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 extern al peripheral leached zone was formed, followed by a calcium carbonate -rich layer, and an intact unleached core. The heavy metals did not su bstitute for calcium in the calcium silicate hydrate gel, rather nicke l and cadmium gradually reacted to form hydroxides. This reaction is d iffusion controlled, and depends on the initial grain size distributio n 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 corros ion. A pH decrease to about 9 in the leached zone can account for the cadmium and nickel concentration in the leachates.