt in these clinkers has been clarified. Several reactions have been followed: formation and decomposition of Klein's salt, melting of aluminates and ferrite, and polymorphic transformations of dicalcium silicate: ‐C₂S →α‐C₂S. Changes in mineralogical phase assemblages at a given temperature owing to the addition of minor amounts of selected elements have also been determined.