Influence of condensed silica fume on characteristics of cement-based solidified heavy metals-contaminated soil for road pavement

The solidification/stabilisation of heavy metals-contaminated soil using ordinary Portland cement and the influence of condensed silica fume (CSF) on the characteristics of solidified soil for possible pavement application were investigated in the present study. The soil to cement ratio was 1 : 12·5 and CSF was added at 3, 5 and 8% of cement by weight to mixtures during specimen preparation. The tests carried out included compressive strength, water absorption and toxicity characteristic leaching procedure, X-ray diffraction and scanning electron microscopy. At early curing ages, heavy metals-contaminated soil intensively decreased the compressive strength from 12·3 N/mm² in cement mixture to 0·14 N/mm² in the soil–cement mixture; and CSF had no noticeable effect on compressive strength but increased the consolidation of the relevant mixtures which satisfied the minimum US Environmental Protection Agency's regulatory limit for safe land disposal. During ageing, CSF gradually enhanced the retarded and diverted cement hydration reactions and the relevant solidified mixtures met the ACI 299R-99 requirement for controlled low strength materials in limited constructions. As an important result, the 28-day-cured soil–cement mixture which included CSF (8% by cement weight) achieved compressive strength of 45 N/mm², which was greater than UK transportation requirement for pavement concrete. Leaching results showed that CSF with a particle size finer than 150 μm and at the added amounts had no significant and noticeable effect on the leaching characteristics. Based on X-ray diffraction patterns, the hydration reactions appeared to be diverted through the production of subsidiary products and as a result, major crystalline phases grew and developed irregularly and consequently coated the aggregates poorly, as observed in scanning electron micrographs. CSF decreased the water absorption of the solidified mixtures in which it was included and their structure became dense.