Leachability of lead from solidified cement-fly ash binders

Abstract

Potential of partly replacing cement with class C fly ash to immobilize lead was investigated. Lead nitrate up to a concentrations of 10% (by weight of binder) was solidified with binders such as Type I Portland cement and a cement-fly ash (equal proportion) mixture. Pore fluid analyses indicated that lead increased the calcium and silicon concentration in the pore fluid during the initial period of hydration of cement and cement-fly ash mixtures. Addition of fly ash to cement reduced the initial and final setting times, but with the addition of lead nitrate setting times were increased. The compressive strength of the solidified cement decreased with the addition of fly ash and lead nitrate. Lead leachability from the solidified binder matrix was studied using the Toxicity Characteristic Leaching Procedure (TCLP) recommended by the U. S. Environmental Protection Agency (EPA). The quantity of Pb(II) leached was dependent on the initial lead nitrate concentration and the binder systems adopted. Lead solidified with cement-fly ash mixture showed a slightly less leaching compared to the cement binder. A simple model has been proposed to predict the leachability of Pb(II) from the binder matrix.