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Ž .Construction and Building Materials 13 1999 329341

The chemical compatibility of cementbentonite cut-off wall material

Stephen L. Garvin, Carolyn S. Hayles

Building Research Establishment, Scottish Laboratory, Kel in Road, East Kilbride G75 0RZ, UK

Received 20 May 1998; received in revised form 4 May 1999; accepted 9 May 1999

Abstract

Containment techniques are amongst the most common methods of remediating land contaminated by previous industrial use. An important part of the containment process is the placing of vertical in-ground barriers to minimise the movement of contamination from site. Self-hardening slurry trench cut-off walls of cementbentonite are barriers that are increasingly beingused in the United Kingdom. The use of cementbentonite slurry trench cut-off walls, particularly in highly aggressiveenvironments, raises concerns over durability and long-term performance. The relatively recent use of such barriers means thatthere is little information on their long-term performance. This paper describes research being undertaken to investigate the

properties of cement

bentonite cut-off walls and to examine potential durability problems. Laboratory immersion tests have beenused to assess the chemical resistance of typical cementbentonite mixes containing ordinary Portland cement, groundgranulated blast-furnace slag and pulverised fuel ash. These mixes showed varying degrees of resistance to chemical attack withpulverised fuel ash mixes more resistant than those with ground granulated blast-furnace slag. The advantages and limitations of such tests are discussed. 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Cut-off wall; Cementbentonite; PC

1. Introduction

It is uncertain as to how much contaminated land

there is in the UK. The House of Commons SelectCommittee estimated in 1990 that there are between50 000 and 100000 contaminated sites. Contaminatedland may pose threats to human health, the naturalenvironment and buildings. It is wrong to categorise allsites of a certain kind as being contaminated, buttypical contaminated sites include domestic and indus-trial landfill sites where leachate and gases may begenerated, and land previously used for industrial pur-poses. Contamination may spread out from these sites

Corresponding author. Tel.: 44-1355-233-001; fax: 44-1355-

241-895.Ž . E-mail address: [email protected] S.L. Garvin

and affect groundwater, surface water and neighbour-ing land and immediate action is required to remediatethe site. On some sites the contamination may not pose

a threat in its existing use, but redevelopment may bedelayed until remediation occurs. The reuse of con-taminated land is a valuable contribution to urbanregeneration, and the number of contaminated sitesused for construction is expected to rise.

The basic remediation options for such sites arecontainment, treatment and excavation and removal tolandfill. The principal encapsulation or containmentmethods involve using appropriate cover or cut-off wall

systems 1 .It is recognised that encapsulation does not necess-

arily result in a long-term solution as the contamina-tion itself still exists, but encapsulation generally offers

both a more cost-effective solution and one which uses

0950-061899$ - see front matter 1999 Elsevier Science Ltd. All rights reserved.Ž .PII: S 0 9 5 0 - 0 6 1 8 9 9 0 0 0 2 4 - 0



( )S.L. Gar in, C.S. Hayles Construction and Building Materials 13 1999 329 341 331

2. leave to hydrate for a period between 4 and 24 h;3. add the cementitious component, in a dry powder

form or as a slurry; and4. pump the material to the trench and allow to set;

insert membrane prior to setting if specified.

2.1.3. Slurry trenches

The majority of cementbentonite slurry trench cut-off walls for containment purposes in the UK areformed using the single-phase method where the trenchis typically 0.6 m wide and is excavated under a self-hardening slurry. The slurry is left in the trench to setand harden over a period of 23 days. Excavation of shallower trenches, up to 12 m depth, in relatively flatground, is usually achieved with a backactor. For deepertrenches and difficult ground conditions, grabs and

cutters are used and a double-phase method is nor-mally used. If continuous excavation through the slurryis likely to cause problems then the cementbentoniteslurry is used to replace a bentonite excavation slurry.If a geomembrane is to be inserted, retarders arenormally used to delay the setting time.

2.2. Specification

Specification of slurry trench cut-off walls is normallymade on the basis of performance as opposed to pre-scription. There is, at present, insufficient publisheddata for specifiers to be more prescriptive.

A specification for a barrier would, typically, includethe following:

specification of fluid properties; specification of hardened properties; durability requirements; quality control on site during construction; and specification of materials as supplied.

2.2.1. Performance specification

The properties of the hardened slurry and durability

are the most important factors that influence long-termperformance.

Permeability: Normally specified as less than 109

Ž .ms drained triaxial cell test . The use of ggbs inthe slurry is normally necessary in order to achievethis permeability. Other cementitious materials may

not give as low a permeability 2 . These tests arecarried out at 28 days andor 90 days. The use of bentonite contents above 40 kg gives a markedreduction in permeability and also increasesstrength. Above 60 kg the slurry becomes thick and

difficult to mix and pump 10 ; Strain at failure: Generally specified because of the

perceived need for a deformable cut-off, which isless likely to crack and leak, particularly if groundmovements are envisaged. A value of 5% in adrained triaxial test is often quoted, however, thisstrain criterion is difficult to achieve with the 109

ms permeability requirement. This is particularlyso where compliance testing is carried out at 28days. Also the 5% strain can generally only beachieved, if at all, by testing at effective confiningstresses much higher than those which exist in situ;

Strength: Where unconfined compressive strengthshave been specified, the values have varied con-siderably, between 100 and 1000 kPa. Sometimes aminimum or maximum value is specified andsometimes a range of values. There is little agree-ment on what is required. Increasing the cemen-titious content from 100 kg to 350 kg can increase

strength by a factor of 4 to 5, but at the highercement contents the mix can be excessively thickand produce a hardened slurry that is strong butbrittle. At the lower cement contents the mix canbe weak. Such mixes also contain more water andso are liable to greater drying shrinkage and crack-ing if left exposed before capping. At cementitiouscontents less than 90 kg set is unlikely to occur.

2.2.2. Effect of materials on specification

The use of ggbs as a replacement material can pro-duce higher early strengths and lower permeabilitiesthan straight PC mixes. However, the hydration charac-teristics of PCggbs mixes can pose difficulties in meet-ing both the permeability and strain specification. Thestrength development is initially more rapid and cont-

inues for longer than for neat PC mixes 12 , so mixescontaining ggbs which have a permeability of 109 msat 28 days will become more brittle and would beunlikely to have a strain at failure greater than 5% atthe same time. If compliance testing is necessary at 28days for contractual requirements, specifying a perme-ability of 108 ms would produce a less brittle mix which is more capable of achieving the required strain

at failure, and which would eventually reach a perme-9 ability of the order of 10 ms 10 .

2.3. Chemical compatiblilty of cement bentonite

The chemical compatibility of cementbentonite de-pends on the behaviour of the cement binder and theclay when exposed to a particular contaminant or mix-

ture of contaminants 13 . The slurry may be suscepti-ble to attack from chemicals which would normallyattack either hydrated cement or bentonite, and thereis a need to identify the contaminants most likely toreduce the effectiveness of the slurry material.



( )S.L. Gar in, C.S. Hayles Construction and Building Materials 13 1999 329 341332

2.3.1. Compatibility of the fresh slurryThere is substantial literature on clay-chemical inter-

actions, but less on the physico-chemical behaviour of bentonite and specifically sodium-exchanged bentoniteused in cut-off walls. However, it is well established

that bentonite consists of a connected and porous network of anisometric particles 8 . The connectivity

and isotropy of bentonite is achieved by the flexibilityof the clay sheets and its lenticular porosity as a resultof this stacking. These factors produce the high swellingcharacteristics and low permeability required to pre- vent seepage of contaminants.

The loss of water, increased permeability, and alter-ation of the microstructure of the cementbentoniteslurries are believed to be the result of calcium ion

concentrations and high pH solutions 8 . Rheologicalproperties of the cementbentonite appear to be con-

trolled by a rise in pH, which is associated with cementhydration reactions. Viscosity is the single most im-portant property controlling the characteristics of thecementbentonite slurry, and it is believed that high

pH values thicken bentonite suspensions 8,14 .Sodium-exchanged bentonite will readily exchange

its sodium ions for other ions such as calcium andaluminium, creating a material that does not have thecapacity to swell to the same extent as the sodium-ex-changed bentonite, but nearer to that of natural sodium

bentonite 7 . The process of calcium-induced aggre-gation is a major factor which controls the unwantedincrease of permeability and water loss. The swelling of

sodium-exchanged bentonite can also be inhibited bythe presence of other sodium salts. The net effect of these chemical reactions will be a fall in the swellingcapacity of the bentonite and thus a reduction in the

effectiveness of the bentonite as a sealant 7 .

2.3.2. Compatibility of the hardened slurryInvestigation of the effect of contaminants on hard-

ened cementbentonite has been addressed by previous workers using several methods. The principalfour methods are as follows:

mixing the material with appropriate volumes of contaminants and analysing reaction products 15,16 ;

immersing specimens in solutions of single or combined contaminants 1517 ;

permeating the material with contaminants in atriaxial cell with a hydraulic head and confining

pressure 13 ; and diffusion tests 16 .

These different test conditions give different infor-mation on the material’s behaviour. Therefore, the testregime used should be considered when analysing, in-terpreting and applying results.

Contaminants which are considered likely to affectthe integrity of cementbentonite barriers include in-organic and organic acids, magnesium and ammonium

salts and sulphates 18 . A description of the principalmechanisms of attack follows.

Acids react with the calcium in the cement hydratesto degrade the binder. The rate of reaction is depen-

dent on the solubility of the reaction product, which if low can inhibit further reaction.

Magnesium and ammonium salts attack the cementpaste fraction. Ion-exchange transforms insoluble cal-

cium in the hardened cement paste into soluble calcium salts which are subsequently leached out of the material.

Sulphates react with the hardened cement paste.The reaction products which form a greater olume

than the reactants. In unconfined samples this leads to disintegration by expansion and cracking.Ž . Phenol weak acid and other organic solutions have

a negligible effect on cementbentonite. Some or -( ) ganic molecules e.g. aniline and ammonium chloride

were found to cause disruption to the structure and porosity of cement bentonite 19 .

The type of cement and proportions of replacementmaterials are thought to affect the chemical resistance.Some authors have suggested that partial replacementof some PC by ggbs is beneficial to sulphate resistanceand also has benefits in lowering the permeability of

the material 2022 . Between 60 and 80% replacementhas been described as the desired replacement level. Alternatively, pfa can be used to replace some of thePC and this may give even better resistance to sulphateattack than ggbs.

The importance of the C A content in PC has been3

described elsewhere 20,23 and it has been recom-mended that at sulphate levels in the groundwater orsoil of between 150 and 1500 mgl, the C A content3

should be no greater than 8%. Above a sulphate levelof 1500 mgl, the C A content should be no greater3

than 5%. However, due to the lack of data in these

publications it was not certain whether these conclu-sions were the result of testing or were adapted fromconcrete technology.

The form of chemical attack may not always lead toa diminution of the physical properties of the material.Pore blocking may occur leading to a reduction of thepermeability. An important function of a cementbentonite barrier would be to remove contaminants byattenuation mechanisms. This is primarily a function of the bentonite fraction which readily exchanges cationsand can bind up heavy metals and other contaminants.

When assessing the effects of reactive chemicals it isimportant to realise that a major source of damage maybe the dissolution of the cementbentonite without any




specific chemical reaction. It has been argued that if aggressive chemicals are present, the cement will be

attacked rather than the relatively inert bentonite 24 .Thus, high bentonite concentrations should be used onaggressive sites. The lower permeability of the higher

bentonite content mix will also help to regulate therate of attack on the cement by the contaminants.

Other work indicated that attack on plastic concrete,made from cementbentonite with aggregate, may bemore severe than on a cementbentonite cut-off bar-

rier without aggregate 22,25 .

2.3.3. Pre ious BRE work

Previous work undertaken by BRE has included testson the chemical compatibility of cementbentonite withcontaminants that are commonly found on old indus-trial sites, and assessing the physical properties of

cement

bentonite both in the ground and using sam-ples retrieved from site. Previous laboratory work,using immersion tests, has highlighted problems associ-ated with the chemical compatibility of cementbentonite mixes with common in-ground contaminants.

This initial work 4,5 included a variety of mixes and arange in the mix proportions including a number of mixes using PCggbs mixes commonly used on site,and PC and PCpfa mixes not typical of those cur-rently used in the UK.

Immersion tests at BRE were undertaken on sam-ples in solutions of both single and mixed contami-

nants. These tests included a set of cement

bentonitemixes immersed in different sulphate solutions. Thereference solution in the sulphate experiments was

Ž 2.sodium sulphate SO , 2500 mgl; class 3 sulphate4

conditions as defined in Digest 363 4,26,27 . It wasassumed that the sodium ion itself would have a nil ornegligible effect on the material. The concentration of sulphate ion in solution ranged from 1000 to 5000mgl. Some of these solutions were found to be par-ticularly aggressive causing degradation of all thecementbentonite mixes.

The degree of attack was linked to the concentration

of the sulphate ion. At lower concentrations, up to2000 mgl, only the weaker mixes or those containing asignificant proportion of ggbs were at risk of degra-dation. The poor performance of ggbs mixes was incontrast to the improved performance that was ex-pected by other workers. At concentrations above 2500mgl the chemical resistance of all mixes decreasedand this was seen most markedly at 3500 mgl andabove. The aggressiveness of the solution depended notonly on the sulphate concentration but also on thecounter ion present. The relative aggressiveness wasMg2

NHNa

Ca2. A combination of sulphate3

Ž .and acid pH 2.5 , gave a greater degree of deteriora-tion than sulphate in isolation.

The overall performance of the PCggbsbentonitemixes in sulphate solutions was poor and immersiontests showed that these more commonly usedcementbentonite mixes were not chemically compati-ble with sulphates, acids and magnesium salts; the

performance of a limited number of PCpfabentonite mixes was better than that of the PC

ggbsbentonite mixes. The proportion of bentonitepresent made a negligible difference to chemical com-patibility results.

3. Experimental

The current laboratory test programme was devel-oped with the aim of investigating the relative chemical

resistance of a number of cementbentonite mixes,specifically mixes with cementitious material of PCpfa. As hardened cementbentonite has a much lowerstrength than concrete and most building mortars, ithas at least as much, if not more, in common with claythan concrete. Normal methods of assessing the dura-bility of cement-based materials, such as measuring thecompressive strength after immersion could not bedirectly employed. Instead changes in weight, lengthand physical appearance were monitored.

Immersion tests were chosen to investigate thechemical compatibility of cementbentonite for the

following reasons:

they can provide a simple and cost effective way of assessing deleterious effects of contaminants on arange of different mixes;

they provide the worst case conditions for chemicalattack on cementbentonite and if the materialperforms adequately well in this test then it can beassumed that it will perform satisfactorily well inthe contaminated ground; and

immersion tests do not, however, represent realisticenvironment. The availability of solution phase con-taminants that could react with the cement

bentonite is much greater than in the ground. Inaddition, the material is free to expand, crack orcontract on chemical attack.

Confined tests have been used to monitor the effectsof sulphate solutions on the material in an environ-ment in which the material could not freely expand.

3.1. The mixes

A number of cementbentonite mixes were made upunder laboratory conditions. These mixes are shown inTable 1 and the mix proportions are expressed as




Table 1aMix proportions used in test programme per 1000 l water

Mix no. Bentonite ggbs pfa PC Totalcement

1A 40 na 36 84 1202A 40 na 60 140 2003A 40 na na 200 2004A 40 40 100 60 2001B 40 na 37.5 112.5 1502B 40 na 60 90 1503B 40 na 75 225 3004B 40 na 120 180 300

a All results given in kilograms.

weights of each material per 1000 kg of water. Thechemical analyses of the PC, the ggbs and the pfa usedare shown in Table 2.

3.2. Preparation

In these tests the quantity of material used to pro-duce the samples in the laboratory was one tenth of the

Table 2Chemical analysis of PC and ggbs

Ž . Ž . Ž .Oxide pfa % ggbs % PC % Bogue equivalentŽ .%

CaO 1.88 42.78 63.57 C S 523SiO 49.41 33.09 22.08 C S 202 2

Al O 24.21 12.59 5.15 C A 102 3 3

Fe O 13.99 0.4 3.55 C AF 82 3 4

NaO 0.57 0.21 0.22 CS 52 2

MgO 1.62 7.59 2.4K O 3.21 0.38 0.72

SO 0.76 3.123

Mn O 0.072 3

LOI 2.89 0.4Insolubles 0.07Free CaO 0

total mix proportions. The procedure for mixing and

curing the slurry was as follows:

1. mix all the sodium exchanged bentonite with all the water for 5 min using a power tool with grout mixerattachment;

Ž .Fig. 1. Samples used in immersion tests examples .




2. leave the bentonite mix to hydrate for 241 h at205C;

Ž3. blend the PC and replacement material either.ggbs, pfa or both, see Table 1 ;

4. mix in the cementitious material as a dry mix to the

hydrated bentonite, mixing time of 5 min;5. check the quality of the mix, i.e. it was homo-

geneous and all solids had been broken down byŽmixing the slurry was mixed further, for 2 min, if

.necessary ; and6. pour the fresh samples of the cementbentonite

into sample tubes for experimentation.

3.3. Experimental methods

3.3.1. Immersion tests

To prepare samples for the immersion tests, the

Fig. 2. Confined attack tests set-up.

Žslurry was poured into moulds sealable sample tubes.100 mm in diameter and 350 mm in length and al-

lowed to set. After 28 days the hardened material wasdemoulded and cured under water until the immersiontest. The samples were then cut into cores, using a

Ž .masonry saw, from a cast cylinder of material Fig. 1 .Samples of each mix were then immersed in solutionsof potentially aggressive chemicals for a maximumperiod of 6 months. The solutions were changed on amonthly basis for the duration of the test.

The visual condition of each sample was recordedmonthly and changes in weight were recorded. Mea-surement of weights were taken after allowing thesample to drain but not to dry to the touch, as this mayhave caused oxidation and drying shrinkage and wouldthus have adversely affected performance. The assess-ment of the performance of each mix in each solution

was derived from a ranking procedure which was previously developed 4,5 . This ranking procedure relieson visual observations and measurement of weightchanges. At the end of the 6-month immersion period aranking was given dependent on performance. A sum-mary of the criteria follows:

Very Poor Sample destroyed within first month of immersion.Ž . ŽVP Weight changes after 1 month if sample still exist-

.ed were generally50%

Ž .Poor P Sample had a very low resistance to chemical attackand lasted less than half the experimental period.Visual damage was evident. Weight changes over

this period were50%

Ž .Moderate M Sample had some resistance to chemical attack. Ingeneral sample survived the duration of the exper-iment. More visual attack was apparent and weightchanges were50% x15%

Ž .Good G Sample was resistant to chemical attack with little visual evidence and weight changes of 15% of original weight

Very Good Attack was at worse marginal during the exper-Ž .VG imental period. Weight changes were5%

3.3.2. Confined contaminant attack

Confined tests involved casting cementbentoniteinto 100-mm diameter plastic pipes and providing con-finement around the material, as shown in Fig. 2. A

Ž .strong sulphate solution 4500 mgl was then placedin contact with the material through a permeable sandlayer. The solution was in direct contact with the ma-terial and could diffuse into it. Outflow was allowedfrom the bottom of the column. Due to the relativelysmall hydraulic head, chemical diffusion from the solu-tion into the material was likely to be as important asadvection in introducing solution to the material. Side- wall leakage was minimised by allowing the material to




set within the plastic pipe and applying a thin bead of sealant, capable of setting under high humidity, betweenthe cementbentonite and the pipe.

The type of test is more representative of the condi-tions found in the ground, where the cementbentonite

experiences a confining pressure, than the immersiontests. However, the confining pressure in the test wouldbe much lower than experienced in the ground. Onlymixes 1A 4A were tested in this manner.

The condition of this confined material was assessedafter 3, 6 and 12 months in three ways:

visual assessment: observation of cracking, soften-ing or discolouration of the material;

chemical profiling: chemical analysis for SO con-4

tent; and SEM analysis: microscopic investigation of reaction

products.

3.4. The solutions

Samples were immersed in a number of solutions which included the following common types of con-tamination:

2 Solution A: sodium sulphate, SO 4200 mgl;4

2 Solution B: sodium sulphate, SO 4200 mgl4

Ž .pH 2 ; 2 Solution C: sodium sulphate, SO 1000 mgl;4

2 Solution D: magnesium sulphate, SO 10004

mgl; Solution E: phenol 10000 mgl; and

Ž . Solution F: sulphuric acid pH 2 .

4. Results

4.1. Immersion tests

Each mix of cementbentonite performed differently

in the solutions. The results are detailed in Table 3using the ranking procedure detailed above. The fol-lowing observations were made:

( Solution A: sodium sulphate 4200 mg l; pH)7 Deterioration was observed for all samples

tested, particularly those with a low PCpfa con-tent. Those PCpfa mixes with highest PC concen-trations were most resistant.

( Solution B: sodium sulphate 4200 mg l; pH)2 Rapid deterioration was observed for all

samples. The PCggbspfa mix had very poorresistance. PCpfa samples with high cement con-tent were slightly more resistant.

Table 3Results of immersion tests by ranking

Mix no. A B C D E G

1A VP VP VP VP M VP

2A VP VP M M M VP3A M P P VP M4A G VP VP G M VP1B VP VP P P M VP2B P VP P P P VP3B G P G VP G P4B VP P G G G P

( Solution C: sodium sulphate 1000 mg l; pH

)7 This solution caused deterioration by expansiveattack, particularly those with a low cement con-tent. Those samples with higher pfa content proved

more resistant than neat PC mixes. However, thePCggbspfa mix was the most resistant to thissolution.

( Solution D: magnesium sulphate 1000 mg l; pH

) 7 This solution caused deterioration to allsamples tested, particularly those with lower pfacontents. Results were similar to those for SolutionC.

( ) Solution E: phenol 10 000 mgl Deteriorationin phenol solution was by discolouration and soften-ing, but not by extensive cracking and disinte-gration. However, all the mixes were resistant to

chemical attack by phenol. ( ) Solution F: sulphuric acid pH 2 Rapid de-terioration was observed for all samples tested. Allmixes showed little resistance to the strong acidconditions and were destroyed within 23 months.

4.2. Confined tests

4.2.1. Visual assessment

Two 100-mm columns of each mix were examinedafter 3, 6 and 12 months under the sulphate solution.

Photographs showing the condition of the materialsfrom the columns are shown in Figs. 35. In all casesthe cementbentonite samples appeared to be in goodcondition. In some cases it can be seen that the contactsurface showed some discolouration and was a dark

Ž .brown colour see Fig. 6 . This may be due to oxidation,leaching out of calcium hydroxide or chemical attack;or indeed a combination of these factors.

3 months: no visible signs of attack on any of thematerials, mixes 1A 4A.

6 months: the contact surface of mixes 1A 3A hadsoftened slightly due to contact with the strongsulphate solution.




Fig. 3. Confined samples, mix 1A, after 3 months.

12 months: similar to 6 months except accelerateddeterioration of mix 3A compared with mixes 1A and 2A. Mix 4A appeared very resistant to chemicalattack with no visible signs of softening or cracking.

4.2.2. Chemical diffusion

Samples were taken from 10 mm, 25 mm, 50 mm andŽ .100 mm depths from the contact surface for chemical

analysis and 10 mm, 50 mm depths for scanning elec-Ž .tron microscopy SEM analysis. The chemical analysis

was a determination of total sulphur content on ovendried samples. Results from samples taken from thecolumns were compared with the results from controlsamples. The results of SO determinations are shown4

in Table 4. Results generally showed an increase nearthe surface of SO concentration after 3 months. How-4

ever, there was considerable variation in the measured values of SO both between mixes and over time. These4

variations may have been the result of inherent vari-ations in the material. It was difficult to determinedefinite accumulation of sulphate at the various depths,but the 25-mm values for mixes 2A and 3A seemed toindicate some accumulation.


4.2.3. SEM analysis

Samples of cementbentonite were prepared forŽ .analysis using the scanning electron microscope SEM .

The SEM used a cryo stage and therefore it was notnecessary to dry the sample, which may have causedcollapse of the microstructure.

The SEM analysis was intended mainly to view theŽ .mineral ettringite AFt as a diagnostic of sulphate

attack. Although AFt is present in hydrated cement





Fig. 7. Mix 3 at 10 mm, angular crystal of Ca AlH.

ground. There have been no known failures of cementbentonite barriers used in the UK, but thesimple immersion test has indicated that the materialcan be degraded by certain contaminants or combi-nations of contaminants.

The immersion tests used were particularly ag-gressive and allowed the material to expand, contractor crack freely and as such it was not representative of the in-ground situation where the material is confined.There are well-documented theories that under con-fined conditions the material will not crack or dissolve

but will soften without increasing the permeability 15 .Whether this self-healing will occur for a wide range of

cementbentonites of various cement types and mix proportions is uncertain. Some work has indicated that

Žcracks can form in plastic concrete cut-off walls with.cementbentonite as the slurry phase and so affect

contaminant transport across the barrier 28 . Whetherthese cracks were the result of chemical attack or otherprocesses was not explained in the publication.

The overall performance of the mixes described inthis paper, immersed in solution, was moderate to very

Fig. 8. Mix 4 at 10 mm, ettringite fibres.

Fig. 9. Mix 1 at 50-mm depth, ettringite and CSH fibres.

poor at high sulphate concentrations. Magnesium sul-phate was no more damaging than sodium sulphate tothe cementbentonite mixes, at sulphate concentra-

Ž .tions of 1000 mgl. Acid H SO was also aggressive2 4

towards the mixes on its own and in combination withŽ .other contaminants sulphate it was able to destroy

most of the mixes in less than 1 month.Sulphate attack was expansive and led either imme-

diately or eventually to disintegration of samples de-pendent on concentration. The mode of attack for acidsolutions was typified by the leaching out of the hy-dration products which were soluble at the low pH of the solution. This leaves the samples with a lightercoloured appearance as opposed to their naturallydarker colour. After sufficient material had leached outthe sample disintegrated. The sulphate ions in thesulphuric acid were, however, able to produce expan-sion in some samples. In combined solutions of sul-phate and acid the expansive reaction dominated, butthe acid conditions accelerated deterioration.

The best performance of these mixes was in phenolsolution, a weak organic acid. The samples showed

Fig. 10. Mix 2 at 50-mm depth, ettringite and hydrating CSH.




negligible signs of attack, only some discolouration andsoftening of the surface was observed. Phenol, as a weak acid, will be reacting in a similar mode to that of the strong sulphuric acid, and a certain amount of thehydration products would have been removed by dis-

solution which gave softening of the surface. The de-gree of attack was, however, much less than that of sulphuric acid attack and the mixes lost an average of 7% by weight during the experimentation period.

The immersion test gives valuable information onthe inherent chemical resistance of a particular mix toa single contaminant or combination of contaminants.It would appear to be an adequate first stage in assess-ing the suitability of a cementbentonite mix for aparticular site. The chemical compatibility of thecementbentonite should be checked with site sol-utions including groundwaters, leachates, artificial

groundwaters and single priority contaminants ident-ified from site investigation reports. A proposed cementbentonite mix may need to be

rejected if there is a sufficiently adverse effect on thematerial in the immersion test. This test would, there-fore, be a screening test prior to further testing forphysical parameters, leachate permeability and dif-fusion tests. Alternatively, if sufficient data on mix performance can be documented, then with aknowledge of site conditions, suitable mixes could beselected. However, this would require that further com-pliance testing be carried out in order to assure perfor-mance.

In confined tests, which were intended to mirrorground conditions, attack was almost entirely elimi-nated for the mixes under the same strong sulphatesolution. The main criteria for the tests was that there

Ž . was contact between the contaminant sulphate andthe material at a surface. The lack of a significanthydraulic head to force solution into the material meantthat contaminant penetration relied mainly on chemi-cal diffusion. There was generally an increase of SO4

concentration near to the surface of the cementbentonite, but the concentration at greater depth didnot show such an increase. In agreement with this

observation there was an accumulation of ettringitecrystals near to the contact surface of the samples. Inthe future, tests will be run with a greater hydraulic

Ž .head to increase contaminant penetration , and forlonger, to assess deterioration patterns over greaterperiods of time.

6. Conclusions

Cementbentonite containment barriers appear tooffer a cost-effective solution to the problems of con-taminated land. There have been no reports of failuresof such barriers and as such their use has increased

particularly to contain contaminated sites. This increas-ing use has initiated the research on chemical compat-ibility, specification and long-term performance.

The following points are drawn from work carriedout to date.

A simple immersion test can be used to assess thechemical compatibility of cementbentonite withcontaminants. Such a test is potentially of use indetermining the suitability of a particular mix for aparticular contaminated site. The lack of publisheddata on mix performance means that some form of compatibility compliance tests should be used. Animmersion test could be used for this purpose.

It has been shown that cementbentonite can bedegraded by some contaminants or mixtures of con-taminants. The degree of attack was dependent on

the mix proportions, type of cement used andnature and concentration of contaminants. The mode of attack was different for each contami-

nant. Acid solutions discoloured and softened thecementbentonite by the leaching out of hydrationproducts, whilst the sulphate solutions producedexpansion.

Immersion tests showed that mixes with higher con-tents of PC or PCpfa had better overall perfor-mance in all the solutions. Results suggest that a

Ž .high PC content e.g. 150 kg may be needed forgood chemical compatibility.

In confined tests, attack was almost eliminated for

the mixes under the same strong sulphate solution.There was a higher concentration of total sulphurnear to the contact surface; SO concentrations4

varied considerably between and within samplesand this may have been due to inherent variationsin the material.

Acknowledgements

The authors would like to thank the Department of

the Environment, Transport and the Regions of theUK for financial support for this research. The workhas been directed by an industry steering group whoare also thanked.

References

1 Wood PA, Bardos RP. Constraints to effective contaminatedland remediation. Stevenage: Warren Springs Laboratory, 1994.

2 CIRIA. Remedial treatment for contaminated land, vol. 6.Containment and hydraulic measures. London: CIRIA SpecialPublication SP106, 1996a.

3 CIRIA. Barriers, liners and cover systems for containment and

control of land contamination. London: CIRIA Special Publi-cation SP124, 1996b.




4 Garvin SL, Paul V, Tedd P. Research into the performance of cementbentonite cut-off walls in the UK. Proceedings of the2nd International Symposium on Contamination in Easternand Central Europe, 2023 September 1994, Budapest, Hun-gary, 1994. p. 414416.

5 Lewry AJ, Garvin SL. Construction on contaminated land.

Proceedings of COBRA, RICS Construction and Building Re-search Conference, HeriotWatt University, Edinburgh,September 1995. p. 203214.

6 Kerr PF. Optical mineralogy. McGraw-Hill, 1977. p. 461. 7 Garner K. Contaminant resistant bentonite. Civil Eng 1982;48. 8 Plee D, Lebedenko F, Obrecht F, Letellier M, Van Damme H.

Microstructure, permeability and rheology of bentonitecement slurries. Cement Concrete Res 1990;20:4561.

9 Jefferis SA. In-ground barriers. In: Cairney T, editor. Contami-nated land-problems and solutions. Blackie Academic and Pro-fessional, 1993.

Ž .10 Building Research Establishment BRE . Slurry trench cut-off walls to contain contamination. Digest 395, 1994.

11 Jefferis SA. Remedial barriers and containment. In: Rees JF,editor. Contaminated land treatment technologies. Elsevier

Applied Science, 1992. 12 Krikhaar HMM, de Vries J. Fundamental behaviour of cement

bentonite. Cement 1993;12:2026. 13 Barker PJ, Esnault A. Developments in techniques of ground

treatment for the protection and rehabilitation of the environ-ment. Proceedings of the International Conference on Con-struction on Polluted and Marginal Land, Brunel University,London, 1992. p. 201214.

14 D’Appolonia DJ. Soilbentonite slurry trench cut-offs. JGeotech Eng Div 1980;April:339417.

15 Jefferis SA. Contaminantgrout interaction. GroutingSoil Im-prove Geosynth 1989:13931402.

16 Brandl H. Mineral liners for waste containment. GeotechniqueŽ .1992;42 1 :5765.

17 Meseck H, Herhans R. Resistance of mineral sealing wall

masses against seepage water from old storages. Conference onContaminated Soil, Hamburg, 1988. p. 585596.

18 Jefferis SA. Bentonitecement cut-off walls for waste contain-ment from specification to in-situ performance. Symposium onManagement and Control of Waste Fill Sites, Leamington Spa,1990.

19 Brown KW, David D. Influence of organic liquids on thehydraulic conductivity of soils. Land Disposal Hazardous WasteEng 1990;88:10311052.

Ž .20 Portland Cement Association PCA . Cementbentonite slurrytrench cut-off walls. Stokie, IL, USA: PCA Concrete Infor-mation, 1984.

21 Spooner PA, Wetzel RS, Grube WE. Pollution migration cut-off using slurry trench construction. Barriers, 1988:191197.

22 Tallard G. Slurry trenches for containing hazardous wastes. ASCECivil Engineering, 1984:4145.

23 Adaski WS, Cavalli NJ. Cement barriers. Proceedings of theFifth International Conference on Management of Uncon-trolled Hazardous Waste Sites, Washington, USA, 1984.p. 126130.

24 Jefferis SA. Bentonitecement slurries for hydraulic cut-offs.

In: Balkema AA, editor. Proceedings of the 10th InternationalConference on Soil Mechanics and Foundation Engineering,1519 June 1981, Vol. 1. Stockholm, 1981. p. 435 440.

25 Jefferis SA. The design of cut-off walls for waste containment.In: Gronow, Schofield, Kain, editors. Land disposal of haz-ardous waste: engineering and environmental issues. Ellis Hor- wood, 1988. p. 225234.

Ž .26 Building Research Establishment BRE . Sulphate and acidresistance of concrete in the ground. Digest 363, 1996.

27 Tedd P, Paul V, Lomax C. Investigation of an eight year oldslurry trench cut-off wall. Green U.K. Conference, Bolton, UK,1993.

28 Weststrate FA, Van Ree CCDF, Meskers CG, Bremner CN.Design aspects and permeability testing of natural clay and

Ž .sandbentonite liners. Geotechnique 1992;42 1 :4956.

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