Hydration, carbonation and thermal stability of hydrates in Ca_7−xSr_xZrAl₆O₁₈ cement

This paper studies hydration stages and phase transformation mechanism of the Sr²⁺-doped calcium zirconium aluminate cement at room temperature. Features of different hydration stages of this cement paste are identified by X-ray diffraction, scanning electron microscopy, differential thermal analysis, thermogravimetric analysis, evolved gas analysis and heat evolution test. It was found that the partial isovalent substitution of Ca²⁺ for Sr²⁺ such as Ca_7−xSr_xZrAl₆O₁₈, with x = 0.3, 0.6, 1.0, reduced the hydraulic reactivity of Ca₇ZrAl₆O₁₈ phase. The course of hydration of mixed oxides of the type 6CaO·SrO·3Al₂O₃·ZrO₂ documented using microcalorimetry was supported by investigations of the solid hydration products. Research showed that the hexagonal hydrates were stable at early and middle curing ages, an indication that including Sr in solid solution could effectively inhibit the conversion from both C₄AH_13–19 and C₂AH₈ to C₃AH₆ and AH₃. Calcium monocarboaluminate ${\text{C}}_4{\text{A}\overline{\text{C}}\text{H}}_{11}$ phases were also found in this system due to the carbonation process of C₄AH₁₃ phase. Substitution of Ca²⁺ ions by Sr²⁺ ions in hexagonal calcium aluminate hydrates causes structural disorder and then contributes to the broadening of lines in the powder X-ray diffraction patterns of layered structures of metastable hydrates. At later curing ages, the formation of two hydrogarnet phases, one Sr-rich (C,Sr)₃AH₆ and the other Ca-rich (C,Sr)₃AH₆, was proved.