THE EFFECTS OF SIMULATED ENVIRONMENTAL ATTACK ON IMMOBILIZATION OF HEAVY-METALS DOPED IN CEMENT-BASED MATERIALS

Environmental corrosion as is manifested in the form of CO2 attack has been found to have a profound effect on the leaching characteristics of heavy metals and the microstructure of the cement binder. An external peripheral leached zone was formed, followed by a calcium carbonate-rich layer, and an intact unleached core. The heavy metals did not substitute for calcium in the calcium silicate hydrate gel, rather nickel and cadmium gradually reacted to form hydroxides. This reaction is diffusion controlled, and depends on the initial grain size distribution of the heavy metal compounds. The incorporation of metals results in a decrease of the Ca(OH)(2) content of the cement paste and increases its vulnerability to carbonation and resistance against acidic corrosion. A pH decrease to about 9 in the leached zone can account for the cadmium and nickel concentration in the leachates.