Fact sheet: Solidification/Stabilization — ex situ

From: Public Services and Procurement Canada

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The ex-situ solidification/stabilization process is used to trap and limit the migration of contaminants in soils. This technology does not include treatment or disposal of contaminated materials, but it does reduce their potential impact on the environment.

The solidification/stabilization process consists of mixing contaminated soils, present at shallow or deep depths, with stabilizers and/or binding agents such as gypsum, lime, bentonite clay, Portland cement and various additives such as pozzolans, fly ash, sulfur and blast furnace slag. Polymers, organic materials (biosolids, manure and compost) and various minerals can also be used as admixtures.    

Stabilization involves a transformation of the chemical properties of the contaminants within the soil matrix by decreasing their solubility in water, their mobility and, therefore, their toxicity. 

In addition, solidification involves a transformation of the physical properties of the matrix to be treated by the addition of binding agents that compact it, modify its pore size and reduce its hydraulic conductivity. The addition of binding agents can maximize the stabilization process.

The method for performing ex-situ solidification/stabilization is to excavate the contaminated matrix, add an agent/stabilizer, mix and then allow it to settle (for setting, curing and hardening) prior to transporting it off-site and disposing of it or reusing it on site as backfill or construction material. The ex situ solidification/stabilization process can be carried out in a processing plant/centre or in a unit (mobile or fixed) set up on site.

Internet links:

Implementation of the technology

Ex situ solidification/stabilization technology can include: 

  • mobilization, access to the site, preparation of the site and installation of temporary facilities;
  • site preparation for excavation of contaminated soils (clearing, grubbing, stripping and storage of topsoil and demolition);
  • layout of the treatment plant or mobile plant;
  • excavation of the soil matrix;
  • dewatering of excavations (sumps, wells, wellpoint, cutoff walls);
  • slope stability control (anchors, berms, stakes, sheet piles, berlin walls, controlled low strength materials);
  • protection of the foundations for the selected structures (shoring, underpinning);
  • transportation of solidified and/or stabilized soil for off-site disposal, or use as backfill and/or construction material on site;
  • surface restoration (grading, paving, hydroseeding or planting).

Materials and Storage

Ex-situ soil stabilization/solidification requires the use of machinery for excavation as well as specialized fixed or mobile equipment for soil treatment. On-site storage may include binding agents, admixtures and water required for the process, as well as fuels, lubricants and other site materials required for the operation of machinery or equipment for the implementation of the process. Temporary piles of contaminated materials may also be located on the site pending treatment.

Residues and Discharges

Treated soils would not be considered waste if they are reused as backfill on the site. Dust may be emitted during excavation, processing, and temporary stockpiling. Water with additives may also be released during the operations. There is a risk of air emissions from equipment exhaust or volatilization of contaminants from fresh excavation walls or temporary piles. If dewatering of the excavation is required, dewatering water may have to be monitored and treated.

Recommended analyses for detailed characterization

Chemical analysis

  • pH
  • Alkalinity
  • Organic matter content
  • Contaminant concentrations present in the following phases:
    • absorbed
  • Groundwater quality upstream, downstream and in the vicinity of the pollution source if stabilized soils are put back in place:
    • Nature and concentrations of contaminants
    • pH
    • Dissolved oxygen
    • Temperature
    • Electrical conductivity
    • Total organic carbon content
    • Concentration of metals

Physical analysis

  • Soil water content
  • Soil granulometry
  • Contaminant physical characteristics including:
    • viscosity
    • density
    • solubility
    • vapour pressure
  • Presence of light or dense immiscible liquids
  • Porosity

Recommended trials for detailed characterization


Other information recommended for detailed characterization

Phase II

  • Presence of potential environmental receptors
  • Presence of above and below ground infrastructure
  • Characterization and delimitation of the extent of the contamination
  • Soil lithologies and stratigraphy
  • Hydrogeological characterization
  • Determination of the risk and monitoring of the migration of contamination from solidification/stabilization systems

Phase III

  • Volume of contaminated material to treat
  • Characterization of the hydrogeological system including:
    • the direction and speed of the groundwater flow
    • the hydraulic conductivity
    • the seasonal fluctuations
    • the hydraulic gradient
  • Hydraulic tests to evaluate dewatering flows, if necessary
  • Evaluation of discharge water quality (if pumping required)
  • Determination of preferential pathways for contaminant migration if soils are reused on site
  • Geochemical and/or hydrogeological modeling if soils are reused on site


A treatability test is recommended to determine the type and optimal amount of binder/stabilizer to add and to validate the stability of the products for several chemical and physical parameters. These tests also establish the geotechnical properties of the stabilized materials.


Solidification/stabilization technology is only applicable to solid contaminated matrices such as soils, sludges and sediments. Solidification/stabilization technology is usually easier to implement in silty, sandy or gravelly soils, as opposed to soils with a high clay content, since it is easier to achieve a uniform mixture. Soils containing clay tend to leave residual clay lenses unmixed and untreated.

Applications to sites in northern regions

  • The technology is achievable in northern environments; however, remote sites require greater mobilization, resulting in higher on-site supervision costs. In addition, equipment availability is limited and work windows are relatively short.
  • Deep freezing and the presence of permafrost may limit the depth at which soil can be excavated for treatment.
  • Soils can be treated with a mobile plant on site, but weather conditions could greatly affect the operation and efficiency.
  • If the treated materials cannot be reused on site, off-site transportation could be difficult and very expensive.

Treatment type

Treatment type
Treatment typeApplies or Does not apply
In situ
Does not apply
Ex situ
Does not exist
Dissolved contamination
Does not exist
Free Phase
Residual contamination
Does not exist

State of technology

State of technology
State of technologyExist or Does not exist

Target contaminants

Target contaminantsApplies, Does not apply or With restrictions
Aliphatic chlorinated hydrocarbons
Does not apply
Does not apply
Monocyclic aromatic hydrocarbons
Does not apply
Non metalic inorganic compounds
Petroleum hydrocarbons
Phenolic compounds
With restrictions
Policyclic aromatic hydrocarbons
Polychlorinated biphenyls


Phenolic compounds refer to pentachlorophenol (PCP) only.

Treatment time

Treatment time
Treatment timeApplies or Does not apply
Less than 1 year
1 to 3 years
Does not apply
3 to 5 years
Does not apply
More than 5 years
Does not apply

Long-term considerations (following remediation work)

If the treated materials are reused on site, long-term monitoring of the ex situ solidification/stabilization performance is generally required to validate its continued effectiveness. Long-term performance monitoring may include groundwater quality monitoring to ensure that contaminants are not leaching from the stabilized soil areas. Long-term monitoring may also include an assessment of the physical integrity of the stabilized area and the maintenance of its geotechnical properties over time, and the integrity of the stabilized soil area may also be validated by long-term monitoring of the vapour emission.

Secondary by-products and/or metabolites

There are no by-products generated during the implementation of the ex situ solidification/stabilization technology. However, this technology does not destroy the contaminants and they are still present following their stabilization. They are retained within the matrix as long as the integrity of the matrix is maintained. 

Limitations and Undesirable Effects of the Technology

  • Solidification/stabilization is a remediation technology that controls only the migration of contaminants in soils.
  • The performance of solidification/stabilization technologies depends on site characteristics and thus cannot be estimated.
  • Solidified/stabilized material may be susceptible to leaching after some time, depending on factors such as climate.
  • Mineral salts and strong acids or bases present in the contaminated matrix can interfere with the solidification/stabilization process.
  • The presence of large debris or blocks can interfere with the solidification/stabilization process.
  • It can be difficult to develop an effective binding agent for heterogeneous mixtures of waste materials.
  • Deeply contaminated soils may be more difficult to excavate for treatment.
  • The use of the site may be changed or restrictions may be imposed as a result of the implementation of the technology (such as, planting of deep-rooted trees or other subsurface activities that would require approval).
  • Solidification/stabilization reduces water infiltration into the soil.  
  • Some stabilization/solidification processes increase the volume of the treated matrix.
  • Large areas are required for the processing and stacking of materials.
  • Frequent rain and freeze/thaw cycles can reduce the life expectancy of solidified/stabilized material and thus increase the mobility of contaminants.
  • The reuse of treated soils as backfill or construction materials will have to take into consideration a change in both geochemical and geotechnical properties.
  • High pumping rates to keep excavations dry may limit the applicability of this technology.
  • The necessity of using slope stability measures and the proximity of infrastructures may limit the applicability of this technology.
  • The use of certain binding agents can cause an exothermic reaction (temperature increase) in the presence of certain contaminants such as waste oil or tar. There will then be a potential for increased volatilization of certain contaminants (vapour emissions).

Complementary technologies that improve treatment effectiveness

Soil screening may be required to separate the uncontaminated soil particle fractions and reduce the amount of material to be processed.

Required secondary treatments

  • Collected vapour emissions (if applicable) may have to be treated with a recovery system before being released to the atmosphere;
  • Drainage water (if applicable) shall be treated prior to discharge.

Application examples

The following links provide application examples:


  • This technique can treat up to 100 m3/d depending on the environmental conditions of the treatment site.
  • Contaminant concentrations in leachate can be reduced by 95% or more after soil stabilization.

Measures to improve sustainability or promote ecological remediation

  • Carrying out treatability tests to optimize the mixture and reduce the need for chemical amendments;
  • Use of renewable energy and energy-efficient equipment for technology implementation;
  • Optimization of the schedule to promote resource sharing and reduce the number of mobilization days;
  • Consideration for locally available and/or recycled materials in the system design.
  • Use of residual materials or by-products from industrial processes, if appropriate, as additives or amendments (e.g., cement kiln dust from cement manufacturing);
  • Process optimization to reduce waste and consumables;
  • Consideration of climate changes is required at the design stage and in the development of the long-term performance monitoring program;
  • Use of drainage water in the soil stabilization process to reduce water requirements and discharge;
  • Use of low-carbon binders with a small environmental footprint (low GHG emissions and low energy demand for their production);
  • Use of the TRIAD approach for the planning and execution of site characterization steps to optimize characterization efforts and reduce the environmental footprint of this work;
  • Technology implementation and site remediation that optimizes the protection of ecological habitats and/or improves the quality of these habitats.

Potential impacts of the application of the technology on human health

Unavailable for this fact sheet


Author and update

Composed by : Mahaut Ricciardi-Rigault, M.Sc., MCEBR

Updated by : Martin Désilets, B.Sc., National Research Council

Updated Date : April 1, 2008

Latest update provided by : Nathalie Arel ing., M.Sc., Frédéric Gagnon CPI., Sylvain Hains ing., M.Sc., Golder Associates Ltd.

Updated Date : March 2, 2022