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Working Report 2002-07
Influence of grout and cement on groundwater composition
· Mel Gascoyne
February 2002
POSIVA OY
T6616nkatu 4, FIN-00100 HELSINKI , FINLAND
Tel. +358-9-2280 30
Fax +358-9-2280 3719
Margit Snellman, POSIVAOY, Toolonkatu 4, FIN -00 1 00 Helsinki, Finland. Fax: 358-9-2280-3719
SUBMISSION OF REPORT
P.O. Box 141 6 Tupper Place
Pinawa, MB ROE 1 LO Canada
Phone 1-204-753-8879 Fax 1-204-753-2292
e-mail: [email protected]
For Expert Review and Assistance in Hydrogeochemistrv Studies (P.O. Number 9575/01/MVS): Report on Influence of Grout on Groundwater Composition
Dear Margit,
Please find enclosed the final copy of the report defined above. The report has been reviewed and approved according to the requirements of my company, Gascoyne GeoProjects Inc. and meets all quality assurance requirements of Posiva.
Yours sincerely,
M~~~~ M. Gascoyne (President and CEO, Gascoyne GeoProjects Inc.)
Working Report 2002-07
Influence of grout and cement on groundwater composition
Mel Gascoyne
February 2002
Working Report 2002-07
Influence of grout and cement on groundwater composition
Mel Gascoyne
Gascoyne GeoProjects Inc.
Pinavva, Manitoba, Canada
February 2002
Working Reports contain information on work in progress
or pending completion .
The conclusions and viewpoints presented in the report
are those of author(s) and do not necessarily
coincide with those of Posiva .
INCLUENCE OF GROUT AND CEMENT ON GROUNDWATER COMPOSITION
ABSTRACT
This report reviews the characteristics of cement chemistry and leaching of concrete and grout that would influence the composition of groundwater surrounding a spent nuclear fuel repository and evaluates the influence of grout and cement on groundwater composition. Aspects of attack of Ordinary Portland Cement by chloride, sulphate and organic species and reaction with active silica in aggregate are described together with the potential for microbial degradation. The use of high-performance concrete is described as a method of improving the properties of grout and concrete and of reducing the rate and extent of chemical attack and the contamination of groundwater.
The variations in groundwater composition of a fracture zone in granite are described for the period of the grouting trials performed by Atomic Energy of Canada Limited in 1987 in Canada's Underground Research Laboratory. The grout used was a low-pH, high-performance cement containing portlandite, silica fume and superplasticizer, to give a high-strength, low porosity and low permeability mixture that was sufficiently fluid to allow grouting of narrow fractures in the rock. Short-term variations in pH, alkalinity, Ca and K concentrations were observed in the groundwater of the grouted fracture zone but, in monitoring for eight years following, only a slight increase in pH (up to 0.5 units) could be distinguished in groundwater sampled repeatedly from the grouted borehole. Implications for grouting and other uses of cement at Olkiluoto are considered.
Keywords: Cement, concrete, groundwater chemistry, superplasticizer, leaching
SEMENTIN JA INJEKTOINTIAINEIDEN VAIKUTUS POHJAVEDEN KOOSTUMUKSEEN
TIIVISTELMA
Tassa raportissa tarkastellaan sementin kemiaa, betonin ja injektointiaineiden liukenemista seka niiden mahdollista vaikutusta kaytetyn polttoaineen loppusijoitustilan ympariston pohjavesikemiaan. Lisaksi tarkastellaan kloridin, sulfaatin, orgaanisten aineiden j a yhteisesti aktiivisen silikan reaktioiden kanssa tapahtuvaa vaikutusta tavalliseen Portland sementtiin. Mikrobiologisen toiminnan mahdollisuutta rapauttaa betoneja kuvataan myos. Korkealaatuisen betonin kayttoa kuvataan eraana ratkaisuna, jolla voidaan parantaa seka injektointiaineen etta betonin ominaisuuksia, pienentaa kemiallisen syopymisen nopeutta ja laajuutta seka vahentaa pohjaveden kontaminaatiota.
Tyossa kuvataan AECL:n (Atomic Energy of Canada Limited) vuonna 1987 Kanadan maanalaisessa tutkimuslaboratoriossa suoritettujen injektointitestien aikaisia pohjaveden koostumuksen vaihteluja graniittisessa rakovyohykkeessa. Alhaisen pH:n injektointiaine koostui korkealaatuisen Portland sementin, silikan ja notkistimen seoksesta, jolla tahdattiin korkeaan lujuuteen, pieneen huokoisuuteen ja alhaiseen permeabiliteettiin, eli seokseen, joka oli riittavan juokseva pienten rakojen injektointiin. Injektoidussa rakovyohykkeessa havaittiin lyhytaikaisia muutoksia pH- ja alkaliteettiarvoissa seka Ca- ja K-pitoisuuksissa. Tyossa tarkastellaan lopuksi sementilla tiivistamisen ja sementin muun kayton seurauksia.
Avainsanat: Sementti, betoni, pohjavesikemia, notkistin, liukeneminen
PREFACE
This work was performed under contract for Posiva Oy, Helsinki. I would like to thank Margit Snellman for supervising this work and her review comments and Simcha Stroes-Gascoyne for writing the section on microbial degradation of concrete.
TABLE OF CONTENTS
ABSTRACT, TIIVISTELMA
PREFACE
1
1. INTRODUCTION .................................................................................................... 3
2. THE CHEMISTRY OF CEMENT ............................................................................ 5 2.1 Pozzolans ................................................................................................... 7 2.2 Superplasticizers ........................................................................................ 8
3. CEMENT-WATER INTERACTIONS ..................................................................... 11 3.1 Dissolution in groundwater ....................................................................... 11 3.2 Dissolution of superplasticizer .................................................................. 13 3.3 C02 interaction (carbonation) .................................................................... 13 3.4 Organic interaction ................................................................................... 14 3.5 so4 interaction ......................................................................................... 14 3.6 Cl interaction ............................................................................................. 15 3. 7 The alkali-silicate reaction ......................................................................... 16 3.8 Speciation and solubility modelling ........................................................... 16 3.9 Interactions with solid materials ................................................................ 18
4. LEACHING STUDIES .......................................................................................... 21
5 MICROBIAL EFFECTS ........................................................................................ 25 5.1 Biodegradation of concrete ....................................................................... 25 5.2 Microbial degradation of superplasticizers ................................................ 25
6. GROUTING EXPERIMENTS ............................................................................... 27 6.1 Grout requirements ................................................................................... 27 6.2 Laboratory studies .................................................................................... 28 6.3 URL grouting trials .................................................................................... 29
6.3.1 Borehole characteristics ................................................................ 29 6.3.2 Background hydrogeochemistry .................................................... 31 6.3.3 Sampling during grouting .............................................................. 31 6.3.4 Grouting of HC9 ............................................................................ 34 6.3.5 Service water contamination ......................................................... 34
6.4 Post-grout analyses .................................................................................. 35 6.4.1 GH1 and GH2 groundwaters ......................................................... 36 6.4.2 The HC6 tracer test.. ............................. ........................................ 36 6.4.3 Long-term changes ....................................................................... 36
7. SUMMARY AND CONCLUSIONS ................................................................... 37 7.1 Concrete types ......................................................................................... 37 7.2 Interaction with groundwater and adjacent rock ........................................ 37 7.3 Use of cement at Olkiluoto ........................................................................ 38
8. REFERENCES ......................................................................................... 39
3
1 INTRODUCTION
Several countries with mature nuclear waste management programs are considering deep geological disposal of nuclear waste in stable shield areas. These include Canada, in the Canadian Shield, and Finland and Sweden in the F ennoscandian Shield. Cement-based sealing materials are being considered as a means of isolating many parts of the waste repository including shafts, tunnels, roof supports and boreholes (Table 1) and, therefore, preventing the migration of radionuclides from the waste. One use of cement that is important for sealing naturally occurring fractures in the repository is the application of grout, a fluid form of cement, that can be pumped at high pressure into boreholes that access water-bearing fractures.
The use of cement in a repository raises a number of concerns that relate to its potential effect on the chemistry of groundwater and minerals in the fracture system and their ability to act as barriers to the migration of radionuclides that may be released from the waste. Although cement (in the form of concrete) has been used for underground constructions for many years, relatively little is known about its longterm stability and the nature of chemical interactions of cement, groundwater, pore fluids and fracture minerals. In particular, very little is known about the effects of saline groundwaters and brines on cement stability in a reducing environment and this is important for the shield disposal programs because, in most cases, once the repository is sealed, resaturation will be by saline groundwaters that are ubiquitous at the depths that are being considered (500-1000 m in crystalline rocks).
This report examines the effect of various types of cement, used as grout, on adjacent groundwater composition and determines the influence of organic compounds and other materials added to cement to increase fluidity, density, strength, etc. The results of a field trial in the Canadian program, are described, in which attempts were made to seal a permeable fracture zone by use of grout.
Table 1. Summary of the potential uses of cement in nuclear waste repositories.
Waste Type Use Examples
Low/Intermediate Level Structural components Walls, floors, roadways, drains
Containers External container, container packing
Wasteform Solidification of sludges, fluids, resins; cementitious buffer and backfill
High Level Structural supports Tunnel supports, shotcrete, shaft lining, rock -bolts
Seals Boreholes, bulkheads in tunnels, shaft plugs
Fracture seals Grout, injection
5
2 THE CHEMISTRY OF CEMENT
Common cement is prepared by grinding various amounts of limestone and clay, heating to about 1400°C, grinding the resulting clinker to a fine powder and adding some gypsum to prevent sudden stiffening when water is added. Portland cement forms the basis of most cements and consists of four materials formed during the calcining process: tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C~)1 . The various grades of Portland cements contain varying amounts of these phases, as shown in Table 2.
Table 2. Composition of various types of Portland cement in terms of constituents C 3S, C 2S, C ~ and C.AF, as defined in the text (from Oscar son et al. 199 7).
ASTM Description C3S c2s CJA C<tAF type (wt.%) (wt. o/o) (wt. o/o) (wt. o/o) I General purpose 45-55 20-30 8-12 6-10 II General purpose & 40-50 25-35 5-7 6-10
moderate heat of hydration
m High early strength 50-65 15-25 8-14 6-10 IV Low heat of 20-25 45-55 4-6 10-14
hydration V Sulphate resistant 40-50 25-35 0-4 10-20
These constituents react with water to produce 40-60% calcium silicate hydrate (commonly referred to as CSH but of variable stoichiometry), 20-25% portlandite (Ca(OH)2 or 'CH' in the above nomenclature), 10-20% hydrated aluminates, ferrites and sulphates (AFm and AFt type phases), 10-20% of pore fluids and 0-5% ofNaOH and KOH, which are generally dissolved in pore fluids. The resulting hardened CSH paste consists of extremely small interlocking crystals and is generally known as a cement 'gel'; it is a relatively strong solid having high porosity and a poor degree of crystallization. The volume of pores is about 28% of the gel (Lagerblad and Tragardh 1995). The mineral phases commonly found in hardened cement are shown in Table 3.
The amount of water added to the cement mix is usually in the range of water/cement ratio (w/c) = 0.35 to 0.5 and this is sufficient to allow the cement to flow. However, the amount of water required to chemically hydrate the cement is less (w/c = 0.22 to 0.25) and so hardened cement contains an aqueous pore fluid, which contains most of the highly soluble alkalis NaOH and KOH.
1 This nomenclature is generally used in cement chemistry: C = CaO, S = Si02, A = Ah03, F = Fe203, H = H20 m = mono t = tri
2 Cement ;efers to th~ paste of C-S-H phases defined above. Concrete consists of large aggregate material (such as sand or stones) bound together by cement.
6
The most common cement in use is Ordinary Portland Cement (OPC, Type 1). This type of cement is generally used when it is not likely to be exposed to S04, either in soil or in groundwater. It may contain free lime (CaO) but this is most often hydrated to Ca(OH)2. The interaction with S04 is mainly with the cement component C3A to form calcium sulphoaluminate ( ettringite) which causes a volume increase of over 200% and, hence, gradual disintegration of the hardened cement. In addition, gypsum may be formed by reaction of S04 with Ca(OH)2 causing a volume increase of over 120% (Neville 1996). Some formulations (such Type 50 cement, used in the Canadian program) have a limited amount of C3A to restrict the formation of ettringite (Stroes-Gascoyne and Johnson 1998).
Because several types of cement will be used in an underground repository to meet the various needs of sealing, a variety of cement compositions may be used. For instance, the cement used in reinforcing rock bolts in the tunnel roof for ground stabilization may be of an expanding cement formulation. Type K cement is typically used for this purpose and contains gypsum which causes formation of the expanding mineral ettringite. There are numerous other cement formulations that are used for specific tasks in the construction and mining industry. They are summarized in Table 3.
Table 3. Principle types of Portland cements and ASJM designation.
Cement Description (U.K.) ASTM Desi2nation
Ordinary Portland Type I Modified Type 11 Rapid Hardening Portland Type Ill Low Heat Portland Type IV Sulphate Resisting Portland Type V Portland Blast-Furnace Type IS Slag TypeS Portland-Pozzolana Types lP, P Water-Reducing Admixtures Type A Retarding Admixtures TypeB Accelerating Admixtures TypeC Water-Reducing/Retarding TypeD Water -Reducing/ Accelerating TypeE High-Energy Expanding TypesK, M High-Energy Expanding Type S(2)
Thermodynamic models have been developed to determine the compos1t1on of complex cement systems. One example is CEMCHEM1, described by Atkins et al. (1994). The model predicts the steady-state solid phase mineral assemblage in cement at 25 °C. Several formulations have been developed to reduce the porosity and the heat of hydration of cement as well as increase its strength and resistance to corrosion. These cements require the addition of pozzolans, which react with free lime in the hardening cement.
7
2.1 Pozzolans
Pozzolans are fine-grained siliceous or aluminous and siliceous materials that can react with hydrated lime (Ca(OH)2) to produce cements. They were originally derived from volcanic soils and sand near Pozzoli, Italy (Lea 1971). Pozzolans react with the Ca(OH)2 that is produced during the hydration of cement and form calcium silicates. Examples of pozzolans include volcanic ash, pumice, opaline shales and cherts, burnt clay, fly ash, and granulated blast-furnace slag. It is important that the silica present in pozzolans is amorphous because crystalline silica (e.g. quartz) has very low reactivity. Modem pozzolans consist of fly ash (also known as pulverized fuel ash) from coal-burning power stations, and silica fume (almost pure Si02 with traces of AI, Fe, Ca, Mg, Na, K, C and S, Onofrei et al. 1992), which is a by-product of the ferro-silicon industry. Fly ash is the silica-rich ash precipitated electrostatically from the exhaust fumes of coal-fired power stations. It is worth noting at this point that fly ash generally contains a few percent of carbon, which may be available to microbes in the corrosion of cement and concrete. In addition, iron blast-furnace slag typically contains~ 1 %sulphide (Glasser 2001) and may influence redox conditions of the hardened cement.
Silica fume has an especially high pozzolanic activity because it is amorphous and has an extremely high surface area(~ 20,000 m2/kg). It is 50-75 times finer than fly ash or cement and is even finer than cigarette smoke (Gray and Shenton 1998)! This greater surface area causes silica fume to be completely reacted within ~ 7 d whereas the reaction of fly ash is much slower. The reactions of fly ash and silica fume have been examined in detail by Weng et al. (1997).
OPC cements modified by addition of pozzolans are known as high performance concrete (HPC). The typical content of pozzolans (e.g. silica fume) in a concrete is from 10 to 30% although some formulations have used up to 55% ofpozzolans (Gray and Shenton 1998). An important advantage of pozzolans, is that they cause cement to hydrate slowly thereby reducing the temperature of the setting process (especially for fly ash). They also show good resistance to S04 attack (because free Ca(OH)2 is now combined with pozzolanic silica and, therefore, is not available) and lower porosity and permeability (because there is less Ca(OH)2 to be removed by leaching). An additional result of using pozzolans is that the high pH effect of ordinary cements (pH> 12) is greatly reduced. Leaching tests on HPC, performed by Oscarson et al. (1996), have showed that pH values of 10.5 can be obtained in distilled water and as low as 8.6 in a synthetic saline groundwater (see section 4). In addition, silica fume helps to reduce 'bleeding' (separation into phases or particle sizes) of the injected cement, probably due to the very fine grain size of the cement and pozzolan.
One disadvantage of pozzolans is the fact that they reduce the 'workability' of the cement mixture because of their surface area, and so extra water has to be added. Conventional cements and grouts use amounts of water to give w/c ratios of up to 2 whereas the stoichiometric requirement for cement hydration is only about 0.2- 0.3 . An additional problem of some pozzolans (particularly fly ash, natural clays, etc. which contain appreciable amounts of Na and K) is that they may contribute significant quantities of these alkalis to the cement and these tend to accumulate in the pore fluid phases as NaOH and KOH thus causing high pH (>13) on leaching (Glasser et al. 1985).
8
2.2 Superplasticizers
Although additives such as silica fume can increase the strength of cement, as described above, the resulting mix is too thick to use with ease. The workability of cement depends on the water content of the mixture and the emplacement conditions. Increasing the w/c ratio improves workability but reduces strength, and increases the hydraulic conductivity and shrinkage. Indirectly, the longevity of the concrete is therefore reduced. The extra water added is not used in cement hydration, but is trapped in the solid structure and increases porosity and so reduces strength. This can be offset by the use of complex organic molecules known as superplasticizers which interfere with the fast hydration reactions of C3A and C3S which cause rapid thickening of the cement.
Prior to the 1970's, lignosulphonates (a waste product of the pulp and paper industry) were used to reduce the water content of cement mixtures and, at a concentration of about 0.1 %, a water reduction of up to 10% was possible (Aitcin et al. 1989). Subsequently, with the development of new, improved superplasticizers, concentrations of up to 2% could be used in high-strength concrete allowing a water reduction of up to 30% without causing significant changes in the resulting concrete (except for some initial retardation of setting).
It is now possible, therefore, to make fluid Portland cements that contain only the amount of water that is required for stoichiometric reactions without need to provide water for fluidity of the mixture. Water/cement ratios as low as 0.20 have been used (Gray and Shenton 1998).
Superplasticizers are typically used in making TypeD cements (water-reducing and retarding, Table 3). They are capable of reducing water content by 25-35%. Superplasticizers do not significantly change the surface tension of water; instead, they disperse cement particles when suspended in water by adsorbing on the surface of the particles causing them to be mutually repulsive due to the anionic nature of the superplasticizers.
One of the most common superplasticizers is sodium-sulphonated napthalene formaldehyde condensate (Na-SNFC). Addition of 0 . 75 wt. % Na-SNFC to a Type-50 cement containing 10% silica fume turns a stiff, viscous paste into a flowing, pumpable fluid for several hours after mixing (Stroes-Gascoyne and Johnson 1998). Other known superplasticizers are polymers of sulphonated melamine formaldehyde and sodium lignosulphate (Aitcin et al. 1989) and, recently, gluconic acid (Schwyn 2001). A summary of currently used plasticizers and their unit cell formulae is given in Figure 1. Using radioactively labelled sulphur e5S) in Na-SNFC, Onofrei and Gray (1989) showed that, after hardening, superplasticizers were strongly bound and immobilized within the hydrated phases of the Portland cement (principally CSH and CAH phases).
Despite these findings, it is possible that superplasticizers could decompose in a repository, if located close to a radiation field. Palmer and Fairhall (1993) have examined the production of gas due to radio lysis of small cylinders of OPC and blast
9
furnace slag grouts that contained Na-SNFC and sulphonated melamine formaldehyde condensate superplasticizers. The radiation field was 104 Gy/hr and total dose was up to 9 Mgy. The results showed that both C02 and H2 were generated by irradiation (up to 6.7 mL gas/g superplasticizer. The authors commented that the radiation did not appear to affect the strength or stability of the grout.
Sodium sulphonated melamine formaldehyde
n
Sodium sulphonated naptbaleoe formaldehyde
n
Sodium lignosulphonate
H OH I I I C-C-C I I I H H
NaS03 OH n
Figure 1. Superplasticizers in current use and their generalized formulae (after Onofrei et al. 1991)
11
3 CEMENT-WATER INTERACTIONS
Cement powder reacts with water during the hydration process as follows:
1)
2)
3)
4)
The principal products are, therefore, various calcium silicate hydrates (CSH) and portlandite, (Ca(OH)2). The chemistries of these components must be considered when determining the stability of cement and concrete.
There are number of processes that can lead to degradation of cement. These include dissolution in groundwater, carbonation, sulphate and chloride attack and the stability of the aggregate mixed in to the cement paste. In this section, emphasis is given to how these processes affect the composition of the surrounding water.
3.1 Dissolution in Groundwater
Hardened cement is generally slow to react with water unless it is porous, in which case large amounts of water may be able to flow through it and dissolve the sparing! y soluble components. The breakdown of cement and concrete in aqueous environments produces a high-pH plume, which is derived, initially, from pore fluids in the cement containing strong alkali (NaOH, KOH). These pore fluids may contain Na and K concentrations as high as 1 - 3 % (Lunden and Andersson 1989, Glasser et al. 1985). Calcium concentrations range only up to 0.003 M, 130 mg/L, however, because solubility of the portlandite (Ca(OH)2) component is controlled by the concentration ofNa and Kin the pore fluids (at high pH, Ca solubility is reduced).
The pH of water leachates of hardened cement follows a number of well-defined stages (Lea 1971, Glasser and Marr 1983, Askarieh et al. 1997, Oscarson et al. 1997) and is represented graphically in Figure 2:
1) Initially, pH of the small amounts of strong alkali (NaOH, KOH) present largely in the pore fluids dominates and can give values as high as 13.5 to 14. In adjacent groundwater, this often creates a high-pH plume that spreads out from the grouted area in response to the flow conditions and masses/volumes of grout present.
2) Once the alkalis have been leached out, pH is controlled, at about 12.5, by Ca(OH)2. This pH is maintained for a long time after hardening because of the relatively high content of unreacted Ca(OH)2 in the cement. Reactions may occur with Mg or C03 ions in groundwater to give precipitates of brucite (Mg(OH)2) or calcite which can form protective layers. Elevated Ca concentrations will be observed in adjacent groundwaters together with the high pH.
12
• 13 1
t
pH • KOH I t
12 • I + ' I
NaOH t Ca(OH)a I 11 f t
1 f
10 I CSH r I : with I I 1. 7>C/S>0.85 9 ! • I I a tCSH
8 'with CIS=0.85
3 4 5 8 7 8
log,o time (yeafs)
Figure 2. Predicted evolution of the pH of groundwater in a UK intermediate radioactive waste repository (in Miller et al. 2000).
3) When all unreacted Ca(OH)2 has been removed, theCa/Si ratio will have fallen to about 1.8 (from an initial value of about 4.5). Incongruent dissolution ofCSH then begins, with preferential removal ofCa. The pH gradually decreases to about 10.5 and Ca/Si reaches 0.85 when congruent dissolution ofCSH begins.
4) Two processes may now take place depending on the Ca and Si content of the leachwater: a) in distilled water, or low-salinity groundwater, slow congruent dissolution of CSH controls pH, at about 9-10, until all CSH is removed, a process that takes considerable time, or b) in groundwater containing significant Ca and Si, congruent dissolution is not achieved and the CSH gel continues to dissolve incongruently until completely removed. This causes a further decrease in Ca/Si ratio and gradual decrease in pH, together with dissolution of precipitated minerals such as ettringite and brucite, until the alkaline buffering capacity of the cement is consumed. Thus, the leaching processes are prolonged in saline groundwaters with high Ca. A more detailed examination of the effects of cement leaching by ground water is given in section 4.
The high pH values of groundwater persist throughout this alteration process and are effective at reducing the solubility and increasing the sorption of most radionuclides (possibly with the exception of alkaline earths such as Cs and Sr which are very poorly sorbed onto cement, Miller et al. (2000)) thus preventing radionuclide migration from the repository. In the process of dissolution of the CSH matrix, silica and alumina tend to be left as hydrated residual grains. At high pH, these become sparingly soluble and are able to migrate from the leaching interface. In the case of cement or grout injected into permeable fractures in bedrock, any dissolved silica and
13
alumina will migrate until pH decreases due to dilution or water-rock interaction. These components will then precipitate from solution and may coat fracture surfaces and block the narrower aperture fractures and matrix pores. Because a large percentage of the cement has been dissolved from the initially grouted fracture, these residuals are unlikely to reduce the permeability of the fracture from its pre-grouted condition but they may cause a reduction in permeability of pores in the rock matrix and of open fractures some distance from the grouting.
3.2 Dissolution of Superplasticizer
Relatively little work has been done to determine the leaching characteristics of organics bound in the cement (mainly as superplasticizers). Onofrei et al. (1992) described laboratory studies in which a radioactively labelled superplasticizer (NaSNFC, labelled with 35S) was used in the preparation of a Canadian Type 50 highperformance grout. The grout was leached in a series of laboratory tests with three ground waters of different salinity and the content of 35 S measured in the leachates.
The release of superplasticizer was found to be derived from the unadsorbed fraction ofNa-SNFC in the pore space and from concomitant dissolution of the C3S and C3A hydrated phases. The cumulative release over the 3 0-day period of leaching was about 10-16 kg/m2, low in comparison to the loading in the solid phase (10-13 to 10-12
kg/m2). It was also observed that the release rate increased with increasing temperature and salinity of the groundwater. It was concluded that the use of superplasticizers would increase the dissolved organic content of groundwater in the vicinity of a repository but the importance of this increase could not be determined until other factors, such as the concentration of naturally occurring organic materials in the groundwater and in the repository itself, was known.
3.3 C02 interaction (carbonation)
Cement may be dissolved by water that is mildly acidic due to C02 dissolved from the atmosphere or from soils:
5)
The portlandite component is dissolved frrst:
6)
and, if not fully dissolved and removed, may be carbonated by reaction with carbonate ion:
7)
Intermediate complexes such as calcium monocarboaluminate can form at very low C03 concentrations, and generally at higher temperatures (Atkins et al. 1994). Carbonated concrete retains much of its strength but because pH is now lower, reinforcing steel may oxidize and corrode (Lagerblad and Tragardh 1995). At lower pH, however, the calcite may be dissolved by further reaction with C02 and water:
14
8)
Once the portlandite or calcite has been removed, the silicate and aluminate phases will break down by Ca loss, but at a slower rate. However, with this loss the physical strength and resistance to further attack has been reduced and the concrete readily disintegrates. Lagerblad and Tragardh (1995) have estimated that the carbonation depth for good quality concrete in a high-humidity tunnel is ~5 mm after 50 years. The effect of carbonation of cement on adjacent groundwater is seen as a slightly elevated Ca concentration and a higher pH.
3.4 Organic interaction
In low and intermediate low-level waste facilities, organic materials (e.g. cellulose, gloves, resins) will degrade to give acids such as H2C03 (from C02 solution in water), low-molecular weight organic acids and HCl (due to radiolysis of chlorinated polymers such as PVC). These acids tend to react with OH groups and lower the pH. Holgersson et al. (1989) have summarised the interaction of radionuclides in alkaline conditions with the complexing agent gluco-isosaccharinic acid (from the degradation of cellulose). Superplasticizers included in grout probably do not tend to interact chemically with cement once released by leaching although no specific data about this aspect has been found so far. The ability of superplasticizers to sorb onto cement, bentonite buffer and host rock and their influence on sorption is currently being examined by both laboratory and field experiments in the Swiss program (Schwyn 2001).
3.5 504 interaction
Cement may react with S04 derived from within the cement matrix (known as 'internal' sulphate attack) or from environmental sources such as S04 that occurs naturally in groundwater ('external' sulphate attack) as described by Ouyang et al. (1988). Both mechanisms will produce gypsum from the reaction with free portlandite:
Ca(OH)2 + MS04 = CaS04 + M(OH)2 9)
where M may be a monovalent or bivalent cation. Alternatively, S04 may react with the hydrated calcium aluminates to form calcium sulphoaluminate (so-called 'monosulphate') followed by sparingly soluble ettringite:
The gypsum produced in equation 9 may further degrade the cement by reaction with CSH to produce ettringite. In equation 9, when M is an alkali element (Na or K) the forward reaction may be limited by the accumulation of strong alkali if flow through the cement is slow. If M is Mg, however, the reaction proceeds rapidly because the hydroxide produced (brucite, Mg(OH)2) is relatively insoluble and induces a lower pH (~10.5), at which the hydrated calcium silicate becomes unstable (Lea 1971) and the concrete cracks from expansion caused by ettringite formation. Thus it is common to find gypsum crystals on exposed concrete surfaces where the calcium sulphoaluminate component has been removed.
15
At temperatures of about 20 °C, ettringite is stable but above 50 oc the reaction in equation 10 moves to the left, monosulphate becomes stable and S04 is released to the pore fluids (Atkins et al. 1994). Sulphate attack is prevented by using cement with low ( <5%) calcium aluminosilicate (C3A) content (Neville 1996).
The effects and mechanisms of S04 attack of cement have been described in detail by Lawrence (1990) both for synthetic solutions in laboratory studies and for S04-rich groundwaters and seawater. An important conclusion of this work is that if so4 is naturally present in groundwater, C3A-free cements should be used which are most resistant (up to 6000 mg/L S04).
3.6 Cl interaction
The principal reaction between concrete and dissolved Cl is with any steel reinforcement rather than with the concrete itself. Unless the cement is made up with seawater or saline groundwater or uses sea-dredged aggregates, the resulting concrete should have a very low Cl content. Any Cl in the mixture generally enters the AFm (ferroaluminate) phase with 60% of the Cl bound to the cement paste as monochloroaluminate and the remainder dissolved in the pore water (Lagerblad and Tragardh 1995). At high Cl concentrations, other salts such as 3CaO.CaCb.15H20, can form (Atkins et al. 1994). When Cl is combined with the AFm phase, the resulting compound is known as Friedel's salt (4Ca0. Ab03. (Cl, OH)w) which is stable above 40 °C. Below 20 °C, the trichloride complex (AFt) is formed.
Chloride attack of cement results in conversion of all aluminate phases (C4AH19, C2AH8) to become part of a S04 or Cl complex (Lagerblad and Tragardh 1995) with eventual disintegration of the concrete. By immersing cured concrete specimens in NaCl solutions, Kayyali and Haque (1995) observed that samples containing superplasticizer absorbed more Cl than those with no superplasticizer. The Cl was found to migrate into the pore fluids. The effect of Cl and S04 on cement has been found to be highly variable from one experiment to another (see review in Al-Amoudi et al. 1994). In many cases, the presence of Cl reduced the extent of S04 attack of cement but in others, the opposite was found.
In a series of specially designed experiments, Al-Amoudi et al. (1994) found that Cl did in fact appear to inhibit S04 attack particularly in OPC and fly ash cements. In silica fume and blast-furnace slag cements the effect was marginal. The penetration of Cl from external sources into hardened cement has been found to be dependent on the composition of the cement. Reported values for Cl diffusion coefficients are 0.1 to 1.2 x 10-12 m2/s for the various types of cement (cited in Lawrence 1990). The effects ofNaCl and CaCb in cement have been considered further by Al-Hussaini et al. (1990).
In summary, therefore, leaching of cement by groundwater may only give an increase in Cl concentration of the groundwater if saline fluids or materials were used in construction of the cement. The long-term effect of this type of cement is still dominated by the leaching of hydroxides and creation of a pH plume in the groundwater. Nevertheless, the expected dissolution rate in saline groundwaters is lower due to the higher ionic strength of the water, especially if Ca concentration is high.
16
3.7 The Alkali-Silicate Reaction
The alkali-silicate reaction (ASR) occurs in the finished concrete when alkalis in the cement pore fluid (NaOH, KO H) react with active forms of silica (e.g. opal, chalcedony, glassy felsic volcanic rock) present in the aggregate (Lea 1971). An alkali-silicate gel is formed which causes expansion and cracking of the concrete. The reaction is further enhanced by the presence of moisture and higher temperatures. Because the ASR consumes alkalis in the cement, pH of the pore fluids will decrease to the point where Ca(OH)2 becomes more soluble and Ca will exchange with Na or Kin the gel to give greater swelling (Wang and Gillott 1991). ASR can be avoided by ensuring either that inactive silica (e.g. quartz) is used as aggregate or the alkali content of the cement is low(< 0.6%). An alternative method of preventing ASR and the resulting expansion and cracking is by addition of lithium (in the form of LiOH.H20) in the molar amount Li = 0.74/(Na + K), which serves to limit the activity of the alkalis, although the exact mechanism is not clear (Blackwell et al. 1997).
3.8 Speciation and Solubility Modelling
Several attempts have been made to model the interaction of hardened cement with water, using various degrees of sophistication to determine the long-term stability of concrete. Very few attempts have been made to examine the changes in water composition as the concrete dissolves, however. The main problems in modelling cement-groundwater interactions are the lack of a high-quality thermodynamic database for cement phases, particularly CSH, and the absence of data on rates of chemical reaction involving cement phases. Models are usually limited to a restricted range of possible CSH compositions that are required for evaluation of the long-term stability of cement in groundwater. However, recently, a unified database for cements has been made available (Kim et al. 2000) and this should allow for better intercomparisons of data and greater accuracy of prediction.
In early studies, Glasser et al. (1985) developed two models, one based on experimental data and the other on theoretical considerations, to describe the effects of high-alkali content pore fluids, the long-term alkalinity controlled by portlandite dissolution, and the influence of various types of pozzolans (fly ash, silica fume, slag). Bemer (1987) developed an empirical model describing incongruent dissolution of CSH phases and later (Bemer 1992) developed a series of submodels defining cement phases and their behaviour in dilute ground water including a 'mixing tank model' in which the longevity of the cement and the evolution of pore fluid composition could be determined. Estimates of the time required before all the portlandite has been dissolved have been made in the form of the number of pore volumes exchanged. Bemer (1992) has estimated 1000 pore volumes are required whereas Engqvist et al. (1996) calculates that 2000 volumes are needed. Lunden and Andersson (1989) used a thermodynamic model to simulate the mixing of cement pore fluids with groundwater and observed that the resulting solution was supersaturated with respect to calcite, hydroxyapatite, chlorite and laumontite.
Various computer models have been used or derived to describe changes in cement composition due to leaching including EQ3/6 (Alcom et al. 1992, Savage and
17
Rochelle 1993), HARPHRQ (Haworth et al. 1995), CHEQCONC (a version of the coupled chemistry/transportation model CHEQMATE, Haworth and Smith 1994). Other codes for modelling cement-water interaction include CEMENT and l\11NEQL/ AU which have been used for modelling at ambient temperature.
The equilibrium chemistry of cement-water interactions has been studied by Reardon (1990). Several different aqueous species are formed when water contacts cement. These are summarised in Table 4 and apply to a pH regime of>8. Reardon has used the Pitzer Ion Interaction Model to calculate activity coefficients and determine reaction mechanisms. The CSH gel phase is difficult to model because it is not a pure phase with a unique solubility product. It has large variations in Ca/Si ratio and a number of distinct forms each of varying stability. Reardon used compositional and solubility data of Gartner and Jennings (1987) to construct a model describing the behaviour of the more stable CSH forms in terms of water chemistry. Using the Pitzer Model, solubility products were calculated for CSH forms in equilibrium with various water compositions. Generally good agreement was found for available measurements of Si02 and CaO contents of solutions saturated with respect to CSH compared with calculated values (Figure 3).
Unfortunately, the model is limited by the lack of experimental data on CSH stability above pH 12.5 (the limit for portlandite solubility) and the speciation of Si02 above pH 13 . A further limitation is the understanding and formulations of the ASR, discussed in section 3.6. These fluids aggressively dissolve siliceous materials such as chert, tuff, glass, Si02-rich limestone, feldpars and quartz. The dissolution of Si02 causes precipitation of several mineral phases containing Ca, Si, 0 , AI and, in some cases, K. Reardon (1990) points out that exact modelling of a concrete-water system is difficult because chemical equilibrium will not exist due to the heterogeneous nature of concrete (CSH gel, alkaline pore fluids, variable aggregate composition and dissolution-precipitation reactions) which causes a dynamic, evolving chemical system. These difficulties are dealt with in more detail, together with a possible solution using an empirical model by Reardon (1992).
Table 4. Principal aqueous species in cement-water systems (from Reardon 1990)
Cations Anions Neutral species Ca2+ er ~Si04° Mg:l+ so4z- H2C03o Na+ HC03-K+ Fe(OH)4-w Al(OH)4-MgOW H3Si04
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30
Figure 3. Comparison of Si02 and CaO concentrations in water saturated with CSH and published solubility data (from Reardon 1990). The upper curve is the average of solubility for me tastable (young) CSH while the lower curve is the predicted solubility of CSH in this classical model.
3.9 Interactions with solid materials
In an attempt to understand the changes in chemistry of the host rock to a cementitious repository, Savage and Rochelle (1993) examined the potential changes in mineralogy and physical properties of a fractured, saturated crystalline rock using the EQ3/6 code. In this study, they reacted quartz and albite minerals (from the rock) with a mixed alkali (Na, K, Ca) fluid and with a saturated Ca(OH)2 fluid. Results indicated that, for both fluids, small increases in rock porosity would occur together with rapid loss of Ca and gain ofNa and Si and little change in pH. Both CSH phases and zeolites would be precipitated in the rock. Similar results were observed by Mader (200 1) in laboratory experiments involving reaction of rock from a shear zone in the Grimsel facility in Switzerland with an alkaline solution with pH 13 .2. Calcium was observed to react with the plume to form CSH phases and Mg in the rock was inferred to have formed Mg(OH)2.
The production of high-pH leachates from OPC may cause changes in redox conditions because the pH-Eh relationship for water shows that water stability at high pH can only be maintained if Eh is low. A low Eh may cause reduction of organics in the cement (mainly the superplasticizer) and produce gases such as C~ or H2S (from S04, due to microbe activity). This process has been postulated by Karlsson (200 1 ).
19
The interaction of high-pH plume groundwaters with the bentonite buffer in a repository has been of considerable concern because of the potential for reducing the swelling capacity and the sorptive properties of the clay minerals by illitisation or zeolite formation. Experiments by Jefferies et al. (1998) have indicated that Ca will saturate the exchange sites of the clay and causing calcite precipitation. Oscarson et al. (1997), however, shows that using high-performance (low-pH) concrete will release much less OH and this can be buffered by the bentonite and cause no significant changes to the properties of the clay barrier.
Shibata (200 1) has examined the effect of high pH solutions on the stability of bentonite buffer. In the experiments with a diluted bentonite suspension at low temperatures (< 100°C), smectite was found to dissolve at very high pH (13) but appeared to be stable at lower pH (10.5). CSH and CASH (containing AI) were found as secondary minerals. In the experiments with a bentonite slurry at 200°C, however, the observed secondary mineral was a zeolite (analcite). In the experiments with compacted bentonite, swelling was observed and evaluated using the PRECIP code. Current studies on cement-buffer interactions by Atomic Energy of Canada Limited (AECL) at the Underground Research laboratory (URL) in Canada, have shown no discernible change in Ca concentration of a sand-bentonite buffer in contact with a cement plug over a 7 -year period (Dixon and Chandler 2000).
Because of the complexity of the system and lack of thermodynamic data in the investigation of cement-groundwater-buffer-host rock interactions, numerous studies have been conducted on natural analogues to determine the probable long-term behaviour of concrete and associated groundwater. Again, these have tended to concentrate on the stability of the cement but some data on the effects of the high-pH plume on groundwater composition and interactions with the host rock have been obtained. A useful summary of the application of natural analogues to understanding long-term cement degradation and water-cement interactions has been given by Miller et al. (2000). A summary of recent work on the high-pH natural analogues at Maqarin (Jordan) and Oman (Cyprus) has been given by Mader (2001). In this study, hightemperature metamorphic zones at Maq arin cause discharge at the surface of hyperalkaline groundwaters (pH up to 12.8) and the precipitation of CSH minerals, causing self-sealing of the fracture system. This suggests that leaching of cement in a repository environment may lead to eventual sealing of groundwater flow paths and greater stability of the repository.
21
4 LEACHING STUDIES
Two situations might arise in the use of cement in an underground facility where concrete might be dissolved by water flow: 1) advective flow through interconnected pores and microcracks in injected grout in rock fractures or through shotcrete on tunnel walls, and 2) diffusive flow through tunnel bulkheads and plugs when the repository is fully resaturated. The latter is an extremely slow process and it is likely that other, chemical, effects would be more important here. These processes have been addressed in studies of the leaching characteristics of cement in water.
A number of studies have been made of the interaction of hardened cement and grout with fresh and synthetic saline groundwater. Greenberg and Chang (1965) performed one of the most detailed of the early leaching studies of the CSH system. They showed that the aqueous phase could be characterized by total Ca and Si02 in solution together with the pH of the phase and the Ca/Si ratio was found to determine the composition of the solid phase.
Atkinson et al. (1989) examined the leaching process in detail to determine the longterm evolution of cement in a radioactive waste repository. Their experiments indicated that the pH in OPC or in cement modified by addition of blast-furnace slag was likely to remain high (> 10. 5) for about one million years in a low-flow rate repository but chemical reactions with ions dissolved in groundwater (particularly C03) could shorten this lifetime considerably.
Various studies have been made at ambient groundwater temperatures to determine conditions in a saturated, potentially saline environment such as could occur in Canadian, Finnish and Swedish repositories. Engkvist et al. (1996) have analysed major ions and pH in replenished solutions of fresh and saline groundwaters in contact with crushed OPC for about 1.5 years. Trends in pH and the major ions over this period are shown in Figure 4 for fresh water and saline water (the synthetic solutions Allard and NASK standard waters, respectively; their composition is given in Table 5, from Lagerblad and Tragardh (1995). Although these are simple experiments, they give a good insight to the potential changes in groundwater composition in contact with grout or concrete over the short term.
The pH, shown as a solid line in all graphs in Figure 4, was calculated from OH titration data and OH complexes. Initially, high concentrations ofNa and K were seen (due to leaching of alkaline pore fluids) but these rapidly decreased while Ca remained constant and at a pH of about 12.5. A constant ion composition was maintained until leaching step 45 when pH and OH and Ca concentrations began to decrease indicating removal of the portlandite. The only significant differences between fresh and saline ground waters from this work are 1) the greater buffering of Cain saline water which causes a less steep decline in Ca concentration after step 45 (Figure 4a), 2) an earlier start ( ~ step 15) to S04 dissolution in saline water and 3) the dissolution of AI in saline water is repressed compared to fresh water (Figure 4b).
In the Nirex (U.K.) program, considerable attention has been given to the development and effects of cracks in the cementitious barriers in a repository (Harris et al. 1997). Leaching experiments of OPC using distilled water resulted in an
22
Table 5. Model groundwater compositions simulating groundwaters in granitic terrain (A/lard water) and deep saline groundwater (NASK water), in mg/L (mMIL), from Lagerblad and Tragardh (1995).
Allard water NASKwater Na 65 {2.8) 3220 (140) K 3.9 (0.1) 80 (2.1) Ca 18 (0.5) 800 (19.9) Mg 4.3 (0.2) 10 (0.4) Cl 70 {2.0) 6390 (180) HC03 123 (2.0) 120 (2.0) so4 9.6 (0.1) 380 (4.0) Si02 12 (0.2) -pH 8.2 7.7
increase in cement permeability throughout the experiment but when a slightly brackish groundwater was used (TDS ~ 0.8 g/L), permeability decreased by over an order of magnitude due to the precipitation of Mg(OH)2. To attempt to understand the influence of high-pH solutions on clay to be used in a potential repository in Switzerland, Chermak (1992, 1993) performed batch experiments in which Opalinus Clay samples were reacted with high-pH NaOH and KOH solutions (~0.1 and 0.01 M) for up to 51 days at temperatures of 150, 175 and 200 °C. The high temperatures used were largely to induce chemical reaction within a reasonable time frame. The initial pH values were 12.9 for 0.1 M and 11.9 for 0.01 M solutions. All reactions at 150 and 175 oc showed a gradual reduction to pH~ 11.5 but a more rapid reduction (to as low as 6.7) at 200 °C. Supporting XRD analyses showed that dissolution of kaolinite and quartz caused an initial decrease in pH and increase in Si02 concentration with production of analcime or phillipsite (Na- and K-bearing xeolites, respectively) followed by vermicullite or K-feldspar. These observations are significant because these minerals have low or no swelling capacity and, hence, the desirable swelling and transport-limiting properties of compacted, clay-based barriers could be adversely affected.
Other experiments on the interactions of groundwater with cement and rock have been performed at higher temperatures either to induce chemical reaction in short-term laboratory studies or to simulate actual repository conditions. For instance, in the case of nuclear waste disposal in the unsaturated zone of Yucca Mountain, USA, temperatures at the container surfaces in excess of 100 oc may occur. Chemical reactions of cement, seepage water and the tuffaceous host rock have been modelled by Bruton et al. (1994) and Carrell et al. (1998) and the mineral phase IIAtobermorite (CasSi02)6(0H)IO.O.SH20) was found to be the stable phase at these temperatures together with calcite and quartz.
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25
5 MICROBIAL EFFECTS
5.1 Biodegradation of concrete
Biodegradation of concrete materials under aerobic conditions is a well-known process (e.g., Diercks et al. 1991). Sulphate-producing bacteria (Thiobacil/us sp.) are capable of oxidizing sulphur, sulphides and thiosulphates to sulphuric acid in a relatively short period of time under aerobic conditions. Nitrifying bacteria (Nitrosomonas, Nitrobacter) can transform ammonia into nitric acid under aerobic conditions. The inorganic acids produced attack the concrete by dissolving Ca(OH)2
and CSH gel from cement. During the pre-closure phase of a repository, which may span tens of years, aerobic degradation of concrete surfaces could occur and precautions should be taken to prevent this (Diercks et al. 1991). Fungi could also be involved, by producing organic acids from cellulose degradation (Libert et al. 1993).
After closure, a repository environment would evolve towards anaerobic conditions relatively rapidly and aerobic degradation of concrete may no longer occur. Under anaerobic conditions, some microorganisms are capable of producing organic acids. However, effects of relatively weak organic acids on the high-performance concrete materials that are proposed for use in nuclear waste repositories are likely to be negligible (Stroes-Gascoyne and West 1994).
5.2 Microbial degradation of superplasticizers
A potentially more significant aspect of microbial degradation of cement and grout is the possibility of microbial degradation of the organic superplasticizers that may be included. It has been established that microbial life exists naturally in deep groundwaters, including oligotrophic granite groundwaters. Although subsurface microbes generally subsist at very low metabolic levels, if given appropriate nutrient sources, their growth and metabolism could increase substantially. In particular, any organic carbon is a prime source of nutrients and energy for these nutrient-starved microbes. Therefore, the addition of superplasticizer to concrete should be examined with respect to its potential availability for microbial consumption, because it has been determined that superplasticizer can be leached from grouts in small amounts (about 10-16 kg superplasticizer/m2 grout/30d, Onofrei et al. 1992). Even such small amounts could potentially result in increased microbial growth in a repository environment over geological time scales if microbes are capable of generating energy from it. Some of the effects of increased microbial activity could include enhanced corrosion of waste containers and production of acids, gases and biofilms. Also, radionuclides may sorb onto microbes and this could result in promotion or retardation of radionuclide migration in a repository environment.
Biodegradation of superplasticizers is possible in principle, although it may require an adaptation period for microbes to acquire this characteristic because of the polymeric nature of the material. The biodegradation of the superplasticizer Disal was studied in the Canadian program (Haveman et al. 1996). Disal is naphthalene-based and the biodegradation of naphthalene by members of the genus Pseudomonas is well documented in the literature (Rossello-Mora et al. 1994, Sanseverino et al. 1993, Yen and Serdour 1988), for instance in the bioremediation of gasoline-contaminated sites. Pseudomonads are common subsurface bacteria and have been found to dominate
26
microbial populations in granitic groundwater (58% of the total bacterial population in granitic groundwater from AECL's Underground Research Laboratory was found to belong to the genus Pseudomonas, Haveman et al. 1995). However, Pseudomonads are not normally able to degrade polymers into monomers, and may, therefore, need to cooperate with another type of bacteria in order to use polymers as a food source (Brock and Madigan 1991).
The Canadian study was of a preliminary nature and involved incubation of groundwater bacteria with Disal alone, with Disal and N03 or P04 and with Disal and N03 plus P04, because these nutrients have been found to be limiting factors to microbial activity in granitic groundwaters (Stroes-Gascoyne 1989). Results from aerobic incubations at 10 oc and 3 0 oc showed that the concentrations of added Disal remained unchanged and bacteria numbers did not differ significantly from controls without Disal over the 7- week course of the incubations.
However, anaerobic incubation of ground water bacteria with Disal and N03 at 10 oc resulted in an increase of an order of magnitude in the bacterial population compared to the control which contained groundwater and N03. This effect was not seen in the equivalent aerobic test. This result suggested that Disal could stimulate microbial growth under denitrifying conditions, which concurs with several reports in the literature on microbial degradation of naphthalene under denitrifying conditions (Mihelcic and Luthy 1988, 1991). It also appears possible that naphthalene degradation can be coupled to S04 reduction (Bedessem et al. 1997) or Mn or Fe reduction (Langenhoff et al. 1996).
Identification of 16 randomly chosen strains of bacteria from the incubations that could grow on a medium for hydrocarbon-degrading bacteria containing Disal as the sole carbon source showed that most indeed belonged to the genus Pseudomonas (Haveman et al. 1996).
Although the results from this preliminary study were not entirely conclusive, they suggested that microbes would be able to use leached naphthalene-based superplasticizer in an anaerobic vault environment provided they could break down polymeric structures fully and provided N03 or other electron acceptors were available as nutrients. Nitrate would probably be present in a vault as it is readily introduced as a contaminant (e.g., from blasting practices during excavation of a repository, Stroes-Gascoyne and Gascoyne 1998), and S04 is present in bentonitebased buffer materials (Stroes-Gascoyne 1989). However, the efficiency of microbes in breaking down the polymeric structure of naphthalene-based superplasticizers was not established in the study by Haveman et al. (1996). Since Pseudomonads are not normally able to degrade polymers into monomers, cooperation with another type of bacteria may be required in order to fully use polymers as a food source. The full adaptation of a microbial community to degradation of polymers may take longer than the 47 days in the Canadian study.
Because it appears that only very small amounts of superplasticizer leach readily from grout and concrete, it is not expected that superplasticizer -enhanced microbial activity would have a large effect on microbial biomass in a repository environment.
27
6 GROUTING EXPERIMENTS
A number of attempts have been made to determine the ability of various cement formulations to grout narrow fissures in fractured crystalline rock. They have ranged from simple laboratory studies to full field demonstrations in underground facilities. One example of this is the work done in Stripa in 1988 (Pusch 1988). This section describes the work done by AECL as part of the Canadian Nuclear Fuel Waste Management Program at the URL, in Manitoba in 1987.
6.1 Grout Requirements
Cement formulations can be customized for specific tasks in the construction of an underground repository for nuclear waste. For use in grouting the narrow-aperture fractures found in crystalline rock, a grout must have a fine particle size and sufficient fluidity to be pumped into these restricted openings. For increased longevity of the grout, the quantity of free portlandite must be limited and this is best accomplished by using silica fume to replace some of the portlandite. The fine particle size of the silica fume and the low w/c ratio prevents the grout from separating into phases ('bleeding') during injection and aids the penetration into narrow fractures.
Conventional grout has a w/c ratio of over 2. When such a grout hardens, the extra water not involved in cement hydration is trapped in the solid structure and increases the porosity. This causes a reduction in strength, an increase in hydraulic conductivity and shrinkage, and a decrease in longevity. The use of superplasticizers allows workable cement-based grouts to be made with a w/c ratio of 0.4 or less (Onofrei et al. 1991, 1997).
Grouts and concretes are known to self-seal in certain circumstances after initial fracturing due to shrinkage or other processes. This process is known as 'autogenous healing' and is performed largely by Ca(OH)2 leached from the matrix with formation of calcite by C02 absorption. Various experiments of thin cement films and bulk grouts have been performed by Onofrei et al. (1997) to determine the permeability, porosity, micro-cracking characteristics and self-sealing capabilities of the grout forms.
Three types of commercially available cements were considered for use in grouting operations in the Canadian program (Gray and Keil 1989):
1) S04-resistant Portland cement (Canadian Type 50). This was considered to be the most appropriate for use in granitic rock of the Canadian Shield (Burnett et al. 1985), where some groundwaters contain sufficient S04 to be considered aggressive. This cement is also the most chemically stable Portland cement type.
2) Expansive cement (Canadian Type K). Although Portland cement grouts injected into saturated fracture zones were not expected to significantly shrink during setting and hardening, the use of expanding cements was thought to be advantageous in special sealing applications as they may improve the contact at the rock-cement interface.
28
3) Micro-fine MC-500. This is a slag cement with extremely fine grain size. The Canadian Type 50 cement is a high-performance, low pH cement and is composed of 90% S04-resistant Portland cement, 10% silica fume and 1% superplasticizer. The detailed composition of Type 50 and other grouts is shown in Table 6. Both the Type 50 and Type K were reground (to a Blaine fineness of 600 m2 /kg) to improve their fineness.
Table 6. Chemical composition of cements studied in the Canadian program (from Gray and Kei/1989).
Type of Cement Microfine MC-500 TypeK Type 50
Chemical Mass% Mass% Mass o/o Constituent Si02 30.0 18.5 21.6 Ah03 11.2 5.0 3.1 Ti02 0.6 0.3 0.2 P20s 0.2 0.1 0.1 Fe203 1.3 2.9 4.0 CaO 47.2 59.7 61.4 SrO 0.1 0.1 0.0 M gO 5.4 4.2 4.4 Na20 0.2 0.2 0.4 K20 0.4 1.1 0.5 so3 3.0 5.7 2.1
Loss on ignition 0.5 2.8 1.3
TOTAL 99.9 100.7 99.2
6.2 Laboratory studies
Various laboratory tests have been performed on the cements in Table 5 to determine the grout with optimum properties. These studies showed that low viscosity, non segregating (bleeding) grouts could be prepared from any of the three cements (Gray and Keil 1989). The amount of superplasticizer could be varied to achieve the desired viscosity without significant impact on the setting time. However, the reground Type 50 cement with 10 % silica fume appeared to require slightly less water for the same viscosity than the others and it was widely available and its properties welldocumented. Moreover, studies have indicated that Type K cement is less thermodynamically stable than the other cements (Gray and Kiel 1989) and so the Type 50, a high-performance, low pH cement, was selected for further trials. The superplasticizer 'Disal' (Na-sulphonated napthalene formaldehyde condensate) was used in all field trials.
The Type 50 grout has also been shown to be capable of sealing very fine fractures ( < 20 Jlm) in granite and to reduce the hydraulic conductivity of a fracture zone from 10·7 to 10·9 m/s (Onofrei et al. 1997). The grout itself has been found to have an extremely low hydraulic conductivity (< 10-16 m/s) under hydraulic gradients far
29
greater (I= 35,000 m/m) than those anticipated in a repository (I= 0.01 m/m). The low hydraulic conductivity, therefore, does not allow significant pore-water exchange and leaching or associated cement dissolution. Thus, determination of the long-term performance of this cement grout cannot be readily made because models assume cement degradation by pore-water exchange and cement leaching.
For the full-scale field grouting trials in the URL, the following reference grout was used (Gray and Keil1989):
S04-resistant, cement (Canadian Type 50) reground to a fineness of600 m2/kg 10 % silica fume (by mass) Disal superplasticizer ( < 2 % by mass of solids) Water (w/c ratio ofbetween 0.4 and 0.6, by mass).
Once set, this grout has a strength of> 40 MP a and a hydraulic conductivity of~ 1 o-14
m/s (a value comparable to that of the unfractured granite in which it was to be used). In contact with groundwater, it gives an expected pH of< 11.0 (Oscarson et al. 1997).
6.3 URL grouting trials
Grouting of three boreholes drilled into parts of Fracture Zone 2 (FZ2) at the 240-m level of the URL was performed in April-May 1987. The region of FZ2 that was grouted is shown in Figure 5 and details of the boreholes with respect to the 240-m level of the URL, and their intersection with FZ2, are shown in Figure 6. The boreholes were located in the HC9 'basin' near to the main shaft of the URL so that once the shaft was excavated through FZ2 the effects of grouting could be observed in the shaft exposure and samples taken for analysis. The HC9 basin is a relatively isolated part of FZ2, having a high permeability but low storage and low rate of supply from the main part ofFZ2. It is elliptical in shape and, as defined by boreholes is approximately 30 m long and 10 m wide (Figure 6).
6.3.1 Borehole characteristics
The first two boreholes to be grouted (GH1 and GH2, Figure 6) had very low hydraulic conductivities (~5 x 10-8 m/s) prior to grouting and, because conventional grouting has a lower limit of 10-7 m/s, very little grout penetrated the fractures in these holes (31 and 188 L of grout, respectively, were injected until refusal at a ;ressure of about 3 Mpa). An adjacent borehole, HC9, was more permeable (10 m/s) and accepted larger volumes of grout (700 L of grout were injected and refusal was never achieved). The boreholes were grouted sequentially and then reamed out and hydraulic conductivity re-determined. Details of the mechanical and engineering aspects of the grouting trials are described by Gray and Keil (1989). During, and for some time after the grouting, characteristics of the groundwater chemistry were monitored in an adjacent permeable borehole, HC6, which also intersected the same basin of FZ2, as shown by hydraulic response testing. The results of this work are described below.
0-
-100 -
ii f5 -200 -0
'a .t::. (I)
E -300 -0 .c
g -6 i -400 -c
-500 - L__j Pink Granite
~ Grey Granite -600 -
30
1
~ Fracture Zones I
0 100 200m [J!Yil] Fractures
Figure 5. Schematic section through the URL area showing the location of the shafts and levels with respect to the fracture zones. The 240-m level is located just above FZ2.
0 5 L--·---,-'
m
1.., INTERCEPTION OF DRILL
HOLE WITH FRACTURE ZONE BELOW
Figure 6. Plan of part of the 240-m level used for the grouting trials, showing the location of the shaft, the borehole collars (dots) and intersections with FZ2 (crosshatched boxes), and the HC9 basin (cross-hatched area), after Gray andKiel (1989).
31
6.3.2 Background hydrogeochemistry
The groundwater composition in FZ2 ranges from a dilute Na-Ca-HC03 water with total dissolved solids (TDS) content as low as 400 mg/L, to a saline Na-Ca-Cl-S04
water with TDS content of up to 6000 mg/L, depending on location in the fracture zone. Groundwaters were analysed for pH and Eh by passing them through a flow-cell that was sealed to the atmosphere. The pH values for both water types range from 8 to 9. In the area of the grouting trials, ground waters tend to lie closer to the dilute composition, as shown by a set of periodic samplings and analyses of HC9 basin groundwaters in the upper part of Table 7. The chemical changes in groundwater from borehole HC9 are described in the remaining sections of 6.3 and section 6.4, below.
6.3.3 Sampling during grouting
During the grouting of the three boreholes in the HC9 basin, groundwaters from adjacent boreholes that are also isolated in the permeable part ofFZ2, HC24 and OBS (drilled as an observation hole), were sampled regularly during a period of continuous flow from these boreholes. Analytical data for the samples are shown in Table 8. These boreholes remained open over the period of grouting (1 to 3.5 hours) to reduce hydrostatic pressure in this part of FZ2 to facilitate grout injection. Once grouting ceased, flow from both boreholes was turned off because continued flow could have compromised the set of the grout.
Sampling ofHC24 and OBS boreholes during the grouting of GH1 and GH2, together with monitoring groundwater pH, showed no evidence of grout contamination of ambient groundwater. The lack of apparent migration of the grout front was probably due to low volumes of grout injected in each of the GH boreholes and a large dead volume in the monitoring boreholes.
During the grouting of GH2, in a parallel experiment, an estimate of the effect of grout on groundwater pH was made by adding freshly mixed grout drop-wise ( ....... 0.06 mL/drop) to 0.5 L ofHC24 groundwater. After each addition, the sample vessel was shaken and pH measured. A graph of the observed trend is shown in Figure 7. This test was not fully representative of the effect of grout on groundwater composition because insufficient time was allowed for equilibration and dissolution, and the test allowed C02 absorption from the air (which could not happen in FZ2). Nevertheless, the large effect on pH of even a small amount of grout is evident in Figure 7. With increase in grout addition, pH stabilized at about 11.5. The pH of the undiluted grout was measured at about 12.6. It should be noted, however, that the pH of water in contact with the hardened grout will be much lower, < 11.0 in dilute ground water and ....... 9.0 in saline groundwater (Oscarson et al. 1997).
Table 7. VcJricJtion in compos.ific,n of HC9 ~~rounc)~vater before and after the ~1routing triols
·-FLOW SAMPLE DATE No ~: ea Mg Si Sr HCC>3 Cl S04 f 8r U03 a •:Ell TDS CHARGE
NAME No. pH BALANCE PRE·-GROU r A \JALYSES
204·008-HC9-2 -2 17-Dec-86 .L60.0 3.09 12.:< 1.52 3.~i() 0 .152 30:•.0 75.8 6.4 0.46 <0.0'1 1.2 8.50 ~.53
204-00B-HC9·An -1 ,!1, JJ .. Jan-87 ::.1o.oo 3.00 12.20 1.46 3.t.() 0 .158 1 7! .00 328.00 -~3.0 6.30 0.6C 0..45 ' .1·7 7.50 S'l5 1 . 4..~{
204-{)(6-HC9-Al2 -1 C JJ .. Jan-87 .~co .oo 2.e 11 .2 1.35 3.2 0.1·l4 174.7 ~-108 -33..4 5.1 l.l9 0.24 J.:).: 881 1.8'%
204-0C~HC'~-Bl.2 -2•\ J.) .. Jan-87 .300.00 2.(.(• 11 .:to 1.47 3.40 0 .141 1 77 .('() 3:2:l .OO 88.6 7.00 0.42 <0.0'! 1.17 7.80 901 0 .06~{,
£04-0CB--H ::<~-BZ2 -2C 1:5-Jon-87 .310.00 2.LC lO..SO 1.31 3.~0 0 .137 1.2'2 325 Zl4..0C8-H::9-BZ2 -2F 1:7-Jon-87 .300.0(1 2 .3 10.5 1 . 2~1 3.6 0.132 1?9 312 as·.2 7.3 0.42 <).Q' 1.L2 8.00 894 0 49~r.:
204-008-HC9 .. 2 -2C 17-Jor ... 87 8 .60 204-008-HC9-2 -3 02-Fel::-87 310.0 2.1C 9.80 1.W 3.70 0.129 17-4.7 289.:3 78.20 7.AO 2.17 <0.01 1.1'~ 8.00 f..65 5 .43%
204-008-~C9-2 -0 l3-Feb-87 8 .40 204·008-~C9-2 -4 04- ~ar·-87 240.0 2 .13 11.10 C.97 4.00 0.070 171.6 :;!73.L 87.2 7.'29 1.53 1.22 8.90 786 -5.20% 204-008-~C9 .. 2 ·5 01 -Apr~87 220.0 1.65 8.80 G.76 3 •• )0 0.100 182.5 244.9 78.4 - 8.60 737 -6.!•7%1
POST -GR'OIJT ANA_ YSES J 204-003-rlC9-2 -7 01 -Sep .. 97 163.0 2.13 3.00 0.31 4.60 0.05~! 226.3 115.0 48.90 6.33 0.62 1.2 559 -·$.01,~
204·003-HC9-2 -8 OS-Nov-87 190.0 1.52 3.20 0.34 4.CO 0.048 221 .0 11' .0 45.7 6.22 0.33 <0.20 1.08 9.05 573 4.~;
204-008-HC~-2 -~) 09-De-:·-87 190.0 1.34 3..10 0.34 4.:?0 0.047 231.0 110.0 47.4 6.6 0.37 <0.1~. 9 . 0~i 563 3.76': 204-008--HCr-2 -1(1 29-.lan-88 190.0 1.13 3.30 0.30 AI.()O 0.042 223.9 115.0 46.5 6.46 0.3 <0.2(1 9 .10 500 3.65,;
20-H>C8-HC9-2 -· 1 02-Mar-88 180.) 1.09 3.20 0.29 3.80 (1.041 235.5 95 .9 46.80 6.Jo) () .. 3() <0.''0 1 .1)4 9.0) 563 3. 2(~;
204-X8-HC9-2 -·. ~· O.S-Jl,;n-88 184.·) 1.13 ~1. 10 0.28 3.HO 0.040 231.:?. 93.0 46.20 6.1•) 0 .. 30 l.l 8 .80 559 5 . 34~)
204-ns-Hc 9-2 -1 2 09-.Sep-88 170.0 0.78 :l'.90 0 .24 3.00 0.036 2.36.0 HX:l.O 5'-.00 6.60 0.20 <0.1C 1.1 8.80 564 -1 . 55~)
204-J08-HC9-2 -1-4 22-Nc-v-88 160.0 (1.76 2.i'5 0 .22 3.i'C· 0.033 2·~3 .0 90.0 t.C.9 6.6 0 .3 <0.05 !.OS 7.40 5513 -3.84~;
2Q4 ... )08-HC9-2 -17 15-Jt,n-89 160.0 0.813 2.:;7 0 .1b 3.S'8 0.032 25 ' .0 n.9 39.9 6.25 <0.10 <0.1C 1.02 8.40 534 ... ().63~)
2Q4.J)()8-HC9-2 -18 21-Ncv-89 157.0 0.84 2.26 0.11 3.el0 o.:o2 244.0 72.7 .56.9 6.7 0.26 <0.10 1.05 8.00 514 0.13~·;
204·{)08-HC9-2 -19 14-Jun-90 158.0 0.63· 2.08 0.15 3.EIO 0.027 ~~44.0 62.9 37.20 6.3 0.22 0.03 l a.c;o 5'J5 .3.34~i
204-009-riC9-2 .. 2() 23-0d-90 153.0 0 .71 2.21 C•.1t. 4.10 0.028 248.0 64.4 50.7 6.45 C.22 <0.05 1.00 9.30 519 -1.(16~;
204-009-HC9-2 .. 21 '0-Ja~-91 158.0 0 .67 2.10 0.11 4.CO 0 .027 2-10.0 64.2 24.3 6AO ·::0.06 1.02 9.26 499 4.01~.
204-003-HC9- :~ -23 03-Dec-•]1 156.0 0.55 1.€0 0.1~· 4.04 O.C2A 251 .0 40.o 35.7 6.6 0.2 <0.02 •).941 9.26 486 .~.95,,
204-003-HC9-2 -24 l O.Jun-:n 1.50.0 0.65 2.19 0.20 4.1~ O.C2i 2·17 .0 5~1. : 35.4 !J.75 C•.20 <0.02 0.95 9.39 495 1 .~.
204-C03-HC9-:2 -25 ~~oct-n 14~.0 0.72 1.85 0. ]:;;· 4 .71 C.C2L 2.:.0 .0 54.5 33.0 6.15 C.21 ·~0.02 0.93 9.4fi 473 '1,4..~.
204 .. ()().3--HC~-:2 ··26 16--.lu"''-n 14&.5 0.72 1.9.3 O.J~· 4.64 C.C2~· 2420 s.~.i' 35.4 ~· . 1 0.653 0.0.34 0.?53 9.44 485 :2,1~.
~~M-00i3-HC~-:2 .. 27 22-Nov-'t3 143.? 0.59 1.as O.lL 5.C1 C.023 207.0 5i3.~! 36.:3 •S C.7 <J.l 0.937 ~)_1.9 448 ·t81~.
~04-003 .. 1-fCt-:2 -30 2fk'ul-'7.5 144.1 C•.63 2.06 0.1:t 4.12 o.o2e 243.(• 50.t· 32.2? 5.3 0.2 <J_, :J.97.4 478 2.:10%. - -
w N
Table 8~ Compositions of local groundwaters during and immediately after grouting trial~
GROUTING SRt1PL'E DATE riHE : Ho~~ K C.s Hg Si Sr HC~ Cl SO•I F~o(f) fQ2't pM Eh Elal.mc:o fDS
;~~;;====~~~~~·~-===========~===~::::======~:::;:;;:::~:;::===~!~-=~~~1=~~~~;=~===~===::;:::;:::_~===========~==~~~-======~========= GH-1 HC21-G'i-1 11-H•«J-87 1110 : 2'90 2.1 9."J 0.6'9 3.80 11.13 180 267 19 O.O~l ?.6 t2l0 "'-3 829
HC21-G~2 11-Ha~-8? 1510 : 1'? ~7 75
GH-2
HC-9
OH-1
BH-~
HC-'9
1'RHC£R TEST
HC21-G~3 11-Hay-87 1510: 280 1.9 8.1 0.65 3.80 0.11 170 251 ?2 0.0~ 7.8 t1~S 5.1 767 HC21-GS-1 1~-H~~-8? 1130 : 2?0 1.'9 8.0 0.6, 3.90 D~ll 21~ 211 0.011 B.L tS G.2 739
HC~-G&-1 22-tt~87 1033 2:10 HC~-G6-2 22-tt~ ... e? 1055 H~~-Gi-3 22~-81 1110 230 HC21-G6-1 22~-8~ 1230 HC21-Gi-S 22-Hay-81 1100 210
HC21-61' ... 1 26-ttay-81 1305 2"W HC21-G7-2 2&~"ay-87 1100 HC21-G7-3 26-"-y-87 1110 '00 HC21-G7-1 Z6 .. "•tr8i' 1.530
HC21-G8~ l'~Jun-8i' 1~26 H~-68-8 22-Ju...-8'1 1"120
IIC2-1-& 21-Jun-81 1530
OBs-61-1 11-Hatj•$1' 1515 OBS-G1--2 11-Hfl~-8i' 15 LS OBS-61•3 11-Ha~-8? j510
220 210
250
250
OSS-G&-1 22-H~,-8? 1025 210 OSS-Gi-2 22-nMJ-&? 1055 OBS-Gi-3 22-nMJ-87 1135 250 . .
1.7 6.0 0.52
1.6 ~-1 0-~8
1.6 6.1 0.53
1.& 6.3 0.51
1.1 23.3 o,ga
e.o 5.2
l.S
s.s J.S
1. 7'
3.9 2.0 2.0
0.1'2 0.1.1 0.06
5 .. 3 O.Si'
5.1 o .. ss OSS-07-1 2&-n~-97 JlJO : 330 2?.0 271.0 0.0~
c.oe o.oa o.oe O.D9
0.:?:0
0.;.10 0.06 o.oa
0.111
0.08
1~'7 1'9? 1'9i' 198 1'95
153 181 25>'9 t6
. 192 1£6 21)1
1'9~ 18:) 21&
203 201 203
198 191 161 18-i 180
183 1~ 1'98 202
181 203 201)
232 211 216
103 187 184)
238
1' 0.015 ?8 ?& <0 .. 010 7~ 95 <0.010
108 <0.010 117 296 0 .. 018 153
78 <0 .. 030 as o.oeCJ ')' 0 .. 0£7
7£ ?S 76o
70 0.01'9 6.$ 71 <0.010
:Z16o 0.013
0.01 o.ol o.os
t3e o.o98 o.1o
8.i" t225
El. i" + 130
8.7. HIS
9.9 t325 e.7 ugo '9.5 •55 '9.5 f9S
u.s •es t.s •15 e.s -1s e.a +20
8.9 +200 ,.o +210 '·1 t230
u.o OBS-0$ 1~-8? 2130 : 310 18.0 ?9 .. 0 0.03 O~i 21-Jun-87 1S55 : 67.0 0.02
2 .. 61 (llOS)
0. ' ' (1)25) o.e... ci52)
18'l 176 120 0.095 0.10 lJ.? -?5
78 <0.030 0.01 GH1-G8 t8-Jun•8? 11~3 GH1-6 2~·Jun•87 16~5
121 15.0 10.7 1~60 11£ JG.O 11.2 ~.80
0.16 0.17
120 1l0
76 28? 81 0.063 0.05 lj.O
GH2-88 18-Jun-9? 1115 G~-6 21-Jun-8? 1100
3SO 100.0 278.0 0.01 310 100.0 271.0 0.03
1.30 (2103) 1,8 1.~ r!21lG2) · 1&1
102 <0.030 0.02 102 <o .. o~ o.o3
HC6-G'J-l 2S-.Jun-8l' 1220 : 200 1.2 8.!5 0.56 0.12 96o <0.030 HCG-G9-2 25-.Jun-ei' 1-100 : 2?0 1.2 8.1 0 .. 51 O. U 81 321 88 <D.D30 0.00 HCG-G9-3 ~-..Jun-·i' 855 : 2?0 1.1 7.1 0.1i 0.10 102 291 81 <0 .. 030 o .. oo ~'-Gt_.. ~~-~ 11!10 : 2'1J 0.9 S.l 0.29 0.07 91 t!S? 68 <0.030 0.00 tl:l5--et.., .:JO-Jwr-1" ~ : 210 0 .. 9 9.J 0 .. 37 0.07 78 <0.030
8.9
'·2 IJ.7
+15 •10 -~5
2.3 716
1.7 6~1
2.7 718
3.6 702 .
-5.7 1"'2
0.7 2-l
-1.7
-1.1
"1.1
5.1
s.& ... 30.0
•0.1 1.9 2.9
S.81 'lO-'
753
7&0
701
721
1l3 616
71'6 758 ,62
HC$--O't-6 30-.Mt-8? 11l.!i : 24J 0.9 S.2 0 .. 3? O.ll? 90 ~1 79 <0.030 0.00 '·8 -95 3.1 661 --·-·a.-......... - ............. ____ ...,.....,. __ .._ ...... -------·-.---··· ... ··~--·-·--·--------·---· .... -... ____________________ _,.. ....... -..........----.-------··------·--------
w w
34
11
10
9
0 0.2 0.4 0.6 0.8 1.0 1.2
VOLUME OF GROUT ADDED (ml)
Figure 7. Effect on pH of addition of fresh grout to HC24 groundwater (500 mL).
6.3.4 Grouting of HC9
The OBS borehole was initially monitored during the grouting of borehole HC9 but this ceased when a strong grout signal was observed (pH = 11, high Ca, K and alkalinity, Table 8). The chemical trends observed in HC24 groundwater are given in Table 8 and the anion concentrations are shown in Figure 8 (only two cation measurements were made). Alkalinity first increased, then decreased, S04 increased continuously and Cl remained relatively constant. A steady pH of9.5 was attained by the end of the test and Eh showed an increase throughout, probably due to introduction of oxygenated water mixed with the grout. The increase in S04 was also seen in subsequent monitoring of the OBS groundwater (Table 8), accompanied by large alkalinity values. The increase in S04 probably originates from dissolution of gypsum present at low levels in the grout and the increase in pH and alkalinity is due to the leaching of lime and alkaline hydroxides.
6.3.5 Service water contamination
During the period of the grouting trials, some vanatlon of mine service water composition was observed and, in particular, pH was seen to rise from a mean value of 7. 8 to peak at 9. 5 (data from URL Environmental Authority). At the URL, all water draining into the URL is pumped to a holding pond and this water is then used as mine service water. The shift in pH was probably due, therefore, to washing down of mixing areas and equipment used in preparing cement pads at the grout injection holes before and during the trials. The mine service water was used in the trials to
35
500,-------------------------------------------------
o Hco3 A CL
400 0 so 4
,...... ...J .._ C')
E ._...
z 300 0
f-< 0: r-z w (.) z 0 200 ()
100
1305 1400 1440 1530
MAY 26, 1987
Figure 8. Variations in anion concentrations in HC24 groundwater during the HC9 grouting trial.
re-pressurize the portion of FZ2 being used for the grouting experiment (otherwise hydrostatic pressure would build only slowly if left to recover unaided). The water was injected through one of the open observation boreholes. Therefore, it is possible that this water mixed with ambient groundwater and caused some contamination of the samples.
6.4 Post-grout analyses
Following the grouting trials, boreholes in the area ofHC9 were sampled to determine the effects of grouting on groundwater in adjacent borehole waters and a 'tracer' test was performed using the products of grout-groundwater interaction as an identifiable tracer that could migrate under flow induced by discharge from a nearby borehole.
36
6.4.1 GH1 and GH2 groundwaters
These boreholes discharged low flows of ground water following re-drilling and were sampled after all trials were completed (Table 8). High pH was observed in GH2 (pH 11. 0) although no measurement was made for GH1. Of note are the high K and Ca concentrations, especially in GH2. Very high alkalinities were also measured but S04 concentrations were close to background values. These results suggest that S04 loss has been stopped by fixation within the hardened cement but K and Ca leaching continued. These results generally agree with those of Ballivy (1987) in tests where solidified grout infillings in granite were leached. The mobility of K under these circumstances was noticeable because K has a very low concentration in ambient groundwater.
6.4.2 The HC6 tracer test
Over a 5-day period June 25-30, 1987, once the grouts had set, groundwater was allowed to flow from borehole HC6 (which enters FZ2 close to the grouted and monitoring boreholes, Figure 6) through a flow cell with monitoring of pH by a timerprinter unit and periodic sampling for analysis. This was done to determine if a highpH plume and chemical grout front existed in the HC9 basin and if it could be mobilized by a withdrawal test such as this. Flow was constant at about 0.5 L/min for the duration of the test and about 3 600 L of water was released.
During the test, pH rose from 8.9 (a typical background value in this area) to 9.8 (Table 8) whereas alkalinity remained fairly stable, at background values, and Na, K, Ca, Mg, S04 and Cl all decreased slightly. Although the pH trend (confirmed by sensor recalibration and a separate measurement) suggests that the grout chemical front had been drawn towards borehole HC6, the ionic data indicate that groundwater composition was relatively unchanged. This may be due to buffering reactions with rock in the flow path or to the fact that the grout, once set was relatively inert to groundwater leaching. It is possible though that the mine service water caused the pH increase although the quantity of water discharged in this test should certainly have withdrawn some from grouted areas of the HC9 basin.
6.4.3 Long-term changes
Periodic sampling of groundwaters in the grouted area resumed one month after the grouting trials were completed. Variations in the groundwater composition for the major ions and pH for borehole HC9 are given in Table 7. It is difficult to resolve the effect of the grouting on major ion composition on a long-term basis because hydraulic testing in FZ2 in the URL area caused significant dilution of ambient FZ2 groundwaters over the next 8 years (as can be seen in Table 7). However, it is significant that the pH of HC9 ground water was frequently >9. 0 following the grouting trials whereas before the trials pH was always <9. 0. Other borehole groundwaters accessing FZ2 do not show this shift so it is possible that the presence of grout in the HC9 area is causing pH to increase by about 0.5 units and this is persisting over the long term.
37
7 SUMMARY AND CONCLUSIONS
7.1 Concrete types
There are clear differences between Ordinary Portland Cement (OPC) type concretes and those containing OPC mixed with pozzolans and various chemical additives, the high-performance concretes (HPC). The reaction rates of HPC are much lower than for concrete made with OPC because permeability is lower and the free lime content is much lower. The use of OPC in concrete in a potential repository environment leads to high-pH(> 13) leachates and pore solutions and the probable generation of a high-pH plume in surrounding groundwater. Advantages of OPC are its rapid setting time, minimal shrinkage, rapid attainment of full strength, low cost and availability in large quantities.
Disadvantages of OPC include silicate mineral dissolution (especially of bentonite and glass waste-forms), ease of attack by S04-rich solutions, release of K+ causing illitisation, the potential for Ca loss, Mg gain, ion-exchange, colloid formation, and high heats of hydration (curing). Other potential disadvantages of OPC are 1) the lower Eh that could be attained within the stability field of water at high pH which may cause microorganisms to metabolise carbon in the cement and reduce sulphate to sulphide which would, in turn, attack a copper container, 2) a decrease in the sorptive ability and swelling capacity of a bentonite buffer, 3) a gradual reduction of matrix diffusion due to plugging of pores by CSH minerals (with consequent increase in (rapid) flow through fractures) and 4) removal of portlandite in OPC causes pH to decrease and CSH phases then become unstable and dissolve causing the concrete to deteriorate.
7.2 Interactions with groundwater and adjacent rock
The degree of interaction of cement with groundwater will depend on c/w ratio, porosity, permeability and, most of all, the composition of the cement. If OPC is used in freshwater environments, leaching of Ca(OH)2 will occur leading eventually to dissolution of CSH phases and disintegration of the concrete. Low S04, HC03 and co3 concentrations in the groundwater will stabilize the cement and maintain a high pH(---12.5) which will tend to reduce the mobility of many radionuclides. In saline groundwaters, the dissolution rate is lower due to the higher ionic strength of the water, particularly of Ca.
Most work has been done on the presence and effect of high-pH pore fluids in OPCtype cements in freshwater (i.e. of low salinity) environments. The influence of salinity in the plume is not well known despite the fact that several countries (Canada, Finland and Sweden) are planning to dispose of high-level radioactive waste in what is likely to be a saline environment.
Several countries currently have testing programs for low-pH, high-performance concrete, which is essentially composed of silica fume, fly ash, a superplasticizer (to allow reduction in water content yet maintain fluidity), and Portland cement. The pH range of these low-alkaline cements is generally between 10 and 11.5 although some formulations give values as low as 9.6. Additional advantages of low-pH concrete are
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low leachability, high strength (when cured), resistance to sulphate attack and low heat of hydration.
Disadvantages of low-pH concrete are a tendency to shrink during setting (and, therefore, create poor host rock-concrete seals) and the general lack of experience with its fabrication, properties, long-term chemical stability and the lack of natural analogues (the Maqarin analogue is the best at present). There appears to be no studies in progress on the influence of saline groundwater on low-pH concrete.
Several studies have shown that CSH phases and Mg(OH)2 can precipitate in the host rock due to leaching of OPC, and there is a potential for alteration of bentonite to illite or zeolites, thus reducing the sorptive capacity of the buffer. Studies of low-pH HPC, however, indicate that HPC will release much less OH and this can be buffered by the bentonite and cause no significant changes to the properties of the clay barrier.
7.3 Use of cement at Olkiluoto
It is possible that cement will be used in a number of forms in the excavation of a repository at Olkiluoto. Because the ambient water table is close to the surface, excavation will cause inflow to the facility and draw-down of the local water table. Estimation of the amount of draw-down will vary with groundwater inflow; for instance, at the URL in Canada, about 100 m of draw-down was caused by a total inflow rate of about 10 L/min. Inflows from the fractured rock at Olkiluoto will likely be greater and so cement will be needed as grouting for boreholes, permeable fracture zones, shotcrete for small tunnel-wall inflows, as seals and plugs in tunnels, and as roof supports, floors, etc. The cement formulation for each application is likely to differ because of the need for specific rheological properties, longevity, porosity, permeability, etc.
Although the amount of cement to be used will depend very much on the frequency and permeability of fractures and loose ground encountered during excavation, it is possible to estimate cement requirements. For instance, Petterson (200 1) has estimated that a spent fuel repository in Sweden might require up to 1 and 3 tonnes/m of tunnel of shotcrete and concrete, respective! y.
The use of low-pH high-performance concrete clearly appears to be preferable to the use of OPC (high-pH concrete) in most repository situations. However, the problem of shrinkage of low-pH concrete needs to be examined and resolved. Detailed testing is needed to optimize the recipe for making low-pH concrete, the mixing procedure, long-term physical and chemical stability, and the influence of saline water on its stability. If possible, to minimize the effects of concrete in a repository, the irreversible use of cement-based materials should be avoided.
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