environment magazine

Stabilisation and Solidification of Contaminated Soil and Waste Part 2:Soil Properties and the Soil-Binder System

Dr Colin Hills Director of the Centre for Contaminated Land Remediation at the University of Greenwich

Scope

In the first part of this occasional series of articles on s/s technology, I introduced stabilisation/solidification (s/s) as a risk management strategy. This was done in the context of the Environment Agency Guidance of 2004 [1] and with reference to recent projects, including the on-going $400M Sydney Tar Ponds remedial operation in Nova Scotia, Canada.

In this second article, co-authored by Dr Peter Gunning of Carbon8 Systems Ltd, the science behind s/s is introduced with reference to key soil properties and the interactions between soil and the binders, and reagents used for stabilisation and solidification. This article sets the scene for subsequent papers in this series, which examine the s/s treatment of inorganic and organic contaminated soils, and the long-term performance of s/s waste forms.

Introduction

Knowledge of the soil/matrix type is important when stabilisation/solidification (s/s) is planned for the remediation of contaminated soil. The characterisation of soil allows a prediction of the performance of the soil-binder system, and any adverse impacts (of the soil being treated) on the waste form. ‘Knowing’, your soil, also aids the selection of a binder system to be employed.

The nature of soil can vary considerably from one contaminated site to another and within a particular site. The chemical and physical properties of a soil have an important influence on both contaminant mobility and short-term and the final waste form properties, including the engineering behaviour of a material, both in terms of its strength/performance and leaching behaviour.

Table 1 gives soil characteristics and their implications on s/s, whereas Figure 1 shows initial works at the Sydney Tar Pond Site, in Nova Scotia. The soils encountered, included made ground and estuarine silts and clays.

start of tar ponds works.

Key soil characteristics

The particle size distribution of a soil to be stabilised can influence the final strength of the product. Well-graded materials tend to exhibit a linear increase in unconfined compressive strength (UCS) with increased addition of cement binder content. However, when small quantities of binder are used, a soils grading may unduly affect strength development.

The clay minerals give unique engineering properties to clayey soils: cohesion and plasticity. Cohesive material can be considered to form a coherent mass. Non-cohesive (granular) material, on the other hand, will not [2]. BS 5930 (1999) allows for the classification of soils [3].

Figure 1: The start of remedial works at the Sydney Tar Ponds, Nova Scotia (image courtesy of the Sydney Tar Ponds Agency)

Knowledge of the cohesivity of a soil guides the approach to treatment by s/s. For example, cohesive soils mix poorly and an additional (lime-treatment) step to decrease plasticity can significantly improve the workability of the material; enabling ex-situ treatments to be used.

Soils containing expansive clay minerals with high liquid limits (40-60%), the liquid limit can be used to gauge the amount of cement required to stabilise a soil. Soils with higher liquid limits may require too much cement and be uneconomical to treat [4].

Table 1: Soil classifications and properties (modified from [5])

The moisture content of a soil is important [2]. If a soil or waste contains too much water then the porosity and permeability once treated is likely to high, and the density and strength will be lower. The moisture in a soil can be adjusted by stockpiling and draining or alternatively, water can be added to soil that is too dry. Soil can also be dried using lime, but more immediate adjustment and compaction facilitates longer-term strength gain [6,7].

The permeability of a soil results from the size and interconnectivity of its pore structures, and is in turn related to the soils particle size distribution, and particle shape and soil microstructure [8]. In general, the smaller the particles, the smaller are the average size of the pores and the lower the coefficient of permeability. The transport of water through a soil will be faster if the soil has a higher coefficient of permeability than if it has a lower value. However, the rate of transport of contaminants can depend upon a number of ‘other’ factors, as discussed later in this series of articles.

The strength of a soil measures its capacity to withstand stresses without collapsing or becoming deformed [8,9]. In general, coarser textured materials have greater soil strengths than those with small particle size. For example, quartz sand grains are subject to little compressibility, whereas clays are more easily compressed.

Some soils are susceptible to volume change as moisture content varies. For example, loess deposits are incompressible when dry, but when wet or subjected to shock or dynamic loading they can suddenly collapse. Soils can swell due to rebound after a period of compression, as a result of the introduction of water, or swell due to the action of frost or from the exposure to air and moisture, as in the case of some shales. Swelling test requirements exist for stabilised soils [10].

Binders for use in s/s

Binders can impart both chemical and physical stability to the treated product. Binders can be considered as falling into two broad groups: primary agents e.g. cement and lime and additives, including pulverised fuel ash (PFA), ground granulated blastfurnace slag (GGBS), silica fume and natural pozzolans. Other additives include bitumen, polymers and modified clays (See Table 2).

Table 2: Common binders used in S/S (modified from [11])

The interaction of primary binders and additives with soil is described below. However, the stabilisation of contaminants in a bound system also depends on the speciation of the contaminants involved [12,13].

Primary binders can be used alone to encapsulate contaminants chemically and (or) physically in the treated waste form.  Additives may not be effective on their own, but when used in conjunction with small quantities of lime or cement can constitute a blended-binder. Additives may be used to ‘tailor’ the redox environment of an s/s system for particular contaminants and to meet quality criteria, such as reduced permeability [14].

Cement is commonly used for s/s applications and can be used with a number of additives including fly ash, soluble silicates and organophilic clays [15]. Cement stabilisation is best suited for inorganic wastes and contaminants, although organic compounds can also be treated [16]. Conversely, many organic compounds and certain inorganic contaminants, e.g. borates, sulfates and some heavy metals, can adversely affect the setting of cement due to reduction in the formation of the crystalline structure, resulting in a more amorphous material [12]. 

Lime is typically used in s/s applications in the form of quicklime (CaO) and hydrated lime (Ca(OH)₂). It is normally applied to fine grained soils for the purposes of modifying pH to stabilise contaminants [16], and the dehydrating effect of quicklime is beneficial for improving the engineering properties of waterlogged ground, sludge’s and slurries. The hydration reaction of quicklime increases the surface area of the reagent assisting in the encapsulation of organic contaminants [15]. Lime is normally used in conjunction with additives such as surfactants and silicates to improve properties and reduce permeability, or with pozzolanic materials to form composites with cementitious properties [12].

Excessive carbon in some PFA may have advantages when used for s/s of certain contaminants, as the carbon surfaces are sites for adsorption and decrease contaminant mobility. PFA may be used with lime or cement, but the former may have a higher leaching rate than cement. It is worth noting that the rate of formation of certain cement minerals such as ettringite and monosulfate may be important as these minerals can readily chemically combine certain metallic and anionic contaminants. PFA has been used to form zeolitic structures for the stabilisation of chromium and cadmium containing residues [17], whereas other work using PFA involved the successful stabilisation of ferro-vanadium wastes [18], lead and chromium [19] and boron [20].

Both cement and lime are sufficiently alkaline to activate the cementitious properties of GGBS. Several other materials, including sulfates, chlorides and alkali-silicates, are also activators [21]. Many waste-streams contain components that will activate GGBS [18] including sodium salts e.g. OH, CO₃, SiO₃, PO₄, HPO₄ and F. Talling and Brandster (1989) reported that alkali-activated GGBS has enhanced resistance to waste/binder interference effects caused by, for example, the presence of organic compounds known to interfere with cement hydration [21]. Binders incorporating GGBS may have improved durability performance compared to conventional cement-based materials.

Interaction of soil and binder

Soils are complex mixtures comprising solid, liquid and gaseous phases. The nature of the soil matrix can influence the efficacy of s/s and any pre-treatment that may be required. It is well established that soil organic matter (SOM) may affect the cementing of soils [22]. The presence of clays may lead to early stiffening of the hardening mix requiring use of a plasticising agent, but clays may also adsorb organic compounds considered responsible for retardation of cement set.

During the hydration of cement, C-S-H is produced and a pH of 12-13 results. At this high pH, clay minerals can react to produce a gel-phase, which further cements the soil matrix. However, silt particles can cause an initial reduction in strength of the waste form during the first 3-6 months, although after a year strength is likely to be greater than if silt was absent.

The interactions of cement and lime on soils are broadly similar. Guidance on the various applications of lime for soil stabilisation is readily available [23], and its use as CaO or as Ca(OH)₂ results in a two-stage reaction.

The first-stage reaction of lime with soil changes soil properties in minutes to hours. There is a decrease in the plasticity of a clayey and an increase in strength, soil dehydration, and change in clay particle chemistry as Na⁺ and H⁺ (from the clay minerals) are exchanged for Ca²⁺(from the lime). The reaction of soil with Ca(OH)₂ is similar.

The second stage of the reaction process involves solidification over days and weeks as a result of pozzolanic reactions [24]. As described, the addition of lime to clay promotes the dissolution of silica and alumina (particularly at the edge sites of the clay particles) producing C-S-H and C-A-H gels [25]. 

As clay minerals agglomerate and flocculate (and plastic index decreases) the soils shear strength increases, which facilitates handling. However, depending on soil type, both an increase and decrease in liquid limits has been observed [26].

Thompson [27] showed that organic matter with a high cation exchange capacity retarded the strength producing pozzolanic reactions. However, Arman and Munfakh [26] concluded that organic soil-lime mixtures do retain strength characteristics, but in the context of contaminated land, the most significant physical change is in permeability, which may initially increase depending on the time taken between mixing and compaction, and curing.

Both quicklime and hydrated lime have been widely used in the USA as part of an s/s remediation strategy, including in conjunction with other materials such as PFA. It should be noted that the addition of lime or cement has a profound effects on SOM, as the increase in pH leads to the mobilisation of humic and fulvic acids. If a soil has a high organic content, humic and fulvic acids may to interfere with the setting of the cementitious binder [28]. It should also be established that some metal contaminants, such as Cu, can readily complex with SOM and become mobilised during s/s [25]. Guidance on how to improve the effectiveness of s/s in respect of the additives and binders available has been summerised by Conner [29].

Summary

The properties of soils can have an important influence on the physical and chemical characteristics of the mature waste form. Soils are themselves complex mixtures of organic and mineral matter that may contain contaminants that are present in pore space or bound to SOM or clay particles.

A wide variety of binders can be used for s/s, including lime or cement. The high pH environment resulting from binder addition causes soils to undergo a 2-stage reaction process leading to s/s. Contaminants are adsorbed, precipitated and physically trapped and/or incorporated into crystalline phases in the hardened waste form.

References

[1]   Environment Agency. (2004). Guidance on the use of Stabilisation/Solidification for the Treatment of Contaminated Soil. Science Report SC980003/SR2. http:// publications.environment-agency.gov.uk/pdf/SCHO0904BIFO-e-e.pdf. Accessed 24.03.2011.

[2]   British Standards Institution. (1990) BS EN 1924: Part 1. Stabilized materials for

civil engineering purposes. Part 1: General requirements, sampling, sample preparation and tests on materials before stabilization.

[3]   British Standards Institution. (1999) BS 5930. Code of practice for site investigations

[4]   Stavridakis, E.I. and Hatzigogos, T.N. (1999 Influence of Liquid Limit and Slaking on Cement Stabilised Clayey Admixtures. Geotechnical and Geological Engineering 17, pp. 145-154.

[5]   Townsend, W.N. (1973) An introduction to the scientific study of the soil. 5^th edition. St. Martin's Press  (New York).

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[8]   British Standards Institution. (1990) BS 1377. Methods of test for Soils for civil engineering purposes.

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[21] Talling, B. and Brandster, J. (1989) Present State and Future of Alkali-Activated Slag Concretes. American Concrete Institute Special Publication, SP-114, pp. 1519-1545

[22] Tremblay, H., Duchesne, J., Locat, J. and Leroueil, S. (2002) Influence of the Nature of Organic Compounds on Fine Soil Stabilization with Cement. Canadian Geotechnical Journal 39, pp. 535-546.

[23] Angel, S., Bradshaw, K., Clear, C.A., Johnson, D., Kenny, M., Price, B.W. and Southall, M. (2004) Essential guide to stabilisation/solidification for the remediation of brownfield land using cement and lime. British Cement Association.

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[25]McKinley, J.D., Thomas, H.R., Williams, K.P. and Reid, J.M, (2001) Chemical Analysis of Contaminated Soil Strengthened by the Addition of Lime. Engineering Geology, 60, pp. 181-192

[26] Arman, A. and Munfakh, G.A. (1970) Stabilization of Organic Soils with Lime. Division of Research Engineering, Louisiana State University, Baton Rouge, USA, Engineering Research Bulletin 103.

[27] Thompson, M.R. (1965) Influence of Soil Properties on Lime-soil Reactions. Public Works 96, 8, pp. 120-123

[28]Vipulanandan, C. and Krishnan, S. (1993) Leachability and Biodegradation of High Concentrations of Phenol and O-cholorophenol. Hazardous Waste and Hazardous Materials 10, pp. 27-47.

[29] Conner, J.R. Guide to improving the effectiveness of cement-based stabilization/

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