Special speleothems in cement-grouting tunnels and ... - Springer Link

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Special speleothems in cement-grouting tunnels and their implications of the atmospheric CO2 sink Zaihua Liu 7 Dianbin He

Abstract Based on the analyses and comparisons of water chemistry, stable carbon isotopes and deposition rates of speleothems, the authors found that there are two kinds of speleothems in the tunnels at the Wujiangdu Dam site, Guizhou, China, namely the CO2-outgassing type and the CO2-absorbing type. The former is natural, as observed in general karst caves, and the product of karst processes under natural conditions. The latter, however, is special, resulting from the carbonation of a cementgrouting curtain and concrete. Due to the quick absorption of CO2 from the surrounding atmosphere, evidenced by the low CO2 content in the air and the high deposition rate of speleothems (as high as 10 cm/a) in the tunnels, the contribution of the carbonation process to the sink of CO2 in the atmosphere is important (in the order of magnitude of 10 8 tons c/a) and should be taken into consideration in the study of the global carbon cycle because of the use of cement on a worldwide scale. Key words Speleothem 7 Cement grouting 7 Atmospheric CO2 sink

Introduction The Wujiangdu Hydropower Station of Guizhou is one of the biggest hydroelectric projects built in karst areas in China. The dam is 165 m high, with a capacity of 630 000 kw. Its electric energy production is 3.34 billion kw7h/a. Since its foundation in 1979, the station has contributed much to the economic and daily life in southwest China. In the course of the station’s construction, huge curtains, located in the dam base and on both banks of the Wujiang River, were built by means of high-pressure cement grouting to prevent potential leakage (Fig. 1). Practice

shows that they are leakproof on the whole. However, in the tunnels with a concrete ceiling for curtain grouting rich speleothems formed ( 1 90% CaCO3, with very high deposition rates) that resembled soda straw (Fig. 2), stalagmite, etc. Such speleothems are widespread in tunnels with a concrete ceiling in the world (see the paper “A cave is a cave” at http://www.orbonline.net/Ftcg/acaveis.htm). The question is: Are these secondary deposits related to the aging of the grouting curtain and concrete, or are they a normal phenomenon in karst areas? Answering this question is important and helpful in predicting the trend of leakage prevention of curtains and in selecting effective measures (Liu 1996). In addition, it may provide a new field for karst and global change study.

Study methods The following methods were mainly used in this study: The compositions (d 13C) of carbon isotope of speleothems, limestone and water samples (Fig. 1) were measured in a laboratory. The CO2 content in the atmosphere was obtained by using a GASTEC CO2 meter. Temperature, conductivity, pH-value, Ca 2c, HCO3 –, CO3 2– and OH – of water were measured in situ with portable meters (Liu 1990); and the deposition rate of speleothems were obtained by a ruler in a given time period. K c, Na c, Ca 2c, Mg 2c, Cl –, HCO3 –, SO4 2–, CO3 2– and OH – of water samples were analysed in a laboratory according to standard procedures. Saturation index (SI) of minerals and partial pressure (Pco2) of CO2 in water were calculated by computer software SOLMINEQ.88 (Kharaka and others 1988). Tables 1, 2 and 3 show the major analytical results of this study.

Discussion and conclusions Received: 21 July 1997 7 Accepted: 13 January 1997 Z. Liu (Y) 7 D. He Institute of Karst Geology, 40 Qixing Road, 541004, Guilin, P.R. China Fax: c86-773-581-3708 7 e-mail: zliu6mailbox.gxnu.edu.cn

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Environmental Geology 35 (4) September 1998 7 Q Springer-Verlag

Classification of speleothems and their geochemical conditions of formation Table 1 shows that there are two kinds of speleothem in the grouting tunnels at Wujiangdu Hydropower Station.

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Fig. 1 Plane of grouting curtain for leakage control of Wujiangdu Dam, showing also some sampling points. T1 2: Triassic limestone; T1 1: Triassic shale; P2 2: Permian limestone; P2 2: Permian coal formation, 1– stratigraphy boundary; 2– groundwater contour (m) before reservoir construction. SW1, SW2 : points for surface water sampling; GW1, GW2 : points for groundwater sampling; L.W.-N, L.W.-A: points for leakage water sampling; B.W.-N, B.W.-A: points for borehole water sampling

Of the two, the deposition rate of type A is much higher than that of type N, but the former is rich in light stable isotope of carbon, with d 13C of –24.0‰ in average, and the latter is rich in heavy stable isotope of carbon, with d 13C of –11.0‰. These results possibly reflect a difference in the formation mechanism of the two kinds of speleothem. It can be seen from Table 3 that the hydrochemistry and carbon isotope composition of water, from which the two kinds of speleothem are formed, are also different. The A type of water, from which A-type speleothem formed, has the following characteristics: 1. High pH-value, usually above 12.00. 2. High K c and Na c, but K c 1 Na c. 3. Higher Ca 2c. 4. Being rich in OH – and CO3 2–, but HCO3 – vanished. 5. OH – and Ca 2c are the major anion and cation in water, respectively, and the hydrochemical type is OHCa. 6. High SIc and SIp, and SIc 1 1.0, which is the major reason why the A type of speleothem has a high deposition rate. 7. Very low Pco2, generally near 0 pa, which made the CO2 in the air (with the Pco2 of 35 pa in average) of the tunnels sink into water, and thus as low as 15 pa of Pco2 was measured in some parts of the tunnels (Fig. 2).

So, A-type speleothem can also be called the “CO2-absorbing” type. 8. The average d 13Cp–22.1‰, which is basically identical to that of A-type speleothem. These characteristics are tremendously different from those of surface water and groundwater (Table 2). This reflects that the A type of water and speleothem is not the result of natural processes. The N type of water, from which the N-type speleothem formed, has the following features:

Fig. 2 CO2-absorbing type of soda straw in cement-grouting tunnels at Wujiangdu Hydropower Station of Guizhou, China. These soda straws were formed over 12 years, and the maximun length is 126 cm. Due to the quick absorption of CO2 from the surrounding atmosphere, a CO2 content as low as 150 ppmv was measured in some places, such as in the small holes for water-leading and decompression in the figure. This low CO2 content, compared to the local atmospheric CO2 concentration (350 ppmv), indicates that there is an important CO2 sink during the formation of the special speleothems


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Table 1 Carbon isotope and deposition rate of speleothem in the tunnels at the Wujiangdu Dam site. (* A and N types of speleothem are from A and N types of water in Table 3, respectively. ** Ad, Ac, and Aw represent the speleothems in drainage ditch in the tunnels, the stone curtain and the speleothem on the wall of deposit type

stalagmite

d 13C (‰, PDB)

borehole for curtain dewatering, respectively, and they fall in type A. The table shows that the deposition rates of spleothems of type A is much high than those for type N, and the type A speleothem is rich in light carbon isotope, whereas type-N speleothem is rich in heavy carbon isotope)

soda straw

Ad**

Ac**

Aw**

A*

N*

A*

N*

P28.7

P11.1

P19.5

P10.9

P21.6

P23.9

P26.1

20.0

~ 1.0

40.0

3.5

25.0

20.0

18.0

deposition rate (mm/a)

Table 2 Characteristics of hydrochemistry and carbon isotope of surface equilibrium constant. The solution is undersaturated for SI~0, water and groundwater at the Wujiangdu Dam site. (* SI (satura- saturated for SIp0 and supersaturated for SI 1 0. SIc and SIp are tion index)plg(QIAP/K), where QIAP is ion activity product, and K the saturation indices of calcite and portlandite, respectively) water type

pH

Kc

Na c

Ca 2c

SO42P (mg/l)

HCOP 3

CO32P

OH P

surface water groundwater

8.30 7.70

1.45 1.49

4.52 2.54

60.42 117.23

67.00 101.58

159.96 272.87

0.00 0.00

0.00 0.00

water type

conductivity (ms/cm)

hydrochemical type

SIc*

SIp*

Pco2 (pa)

d 13C (‰, PDB)

surface water groundwater

353 596

HCO37SO4-Ca HCO37SO4Ca

c0.60 c0.30

P10.07 P11.11

69 981

P 8.7 P13.4

Table 3 Characteristics of hydrochemistry and carbon isotope of leakage water and borehole water in the tunnels at the Wujiangdu Dam site. [* L.W.-N: Leakage water with type-N issues from limestone fissures or from ironmade pipe through concrete wall, showing similar hydrochemical and isotopic features to normal karst water; L.W.-A or leakage water with type A is mainly related to

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concrete contact, showing different hydrochemical and isotopic features than normal kast water; ** B.W.-A: Borehole water with type A is related to cement grouting curtain; B.W.-N or borehole water with type N has no connection with the curtain according to drilling data (Fig. 1)]

water type

pH

Kc

Na c

Ca 2c

SO42P (mg/l)

HCOP 3

CO32P

OH P

L.W.-N* L.W.-A*

7.57 12.56

1.33 90.65

3.19 32.42

99.34 399.00

41.15 2.94

290.02 0.00

0.00 39.68

0.00 381.74

B.W.-N** B.W.-A**

7.17 12.02

0.91 18.26

4.82 9.57

91.17 135.32

63.69 23.52

282.46 0.00

0.00 34.72

0.00 113.19

conductivity (ms/cm)

hydrochemical type

SIc

SIp

L.W.-N L.W.-A

499 5797

HCO3-Ca OH-Ca

c0.44 c3.17

P10.82 P 1.05

7000 0

P12.7 P21.1

B.W.-N B.W.-A

506 1587

HCO3-Ca OH-Ca

c0.00 c1.77

P11.57 P 1.82

17 300 0

P11.3 P23.0


Pco2 (pa)

d 13C (‰)

water type

Cases and solutions

Gas phase

CO2(g)

Liquid phase

CO2(l) H2O

H2CO3

Gas phase

CO2(g)

Liquid phase

CO2(l)

2CO3

H2O

H2CO3

2H+

HCO3-

H+

Liquid phase 2H2O Liquid phase

2OH-

Solid phase

Ca (OH)2(s)

Ca2+

Ca2+ Solid phase CaCO3(s)

A

Fig. 3A, B Three-phase models for the formation of speleothem with A “CO2-absorbing” type and B “CO2-outgassing” type in the grouting tunnels at Wujiangdu Hydropower Station of Guizhou, China. Note that there is a CO2 sink from gas phase to liquid phase in the formation of the “CO2-absorbing” type speleothem (CaCO3), whereas there is a CO2 source from liquid phase to gas phase during the formation of the “CO2-outgassing” type speleothem (CaCO3)

1. 2. 3. 4.

Medium pH-value. Low K c and Na c, but K c~Na c. Medium Ca 2c, generally about 90 mg/l. Being rich in HCO3 –, but CO3 2– and OH – disappeared, with hydrochemical type of HCO3 –Ca. 5. Low SIc and SIp, 0.00~SIc~1.0. 6. High Pco2, usually above 1000 pa, being much higher than that in the air of the tunnels, which made CO2 escape from the water. So, the N type of speleothem belongs to the “CO2-outgassing” origin. 7. The average d 13Cp–12‰, being close to that of the N type of speleothem. In addition, from the comparison between Tables 2 and 3, one can find that N-type water is similar to groundwater, i.e. it is normal karst groundwater. So, the speleothem deposited from this kind of water is natural. To sum up, the speleothem of “CO2-absorbing” type is related to the aging of the cement grouting curtain and concrete. It is the result of a series of physicochemical processes in the artificial system, such as dissolution of Ca(OH)2 – the product of cement hydration (Papadakis and others 1992; Barnes 1983); migration of the dissolved ions; absorption of CO2 in the air; and the precipitation of CaCO3. These processes can be shown by a three-phase model (Fig. 3A). The overall reaction, generally termed as “concrete carbonation ” (Ho and Lewis 1987; Papadakis and others 1989, 1991), is as follows: Ca(OH)2(s)cCO2(g)cCaCO3(s)cH2O where (s) and (g) represent solid phase and gas phase, respectively. On the other hand, the speleothem of a “CO2-outgassing” type in the tunnels is the result of the karst process un-

2-

CO3

HCO3

CaCO3(s)

B

der natural conditions (Lorah and Herman 1988; Dreybrodt and others 1992; Liu and others 1995), in which CO2 escapes from the water, and CaCO3 precipitates (Fig. 3B). The overall reaction is: Ca 2cc2HCO3 –cCaCO3(s)cCO2(g)cH2O Some measures to prevent aging of grouting curtain and concrete Ca(OH)2 in the grouting curtain and concrete is mainly a product of the hydration of the mineral 3CaO7SiO2 in cement (Papadakis and others 1992; Barnes 1983), so the prevention of aging of the curtain and concrete, the formation of speleothem or the carbonation is to select appropriate types of cement, i.e, to select the cement with low content of 3CaO7SiO2, such as the slag cement. On the other hand, water is a carrier of material, so another method to solve the problem is to improve the quality of curtain grouting and concrete pouring, i.e. to make curtains and concrete waterproof by any means. Contribution of concrete carbonation to the atmospheric CO2 sink It can be seen that due to the quick absorption of CO2 from the surrounding air, which is evidenced by the low CO2 content in the air and the high deposition rate of speleothems in the tunnels, the contribution of concrete carbonation to the atmospheric CO2 sink may be important. This can further be visualized by the following estimation on a worldwide scale. According to Steve Szote of the National Concrete Masonry Association, about 0.2 kg of CO2 per kg of cement in concrete or concrete masonry units will be absorbed over time (“Concrete as a CO2 sink?” at http://www. ebuild.com/archives/othercopy/concreteCO2sink.html). From Mineral Commodity Summaries (“Cement” at http://minerals.er.usgs.gov/minerals/pubs/mcs/cement.txt), it is known that the annual cement production is about 1.4 billion metric tons in the world. So the potential capacity for cement to absorb CO2 from the atmosphere is about 76 million tons c/a, which is more than half of the CO2 emissions from cement production. If one takes the data from McMay (“Concrete as a CO2 sink?”, Environmental Geology 35 (4) September 1998 7 Q Springer-Verlag

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website given at beginning of paragraph), i.e. 1.4 kg of cement absorbing 0.4–0.5 kg of CO2, the potential capacity for cement to absorb CO2 from the atmosphere would be about 109–136 million tons c/a, which nearly balances the CO2 emissions from cement production, or about 10% of the so-called “missing CO2 sink” (Siegenthaler and Sarmiento 1993). Therefore, the sink of CO2 by carbonation of concrete or concrete masonry units should be taken into account in the study of global carbon cycle due to the worldwide use of cement.

References

Barnes P (1983) Structure and performance of cements. Applied Science, London Dreybrodt W, Buhmann D, Michaelis J, Usdoswski E (1992) Geologically controlled calcite precipitation by CO2 outgassing: field measurements of precipitation rates in comparison to theoretical predictions. Chem Geol 97 : 285–294 Ho DWS, Lewis RK (1987) Carbonation of concrete and its prediction. Cem Concr Res 17 : 489–504 Kharaka YK, Gunter WD, Aggarwaol PK, Perkins PH, Debraal JD (1988) SOLMINEQ.88. US Geol Surv Water-ReAcknowledgements This research was supported by National sour Invest Rep Natural Science Foundation of China, Ministry of Geology and Liu Z (1990) Necessities of measuring pH in situ in the study of Mineral Resources of China, and International Gelogical Correkarst hydrogeochemistry. Carsolog Sin 9 : 310–317 lation Program 379. Many thanks are to Prof. Daoxian Yuan Liu Z, Svensson U, Dreybrodt W, Yuan D, Buhmann D from the Institute of Karst Geology, China and Prof. W. Drey(1995) Hydrodynamic control of inorganic calcite precipitabrodt from Bremen University, Germany, for their suggestions tion in Huanglong Ravine, China: field measurements and in this study. The author is also grateful to Mrs. Liu Jiaying, Cai theoretical prediction of deposition rates. Geochim CosmoFangfu, Gong Chaoshu et al. from Wujiangdu Hydropower Stachim Acta 59 : 3087–3097 tion of Guizhou for their help. Liu Z (1996) A study on the aging of the grouting curtain at Wujiangdu hydropower station, Guizhou. Guangxi Teachers University Press, Guilin Lorah MM, Herman JS (1988) The chemical evolution of a travertine-depositing stream: geochemical processes and mass transfer reactions. Water Resour Res 24 : 1541–1552 Papadakis VG, Vayenas CG, Fardis MN (1989) A reaction engineering approach to the problem of concrete carbonation. AIChE J 35 : 1639–1650 Papadakis VG, Vayenas CG, Fardis MN (1991) Fundamental modelling and exprimental investigation of concrete carbonation. ACI Mater J 88 : 363–373 Papadakis VG, Vayenas CG, Fardis MN (1992) Hydration and carbonation of pozzolanic cements. ACI Mater J 89 : 119–130 Siegenthaler U, Sarmiento JL (1993) Atmospheric carbon dioxide and the ocean. Nature 35 : 119–125

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