Solidification/stabilization of a synthetic electroplating sludge in cementitious binders containing NaOH

Solidification/stabilization of a highly concentrated synthetic electroplating waste sludge by cementitious mixtures containing NaOH was studied. The sludge contained 86.2 mg/g Ni, 84.1 mg/g Cr, 18.8 mg/g Cd, and 0. 137 mg/g Hg and before mixing was dewatered to 25% solids. It was stabi- lized by ordinary Type I portland cement (OPC), an OPC/Class F fly ash mixture (referred to as CFA), and a lime/Class C fly ash mixture (referred to as LFA). Mix ratios of OPC: sludge of 0.3: 1, OPC:fly ash:sludge of 0.2:0.5: 1, and lime:fly ash:sludge of 0.3:0.5:1 were used, respectively. NaOH was added to each mix at 0 (control sample), 2, 5 and 8% by weight of solidified binder. A set of binder samples without sludge, but con- taining 8% NaOH, was also prepared. The microstructure, microchemistry, and component phases present in the binders after curing were determined by scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffractometry. The chemical species present in the sludge were not significantly affected by the high pH produced by NaOH; the sample showing the least effect was CFA/sludge. Ettringite was formed in CFA /sludge samples containing up to 5% NaOH, and in CFA/sludge and LFA/sludge containing up to 2% NaOH. NaOH reacted with the binders to produce phases containing Na plus minor amounts of Cr and Ni.