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\underline{\rm S} 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 A. 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: \alpha_{\rm{H}}^\prime-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.