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(NO3)(2)center dot 6H(2)O and ZnCl2, on the hydration of Portland cement and tricalcium silicate (C3S) was investigated both experimentally and by thermodynamic modelling. The reaction of C3S in zinc-rich solutions is strongly retarded and initially results in the formation of Zn(OH)(2), Zn-5(OH)(8)Cl-2 center dot H2O, CaZn2Si2O7 center dot H2O, Zn2SiO4 and (ZnO)(2x)(SiO2)(x)center dot 2xH(2)O, while the pH values remain low. At higher C3S 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 C3S in particular at lower pH values and ii) the lowering of pH by addition of zinc salts.