Cement-based Solidification/Stabilisation Treatment of Organic Chemical Contaminants in Waste
The cement-based Solidification/Stabilisation (S/S) treatment is a viable waste immobilisation treatment process for certain contaminants. This technique essentially involves the use of a binding reagent to micro encapsulate the contaminants with or without the addition of chemical additives and the principal binder used is Ordinary Portland Cement (OPC).
Contaminants (toxic or non toxic) in waste are often not compatible with the cement hydration reactions and can adversely affect the setting of the mix. One should engage a specialist consultant/contractor before conducting a S/S treatment.
This advisory note only deals with the treatment of semi-volatile organic contaminants in contaminated soil or waste sludge. Further advice may be sought from EPA regarding the S/S immobilisation treatment.
Cement & Additives in S/S Treatment
When ordinary cement is mixed with water, its chemical constituents undergo 6 principal hydration reactions. The initial pH of the mix rises quickly to a pH of 12 (alkali). With a proper control of the water to cement ratio, quantities of additives and the use of suitable mixing equipment (see Technical Note 1), the treated waste would set and attain unconfined compressive strength (UCS) greater than 1 MPa within 7 days.
The actual compositions of commercial OPCs can vary from type to type. Examples of OPCs are in Annex A (PDF 18KB).
Supplementary cementitious materials (SCMs) commonly used to enhance the properties of concretes include industrial by-products such as pulverised fly ash (PFA) from coal combustion, blast furnace slag (BFS) from iron making and condensed silica fume (CSF) from the ferrosilicon industry. These by-products can have widely variable chemical compositions depending on the production source. For example, CSF-OPC blend is used in making high strength concrete, whilst PFA has performance-enhancing effects on the OPC mix in terms of workability, pumpability, strength, shrinkage, and permeability. PFA can also serve as a sorbent for some metals and organic chemical compounds in waste treatment. BFS has its own source of lime (CaO) and silicate compounds. It can react with water in the presence of calcium hydroxide to form cementitious substances. Other SCMs include silica fume, granulated furnace slag, metakaolin (e.g. calcined clay). Incorporate SCMs into the cement mix when the modified mix benefits the S/S treatment process.
In treating hydrocarbon contaminated wastes, chemical additives such as surfactants can, amongst other things, greatly improve the mixing performance and setting of the mix. Seek advice of the specialist chemical supplier.
Cement-based S/S Treatment Process
The contaminants present in the waste can interfere with the S/S treatment in many ways. These include
- Inhibits the hydration process by preventing the water molecules from reacting with the silicate compounds.
- Slows down or prevents the formation of water-containing calcium silicate crystalline compounds (silicate gel).
- Reacts with the calcium hydroxide to form undesirable complexes.
- Prevents the coating of the silicate gel on the aggregates and hence the setting of the mix.
- Forms weak mineral complexes which do not bind well with the silicate gel.
- Retards the cement set and the mix takes much longer time to cure.
- Interferes with cement hydration due to incomplete immobilisation of contaminants.
- Excessive water associated with the contaminants or waste can cause problems in setting.
It is imperative to identify and speciate all contaminants (organic and inorganic) present in the waste. Use favourable lab scale/pilot scale trial results only as a guide to test and optimise the mix formulation for the full scale treatment. Use larger quantities of binder/reagents to allow for the less than ideal mixing efficiency at full scale treatment (Jeffrey Means et al. 1995).
The preparation of samples and UCS test method should be based in accordance with EPA’s requirement as per the General Immobilisation Approval 2005/14 (PDF 28KB) for coal tar impacted soil from former gas work sites.
Mixing is a critical element of any chemical fixation and S/S treatment process (J Connors, 1990, J Connors, Portland Cement Association, 1997). The mechanical mixer functions as a chemical reactor. Design and engineer the full scale S/S treatment in accordance with the draft EPA policy for Mixing Plant and Equipment for Immobilisation Treatment, Oct 2007. For example, the following types of mixer may be commissioned for the full scale S/S treatment:
- Pug mill type mixer.
- Paddle type mixer including Ribbon mixer and Turbine/Pan mixer.
Treatment of Organic Hydrocarbons
Organic contaminants known to affect the cement S/S treatment are in Annex B (PDF 12KB). These can affect the cement setting performance even at relatively low concentrations. The cement based S/S treatment is not suitable for fixing volatile organic chemicals (VOCs) because these chemicals are capable of escaping into the environment under ambient conditions. Contaminated soil, waste sludge and residues containing semi-volatile organic chemicals (SVOCs) may be suitable for treatment by the cement-based S/S method.
SVOC means any organic chemical compound based on carbon chains or rings with a vapour pressure less than 2mm of mercury (0.27 kPa) at 25o C and 101.3 kPa, and has a boiling point of greater than 100o C at 101.3 kPa.
The cement treated waste must meet the following UCS criteria.
|General Performance Criteria for S/S Treated Waste|
Polycyclic Aromatic Hydrocarbons (PAHs) and/or toxic organic chemicals
1.5 MPa (14 days)(see note 1 in remarks column)
These organic contaminants are hazardous, toxic or persistent in the environment.
Note 1: With the prior approval of EPA and subject to conditions a minimum UCS of 1.0 MPa (14 days) may be acceptable if PAH concentrations do not exceed 13,000 mg/kg.
Total Petroleum Hydrocarbons (TPHs) C10-C36 (see note 2 in remarks column)
0.35 MPa (28 days)
These hydrocarbon compounds are lowly or moderately toxic and can biodegrade.
Note 2: If the PAHs concentration is more than 200 mg/kg, the UCS of 1.5 MPa applies.
Annex A: Cement Type and Performance (PDF 18KB)
Annex B: Organic Contaminants Known to Affect the Outcome of Solidification (PDF 12KB)