Hydration, carbonation and thermal stability of hydrates in Ca7−xSrxZrAl6O18 cement

Abstract

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 Ca7−xSrxZrAl6O18, 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 C4AH13–19 and C2AH8 to C3AH6 and AH3. Calcium monocarboaluminate C 4A C ¯ 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.