Definitions and EU Status

Stabilisation/solidification (S/S) comprises a wide range of techniques and methods for the treatment of hazardous waste. As the name suggests, it includes two processes (solidification and stabilisation) that can sometimes be interrelated. These terms are occasionally used interchangeably, however they are distinctly different.

o Solidification refers to techniques that aim to encapsulate the waste in a monolithic solid with structural integrity. The encapsulation may be of fine waste particles (micro-encapsulation) or of a large block or container of wastes (macro-encapsulation) (Chandler et al, 1997). Contaminant migration is restricted due to a dense matrix with low porosity and high tortuosity, as well as reduced surface area available for leaching.

o Stabilisation refers to techniques that aim to reduce environmental impact by converting contaminants in the waste material into less soluble, mobile or toxic forms.

They do not necessarily involve a change in the physical nature of the waste.

These two processes are often used in combination such as in S/S using hydraulic binders, altering both the physical and chemical characteristics of the waste. This provides an additional barrier in case for example that the solidified matrix deteriorates.

S/S using hydraulic binders was used to treat nuclear wastes in the 1950s and was widely applied to hazardous wastes in the early 1970s (Conner, 1990). It has been used both to treat wastes as a pre-treatment stage prior to disposal, as well as soils and sediments contaminated by previous improper disposal. According to Batchelor (2006) S/S has been identified by the US EPA as the Best Demonstrated Available Technology for 57 regulated hazardous wastes and is one of the most commonly applied technologies at Superfund sites in the US, being used at 24% of sites between 1982 and 2002.

50 S/S has also been adopted by certain EU Member States as a pre-treatment method for waste prior to landfill to comply with the requirements of the Landfill Directive (1999/31/EC).

Figure 2.1 provides information on the quantities of hazardous waste treated using S/S in the EU as well as selected Member States (Eurostat). It is noted that quantities presented in Figure 2.1 pertaining to treatment via S/S, may be skewed as Eurostat includes quantities of vitrified waste in the same treatment category as S/S waste. Quantities of hazardous waste treated via S/S have increased in the EU-27 between 2004 and 2008¹.

Figure 2.1 S/S (including vitrification) in the EU (Eurostat, 2013)

Germany and France have been utilising S/S extensively for the treatment of hazardous waste.

In contrast, the UK has been fairly slow in the adoption of S/S as a treatment method compared to the other Member States. A sharp increase was however observed in the use of S/S in the UK between 2006 and 2008 according to Eurostat data, with quantities increasing from 3,225 to 65,077 tonnes. A guidance document on the cement S/S of contaminated soils published in 2004 by the UK Environment Agency (Environment Agency, 2004) explains that the poor uptake of S/S technologies up to 2004 was due to the following barriers:

o Relatively low cost and widespread use of disposal to landfill

o Lack of authoritative technical guidance on S/S in the UK

1 2010 Eurostat data for S/S waste were not available by the time this thesis was finalised 0

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o Uncertainty over the durability and rate of contaminant release from S/S-treated material

o Experiences of past poor practice in the application of cement stabilisation processes used in waste disposal in the 1980s and 1990s

o Residual liability associated with immobilised contaminants remaining on-site, rather than their removal or destruction

Recent developments in EU waste management policies may pose an additional barrier to the application of S/S as a treatment method for hazardous wastes. In particular and as described in Chapter 1, treatment methods that do not involve material reuse or recovery will be given less priority compared to other alternatives. Hence, S/S prior to landfill disposal may be a less preferred option as Member States are aiming to move up the waste management hierarchy included in the Waste Framework Directive.

S/S using cementitious binders has the advantage that it may provide a route for utilisation of certain types of waste in cement or concrete matrices. Such an approach is dependent on the type of hazardous waste and the effect of constituents on cement hydration and would require rigorous testing for compliance with international standards. The economics of a working process also play an important role as well as the presence of a market for the products of the treatment process. The nature of S/S using cementitious binders is such that product properties could be tailored by varying key parameters of the mix formulations, assuming that this results in environmentally and economically viable products.

2.1.1 Binders used in S/S

S/S using hydraulic binders has previously been used for the treatment of wastes containing a variety of organic and inorganic contaminants such as metals and metalloids, asbestos, inorganic cyanides, radionuclides, solid organics, polyaromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and dioxins (Environment Agency, 2004). S/S binders used for treating various types of wastes include Portland cement, lime, pulverised fuel ash, ground granulated blast furnace slag (GGBS), and these are often used in combination.

Lime is available in the form of quicklime (calcium oxide), hydrated lime (calcium hydroxide) or milk of lime (calcium hydroxide suspension). Lime can be used to raise the pH of the S/S

52 matrix with favourable effects on the solubility of contaminants as will be demonstrated in the following sections. It can be used either alone or in combination with cement or other pozzolanic materials.

PFA is a by-product of coal fired electricity generation with pozzolanic properties due to the amount of silica (SiO2) and alumina (Al2O3). It has a history of use spanning over 50 years, being used as a concrete additive, structural fill, for grouting, in soil stabilisation and for the production of lightweight aggregates. Different types of PFA include dry, classified and conditioned PFA.

GGBS is a by-product from the steel industry. The molten slag from a blast furnace is quenched in water and this produces amorphous calcium silicate and aluminosilicates. This is ground to a fine powder to form GGBS which has excellent pozzolanic properties and is increasingly used to partially substitute Portland cement in concrete.

Thermoplastics such as bitumen or bitumen emulsions and asphalt can also be used for the treatment of waste. Bitumen is thought to have minimal interaction with waste constituents (Environment Agency, 2004) and therefore the main contaminant retention mechanism is physical encapsulation. The physical integrity of the matrix is therefore critical when such binders are used.

S/S technologies provide several routes for contaminant immobilization. The most critical parameter in most S/S systems for determining if inorganic contaminants are in a mobile or immobile form is pH. Chemical mechanisms such as precipitation, adsorption and ion exchange produce immobile contaminant forms and are strongly influenced by pH. The success of S/S is dependent on the interactions between binder and waste resulting in the formation of pH-controlling hydration phases as well as the pH-dependent reactions that determine the extent of contaminant immobilisation (Batchelor, 2006).

53 2.2 Chemistry of Cementitious Stabilised/Solidified Waste Forms

Cement chemistry and chemistry of S/S waste forms have been fields of extensive study. An extensive review of cement chemistry is not in the scope of this study, therefore only fundamentals are presented and focus is given on the contaminant binding capacity of cements. Hewlett (1998) and Taylor (1997) provide more details on cement chemistry.

Portland cement is a mixture containing predominantly tricalcium (C3S) and dicalcium (C2S) silicates, with smaller amounts of tricalcium aluminate (C3A), calcium aluminoferrite (C4AF) and gypsum (CaSO4). The reaction of cement with water is referred to as hydration during which each constituent reacts (hydrates) at a different rate to produce a range of hydration products. According to Bullard et al (2011), hydration involves a collection of coupled chemical processes, the rate of which depends on their nature, as well as the state of the overall system at a given time. These processes fall into one of the following categories:

1. Dissolution/dissociation involves detachment of molecular units from the surface of a