The chemical compatibility of cement–bentonite cut-off wall material

Ž .Construction and Building Materials 13 1999 329]341

The chemical compatibility of cement]bentonite cut-offwall material

Stephen L. GarvinU, Carolyn S. HaylesBuilding Research Establishment, Scottish Laboratory, Kel̈ 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 ofcontamination from site. Self-hardening slurry trench cut-off walls of cement]bentonite are barriers that are increasingly beingused in the United Kingdom. The use of cement]bentonite 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 theproperties of cement]bentonite cut-off walls and to examine potential durability problems. Laboratory immersion tests have beenused to assess the chemical resistance of typical cement]bentonite 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 ofsuch tests are discussed. Q 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Cut-off wall; Cement]bentonite; PC

1. Introduction

It is uncertain as to how much contaminated landthere is in the UK. The House of Commons SelectCommittee estimated in 1990 that there are between50 000 and 100 000 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

U Corresponding author. Tel.: q44-1355-233-001; fax: q44-1355-241-895.

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 posea 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

w xsystems 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 offersboth a more cost-effective solution and one which uses

0950-0618r99r$ - see front matter Q 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. Gar̈ in, C.S. Hayles r Construction and Building Materials 13 1999 329]341330

established engineering techniques of construction;most importantly it avoids disturbing the contaminatedarea. A review of containment and control measures

w xhas recently been produced in the UK 2,3 .One of the most common forms of vertical barrier in

the UK is the cement]bentonite slurry trench cut-offwall, an engineering-based remediation method whichrepresents a relatively low-cost solution for contami-

w xnated land sites 4 . These systems, consisting of self-hardening cement]bentonite, were initially developedfor water exclusion or as a hydraulic cut-off, forexample, in dam foundations. However, they are nowmore commonly used as barriers to control lateralmigration of contaminants from former industrial sitesand landfill sites. In situations where barriers are builtadjacent to landfill sites and control of gas migration isrequired, geomembranes, usually high-density polyethy-

Ž .lene HDPE , are frequently incorporated into thebarrier.

There is concern that leachates generated from someold industrial sites, such as chemical and gasworkssites, or toxic wastes disposed in shallow areas of a site,may have an adverse effect on the performance of acement]bentonite barrier. There are potentially sev-eral physio-chemical mechanisms by which contami-nants in the groundwater can affect the swelling of thebentonite and thus the performance of the slurry. Inaddition, and perhaps more importantly, the contami-nants may attack the set material and affect long-termdurability and performance. The aims of the researchwere as follows:

v to investigate the fundamental chemical compati-bility of cement]bentonite with common contami-nants through tests on laboratory samples; and

v to improve the understanding of the materials as acontribution to validating their use and improvingspecification.

The results detailed in this paper are based on testson mixes based on Portland cement and pulverised fuelash. While it is more common to specify mixes withPortland cement and blast-furnace slag, these mixes

w xwere the subject of previous papers 4,5 . The mixes ofPortland cement and pulverised fuel ash also showedpotentially better chemical compatibility in previoustests and a more thorough study was required.

2. Cement–bentonite slurry trench cut-off walls

2.1. Materials and construction

2.1.1. Material propertiesThe range of materials used for slurry trench cut-off

walls in the UK is not extensive. The following ma-

terials have been commonly specified and used:

v bentonite, normally sodium exchanged, described asŽ .CE civil engineering grade;

Ž .v Portland cement PC ;v partial PC replacement materials; this is mainly

Ž .ground granulated blast-furnace slag ggbs , butŽ .pulverised fuel ash pfa has also been used and

microsilica has potential; andv admixtures, these include set retarders and disper-

sants.

Bentonite is a clay composed largely of dioctahedralw xsmectites, usually montmorillonite 6 . The exchange-

Žable cations are calcium, sodium and to a lesser extent.aluminium , giving rise to the classification of calcium

or sodium bentonite. In the UK, the most commonbentonite used is a civil engineering grade sodium-exchanged bentonite; this is calcium bentonite that hasbeen treated with sodium carbonate and is otherwiseknown as sodium-activated bentonite. Sodium-ex-changed bentonite has an outstanding ability to swellon uptake of water, expanding to between 10 and 15

w xtimes its original volume 7 , which gives rise to com-plete dissociation of the smectite crystals. Other im-portant aspect of bentonite suspensions are that it hasa high sorptive capacity, a thixotropic nature and a high

w xdispersibility 8 . The microstructure of hydrated ben-tonite suspensions is a connected and porous networkof anisometric particles which assist the high sorptivecapacity.

The use of ggbs reduces the overall cementitiousquantity required to achieve a given permeability andnormal replacement levels are 70]80%. Alternatively,pfa has been used as a partial replacement material.Replacement levels between 10 and 30% are normal.Above 30% the resulting hardened slurry may be too

w xweak for this purpose 9 .w xTypical mix proportions 10,11 used in barriers for

various purposes lie in the range:

Typical mix proportions Bentonite 30]60 kgCementitious material 100]350 kgWater 1000 kg

Cementitious component PCrggbs 90]200 kgPCrpfa 150]350 kgPC 90]200 kg

2.1.2. Mixing procedureMixing is generally carried out on site with the

cement]bentonite slurry later pumped from the mixingplant to the trench. The basic mixing procedures can bebriefly summarised as follows:

1. mix the bentonite and water;

( )S.L. Gar̈ in, C.S. Hayles r 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 trenchesThe majority of cement]bentonite 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 2]3 days. Excavation ofshallower trenches, up to 12 m depth, in relatively flatground, is usually achieved with a backactor. For deepertrenches and difficult ground conditions, grabs andcutters are used and a double-phase method is nor-mally used. If continuous excavation through the slurryis likely to cause problems then the cement]bentoniteslurry 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:

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

2.2.1. Performance specificationThe properties of the hardened slurry and durability

are the most important factors that influence long-termperformance.

v Permeability: Normally specified as less than 10y9

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

w xnot give as low a permeability 2 . These tests arecarried out at 28 days andror 90 days. The use ofbentonite contents above 40 kg gives a markedreduction in permeability and also increasesstrength. Above 60 kg the slurry becomes thick and

w xdifficult to mix and pump 10 ;v 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 10y9

mrs 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;

v 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 increasestrength 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 specificationThe 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 PCrggbs mixes can pose difficulties in meet-ing both the permeability and strain specification. Thestrength development is initially more rapid and cont-

w xinues for longer than for neat PC mixes 12 , so mixescontaining ggbs which have a permeability of 10y9 mrsat 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 10y8 mrs would produce a less brittle mixwhich is more capable of achieving the required strainat failure, and which would eventually reach a perme-

y9 w xability of the order of 10 mrs 10 .

2.3. Chemical compatiblilty of cement]bentonite

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

w xture 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.


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

actions, but less on the physico-chemical behaviour ofbentonite and specifically sodium-exchanged bentoniteused in cut-off walls. However, it is well establishedthat bentonite consists of a connected and porous

w xnetwork of anisometric particles 8 . The connectivityand 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 cement]bentoniteslurries are believed to be the result of calcium ion

w xconcentrations and high pH solutions 8 . Rheologicalproperties of the cement]bentonite 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 thecement]bentonite slurry, and it is believed that high

w xpH 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

w xbentonite 7 . The process of calcium-induced aggre-gation is a major factor which controls the unwantedincrease of permeability and water loss. The swelling ofsodium-exchanged bentonite can also be inhibited bythe presence of other sodium salts. The net effect ofthese chemical reactions will be a fall in the swellingcapacity of the bentonite and thus a reduction in the

w xeffectiveness of the bentonite as a sealant 7 .

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

ened cement]bentonite has been addressed by pre-vious workers using several methods. The principalfour methods are as follows:

v mixing the material with appropriate volumes ofcontaminants and analysing reaction productsw x15,16 ;

v immersing specimens in solutions of single or com-w xbined contaminants 15]17 ;

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

w xpressure 13 ; andw xv 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 cement]bentonite barriers include in-organic and organic acids, magnesium and ammonium

w xsalts and sulphates 18 . A description of the principalmechanisms of attack follows.

v 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 iflow can inhibit further reaction.

v Magnesium and ammonium salts attack the cementpaste fraction. Ion-exchange transforms insoluble cal-cium in the hardened cement paste into soluble cal-cium salts which are subsequently leached out of thematerial.

v Sulphates react with the hardened cement paste.The reaction products which form a greater ¨olumethan the reactants. In unconfined samples this leads todisintegration by expansion and cracking.

Ž .v Phenol weak acid and other organic solutions havea negligible effect on cement]bentonite. Some or-

( )ganic molecules e.g. aniline and ammonium chloridewere found to cause disruption to the structure and

w xporosity 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

w xthe material 20]22 . 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 been3w xdescribed elsewhere 20,23 and it has been recom-

mended that at sulphate levels in the groundwater orsoil of between 150 and 1500 mgrl, the C A content3should be no greater than 8%. Above a sulphate levelof 1500 mgrl, the C A content should be no greater3than 5%. However, due to the lack of data in thesepublications 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 cement]bentonite barrier would be to remove contaminants byattenuation mechanisms. This is primarily a function ofthe 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 cement]bentonite without any


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

w xattacked rather than the relatively inert bentonite 24 .Thus, high bentonite concentrations should be used onaggressive sites. The lower permeability of the higherbentonite 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 cement]bentonite with aggregate, may bemore severe than on a cement]bentonite cut-off bar-

w xrier without aggregate 22,25 .

2.3.3. Pre¨ious BRE workPrevious work undertaken by BRE has included tests

on the chemical compatibility of cement]bentonite withcontaminants that are commonly found on old indus-trial sites, and assessing the physical properties ofcement]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 cement]bentonite mixes with common in-ground contaminants.

w xThis initial work 4,5 included a variety of mixes and arange in the mix proportions including a number ofmixes using PCrggbs mixes commonly used on site,and PC and PCrpfa 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 2y.sodium sulphate SO , 2500 mgrl; class 3 sulphate4w xconditions as defined in Digest 363 4,26,27 . It was

assumed that the sodium ion itself would have a nil ornegligible effect on the material. The concentration ofsulphate ion in solution ranged from 1000 to 5000mgrl. Some of these solutions were found to be par-ticularly aggressive causing degradation of all thecement]bentonite mixes.

The degree of attack was linked to the concentrationof the sulphate ion. At lower concentrations, up to2000 mgrl, 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 2500mgrl the chemical resistance of all mixes decreasedand this was seen most markedly at 3500 mgrl andabove. The aggressiveness of the solution depended notonly on the sulphate concentration but also on thecounter ion present. The relative aggressiveness wasMg2q)NHq)Naq)Ca2q. A combination of sulphate3

Ž .and acid pHs2.5 , gave a greater degree of deteriora-tion than sulphate in isolation.

The overall performance of the PCrggbs]bentonitemixes in sulphate solutions was poor and immersiontests showed that these more commonly usedcement]bentonite mixes were not chemically compati-ble with sulphates, acids and magnesium salts; theperformance of a limited number of PCrpfa]bentonite mixes was better than that of the PCrggbs]bentonite 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 chemicalresistance of a number of cement]bentonite mixes,specifically mixes with cementitious material of PCrpfa.As hardened cement]bentonite 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 cement]bentonite for thefollowing reasons:

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

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

v 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 cement]bentonite 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 nra 36 84 1202A 40 nra 60 140 2003A 40 nra nra 200 2004A 40 40 100 60 2001B 40 nra 37.5 112.5 1502B 40 nra 60 90 1503B 40 nra 75 225 3004B 40 nra 120 180 300

aAll 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 2Al O 24.21 12.59 5.15 C A 102 3 3Fe O 13.99 0.4 3.55 C AF 82 3 4NaO 0.57 0.21 0.22 CS 52 2MgO 1.62 7.59 2.4K O 3.21 0.38 0.72SO 0.76 3.123Mn O 0.072 3LOI 2.89 0.4Insolubles 0.07Free CaO 0

total mix proportions. The procedure for mixing andcuring the slurry was as follows:

1. mix all the sodium exchanged bentonite with all thewater 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 24"1 h at20"58C;

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 thehydrated 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 by

Žmixing the slurry was mixed further, for 2 min, if.necessary ; and

6. pour the fresh samples of the cement]bentoniteinto sample tubes for experimentation.

3.3. Experimental methods

3.3.1. Immersion testsTo 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 solutionwas derived from a ranking procedure which was pre-

w xviously 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 generally "50%

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 overthis period were "50%

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 were "50%)x-"15%

Ž .Good G Sample was resistant to chemical attack with littlevisual evidence and weight changes of "15% oforiginal weight

Very Good Attack was at worse marginal during the exper-Ž .VG imental period. Weight changes were "5%

3.3.2. Confined contaminant attackConfined tests involved casting cement]bentonite

into 100-mm diameter plastic pipes and providing con-finement around the material, as shown in Fig. 2. A

Ž .strong sulphate solution 4500 mgrl 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 ofsealant, capable of setting under high humidity, betweenthe cement]bentonite and the pipe.

The type of test is more representative of the condi-tions found in the ground, where the cement]bentoniteexperiences 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:

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

v chemical profiling: chemical analysis for SO con-4tent; and

v SEM analysis: microscopic investigation of reactionproducts.

3.4. The solutions

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

w 2yxv Solution A: sodium sulphate, SO s4200 mgrl;4w 2yxv Solution B: sodium sulphate, SO s4200 mgrl4

Ž .pHs2 ;w 2yxv Solution C: sodium sulphate, SO s1000 mgrl;4

w 2yxv Solution D: magnesium sulphate, SO s10004mgrl;

v Solution E: phenols10 000 mgrl; andŽ .v Solution F: sulphuric acid pHs2 .

4. Results

4.1. Immersion tests

Each mix of cement]bentonite performed differentlyin the solutions. The results are detailed in Table 3using the ranking procedure detailed above. The fol-lowing observations were made:

(v Solution A: sodium sulphate 4200 mgrrrrr l; pHG)7 }Deterioration was observed for all samples

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

(v Solution B: sodium sulphate 4200 mgrrrrr l; pHs)2 }Rapid deterioration was observed for all

samples. The PCrggbsrpfa mix had very poorresistance. PCrpfa 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 VP2A 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

(v Solution C: sodium sulphate 1000 mgrrrrr l; pHG)7 }This solution caused deterioration by expansive

attack, particularly those with a low cement con-tent. Those samples with higher pfa content provedmore resistant than neat PC mixes. However, thePCrggbsrpfa mix was the most resistant to thissolution.

(v Solution D: magnesium sulphate 1000 mgrrrrr l; pH)G7 }This solution caused deterioration to all

samples tested, particularly those with lower pfacontents. Results were similar to those for SolutionC.

( )v Solution E: phenol 10 000 mgrrrrr l }Deteriorationin phenol solution was by discolouration and soften-ing, but not by extensive cracking and disinte-gration. However, all the mixes were resistant tochemical attack by phenol.

( )v Solution F: sulphuric acid pHs2 }Rapid de-terioration was observed for all samples tested. Allmixes showed little resistance to the strong acidconditions and were destroyed within 2]3 months.

4.2. Confined tests

4.2.1. Visual assessmentTwo 100-mm columns of each mix were examined

after 3, 6 and 12 months under the sulphate solution.Photographs showing the condition of the materialsfrom the columns are shown in Figs. 3]5. In all casesthe cement]bentonite 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.

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

v 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.

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

4.2.2. Chemical diffusionSamples were taken from 10 mm, 25 mm, 50 mm and

Ž .100 mm depths from the contact surface for chemicalanalysis and 10 mm, 50 mm depths for scanning elec-

Ž .tron microscopy SEM analysis. The chemical analysiswas 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 shown4in Table 4. Results generally showed an increase nearthe surface of SO concentration after 3 months. How-4ever, there was considerable variation in the measuredvalues of SO both between mixes and over time. These4variations 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 analysisSamples of cement]bentonite were prepared for

Ž .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. 6. Discolouration typical after confined tests, mix 4A.

materials, its quantity is normally restricted due to thelow volume of sulphate in the cement. However, sul-phate introduced from external sources can react withcalcium-alumino-hydrates in the paste and result inhigh quantities of AFt being formed.

AFt was found in quantities of ‘none’ to ‘easily seen’in control samples, therefore an abundance of AFt wasconsidered to be associated with sulphate attack fromthe contaminant solution. A summary of the SEManalysis is shown in Table 5 and examples are repro-duced in Figs. 7]10. In most of the samples AFt was

Ž .abundant near to the contact surface 10 mm depthŽbut at greater depth it was at background levels i.e.

.easily seen or less . These results partly corroborate theresults of chemical tests for total sulphur that indicateda higher concentration of sulphur near to the contactsurface and lower concentrations within the material.

5. Discussion

The durability and chemical compatibility of thecement]bentonite materials used as cut-off walls areimportant factors in determining their potential servicelife and how long they will function properly in the

Table 4aChemical analysis for SO in cement]bentonite samples4

bDepth Mix 1A Mix 2AŽ .mm 3 months 6 months 12 months 3 months 6 months 12 months

10 11.85 7.1 7.02 6.78 6.52 6.925 12.56 7.24 7.23 1.98 3.47 4.850 12.78 6.35 6.6 1.84 1.78 1.8

100 10.43 1.79 4.84 1.88 1.79 1.8

Mix 3A Mix 4A

3 months 6 months 12 months 3 months 6 months 12 months

10 7.31 9.28 9.2 10.17 8.11 725 3.45 8.33 8.1 10.38 8.25 750 2.21 7.76 7.6 10.19 8.51 6.4

100 2.15 2.28 2.4 8 7.84 5.5

a Values are % by weight of dry material.b100-mm columns.

Table 5Ž .Three-month SEM analysis to monitor hydration: a summary of mix constituents present X

Feature Mix 1A Mix 2A Mix 3A Mix 4A

Talc CSH X X X XFibres AftrAFm XHexagonal AftrAFmClinkerrslag grainspfa X XCaOHCa]Al]H X

Ž .SEM analysis for ettringite in chemically attacked samples XŽ .Depth mm Mix 1A Mix 2A Mix 3A Mix 4A

10 X X X X50 X X X


Fig. 7. Mix 3 at 10 mm, angular crystal of Ca]Al]H.

ground. There have been no known failures ofcement]bentonite 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 ofthe 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

w xbut will soften without increasing the permeability 15 .Whether this self-healing will occur for a wide range ofcement]bentonites of various cement types and mixproportions is uncertain. Some work has indicated that

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

w xcontaminant 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 cement]bentonite mixes, at sulphate concentra-

Ž .tions of 1000 mgrl. Acid H SO was also aggressive2 4towards the mixes on its own and in combination with

Ž .other contaminants sulphate it was able to destroymost 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 ofthe 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 aweak acid, will be reacting in a similar mode to that ofthe 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 ofsulphuric acid attack and the mixes lost an average of7% 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 cement]bentonite mix for aparticular site. The chemical compatibility of thecement]bentonite should be checked with site sol-utions including groundwaters, leachates, artificialgroundwaters and single priority contaminants ident-ified from site investigation reports.

A proposed cement]bentonite mix may need to berejected 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 mixperformance 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 SO4concentration near to the surface of the cement]bentonite, but the concentration at greater depth didnot show such an increase. In agreement with thisobservation 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

Cement]bentonite 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.

v A simple immersion test can be used to assess thechemical compatibility of cement]bentonite 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 ofcompatibility compliance tests should be used. Animmersion test could be used for this purpose.

v It has been shown that cement]bentonite can bedegraded by some contaminants or mixtures of con-taminants. The degree of attack was dependent onthe mix proportions, type of cement used andnature and concentration of contaminants.

v The mode of attack was different for each contami-nant. Acid solutions discoloured and softened thecement]bentonite by the leaching out of hydrationproducts, whilst the sulphate solutions producedexpansion.

v Immersion tests showed that mixes with higher con-tents of PC or PCrpfa 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.

v In confined tests, attack was almost eliminated forthe mixes under the same strong sulphate solution.There was a higher concentration of total sulphurnear to the contact surface; SO concentrations4varied considerably between and within samplesand this may have been due to inherent variationsin the material.

Acknowledgements

The authors would like to thank the Department ofthe 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.

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The chemical compatibility of cement–bentonite cut-off wall material