Ceramics-Silikáty 64, (1) 18 - 28 (2020)

Ye‘elimite (Ca₄Al₆SO₁₆) is one of the major components of calcium sulfoaluminate cements with a rapid hydration, high early strength, and corrosion resistance. In this study, the mechanism of the phase transition between two phases (Pcc2 and I43m) was examined by controlling the substitution rate of the Fe³⁺ ions. The results indicated that Ca₄Al₆SO₁₆ was the main product of all the samples at 1250 ̊C in a range from 0 to 25 at.%, which was accompanied by minor calcium aluminate (CA and C₁₂A₇) phases. The substitution of the Fe³⁺ ions into Ca₄Al₆SO₁₆ promoted the transformation of the crystal structure from Pcc2 to I43m. With the substitution rate reaching 10 at.%, the transformation from Pcc2 to I43m was effective, according to space group of I43m with a=b=c=9.2147(1) Å and V=782.4313(3) ų (Z=2). This effective transformation was formed without a structural change in the temperature dependent XRD analysis. The morphology change in the SEM and the structural disorder in the IR confirmed that there was a change among the different samples doped with the Fe³⁺ ions. Substitution of the Fe³⁺ ions for the Al³⁺ ions revealed that the early hydration kinetics were faster for the Fe³⁺-doped ye‘elimite phase compared to the pure ye‘elimite phase, which was explained by the presence of minor amounts of the C₁₂A₇ phase in the Fe³⁺-doped ye‘elimite phase.