Stabilization/solidification of electroplating sludge in cement based mortars: influence of rice husk ash on compressive strength and heavy metal leaching

Abstract

Electroplating generates large volumes of heavy metal bearing sludge whose secondary environmental burden can exceed that of the wastewater itself, yet a unified, reliably safe disposal route is still lacking. This study examined the stabilization/solidification (S/S) of electroplating sludge (ES) in cement based mortars and assessed rice husk ash (RHA) as a complementary matrix for immobilizing the contained metals. Eleven mortar mixes were prepared in three groups: ES as a partial cement replacement (0-75 % of cement) combined with RHA at 5 % (Group A) or 10 % (Group B), and ES as a partial fine aggregate replacement at 15-35 % (Group C). Fresh state behavior (slump, setting time, unit weight), unconfined compressive strength at 7, 28 and 90 days, and the leaching of Cd, Pb, Ni, Cr, As and Cu were measured. Replacing fine aggregate with ES increased the slump, whereas increasing the ES content of the binder progressively retarded setting, an effect amplified by a higher RHA content. Compressive strength fell sharply once ES entered the binder, from about 20-28 MPa for the sludge free controls to 1-6 MPa at 25-75 % ES and recovered only modestly with curing age. Leaching tests showed that Cd, Pb and As were immobilized below their US EPA thresholds, whereas Cr remained above the regulatory limit in every sludge bearing mix. The results delimit the replacement levels over which ES can be incorporated into cementitious materials and identify chromium leaching as the principal constraint on safe reuse.