Three calcium aluminate (C(3)A) and calcium sulphoaluminate (C(4)A(3)(S) over bar) as clinker minerals were synthesized to investigate the solidification/stabilization (S/S) mechanism of lead and chromium ions on cement matrix. Effects of AFt phases on S/S mechanism were analyzed using FT-IR, SEM-EDS and heat evolution. Lead and chromium leachability from solidified AFt phases was also investigated using the toxicity characteristic leaching procedure (TCLP) recommended by the U.S. Environmental Protection Agency (EPA). The results showed that clinker mineral types considerably affected the S/S mechanism of AFt and AFm phases incorporating chromium and lead ions. In the case of adding chromium, both AFt and AFm phases based C(3)A and C(4)A(3)(S) over bar allowed ionic substitution Cr3+ for the trivalent ion as Al3+ and CrO42- for SO42-. Moreover, chromate hydrocalumite [3CaO center dot Al2O3 center dot CaCrO4 center dot nH(2)O] was also observed. However, in the case of adding lead, it was found that when C(3)A and C(4)A(3)(S) over bar were hydrated, the formation of AFt and AFm phase based C(4)A(3)(S) over bar was faster than C(3)A. Especially, AFt phases and the calcium lead hydroxides (Ca[Pb(OH)(3)center dot H2O](2)) incorporating lead ions were formed during hydration of C(4)A(3)(S) over bar in the early stage compared to the C(3)A case. This showed that AFt and AFm phase based C(4)A(3)(S) over bar for S/S of lead were more effective than that of C(3)A.

Keywords: C(3)A; C(4)A(3)(S)over-bar; AFt phases; Lead; Chromium (III)