Abstract
Although stabilisation/solidification has been described as one of the Best Demonstrated Available Technology (BDAT) in the USA, in the UK there is a general reluctance to adopt the technology. This is caused partly by uncertainties pertaining to the long-term stability of the treated materials due to lack of adequate field performance data. Most validation work carried out so far has focused on testing waste forms rather than soils, cured at ambient temperatures for periods up to 28 days. There is therefore a need to develop predictive methods using accelerated ageing techniques.
The objective of this research was to assess the potential use of elevated-temperature accelerated ageing to provide an insight into the long-term stability of S/S treated contaminated soils. The work reported here details the laboratory experiments performed using model soils to “simulate” the contaminated soils at the MoD West Drayton site in Middlesex near Heathrow Airport, UK. It was found that the effectiveness of cementitious stabilisation/solidification lies in the ability of the cementitious materials to harden over time, creating the required hydration products which immobilise the contaminants. Some of the characteristics of the hydration products are known to change with time, for example, the degree of silicate polymerisation and pH. The literature review showed that elevated temperatures can reproduce some of these effects. Accelerated ageing tests using elevated temperatures were therefore conducted up to 3 months, to investigate the various physicochemical characteristics of both un-aged and aged treated soils.
The UCS, permeability, durability and leaching properties of the materials were studied and were generally found to improve as a function of temperature and time. Although microstructural examinations using X-ray diffraction and scanning electron microscopy demonstrated few hydration products in the multiple-contaminant mixes, there was evidence from the UCS and durability tests that normal hydration was taking place at 3 months. Mineral oil and lead nitrate alone were found to have an insignificant effect on the development of strength. In freeze-thaw durability tests some multiple-contaminant mixes performed better than the uncontaminated mixes.
The results showed that it is possible to predict the service life of S/S materials based on the Arrhenius equation. The correlation with the field results produced mixed results with deviations attributed to differences in soil heterogeneity between the model and the site soils.