The hard facts of remediation
Cementitious remediation offers a realistic alternative to 'dig and dump' in the treatment of contaminated land, according to Michael Southall of Castle Cement
Cement (and here the term is used in the loosest sense to include a number of different powders which react with water to develop cementitious properties) is a most effective means of remediating contaminated land. After landfill, cementitious remediation is arguably one of the most flexible techniques available to manage a wide range of different contaminant types, including recalcitrant organic species such as PCBs and PAH.
In addition to dealing with ground chemistry, cement is able to deliver a significant improvement to the engineering behaviour of the ground. Therefore this technique provides an opportunity to deal effectively with contamination and physically marginal land in one combined process, saving time and money. Although relatively unused in the UK, cementitious remediation is firmly established within the United States where up to 25 per cent of land remediation is undertaken using cement-based techniques for a wide portfolio of contamination scenarios under the USEPA 'Superfund' programme. In mainland Europe the technique is also widely accepted and used. It is not difficult to see that the opportunity for this technology in the UK is very significant and there is an ever-increasing acknowledgement of its benefits and a growing willingness to consider it as a realistic solution.
The process of using cement to remediate contaminated soils is termed 'solidification/stabilisation', or 's/s' for short. It is a relatively simple, engineering based technique involving the addition of inorganic hydraulic binders (cements) to potentially hazardous materials to create new solids, in which the contaminants are rendered practically immobile and 'non-leachable'. Although not removed (or destroyed), the contaminants are prevented from being able to cause harm and as such the pollutant linkage is broken. The addition of the binder involves two separate simultaneously acting mechanisms, namely:
Solidification - Physical alteration of the material to make it more stable and less prone to being affected by external agents that may mobilise the contaminants: Stabilisation - Modification of contaminant species present in the material to make them more chemically stable and less mobile.
As each site is different the remedial objectives will be particular to that site and the remedial solution will always be bespoke. Each s/s application is designed to take into account the site conditions and characteristics of the contamination profile, the remedial objectives as determined with reference to a conceptual risk model, and the overall requirements of the stakeholders involved with the project.
The binder used will be project specific and designed through a programme of laboratory and field 'treatability' studies used to verify binder composition, application dose rate and optimal method of application. The parameters assessed include engineering criteria and chemical stability/contaminant mobility using an appropriate leaching test protocol.
Almost invariably the binder will comprise a principal hydraulic powder, which does most of the work and a minor constituent used to modify and augment the principal binder and fine-tune the efficacy of the process. There are four principal hydraulic binders: Common cements (complying with BS EN 197 Pt1 and include ordinary Portland cement and replacements), lime (quick lime and hydrated lime), ground granulated blast furnace slag (ggbs) and pulverised fly ash (pfa). These may be used either on their own or in combination. The minor constituents are chosen carefully to enhance the main s/s mechanism in respect to the particular material being treated and to bestow particular properties on the treated materials. The list is infinite but includes such things as pH and redox modifiers, wetting agents, flocculants, sorbents, fillers, accelerators, hydrophobic additives and components to produce additional chemical reactions (e.g. formation of sulfides, metal complexation using chelating agents).
S/s works well with many different contaminants. It is best noted for its treatment of heavy metals and there are many examples of its success in these situations. The technique also works well with organic contaminants and there is a growing numbers of cases where s/s has been used to remediate PCBs, PAHs, VOC/BTEX and petroleum hydrocarbons. Figure 1 shows the relative effectiveness of s/s with common contaminants.
S/s is very flexible and can be carried out either as an ex-situ or an in-situ process depending on the site conditions, nature of the contamination and other factors such as access, handling constraints (odour and/or consistency of the material to be treated) and future intended use of treated materials. Notwithstanding the benefits that s/s has to offer, the greatest benefit comes from its contribution to sustainable construction. S/s can avoid disposal of contaminated materials to landfill, a finite and diminishing resource, and provides collateral benefits. These can include a reduced environmental impact from traffic movements, less noise and less dust. In other cases this technique can reduce the volume of material to be landfilled, with the material being placed being rendered more inert and presenting less of a long term environmental risk. In addition the technique reduces the use of unbound granular fill.
As far as the future is concerned, it is anticipated that s/s will become a mainstream technique within the UK. This will be helped by the growing knowledge of this technique, the longevity and durability of the process, the mechanisms that allow it to work, and predictive methods for the long-term performance of treated waste forms.