Abstract
Portland cements (PC) and blended PCs are the most commonly used binders in stabilisation/solidification (S/S) applications. However, such systems have found limited suitability with organic contaminants. Moreover, the high alkalinity associated with PC militates against the soil microbes and hence, hinders the natural attenuation of the organics. Furthermore, the production of PC is not only an extremely resource and energy intensive process but also has significant negative environmental impacts. Thus, with the aim of tackling the above issues, this research has been focused on firstly developing and applying a low-pH binder system for facilitating microbial activity in parallel to the S/S of heavy metals, and additionally on investigating the application of less environmentally damaging cement in S/S. These were addressed employing two novel binders; viz. low pH magnesia phosphate cements (MPCs) and reactive magnesia (MgO) cements respectively.
The first part of this investigation involved the initial material characterisation and MPC formulations using different phosphate and retarder sources. The suitable mixes were primarily short-listed based on their pH development along with the other intrinsic physical, mechanical and microstructural properties of the cement pastes. The successful mixes were initially applied to the S/S treatment of model soil contaminated with different individual heavy metals contaminants (i.e. Pb and Zn) and their performance was compared with PC-based binders. Finally, the performance of selected MPC mixes was evaluated in addressing the S/S of the above heavy metals in the presence of an organic (i.e. 2-chlorobenzoic acid). The short and long-term effectiveness of the S/S treatment was examined through results of batch leaching test, chemical, mechanical and microstructural properties.
Furthermore, two reactive MgO-based formulations were employed for S/S of model soil spiked individually with two different concentrations of three types of contaminants viz. heavy metal (i.e. Zn), chlorides and organic (i.e. paraffin oil) and the results were compared to that of PC-based binder. The experimental techniques employed to assess the long-term effectiveness of S/S treatment up to 448 days of curing included a range of leaching tests along with the investigation of various physical, mechanical, chemical and microstructural properties. Finally, the impact of accelerated carbonation up to 28 days on the S/S properties of the mix with only reactive MgO treating high heavy metal contamination was briefly investigated. This study was successful in formulating MPC mixes which not only developed low-pH ranges favourable to soil microbes but were also more effective in immobilising heavy metals than PC-based binders in an individual contamination scenario. Furthermore, the heavy metal stabilisation performance of the MPC mixes suffered negligible impact in the presence of organic contaminant.
The leaching performance of reactive MgO mixes was found to be superior to PC-only mix in terms of Zn immobilisation and was successful in reducing Zn to inert levels compliant with the UK acceptance criteria even at high levels of contamination at all ages. Furthermore, mixes with reactive MgO demonstrated higher buffering capacities than PC. On the other hand, the chloride as well as organic retention exhibited by most binders was observed to be poor. The mechanical properties improved with the inclusion of PC in the reactive MgO cements and also with curing age whilst the analogous permeabilities reduced. The accelerated carbonation was found to considerably improve the mechanical properties of the S/S material treated using only reactive MgO without significantly affecting the metal leachability.
This research established that the two novel magnesia-based cements have the potential to offer a more effective and sustainable S/S systems compared to the conventional PC-based systems.