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
P.O. Box 141 6 Tupper Place Pinawa, MB ROE 1LO Canada Phone 1-204-753-8879 Fax 1-204-753-2292 e-mail:
[email protected]
Margit Snellman, POSIVAOY, Toolonkatu 4, FIN -00 100 Helsinki, Finland. Fax: 358-9-2280-3719
SUBMISSION OF REPORT
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.
1
TABLE OF CONTENTS
ABSTRACT, TIIVISTELMA PREFACE 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 Structural supports
Solidification of sludges, fluids, resins; cementitious buffer and backfill Tunnel supports, shotcrete, shaft lining, rock-bolts
Seals
Boreholes, bulkheads in tunnels, shaft plugs
Fracture seals
Grout, injection
High Level
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 (C 2 S), 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 C3S, C2S, C~ and C.AF, as defined in the text (from Oscarson et al. 199 7).
ASTM type I II
m IV V
Description General purpose General purpose & moderate heat of hydration High early strength Low heat of hydration Sulphate resistant
C3S (wt.%) 45-55 40-50
c2s (wt. o/o) 20-30 25-35
CJA (wt. o/o) 8-12 5-7
C 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 Type50 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 5 S) 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).
e
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
I H
I
I H
C-C-C 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 in the pore fluids dominates and can give values as high as 13.5 to 14. 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
•1
13
t
pH KOH
12
+
NaOH
11
I
•
f
I t
• 't
t I
I
Ca(OH)a
1 f
10
CSH r : with I
I I I !
9
1.7>C/S>0.85
•I
I
tCSH
a
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 groundwater 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 35 S) was used in the preparation of a Canadian Type 50 highperformance grout. The grout was leached in a series of laboratory tests with three 35 groundwaters of different salinity and the content of 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 30-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 (
