Impact of Ni(Ⅱ) and Cd(Ⅱ) on the hydration and microstructure of cement pastes for immobilization: C-A-S-H composition and binding characteristic

Abstract

In this work, the impact of two heavy metal cations, Ni2+ and Cd2+, on the hydration and microstructure of cement pastes were studied to better understand their binding characteristics in immobilization process. The addition of Ni2+ and Cd2+ exhibits an accelerating effect on early hydration kinetics. The compressive strengths of cement pastes incorporated with Ni2+ closely correlate to the capillary pore volume. The increase in the size and volume of large pores in the presence of Cd2+ leads to a considerable reduction of compressive strength. Ni2+ and Cd2+ are mainly bound into the outer products (OP) of hydrated cement pastes and the binding characteristics are connected with the OP composition. Ni2+ could precipitate as hydroxides and intermix with C-A-S-H or be adsorbed by deprotonated silanol groups. Cd2+ are mainly bound with two deprotonated silanol groups at the bridging position. The binding of Ni2+ and Cd2+ on the deprotonated silanol groups in C-A-S-H substitutes partial Ca2+ for charge compensation. More Ca2+ in the pore solution are available for the nucleation and growth of hydration products to accelerate the early hydration kinetics.