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-Al(2)O(3)-ZrO(2) 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, (27)Al MAS NMR and SEM-EDS. The substitution of Sr(2+) for Ca(2+) in the Ca(7)ZrAl(6)O(18) lattice decreases the reactivity of Sr-substituted Ca(7)ZrAl(6)O(18) in the presence of water. Since the original cement grains remain unhydrated up to 3 h (Ca(7)ZrAl(6)O(18)) or 72 h (Sr(1.25)Ca(5.75)ZrAl(6)O(18)) 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 (27)Al(IV) to hydrated (27)Al(VI) species was achieved during the first 24 h (Ca(7)ZrAl(6)O(18)) or 7 d(Sr(1.25)Ca(5.75)ZrAl(6)O(18)) 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)(3)AH(6) hydrate into the cubic phase Ca-rich (Sr,C)(3)AH(6) or pure C(3)AH(6) in the hardened Sr-doped cement paste at the age of 7 d. The same (27)Al NMR chemical shift was detected at the age of 24 h for the reference hardened undoped Ca(7)ZrAl(6)O(18) cement paste.