Stabilisation of contaminated materials using inorganic cementitious agents is becoming more widely used in the UK. The method has particular advantages for bulk fill operations such as highway earthworks. The Transport Research Laboratory is currently undertaking a large research project for the Highways Agency on processing contaminated land for use as backfill in highways schemes, including long term dynamic leaching tests. Cardiff University's Geoenvironmental Research Centre was approached to undertake fundamental research into the processes taking place inside lime stabilised contaminated soils, with particular regard to the chemical aspects and their effect on contaminant mobility. The contaminated material is a mixture of lightly contaminated river silt to which pulversied fuel ash has been added, sourced from the A13 highway scheme at Rainham Marshes, to which heavily contaminated sewage sludge was added and the resulting mixture stabilised with lime. The Research Centre undertook: a review of the literature relating to stabilisation of contaminated soils and waste sewage sludges with lime; a detailed chemical analysis of the leachates, stabilised solid and unstabilised solid samples supplied by the Transport Research Laboratory; geochemical assessment modelling of the leachates; and this interpretative report on the work. Lime stabilisation of soil and waste sludges results in an increase in material strength, due to cementing between the particles, and a pH greater than 12. In materials with a low organic content heavy metal mobility is typically reduced due to physical micro-entrapment and not to hydroxide formation. The principal minerals found in the solid samples examined in this study are silica, calcite and hematite, none of which generally has a strong affinity for heavy metals. This suggests that the heavy metals present in the parent solids are not sorbed on to mineral surfaces in the soil. In the leachates from the lime stabilised material none of the heavy metals tested is present at a concentration likely to pose a significant environmental threat, and the concentrations are likely to decline over time. However, some heavy metals, particularly copper, nickel and lead, were present at higher concentrations than expected based on the pH and the inorganic chemistry. It is suggested that this is a result of complexation of these heavy metals with the organic matter; where this organic matter dissolves in the highly alkaline environment resulting from the lime stabilisation the metal moves into solution as a metal-organic complex. The principal substances of concern in the leachates would appear to be the organic content, the copper and the nickel. In addition, significant bacterial growth was observed in the leachates, which may cause degradation of the organic matter and long term release of complexed heavy metals. Further work should be done to assess the effects of microbial action within the stabilised materi

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