Immobilization of PB(II) and CD(II) in Blasting Wastes by Portland Cement Compounds

This report addresses the effectiveness of Portland Cement-Soluble Silicates (PCSS) blasting additives for reduction of TCLP metals and for reduction of long-term liabilities upon disposal. The work was performed on blasting wastes from the Marine Corps Logistics Base (MCLB) in Albany, Georgia. TCLP Cd exceeded TCLP limits in half of Black Beauty samples without PCSS or with only 10% (w/w) PCSS. Cd exceedances were eliminated with 15 or 20% PCSS. Pb exceeded TCLP limits on 15% of regular blasting wastes, but 10% PCSS eliminated all Pb exceedances. Steel blasting wastes had 83% exceedances for TCLP Cd, and plastic or glass wastes frequently exceeded TCLP Pb and Cd.

Leaching experiments were performed on each blasting waste with and without laboratory addition of PCSS. If pH was continuously adjusted with rapid mixing so that the leaching environment never rose above pH 5, then every sample (including those with PCSS) exceeded TCLP standards for Cd and there were several exceedances for Pb.

These results are consistent with thermodynamic principles modified by chemical kinetic considerations. High temporary pH permits instantaneous precipitation of PbSiO₃(s), CdSiO₃(s), or calcium silicate hydrate (CSH). These materials then dissolve very slowly. PCSS is very alkaline, keeping the TCLP test pH well above 5 and preventing immediate acid titration for all reasonable disposal scenarios.

PCSS was not physically separated from the blasting materials during dry sieving, and inconsequential separation of PCSS materials from blasting wastes are expected during accumulation, storage, transfer, and disposal operations. This was confirmed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).

Recommendations included: (1) segregation of wastes likely to result in TCLP exceedances; (2) reducing contamination through selection of blasting materials and through masking operations; (3) use of PCSS on wastes that may result in TCLP exceedances; (4) mono-fill disposal to maintain alkaline; and (5) studies of the long-term leachability of TCLP metals under disposal conditions.