Retardation mechanism of zinc on Portland cement and alite hydration.

The significant inhabitation of Portland cement hydration by zinc restricts the utilization of zinc-bearing solid wastes in cementitious materials. The influence of zinc, in the form of (Zn(NO 3) 2 ·6H 2 O and ZnCl 2 , on the hydration of Portland cement and tricalcium silicate (C 3 S) was investigated both experimentally and by thermodynamic modelling. The reaction of C 3 S in zinc-rich solutions is strongly retarded and initially results in the formation of Zn(OH) 2 , Zn 5 (OH) 8 Cl 2 ·H 2 O, CaZn 2 Si 2 O 7 ·H 2 O, Zn 2 SiO 4 and (ZnO) 2x (SiO 2) x ·2xH 2 O, while the pH values remain low. At higher C 3 S reaction, Zn2+ concentrations decrease and the pH starts to rise as Ca2+ gradually accumulates in pore solution. Such retarding effect of zinc may be due to i) chemisorption of Zn2+ on the surface of C 3 S in particular at lower pH values and ii) the lowering of pH by addition of zinc salts. [ABSTRACT FROM AUTHOR]