Effect of calcite on lead-rich cementitious solid waste forms

The effect of calcite on lead-rich solidified waste forms generated using Portland cement has been investigated. Samples of cementitious wastes in the absence and presence of Pb and in the absence and presence of calcite were examined separately at 2, 7, 14 and 28 days of hydration by X-ray diffraction and SEM/EDS and for compressive strength. The presence of lead was observed to produce lead carbonate sulfate hydroxide (Pb4SO4(CO3)2(OH)2), lead carbonate hydroxide hydrate (3PbCO3·2Pb(OH)2·H2O) and two other unidentified lead salts in cavity areas, and was observed to significantly retard the hydration of cement. Calcite addition to the Pb wastes was found to induce the rapid crystallization of calcium hydroxide coincident with the onset of C-S-H gel germination. The rapid dissolution of lead precipitates was observed with the subsequent development of very insoluble gel products of the form C-Pb-S-H. These products are formed by chemical incorporation of re-dissolved Pb species into silicate structures.