IMMOBILIZATION MECHANISMS IN SOLIDIFICATION STABILIZATION OF CD AND PB SALTS USING PORTLAND-CEMENT FIXING AGENTS

We have investigated the behavior of Cd and Pb salts toward cement-based solidification using TCLP leaching tests, conduction calorimetry, and solid-state NMR as a function of time. Concentrations of Cd in leachates are very low, while Pb concentrations are considerably higher and would represent a serious threat to groundwater. The Cd/cement system involves Cd(OH)2, which provides nucleation sites for precipitation of calcium silicate hydrate (C-S-H) “gel” and calcium hydroxide, resulting in Cd being in the form of the insoluble hydroxide with a very impervious coating. On the other hand, the Pb/cement system involves hydroxide, sulfate, and nitrate mixed salts, which retard cement hydration reactions by forming an impervious coating around cement clinker grains. However, as pH in the cement pore waters undergoes fluctuations during the progress of hydration, the Pb salts undergo solubilization and reprecipitation on leachable surfaces of the cement matrix. © 1990, American Chemical Society. All rights reserved.