The Maqarin (Jordan) natural analogue for 14C attenuation in cementitious barriers

Waste Management, Vol. 14, No. 5, pp. 467-477, 1994 Copyright © 1994 ElsevierScienceLtd Printed in the USA. All rights reserved 0956-053X/94 $6.00 + .00

Pergamon 0956-053X(94)00040-9

ORIGINAL CONTRIBUTION

THE MAQARIN (JORDAN) NATURAL ANALOGUE FOR 14C ATTENUATION IN CEMENTITIOUS BARRIERS lan D. Clark* Ottawa-Carleton Geoscience Centre, University of Ottawa, 161 Louis Pasteur, Ottawa, Ontario, Canada K I N 6N5

Ramesh Dayal Ontario Hydro, 800 Kipling Ave., Toronto, Ontario MSZ 5S4

Hani N. Khoury Department of Geology and Mineralogy, University of Jordan, Amman, Jordan

ABSTRACT. Carbonation reactions in portland cement grout examined in the laboratory suggest high attenuation of 14C in cementitious barriers for low- and intermediate-level radioactive waste repositories. Natural cementitious environments at two sites, Maqarin and Daba, in Jordan offer evidence that extensive carbonation can occur at field scales under both unsaturated and saturated conditions. Here, in situ spontaneous combustion of bituminous marl in the past has led to calcination and formation of calcium/silica/alumina-oxides typical of portland cement clinker. Retrograde alteration within these metamorphic zones began with hydration and precipitation of portlandite as a rock forming mineral along with ettringite, thaumasite, and other calcium-silica-hydrate-like phases. Metamorphism was a relatively recent event at the Maqarin site. Here hyperalkaline groundwater discharge from the alteration zones with two distinct geochemical facies: (a) higher TDS Ca-K-Na-OH-SOa groundwaters (pH > 12.5) apparently represent the earliest discharge following hydration, and (b) lower TDS Ca-OH groundwaters (pH 12.0 to 12.4) which appear to be later-stage leachates from the alteration zone. Subsequent carbonation has precipitated secondary calcite observed in the Eastern alteration zone. In central Jordan, travertines associated with the Daba marble record a third phase of porewater discharge where silica has been remobilized during carbonation of CSH-like phases. The unique geochemical features of the Maqarin site were examined to evaluate the validity of using it as a potential natural analogue for cement grout carbonation reactions studied under laboratory conditions.

INTRODUCTION

h o w e v e r , for its subsequent release following disposal and ease of assimilation into the biosphere has p r o m p t e d an examination of 14C mobility and fate. Releases of 14CO2 are also e x p e c t e d f r o m direct leaching of spent C A N D U fuel (3). The g e o c h e m i s t r y of inorganic c a r b o n in natural subsurface settings is well u n d e r s t o o d , although the mobility and flux of r e p o s i t o r y - s o u r c e d 14C in natural groundwaters is being examined. One of the principal p a t h w a y s identified for 14C migration is as gaseous diffusion f r o m unconfined aquifers to the a t m o s p h e r e (4). Given the long half-life of 14C and its relatively c o n s e r v a t i v e migration behaviour, it is generally believed that cementitious engineered barriers would enhance the safety of the disposal s y s t e m with respect to 14C containment. In view of these considerations, a considerable a m o u n t of w o r k has fo-

The attenuation and long-term stability of ~4C waste are i m p o r t a n t considerations for the C A N D U reactor low/intermediate w a s t e m a n a g e m e n t program. R a d i o c a r b o n p r o d u c e d in the m o d e r a t o r h e a v y water f r o m the neutron activation of 170 (n,a) is rem o v e d as c a r b o n a t e and b i c a r b o n a t e ions on mixedb e d i o n - e x c h a n g e r e s i n s (1,2). T h e p o t e n t i a l , RECEIVED 3 JANUARY 1994; ACCEPTED 30 MAY 1994. *To whom correspondence may be addressed. Acknowledgments--Tony Milodowski sub-sampled and described mineralogical specimens for calcite and hydration water analyses. Wendy Abdi and Gilles St.-Jean assisted with 2H analysis of hydration waters. Virginia Oversby and Russell Alexander are thanked for their review of this manuscript. This work has been supported by Nagra, U.K. Nirex, SKB, and Ontario Hydro as part of the Maqarin Natural Analogue Programme and by the National Science and Engineering Research Council, Ottawa (NSERC operating grant OGP0042590 to I. D. Clark).

467

468

I.D. CLARK, R. DAYAL, AND H. N. KHOURY

c u s s e d on the a s s e s s m e n t of c e m e n t - b a s e d engineered barriers to provide the desired containment for 14C, as part of Ontario Hydro's Reactor Waste Disposal Program (5). Specifically, research to date has focused on the examination of cementbased waste forms and barriers that provide highly alkaline environments conducive to immobilization of ~4C (2,5-9). While these investigations have examined radiocarbon behaviour and transport in cementitious materials at laboratory time scales, an understanding of these reactions and the mobility of 14C in geologically "aged" cements is lacking. The Maqarin site in northern Jordan together with similar but older sites in central Jordan (Fig. 1) incorporate both prograde and retrograde metamorphic zones in a host bituminous marl as a consequence of spontaneous in situ combustion. The suite of cement and cement-like minerals present in the retrograde zones are surprisingly similar to those found in portland cement grouts (10). The geochemistry of high pH groundwaters discharging at Maqarin (11) is also analogous to the pore waters typical of cement grouts. Further similarities are found in the mechanisms of carbonation and history of calcite replacing after portlandite and calciumsilica hydrate minerals (12). Here are presented the detailed mineralogy and carbonation reactions in these naturally "aged" cementitious settings as a natural analogue to the behaviour, transport, and long-term fate of ~4C in cementitious environments.

Cement Clinker, Grout, and Porewater The principal components of portland cement clinker include calcium and silica oxides with minor Al203 (13). According to the Bogue formula, these are present as calcium-silica (CS) and calciumalumina-iron (CAF) compounds as: C3S (54%), C2S

Western

"~) '11 ff

Seepage From Alteration Zone - - 5o Topographic Contour (masl)

/

\ { ("

~ 1 ~ "

~om

25o

......

~¢ ~ c ~ - ~ ; ~ o n e - ~ - ~ l ~ T "~">' ~ ' , . 31°N l I / .ICIRFIAN ,,~ I

I I

/ ..... P~

/

,..,

I

3;E

(17%), C3A (11%), and C4AF (9%). Minor oxide concentrations are present as impurities (MgO [2.7%] KzO [0.9%] and Na20 [0.15%]) or as additives to enhance the cement's setting properties (Fe203 [2.6%] and S03 [2.3%]). Hydration of these compounds produces calcium silica hydrates (CSH), typically of C3S2H3 average composition with low solubility and high durability characteristic of hardened cement grout. Hydration also generates portlandite as a reaction product, according to the following principal reactions:

2C3S + 6H20

~

C 3 3 2 H 3 q-

3Ca(OH)2 and

2C2S + 4H20 ~ C382H3 q- Ca(OH)2.

[I] [21

Complete hydration of cement clinker is not immediate as hydration products create low permeability reaction rims around oxide grains, which can persist for substantial periods of time depending on particle size (13). The consequences is then a longterm source of Ca(OH) 2 alkalinity in cementitious environments. Hydration of calcium aluminate is very rapid and normally moderated by the addition of gypsum to the clinker. Hydration of C3A in the presence of sulphate produces ettringite [Ca6(AI(OH)6)z(SO4)3" 26H20]. Present then in hydrated cement grout are CSH phases, similar to tobermorite [CasH2(Si309)2 • 4H20] and portlandite [Ca(OH)z]. Also present are ettringite and hydrogrossularite [Ca3AI206 • 6H20 ]. Water to cement clinker ratios in the order of 0.4 reflect the high degree of hydration. Final dry porosity of solid cement grout is in the order of 20% (7). The chemical composition of cement grout porewaters will be controlled largely by the solubility of portlandite and CSH phases as well as by minor hydroxide and sulphate minerals present. Initial discharges from a cementitious repository will also be characterized by high KOH and NaOH alkalinity owing to the high solubility of these hydroxides (14,15). As their oxides are only minor components of cement clinker, alkali concentrations are predicted to be substantially lower in subsequent pore volumes. Also, unlike portlandite, these hydroxides are only initially present, and not produced by continued hydration of cement. Thus, after the initial discharge, groundwaters from cement grout will be buffered near pH 12.5 by dissolution of portlandite (16).

I

FIGURE 1. The Maqarin area of northern Jordan with location of metamorphic zones and sites where hyperalkaline groundwaters discharge. Inset: location of Maqarin area and the Daba Marble zone.

Carbonation of Cement Grout In the presence of C 0 2 , portlandite is highly unstable, and in humid environments it is readily altered to calcite. CSH phases, although kinetically less re-

MAQARIN NATURAL ANALOGUE FOR 14C ATTENUATION

active, also react with C02 (17,18). The reaction involves a potentially significant uptake of C02 by reaction with the grout with mass increases of up to 15% and permeability reductions of several percent (7,9). The importance of such a mechanism for attenuation of '4C in a cementitious barrier are clear (6). The dominant reactions during carbonation of portlandite or CSH are: Ca(OH)2 + C02 ~

CaC03 + H 2 0 and

CSH + C02--~ CaC03 + Silica + H20.

[3] [4]

As pH of the pore waters drops, however, subsequent reactions dominate during the progressive carbonation of cement. Notably, ettringite reacts with CO 2 to produce gypsum and gibbsite, approximated by the following reaction: Ca6(AI(OH)6)2(S04) 3 • 26H20 + 3C02 3CaCO 3 + 3 C a S O 4 . 2 H 2 0 + 2AI(OH) 3

+ 23H20

[5]

As part of the Reactor Waste Disposal program at Ontario Hydro, a variety of laboratory tests were performed on selected grout materials to investigate specific aspects of cement grout carbonation (2,59). As a starting point, powdered grout was used to induce reaction-controlled carbonation, thereby minimizing the constraints the matrix transport parameters are likely to exert in achieving the maximum carbonation. Further tests were designed to derive information on carbonation uptake kinetics, carbonation mechanisms, total carbonation capacities, as well as on the effect of water content on the rate and degree of carbonation. In order to induce, simultaneously, radiocarbon reaction and transport processes, solid grout specimens were carbonated to investigate the effects of matrix transport parameters on the overall carbonation process. Solid grout specimens were carbonated under both batch equilibrium (diffusioncontrolled) and reactant flow-through (advectioncontrolled) conditions. The reactant flow-through test configuration for grout carbonation provides more conducive conditions for the carbonation reaction in that the reactant can be transported through the porous matrix directly the reaction sites. An added feature of the flow-through set-up is that information on progressive changes in material permeability, induced as a result of increasing carbonation, can be obtained. Grout specimens used in the laboratory study were prepared using ordinary Portland cement, quartz sand, and deionized water with a cement/

469

water ratio of 0.40 or 0.50 (5). The major findings of the cement grout carbonation research are: • Mineralogic changes associated with carbonation involve the reaction of aqueous carbon dioxide with the principal hydrated calcium-bearing compounds in grout, yielding calcite that has the potential to immobilize radiocarbon. • The rate of carbonation of powdered grout is very fast, on the order of one day under high Pco2. Depending on the initial grout formulation, the maximum carbonation capacities of the various grout mixes studied range from 15% to 19% by mass of the grout mass. • Maximum carbonation capacities were not attained for solid grout samples because the rate of carbonation was much slower and limited by diffusion of the reactant through the water saturated pore network to the reaction sites. • The geochemical modelling results indicate that the computer simulations provide a good representation of the evolution of a carbonating grout/ water system. The initially stable hydrated minerals such as brucite, portlandite, and CSH phases become unstable with progressive carbonation. Upon complete carbonation, the intermediate reaction products are transformed into stable end-products such as calcite and amorphous silica. • Both water saturated and unsaturated grout reveal a significant reduction in permeability as a result of carbonation. This effect is even more pronounced when the grout is subjected to successive carbonation cycles. • Besides a reduction in permeability caused by the clogging of pores with authigenic carbonate material, the other important carbonation-induced changes in grout properties include a progressive reduction in porosity and average pore size, and an increase in specific surface area. • For solid grout samples, it is the carbonationinduced changes in materials transport properties, together with generation of pore water, that exert a limiting effect on the rate of transport of the reactant (aqueous C02) to the reaction sites. • In a real repository, the beneficial effects of carbonation may be that decreased permeability in the barrier will ultimately result in diffusioncontrolled mass transfer, even under advective flow conditions. Reduced water flow will impede waste leaching and subsequent transport of radionuclides escaping from the waste package. • Carbonation of grout by inactive dissolved inorganic carbon in groundwater, prior to the release of '4C from the waste package, would reduce the effectiveness of the engineered barrier as a geochemical sink for '4C. In other words, the longer the time period between grout backfill emplace-

470 ment and the onset of actual waste leaching, the lower will be the efficiency of the barrier for 14C uptake by carbonation. At that stage, other processes such as isotopic exchange (between 14C and 12C) and coupled dissolution and precipitation of calcite in carbonated grout may serve to provide a certain degree of retardation of ~4C transport in the grout backfill. Laboratory examination of carbonation in cement grouts is fundamental to establishing reaction pathways and constraints; however, problems of scale occur in translating laboratory results to field dimensions and geological time periods. Notably, the use of high C02 pressures to accelerate carbonation, the periodic desiccation of the powdered grout specimens, and the short duration of carbonation experiments may limit extrapolation to field conditions. The unique geochemical features of the Maqarin site in Jordan make it a potential natural analogue for evaluating cement grout carbonation reactions relevant to ~4C attenuation in cementbased systems. This is particularly important given the strong similarities in the mineralogy and geochemistry of the laboratory and natural settings.

THE JORDANIAN NATURAL ANALOGUE Perhaps the most relevant natural analogue to examine transport of safety relevant radionuclides in a cementitious repository is found in the "marble" zones of Jordan (19). Here, naturally occurring cement minerals have been carbonated according to reaction pathways and with reaction products observed in the laboratory experiments. These unusual sites offer a unique field scale example which should indicate the limits of extrapolating from laboratory to field scale.

The "Marble" Zones of Central and Northern Jordan Included in the late Cretaceous to Tertiary stratigraphy dominating the central and northern Jordanian landscape are Maestrichtian to Paleocene bituminous marls that host some unusual " m a r b l e " metamorphic zones. The --50 m thick bituminous marl beds have seen minor deformation with tensional faulting and fracturing. They contain 15-20% organic matter, of which - 1 2 % is S03, authigenic chalcedony, and opaline silica, as well as relatively high concentrations of trace elements (notably the transition metals, Mo, Se and U) (20). The marble zones are characterized by both prograde high temperature arid retrograde low temperature facies consistent with contact metamorphism, although lacking the prerequisite contacts with ig-

I.D. CLARK, R. DAYAL, AND H. N. KHOURY neous intrusives (21) (Fig. 1). The metamorphic agent has been attributed to spontaneous in-situ combustion of the bitumen, likely sustained by O2 supplied by convection of air through fractures. The process has led to metamorphism and decarbonation of the host biomicritic marl and extensive brecciation (22-24). Similar high and low temperature mineral assemblages have been observed in stratigraphic equivalents in Israel and the West Bank (23,25). Although no direct evidence has been found for in-situ combustion today, bituminous marls are found burning ex-situ are Maqarin, where excavated material from dam construction has combusted. In the past, these marls have been used as a source of fossil fuel to power steam engines and produce bitumen for the Turkish Army. Metamorphic sites in central Jordan have been dated to early Pleistocene (26). The analogous "mottled" zones identified in Israel and the West Bank have been dated to the Late Miocene (27). The Maqarin site in northern Jordan differs from these areas in that the metamorphic zones discharge hyperalkaline groundwaters that have been highly altered by the retrograde alteration. While the presence of these solutions suggests that combustion is a more recent phenomenon, no thermal anomaly has been identified at the site, and groundwaters discharge at the ambient temperature of 24° to 26°C.

Mineralogy of the Primary and Altered Metamorphic Rocks The high temperatures attained during in-situ combustion led to the decarbonation of the marl and the formation of a variety of prograde metamorphic minerals, including spurrite [Ca5C03(Si04)2] and larnite [Ca2Si04], which dominate the prograde assemblage, plus wollastonite [CaSi03] and recrystallized calcite, and minor diopside, anorthite, fluoroapatite, graphite, Cu-sulphide, Cu-selenide, and Ca and Fe oxides (21,28). Spurrite and larnite are similar to crystalline polymorphs of the amorphous C3S and CzS phases that dominate portlandite cement clinker. Secondary mineralization occurs within the brecciated zone as vein, vug, and intergranular porosity infilling as well as alteration of the host matrix. This retrograde alteration of the high temperature assemblage is characterized by hydration, carbonation, and sulphatization reactions (Table 1). Dominant minerals in this suite include secondary calcite, thaumasite, portlandite, and CSH phases (tobermorite, afwillite, jennite and CSH-gel). These minerals also dominate the initial mineralogy of cement grout (7). Also found are a host of pure and solidsolution sulphate phases, including gypsum, bassanite, hashemite, eilestadite, barite, and celestite. Thus, the subsequent alteration reactions observed

MAQARIN NATURAL ANALOGUE FOR 14C ATTENUATION TABLE 1 C e m e n t Minerals of the Retrograde Metamorphic Zone Portlandite Thaumasite Ettringite Tobermorite Jennite Afwillite C S H Gel

Ca(OH) 2 Ca6H4(Si04)2S04(C03) 2 • 26H~0 Ca6(AI(OH)6)2(S04) 3 " 261120 CasSi6016(OH) 2 • 2-81120 Ca9HzSi6OIs(OH)8 . 6HzO Ca3Si204(01-1) 6 Amorphous

in these zones may be analogous to the carbonation and aging of cement grout. Although tobermorite, afwillite, and amorphous Ca-silicate have been recorded (21), material examined suggests that thaumasite is the most volumetrically significant Sibearing phase. Thaumasite is not a true CSH phase but rather an ettringite-structured mineral showing solid-solution between silicate and carbonate in thaumasite and sulphate in ettringite. The earliest alteration product appears to be a tobermorite-like CSH phase, which suggests that the earliest alteration process was hydration of spurrite and larnite; however, subsequent replacement by ettringite along with precipitation of BaCa-Sr sulphates signifies that sulphatization reactions are an early alteration process as well. The source of sulphur in these reactions was shown by stable isotopes to be from the bitumen itself, released during combustion as S02 and S04 z-. Values for 834S ( - 1 . 8 to -2.6%0 CDT) are consistent with Mesozoic hydrocarbon, and enriched 8180so4 (9.1 to 12.2%o SMOW) indicates oxidation with atmospheric 02 (12). Additional CSH-like phases include thaumasite and minor afwillite. Carbonate substitution in thaumasite signifies that C02 was present in the reaction zone at an early stage during alteration mineral paragenesis; however, as the most altered rocks are dominated by calcite and ettringite, carbonation is clearly a later stage process. The predominance of secondary calcite in the alteration zone shows that this has been a major alteration process and demonstrates that recarbonation can be extensive on a field scale.

eralogy shows the evolution of porewater chemistry during long-term carbonation and leaching in the underlying reaction zones.

Natural Cement Groundwaters at Maqarin The occurrence of metamorphosed marl and associated discharges of hyperalkaline groundwater at Maqarin were described only recently (10,11,21, 29). The site later gained attention as a natural analogue for safety-relevant radionuclide behaviour in the cement barrier environment of a radioactive waste repository (20). High-pH waters seep from the brecciated alteration zones within horizontal adits constructed in 1980 for dam site investigations (Adit A-6), along a rock face cut circa 1905 for the Damascus-Palestine railway, and naturally along a portion of the Yarmouk River (Fig. 1). The chemistry of these waters is characterized by high hydroxide alkalinity, saturation with calcium sulphate, and elevated concentrations of a host of minor and trace elements. Two distinctly different geochemical facies exist (Table 2). The western seeps are characterized by generally higher alkalinities, pH values up to 12.9, exceeding the portlandite buffering point near pH 12.5 (Kca(OH)2 = 10 -5.02 at 25°C; Table 3), are highly mineralized (-4000 ppm TDS) and have a host of trace elements (30). Dissolution of alkali hydroxides contribute to the high pH and elevated concentrations of K ÷ and Na +. High sulphate concentrations are maintained by dissolution of sulphate minerals, notably gypsum and barite (Table 3). The eastern seeps (Railway Cut and Adit A-6) are slightly below Ca(OH) 2 satTABLE 2 Geochemistry of Maqarin Hyperalkaline Groundwaters (mM • 1 -~) (20,34)

Parameter p/P Eh (mV)

T (°C)

NATURAL "CEMENT" GROUNDWATERS IN JORDAN

Ca 2÷ M g 2÷ Na ÷

K÷

The occurrence of these alteration mineral suites suggests that these zones must have hosted hyperalkaline groundwaters similar to those that characterize cementitious environments. At Maqarin, such groundwaters discharge today from the alteration zones. At older sites in central Jordan, Quaternary travertines capping some of the metamorphic z o n e s are e v i d e n c e for d i s c h a r g e s of hyperalkaline groundwaters in the past. Their min-

471

C032 HC03 CIS042 N O 3Si

Eastern Springs MQ-I

Western Springs MQ-6

12.34 < + 192 24.2 16.29 <0.008 1.82 0.45 0.033 b

12.9 + 127 26.3 27.94 <0.062 8.39 19.72 0.020 b

Local Nonalkaline Groundwater M6 7.22 23.7 1.88 0.22 0.54 0.09 0.00 3.44

1.77 3.04 0.03 <0.001

1.29 17.39 0.62 <0.11

0.10 0.40 -

aField m e a s u r e m e n t , pH calibrated to Ca(OH)2-saturated buffer (12.51). MQ-6 r e m e a s u r e d in 1993. bQuantitatively m e a s u r e d by acidification and CO 2 extraction under vacuum.

472

I.D. CLARK, R. DAYAL, AND H.N. KHOURY TABLE 3 Saturation Indices for Selected Secondary Minerals MQ-2 (Eastern)

logSl[ CaCO 3] logSl[Ca(OH)2] logSl[CaSO 4 • 2H20] logSI[BaS04] logSl[SrSO 4]

-

MQ-5 (Western)

0.70 0.46 0.71 0.88 0.07

0.47 - 0.04 - 0.01 0.13 - 0.09

uration (pH 12.2-12.5) and significantly less mineralized (-1500 ppm TDS). In both waters, the formation of thaumasite and other C S H phases maintains dissolved silica concentrations at less than detection. Mg is not detected because of the low solubility of Mg(OH) z (Ksp = 10 ~L0) at high pH values. Dissolved carbonate is held to <2 mg/l by calcite solubility (Table 3). The two facies are thought to represent varying degrees of dissolution in the alteration zone, where the highly mineralized, high pH western seeps are the initial pore volume following retrograde alteration, and the eastern seeps are subsequent pore volumes from a more evolved alteration zone. This is analogous to early and late stage discharge from a cementitious repository (14). Stable isotope measurements for groundwaters and minerals support this interpretation. In Fig. 2, the stable isotope signature for all high pH groundwaters is enriched above local meteoric waters which suggests modification following recharge. Minor evaporation is evident in the single nonalkaline groundwater sampled in the Maqarin area, although it falls within the range observed for local rain; however, the stronger enrichment observed in the hyperalkaline waters is attributed to mineral hydration. Most hydration waters sampled from alteration minerals have lower 2H contents (Table 4) than associated groundwaters. Accordingly, 2H exchange with mineral hydration waters is likely responsible for the complementary 2H enrichment observed in these groundwaters. Correspond-10

LMWL ~

L~

• o -20

I

~o - 3 0

•

"

D

r,, L ~ C 7,

Western Springs

•0 ,:

Eastern Springs

Mean of Local Rain

L3 [t -40'

EZ

• 8

-6

-5

-4

-3

6 ] 8 0 O/oo

F I G U R E 2. The stable isotope composition of precipitation ( ) hyperalkaline groundwaters (C)---Eastern Springs; O - - W e s t e r n Springs) and neutral pH groundwaters (B) from Maqarin. The local meteoric water line (LMWL) is defined as b2H = 6.5 8180 + 14 from precipitation at Irbid (35).

ing measurements of 180 have not yet been undertaken, although a similar 180 depletion in the mineral phase is anticipated. In highly alkaline groundwaters, strong H20-hydroxide fractionation during hydration (-40%~) may impart isotopic enrichments on 1120. Dakin et al. (31) noted an glSo enrichment of 1 to 2%0 in water during hydration of cement (pH > 10). These data suggest then that these hyperalkaline groundwaters were involved in mineral hydration. The greater enrichment observed in groundwaters from the Western alteration zone, together with the higher pH (12.9) their greater K +- and Na +-hydroxide alkalinity and high sulphate contents, suggests that the earliest pore volume following metamorphism is only now discharging.

Carbonation in the Alteration Zones at Maqarin The extensive formation of secondary calcite in the alteration zones is a clear indication that recarbonation is an important process, although the source of C02, timing, and transport mechanisms were unclear. Stable isotopes in the carbonate phases, examined by laser microsampling, shed some light on these processes (12). In this study, samples were examined from the (a) high temperature prograde metamorphic zone, (b) an early retrograde alteration, and (c) a late stage alteration zone. The evolution in both 13C and 180 show changes in both the source of C02 and temperature during carbonation (Fig. 3). High temperature, partially calcined marble, with recrystallized calcite and polygonal shrinkage cracks, shows ~3C and 180 depletions characteristic of high-temperature metamorphic carbonates. Here, high temperature decarbonation imparts an isotopic depletion for both 13C and 180 in the residual CaCO 3 and an isotopically enriched C02. Early secondary carbonates are highly depleted in t3c and 180, documenting recarbonation in an elevated temperature environment (>150°C), which diminishes 180 fractionation between meteroic waters and calcite. Later stage carbonates were precipitated in a lower temperature environment (> 100°C). The source of C02 involved in carbonation is evident from 13C. A shift to lower ~13C values signifies the participation of organic C from combusted bitumen, as a mixture with minor enriched CO 2 derived from heating calcite. Significantly, the elevated temperature of carbonation indicates that conditions in this near-surface environment were also unsaturated, which are required for C02 transport. As seen in experimental work (7), CO 2 transport is greatly diminished under saturated conditions. The following model outlines the probable sequence of reactions from initial high temperature calcination of the bituminous marls to the subsequent retrograde alteration by a CO2-rich atmo-

MAQARIN

NATURAL

ANALOGUE

F O R 14C A T T E N U A T I O N

473

TABLE 4 2H Contents of Hydration Water From Retrograde Alteration Minerals Dominant Mineral

Secondary Minerals

A 960 CBI

thaum,

A 960 CB2

thaum,

zeolite, g y p s u m calcite zeolite

A 962 CBI A 962 CB3a A 962 CB3b A 965 C B I a A 965 C B I b A 965 CB2 M 39 P A 6.3 Pa A 6.3 Pb

tob. tob. tob. jenn. jenn. ett. ett. ett. ett.

v. minor calcite none none ett., thaum, ett., thaum, none port. thaum, thaum,

Sample

tob. = Tobermorite C a s S i r O l r ( O n ) 2 • 2 ~ H z O . ett. = Ettringite C a r ( A I ( O H ) 6 ) 2 ( S 0 4 ) 3 • 2 6 H 2 0 . thaum. = Thaumasite Ca6(Si(On)4) 2 304(C03) 2 j e n n . = Jennite Ca9H2(Si6On8)2(OH) 8 . 6 H 2 0 . port. = Portlandite C a ( O H ) 2 .

•

Bitumen Ill

~o

. ji!.qb

Late (Low T) Recarbonation

-5

Marl

-10 ~"

~.

15

co °

-20

Decarbonation

Marble

25

•

Early (High Tt • ~

-30 3O

-25

20

Recarbonation -15 (~ 13 C

-10 qoo

5

O

~2H %,

early, immediately following decarbonation veinlet in unaltered host, near A 960 CB1 late stage hydration late stage hydration late stage hydration intermediate intermediate early stage early stage late stage late stage

-38

repeat -31

-44 +92 -126 -75 -36 -26 -41 -90 - 80 - 162

+78 -80 -37 -56 - 100 -86

26H20.

sphere under unsaturated conditions during cooling (Fig. 4). Combustion of bitumen in the reaction zones led to decarbonation and a prograde metamorphic assemblage dominated by isotopically depleted carbonate minerals, including spurrite, larnite, and recrystallized calcite (Bl3C ~-14%o), plus alkalioxides and other minor phases. Retrograde alteration likely began contemporaneously or shortly following combustion, with water vapour in the atmosphere circulating under unsaturated conditions. Hydration of CS-oxides led to the formation of thaumasite and other CSH-like phases, with portlandite as a hydration by-product. Clearly, heterogeneities in permeability and water content precluded complete hydration, which has allowed primary metamorphic minerals to persist up to the present. Under the unsaturated conditions prevailing in the reaction zones, strong temperature gradients

0

Sequence

5

10

PDB

F I G U R E 3. Stable isotope c o n t e n t s for carbonate sampled by laser microprobe. T h e decarbonation trend is defined by analyses of partially calcined marl found burning in a waste rock d u m p site at Maqarin. The marble is highly m e t a m o r p h o s e d marl from the reaction zone. L o w 6~80 c o n t e n t s of the early recarbonation p h a s e s indicates a higher t e m p e r a t u r e of formation in comparison with the 180 enriched carbonates of the later carbonates.

then promoted the circulation of an atmosphere rich in S02 and C02 from combusted bitumen and calcination of the marls. Contributions of atmospheric and soil C02 are also likely to have been present in this setting. Unsaturated conditions in the reaction zone facilitated transport of these gases to reaction sites, which promoted early formation of sulphate minerals and extensive subsequent carbonation of hydroxide and CSH phases. Ettringite and calcite then formed as the final products of these alteration reactions; however, as carbonation proceeded, temperatures in the alteration zones were dropping and permeability for the circulating atmosphere was becoming restricted by accumulation of reaction water and recirculation of local meteoric waters. Under conditions of complete saturation, recarbonation became severely restricted. Secondary alteration in these zones evolved to hydroxide and sulphate dissolution. These reactions characterize the hyperalkaline groundwaters observed discharging today from the metamorphic zones at Maqarin. Radiocarbon measurements were carried out on 4 samples to assess the participation of atmospheric and soil C02 in the recarbonation process. Three samples had low but significant 14C activities (3.73, 4.82 and 19.68 pmC), showing that external sources of CO2 contribute to carbonation. One value of 92.4 pmC for a matrix of portlandite, with in-grown calcite and ettringite, reflects modern (<650 a) carbonation from almost exclusively external sources (12).

Travertines in Central Jordan: Long Term Carbonation and Remobilization of Silica Continued hydration of high temperature CS-oxides or clinker can be a long-term source of CSH phases and portlandite, which sustains a high calcium hy-

474

I . D . CLARK, R. DAYAL, AND H. N. KHOURY

Combustion of Bitumen Calcination Hydration

Recarbonation

~,x,~oil CO2

8 3C 1

%0

CaCO 3

+ C

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+

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/ CSH ~

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~

CaC03 Ca2+

O:

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~15

-

-20

-25

j y / / / / _.--

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Time FIGURE 4. Conceptual model showing sequence of thermal metamorphicand retrograde alteration reactions with changes in temperature and humidityin the reaction zone at Maqarin. droxide alkalinity in groundwaters from cementitious environments. Carbonation of portlandite then acts as the dominant sink for C02; however, following complete hydration, carbonation of ettringite and the less soluble CSH minerals should then become the principal sink for C02. According to reactions (2) and (3) above, calcite, silica, and gypsum are the dominant reaction products. Under laboratory conditions, the carbonation reaction pathways involving portlandite dominates (9), probably because of its higher solubility and hence reactivity at early times. Once the availability of portlandite is exhausted, the pore water pH should then drop and C02 attenuation by reaction with ettringite and CSH phases begins to dominate. As long as Ca 2+ concentrations remain high because of gypsum solubility (after ettringite), silica will be retained by C S H phases. Thus, only when Ca 2+ is consummed by precipitation of CaC03 or less soluble sulphates (e.g. barite), will C02 attenuation be dominated by C SH phases. This late stage of carbonation is then identified by release and precipitation of amorphous silica. This sequence of reactions can be traced in fossil travertines associated with alteration zones in central Jordan. The Daba-Suwaqa marble complex outcrops on the sides and tops of hills throughout central Jordan (Fig. I). Middle Quaternary calcite travertine formations (Khan El Zabib travertines) occur on the higher (>100 m high) summits in units up to several metres in thickness. The morphology of these spring deposits suggests that they are remnants of considerably more extensive formations, reduced by regional erosion and deflation of the landscape.

A variety of textures dominate, including (a) fine horizontal laminations (< 1 m m ) o f cryptocrystalline calcite, (b) calcite molds and calcite replacement of vegetation, and (c) massive, cryptocrystalline calcite occurring in bands up to several cm thick interlayered with porous, friable calcite. Secondary amorphous silica phases, described by Khoury (32), have precipitated in primary porosity within the travertines in the lower parts of the section and within the calcite casts of vegetation observed in texture (b). Stable isotopes in the calcites are depleted from equilibrium values as a result of kinetic effects during precipitation. These characteristics are consistent with an unusual mode of formation for the travertines: C02 uptake and calcite precipitation by hyperalkaline groundwater springs (26). In the context of their geological setting, these travertines demonstrate that "clinker" within the marble zone was actively being hydrated, producing portlandite and sustaining high pH groundwater discharge. As silica has low solubility in high Ca 2+ waters, however, the opaline phases represent a subsequent geochemical facies where groundwater pH was no longer buffered by portlandite dissolution. Also, most ettringite would have reacted and its gypsum flushed from the system. Only as a late stage reaction would carbonation of CSH-like minerals have then dominated in the alteration zone, thus releasing silica. This sequence is also observed in the detailed mineralogy of the alteration zone. Secondary calcite is found in veins as pseudomorphic replacement of ettringite and spurrite and often in association with ettringite, thaumasite, and portlandite.

MAQARIN NATURAL ANALOGUE FOR 14C ATTENUATION Chalcendony is also found as a late secondary mineral in veins cross-cutting earlier calcite and replacing ettringite. Clearly, heterogeneities in this flow system have precluded complete hydration and carbonation. Three important points can be made. The thickness of the travertines suggest that hyperalkaline groundwaters discharged in this landscape for a considerable period of time. Less extensive travertines from hyperalkaline groundwaters in Oman (from serpentinization of ophiolites) are shown to have accumulated over several thousands of years (33). Further, as the Khan E1 Zabib travertines were formed by hyperalkaline groundwaters discharging from the alteration zone, saturated conditions must have prevailed. The release of silica, as a consequence of carbonation of C S H minerals, demonstrates that this phase of recarbonation also took place under saturated conditions. Finally, the persistence of these travertines since the time of these reactions over the past - 1 ma, suggests that calcite is a very stable host for 14C in a high pH environment.

CONCLUSIONS Recent studies in the laboratory have shown that the carbonation capacity of portland cement grout is enormous and offers a potentially effective geochemical sink for the attenuation of ~4C in a waste repository setting; however, whether this capacity can be fully realized at field scales has not been established. The Jordanian sites together offer a natural analogue demonstrating hyperalkaline discharge and carbonation on a large scale and over a long time period. As a natural equivalent to a cementitious environment, its mineralogy is not an exact duplication of portland clinker or grout. Nor does it show quantitative attenuation of the C02 source term. Nonetheless, it is a surprisingly accurate analogue to study cement carbonation reactions on large physical and temporal scales. Formation of CS- and CA-oxides similar to cement clinker took place during in situ combustion of bituminous marl at Maqarin as well as sites in central Jordan. Hydration reactions following combustion are evident at all sites where secondary alteration minerals, including portlandite, ettringite, thaumasite, and other CHS-like minerals are found. The three principal alteration zones in Jordan are analogous to early, later stage and long-term reactions (cement hydration, carbonation and leaching) and discharge from a cementitious barrier: 1. The Western seeps at Maqarin are analogous to the earliest phase of hydration in cement. Here, pore waters are mineralized with high solubility

475

h y d r a t i o n p r o d u c t s i ncl udi ng K- and N a hydroxides and portlandite, which maintain the pH near 12.9. These groundwaters are essentially the first pore volume to discharge from the alteration zone. . Hyperalkaline groundwaters from the Eastern zone at Maqarin are analogous to later stage discharges evolving from hydration with portlandite buffering to carbonation. The pH in these waters is lower (12.0-12.4), and mineralization is considerably less than the Western seeps, signifying discharge of subsequent pore volumes. Secondary calcite in this alteration zone has formed as a result of carbonation under initially non-saturated conditions. . The final phase in the evolution of groundwaters discharging from these alteration zones is observed in the Daba marble and associated travertines of central Jordan. No hyperalkaline groundwaters exist in this Mid-Pleistocene system; however, the calcite travertines signify extensive discharge of hyperalkaline groundwaters at a time when hydration reactions provided a source of portlandite in the subsurface, such as found today at Maqarin. Alteration reactions then evolved towards carbonation of CSH-like phases in the alteration zone. Secondary chalcedony in the travertines records this final phase of silica remobilized by alkaline groundwaters (pH > 10) in which Ca 2+ was now controlled by calcite precipitation rather than by portlandire dissolution. The sequence of geochemical reactions and associated mineral phases observed in the alteration zones at Maqarin and in central Jordan present an interesting natural analogue to similar reactions observed for 14C behaviour in cementitious environments, based on laboratory work and predicted by geochemical modelling. The extent of carbonation and the persistence of hyperalkaline groundwater discharge provides encouraging field evidence for the attenuation of 14CO2 in such environments; however, the Jordanian natural analogue should be used with caution. Carbonation here has not been shown to be quantitative, nor has long-term 14C isolation been addressed. Further, primary oxides and unstable hydration minerals like portlandite can coexist because of heterogenetities in these brecciated alteration zones. Although the cement carbonation processes observed in the field are similar to those studied in the laboratory, specific field data are needed to establish more closely the analogy between the laboratory and field systems with respect to cement carbonation reactions, which are directly relevant to the assessment of ~4C attenuation in cementitious,

476

engineered barrier in low/intermediate level waste repositories. Specific focus in the following areas, comparing the Western and Eastern alteration zone with laboratory results, is required: • Degree of portlandite carbonation and carbonation rates and mechanisms. • Sequences of carbonation of reactive phases (portlandite, ettringite, tobermorite, and other CSH phases) and subsequent silica gel precipitation. • Evidence for effect of carbonation on porosity and permeability and associated changes in mass transport properties. • Field evidence for stability of secondary calcite under hyperalkaline conditions. Since calcite will be the host phase for '4C, it is important to establish its long-term stability in a cementdominated environment.

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