Strontium retention of calcium zirconium aluminate cement paste studied by NMR, XRD and SEM-EDS

This work concerns the hydration mechanism of calcium zirconium aluminate as a ternary compound appearing in the CaO-Al₂O₃-ZrO₂ diagram besides the calcium aluminates commonly used as the main constitutes of calcium aluminate cements (CACs). Moreover, a state-of-the-art approach towards significant changes in hydraulic properties was implemented for the first time in this work, where the effect of structural modification on the hydration behavior of calcium zirconium aluminate was proved by XRD, ²⁷Al MAS NMR and SEM-EDS. The substitution of Sr²⁺ for Ca²⁺ in the Ca₇ZrAl₆O₁₈ lattice decreases the reactivity of Sr-substituted Ca₇ZrAl₆O₁₈ in the presence of water. Since the original cement grains remain unhydrated up to 3 h (Ca₇ZrAl₆O₁₈) or 72 h (Sr_1.25Ca_5.75ZrAl₆O₁₈) of curing period in the hardened cement paste structures, strontium can be considered as an inhibition agent for cement hydration. The complete conversion from anhydrous ²⁷Al^IV to hydrated ²⁷Al^VI species was achieved during the first 24 h (Ca₇ZrAl₆O₁₈) or 7 d(Sr_1.25Ca_5.75ZrAl₆O₁₈) of hydration. Simultaneously, the chemical shift in the range of octahedral aluminum from ca. 4 ppm to ca. 6 ppm was attributed to the transformation of the hexagonal calcium aluminate hydrates and Sr-rich (Sr,C)₃AH₆ hydrate into the cubic phase Ca-rich (Sr,C)₃AH₆ or pure C₃AH₆ in the hardened Sr-doped cement paste at the age of 7 d. The same ²⁷Al NMR chemical shift was detected at the age of 24 h for the reference hardened undoped Ca₇ZrAl₆O₁₈ cement paste.