Fact sheet: Ex situ Solidification/Stabilization

From: Public Services and Procurement Canada

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Ex situ solidification/stabilization is a general term used to describe a group of processes (or technologies) that are often used together (or considered a variation of one technology) to treat an array of wastes including both solids and liquids but for contaminated sites these technologies are primarily used to treat inorganic contaminants in soils and less commonly for sediments. Heavy metal-contaminated wastes/soils are typically the most commonly treated contaminants using ex situ solidification/stabilization given their leaching potential. There are several variations to the solidification/stabilization approach such as solidification of radioactive wastes, mud stabilization and vitrification.

In this process, the soil and the contaminants will be bound either physically (solidification) or chemically (stabilization) which will reduce or eliminate the potential for leaching of the contaminants. This treatment prevents off-site contaminant migration and expansion of the contaminated zone, however, the physical and/or chemical processes in solidification/stabilization do not remove or destroy the contaminant.

Solidification/stabilization consists of mixing excavated contaminated soil with stabilizers or binding agents. The most commonly used additive is Portland cement. However, other additives are also used such as lime/pozzoland (e.g., fly ash and cement kiln dust), etc. These may be used with other reagents such as iron salts, silicates, clays, tar, asphalt, modified sulfuric concrete, polyethylene, soluble phosphate, which are designed to enhance the set/cure time and/or compressive strength of the soil, or to reduce the leachability of metals.

Solidification of excavated material involves transforming the physical properties of the contaminated soil by the addition of a binding agent which compacts the matrix and reduces both the pore volume and hydraulic conductivity of the matrix. The process is designed to encapsulate (or isolate) a contaminant and subsequently restrict or reduce potential for contaminant migration (or volatilization) by decreasing the surface area exposed to leaching and/or by coating the waste with low-permeability solidification materials.

Stabilization involves transforming the chemical properties of the contaminants within the soil matrix. The contaminants are transformed into compounds having lower leachability, water solubility, mobility and toxicity. In certain cases, a binding agent may also be added to the contaminated material during stabilization to maximize the stabilization processes. Certain additives, like hydroxylating substances, contribute to increasing the pH of the environment, which has the effect to reduce the solubility thus the mobility of contaminants. Other bindings agents, like silica, contribute to encapsulate the contaminated particles or to adhere to stabilizing agents and form solid agglomerates.

Both remediation technologies are based on excavation of the soil and its subsequent mixing with additives and/or reagents. The mixed soil is then allowed to rest (setting, harden or cure) before being transported off-site for disposal, or used on-site as fill material. Treated soils, in particular those with greater strengths (i.e. compressive strength), can also be used as construction material.

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Implementation of the technology

The ex situ solidification/stabilization process generally consists of excavation of contaminated soil for mixing with additives. The excavated soil is screened to separate and remove oversized materials, if present, before mixing with additives. It is more common to add dry additive to the soil. This process, however, generates dust that needs to be mitigated. An alternative is the addition of additive to water in order to form a grout or paste before mixing it with the contaminated soil. Depending on the contaminated material and type of additive used, the treated material requires time for curing or setting. There are in general three (3) different mixing methods (all mechanical):

  • Direct mixing (e.g., by using a backhoe);
  • Plant processing using either batch or continuous feed that would be comprised of treatment unit, chemical storage, metering and mixing equipment;
  • In-drum processing, in which the container and mixing apparatus are disposed of along with the treated material. Pugmills are commonly used as blending devices for plant processing.

Following treatment, depending on regulations, the material is either transported off-site for disposal, or used as backfill or construction material on-site. During the stabilization process, increase in temperature from exothermic chemical reactions (for example, quicklime hydrolysis) can result in the increase in mobility of contaminants through increased desorption, solubility, and/or volatilization, and thus should be considered in the design of the stabilization system. Accordingly, additional mitigation measure may be needed to address this potential issue.

  • Excavation and solidification/stabilization treatment systems may include:
  • Mobilization of equipment and construction of temporary facilities and site access considerations.
  • Physical and chemical characterization of the soil or sediment (particle size distribution, composition, bulk density, permeability, plasticity, contaminant types, distribution and concentration).
  • Clearing/grubbing/demolition.
  • Topsoil stripping and temporary stockpiling.
  • Excavation dewatering (sumps, wells, well points, cut-off walls, etc.).
  • Slope stability controls (cutbacks, benches, piling, soilcrete, controlled low strength material, etc.).
  • Foundation protection for retained structures (shoring, underpinning, etc.).
  • Excavation (typically with hydraulic excavators or backhoe; less frequently with a clam shell, drag line, dozer, loader, bucket auger or other).
  • Removal of large rocks or debris.
  • Setup of equipment for the mixing process: plant processing, direct mixing (in place), or in-drum processing.
  • Testing of the solidified and/or stabilized material for leaching (and volatilization) of contaminants and material strength.
  • Transport of solidified and/or stabilized soil for off-site disposal, or use as on-site backfill material and/or on-site construction material.
  • Preliminary trials, bench scale or pilot scale tests to get the right mix and formulations.
  • Treatability studies are generally required.
  • Surface restoration (grading, paving, hydroseeding or planting).

The method relies on traditional/commonly-available civil/earthworks construction equipment and methods for the excavation component. Commercial and transportable units are available for the treatment process that may include metering and blending devices. Chemicals include additives such as Portland cement or lime and water that are mixed with the soil.

Materials and Storage

  • Sufficient storage space is required for the plant processing if used.
  • On-site storage is typically limited to small amounts of fuel and lubricant (daily fuelling of equipment is often from a mobile tank) as well as miscellaneous construction site supplies.
  • Contractor may create temporary stockpiles of contaminated materials pending treatment. To protect from dust and contaminant vapours, the soil stockpiles and feed and mixing equipment need to be covered, or sprayed with water or foam.

Waste and Discharges

  • Treatment systems may generate solid, liquid and gaseous residuals, but the appropriate storage, management and disposal of treatment residuals are part of proper treatment system operation.
  • The treated soil is commonly not considered a residual material although the contaminants that remain immobilized in the soil may be considered as such. The potential concerns for residuals depend on contaminant type, solidification/stabilization method and use of treated materials.
  • Windblown dust can occur from soil loading, track-out or stockpiles, or soil mixing, for example, and may also deposit directly on downwind surfaces and stormwater.
  • Solid discharges from soil mixing process may include particulates.
  • Diverted and collected/treated stormwater are typically passed into the local stormwater system. Use of water with additives can create a wastewater stream that requires proper control and management.
  • There is potential for vapour phase emissions from equipment exhaust and volatilization of contaminants from fresh excavation faces or soil stockpiles. In specific cases, naturally occurring radon may also be mobilized.

Recommended analyses for detailed characterization

Chemical analysis

  • Metals speciation
  • Chemical analysis of each particle size fraction
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
    • free

Physical analysis

  • Soil water content
  • Soil granulometry
  • Contaminant physical characteristics including:
    • viscosity
    • density
    • solubility
    • vapour pressure

Recommended trials for detailed characterization


Other information recommended for detailed characterization

Phase III

  • Volume of contaminated material to treat


A treatability study is recommended before beginning the treatment to provide information on the optimal quantity of stabilizers/binders to add to maximize the stabilization/solidification treatment according to the type of contaminated matrix and volume of material to be treated.


The solidification/stabilization technology is applicable to solid contaminated material only, such as soil, mud and sediment. The target contaminants are generally metals and free radicals. Application of this technology has been successfully performed to treat metals and radioactive materials as well as organic contaminants of concern including non-volatile and semi-volatile compounds such as chlorinated ethenes, petroleum hydrocarbon constituents, polychlorinated biphenyls, pesticides, and dioxins and furans. However, it has not been demonstrated to treat volatile organic compounds. This technology is in constant evolution and may be applicable to a wider range of contaminants in the future.

The solidification/stabilization technology is generally easier to implement in sandy, silty or gravely soils, than in soils with high clay content as it is easier to achieve a uniformity of mixing in the former, while the latter may tend to leave residual clay balls of unmixed and untreated material.

Applications to sites in northern regions

  • Remote sites are prone to high mobilization and on-site monitoring costs, limited equipment availability and short seasonal work windows.
  • Northern systems require climate-appropriate design, including consideration of deep freezing, permafrost, spring melt and frost heave.
  • This technology can be used in isolated regions without services or electricity.
  • Due to permafrost, only very shallow soils may be viable for excavation (active layer).
  • Road transportation of contaminated material for treatment at an off-site facility or road transportation of treated material for re-use off-site is often cost-prohibitive and/or not feasible. Train or barge/boat haul may be economically feasible, but is typically not viable. If multi-mode transportation is used, accumulation, transfer and re-handling sites are subject to dust, vapour, odours, noise, track-out and stockpile leaching considerations.
  • If long-term monitoring is required, it may be challenging and costly. Telemetry can be used for remote monitoring of site conditions.

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


Depending on the volume of the soil requiring treatment, the treatment may take weeks to months. The output can be in the order of several hundred cubic metres per day.

Long-term considerations (following remediation work)

There is potential for leaching of contaminants from treated material if it is used as construction material on-site, hence monitoring is often required. There is also potential for dust generation in the long term if treated material is used at the surface.

Secondary by-products and/or metabolites

  • There is no by-product production when using the solidification/stabilization technology.
  • Contaminants are not degraded by the treatment and are still in the contaminated matrix after treatment.
  • The contaminants are immobilized within the matrix and do not migrate as long as the integrity of the matrix is maintained. To ensure this, leachate from the disposal site must be analyzed to monitor for any contaminant migration.

Limitations and Undesirable Effects of the Technology

  • This technique does not eliminate the contaminants but rather immobilizes them.
  • When the treated material is returned to the site, stabilization/solidification technology is considered as a temporary remediation technique, where control of contaminant migration is the goal.
  • Mixing concrete with used oil and tar requires the addition of specific agents. During mixing of the agents with the matrix, heat may be produced which increases volatilization of certain contaminants.
  • The solidified/stabilized material may be susceptible to leaching over time.
  • This technique only applies to contaminated soils from the vadose zone.
  • Mineral salts within the contaminated matrix may interfere with the solidification/stabilization process.
  • The volume of material for disposal is increased by this technique.
  • If the treated soil is used as construction material on-site, its geochemical properties will be different from the in situ geochemical conditions. For example, it is expected to have significantly lower permeability than the surroundings in situ soil.
  • The physical disruption of excavation is significant. Possible adverse effects from excavation activities include slope failures, equipment collisions, small spills of fuel or hydraulic fluid, damage to utilities, damage to adjacent structures or breathing air quality issues in low-lying areas, such as the excavation pit itself. Landslides collapse and damages to adjacent structures are possible if the geotechnical and civil works are inadequate.
  • Dramatic, although short-lived, changes to site-scale hydrology and hydrogeology are commonly associated with excavations.
  • Some considerations associated with nuisance or safety include noise, odour, light and traffic.
  • Dust production during soil manipulation is a significant issue particularly with fine-grained material in dry and windy conditions. Typically, water spray is used for dust suppression. A number of alternate (more expensive) approaches were used for certain site-specific reasons including polymer sprays, calcium chloride solutions, lignosulfonates, foaming agents, etc. Biodegradable fabric ground cover can be used for controlling both dust and erosion as vegetation re-establishes.
  • Transportation of contaminated material increases the cost of their treatment and is not well accepted in residential areas.
  • Large areas are required for the treatment and stockpiling of materials.

Complementary technologies that improve treatment effectiveness

  • Physical separation to remove the uncontaminated soil particle size fraction(s) will reduce the volume of material to be treated.
  • Addition of specific agents can maximize the stabilization and solidification of the treated matrix.

Required secondary treatments

  • Certain locations require that stabilized/solidified material be disposed of in a leakproof environment.
  • Leachates must be analyzed to monitor potential contaminant migration.

Application examples

An application example is available at this link:


The performance of several stabilizers and binding agents is well documented, having been proven to prevent contaminant migration while being easy to apply at low cost.

Measures to improve sustainability or promote ecological remediation

  • Use of renewable energy and energy-efficient machinery (e.g., geothermal, wind or solar energy).
  • Process optimization to reduce wastes and consumables.
  • Schedule optimization for resource sharing and fewer days of mobilization.
  • Assessment for on-site use of treated material for construction or backfill in order to reduce trucking and landfill disposal.
  • Use of waste or by-products of industrial processes, if appropriate, as additives or reagents. For example, cement kiln dust from cement manufacturing can be used as an additive for solidification/stabilization.

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 : Daniel Charette, P.Eng., ing., Chris T. Kimmerly, M.Sc., P. Geo., exp Services Inc.

Updated Date : March 31, 2017