Retention of zinc and chromium ions by different phases of hydrated calcium aluminate:: A solid-state 27Al NMR study

In cement-based materials, heavy metals are present in trace amounts. Depending on their mode of fixation and exact location in the solid, these metal ions may be released later into the environment. Solid phases formed during the hydration of tricalcium aluminate C(3)A have been studied by Al-27 MAS NMR. It is shown that different layered AFm phases are formed depending on the nature of the charge-compensating anions incorporated into their interlamellar space. C(3)A samples hydrated 15, and 72 h with and without metal ions have been examined and the results compared. Octahedral aluminum present in both the final hydrated form C(3)AH(6) and AFm phases can be distinguished by the values of chemical shift and quadrupolar parameters (nu(q) and eta). Chromium ions substitute Al in both structures. Adsorption of CrO42- and Zn(OH)(4)(2-) ions at the interlamellar or external surfaces of the AFm phases seems to be favored. This leads during hydration to a temporary stabilization of the AFm phases relative to the final stable C(3)AH(6) phase.