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

In cement-based materials, heavy metals are present in trace amounts. Depen ding on their mode of fixation and exact location in the solid, these metal ions may be released later into the environment. Solid phases formed durin g the hydration of tricalcium aluminate C(3)A have been studied by Al-27 MA S NMR. It is shown that different layered AFm phases are formed depending o n the nature of the charge-compensating anions incorporated into their inte rlamellar space. C(3)A samples hydrated 15, and 72 h with and without metal ions have been examined and the results compared. Octahedral aluminum pres ent in both the final hydrated form C(3)AH(6) and AFm phases can be disting uished by the values of chemical shift and quadrupolar parameters (nu(q) an d 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 A Fm phases seems to be favored. This leads during hydration to a temporary s tabilization of the AFm phases relative to the final stable C(3)AH(6) phase .