Effect of inorganic materials on the solidification of heavy metal sludge

Portland cement, cement-fly ash and lime-fly ash binders were used to solidify a synthetic heavy metal sludge containing nitrates of Cr, Ni, Cd and Hg. The sludge to binder (cement, cement-fly ash and lime-fly ash) ratio was kept at 3.33, 1.43 and 1.25, respectively. In addition inorganic substances like Cu, Zn, Pb, Sodium hydroxide and sodium sulfate were added. The molded samples were cured at room temperature for 28 days. The solidified samples with and without interference were examined for the change in their bulk density and compressive strength at definite time intervals during curing. All the metals and sodium salts added increased the average bulk density of the final product with increase in concentration (2% to 8%) with all the binder systems. The samples containing copper and lead decreased the compressive strength at all the concentrations added with CFA and LFA binders. Zn had the largest effect on all the three binder systems lowering the strength of all samples at all the days and concentrations except the 2% Zn with CEM binder. However, Pb had only minor effect on the compressive strength with CEM binder and values remained almost constant at all the times and concentrations studied. In contrast, the effect of sodium sulfate was less marked while sodium hydroxide increased the rate of set and 28-day compressive strength of samples containing cement as binder. These observations confirm the need for specific studies of the waste and binder prior to the selection of a solidification process for the treatment of hazardous wastes. The results provide a better understanding of materials that may interfere with the immobilization of waste constituents and provide information on the possible mechanism of the interfering effects. (C) 2003 Elsevier Ltd. All rights reserved.