Abstract
Stabilisation/Solidification (S/S) is viewed as an established technology in the USA. However, its use in the UK has generally been limited. This is partly due to the uncertainty of the long-term performance of cement-based S/S materials mainly due to the lack of sufficient long-term performance data and with most validation work existing being restricted to testing at 28 days. Hence, there is a need for the formulation of methods for predicting the long-term behaviour of cement-based S/S materials and one such approach is to use accelerated ageing techniques. Processes such as hydration and carbonation could be enhanced for accelerated ageing of such cementitious materials. Hence, accelerated carbonation was adopted as the ageing technique in this research.
The objective of this research was mainly focussed on the investigation of effects from accelerated carbonation on cement-based grout and soil-grout mixes and to understand the impact carbonation would have on the real-time long-term performance of cement-based S/S treated materials. The cement-based grout and soil-grout mixes developed in this research were based on the contaminated soil at the MoD site in West Drayton near Heathrow Airport, UK and the grouts used to treat it. The laboratory developed mixes subjected to accelerated curing conditions underwent exposure to elevated levels of CO2 alone or in combination with elevated temperature at different relative humidity (RH) levels. The naturally-cured (control) and accelerated-cured mixes were then tested to investigate their physicochemical properties at different curing periods.
Carbonated depth and CaCO3 content measurements showed that the degree of carbonation was higher in the accelerated-cured mixes than the naturally-cured mixes, and generally increased with time, reduction in RH and initial exposure to elevated temperature. The NRA leachate pH, x-ray diffraction and scanning electron microscopy results were also found to be good indicators of the degree of carbonation. Although most of the 28-day UCS values of the accelerated-cured mixes showed higher values relative to the naturally-cured mixes, the values of the fully carbonated mixes as a result of levelling off generally fell below those of the naturally-cured mixes which continued to increase in value with time. Therefore, UCS was not considered a good parameter for use in investigating the ageing behaviour of these mixes. The NRA leachability results showed that whilst moderate levels of carbonation improved the metal leachate concentrations of the accelerated-cured mixes that at higher levels of carbonation the opposite was true for all metals investigated except Pb. The TCLP leachability results showed that even a moderate level of carbonation would result in the accelerated-cured mixes having higher leachate metal concentrations compared with the naturally-cured mixes. The leachability results also showed that curing at lower RHs increased the leachate concentrations.
Based on the outcome of the investigation, it is concluded that although carbonation has the potential to improve the performance of cement-based S/S materials in the early part of their service life (i.e. when the degree of carbonation is low) that prolonged exposure (i.e. when the degree of carbonation is high) could lead to the lowering of their metal retention capacity in the long-term. It is also concluded that the in-service curing conditions, such as relative humidity, could have an influence on the latter situation.