Solidification/stabilisation of air pollution control residues using Portland cement: Physical properties and chloride leaching

Portland cement (CEMI) was used to solidify air pollution control (APC) residues from an energy-from-waste plant burning municipal solid waste. APC residue/CEMI mixes were prepared with CEMI additions ranging from 0 to 50 weight% (wt%) of total dry mass and water/solids ratios between 0.40 and 0.80. Isothermal conduction calorimetry was used to assess the effect of APC residues on the hydration of CEMI. Although up to 30wt% additions of APC residues accelerated CEMI hydration, the total heat of hydration during the initial 98h was significantly reduced. Higher levels of APC residues severely inhibited CEMI hydration. The consistence, setting time, compressive strength, porosity and chloride leaching characteristics of the solidified products were determined. As might be expected, increasing the CEMI addition and reducing the water content resulted in increased compressive strengths. All mixes achieved compressive strengths greater than 1MPa at 7 and 28days but only 50wt% samples did not show significant strength reduction when tested after immersion in water. Monolithic leaching tests indicated low physical immobilisation of chloride in the CEMI solidified APC residues, with chloride leaching in excess of relevant UK landfill waste acceptance criteria (WAC). The results of this study show that greater than 50% CEMI additions would be required to effectively treat APC residues to meet current WAC limits.