Leaching behavior of carbonate bearing backfill material – An experimental and modelling approach

Construction and Building Materials 223 (2019) 254–264

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Leaching behavior of carbonate bearing backfill material – An experimental and modelling approach Rita Fuchs a,b, Florian Mittermayr c,⇑, Claudia Baldermann c, Stephan J. Köhler d, Albrecht Leis e, Hanns Wagner f, Martin Dietzel a a

Institute of Applied Geosciences, Graz University of Technology, Rechbauerstraße 12, 8010 Graz, Austria Institute of Earth Sciences, The Hebrew University of Jerusalem, Campus Edmond J. Safra, Givat Ram, 91904 Jerusalem, Israel Institute of Technology and Testing of Building Materials, Graz University of Technology, Inffeldgasse 24, 8010 Graz, Austria d Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Lennart Hjelms väg 9, 75007 Uppsala, Sweden e JR-AquaConSol GmbH, Steyrergasse 21, 8010 Graz, Austria f ÖBB-Infrastruktur AG, Europaplatz 2/I, 8020 Graz, Austria b c

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Advanced flow through experimental Pea gravel dissolution experiments

peristaltic pump

Backfill: Pea gravel

three-way-valve sample: out

output solution magnetic stirrer

i n f o

Article history: Received 11 December 2018 Received in revised form 18 June 2019 Accepted 20 June 2019

Keywords: TBM Pea-gravel Calcite Backfilling Durability Tunnel-stability

10000 1000

.5 :0

Flo

l/s

l/s

w ra

-1.05

-0.75

-0.45

-0.15

SI calcite (initial)

a b s t r a c t Carbon dioxide is known as an important agent in aqueous media to induce chemical attack on building materials. The leaching behavior of pea gravels, which are used as backfill material in continuous tunneling, is not entirely resolved until now. Evaluating the durability and economical advantage of individual backfill materials requires a proper experimental design to survey the dissolution reactions and to develop a useful modelling approach to calculate dissolution for carbonate bearing pea gravels. In this study a combined flow through reactor unit was developed, where conditions for chemical attack on gravel material can be simulated by changing flow rate and/or CO2 partial pressure. The addition of CO2(gas) was adjusted by pHstat conditions. Solution chemistry was monitored in-situ and by analyzing samples throughout experimental runs. For 5 natural gravels with different calcite(dolomite)/quartz ratios, the dissolution rates of Ca2+ for carbonates (RCa_cc; normalized on exposed carbonate surfaces) were found to reflect mineralogy, carbonate content, flow rates and thus saturation state conditions. RCa_dol values for dolomite are significantly lower (1012 < RCa_dol < 1013 mol cm2 s1) compared to calcite (109 < RCa_cc < 1012 mol cm2 s1). The experimentally obtained RCa values from the pea gravels and literature data were used to develop a model that estimates the durability of pea gravel and Ca2+ transfer to a drainage system for potential carbonate scaling at various environmental conditions. Our contribution highlights that the application of limestone and dolostone as backfill material can only be reliably assessed by considering the local hydrological and hydrochemical conditions. Ó 2019 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. E-mail address: [email protected] (F. Mittermayr). https://doi.org/10.1016/j.conbuildmat.2019.06.168 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

rate

8 te:

-1.35

magnetic stirrer

w Flo

100

SpC

years (for 100% calcite dissolution)

input solution SpC pH

10

CO2-controlled pHstat=4.5

1m

a r t i c l e

Pea gravel dissolution modelling

Groundwater

CO2 tank

set up for leaching behavior from pea gravel.
Monitoring of reaction mechanisms and kinetics for carbonate mineral dissolution.
Modelling and assessment of gravel durability for construction purpose.

R. Fuchs et al. / Construction and Building Materials 223 (2019) 254–264

255

tunnel constructions using TBM, distinct dissolution behavior and durability of individual backfill materials are not well established.

1. Introduction 1.1. Tunneling and backfill material In general, two methods are frequently used for construction of tunnel buildings: (i) conventional tunneling by sequential drilling and blasting, known as the New Austrian Tunneling Method (NATM) and (ii) continuous advance using a tunnel boring machine (TBM). In NATM the building process can be readily adjusted to the different conditions within the locally occurring rock masses as the tunnel walls are secured immediately after excavation by shotcrete and other temporary support like anchors. Using TBM requires less manpower and in favorable rock masses the excavation process is faster (>20 m/d) compared to NATM, but TBM presumes rather constant and inherent construction conditions [1]. A schematic view of the Koralmtunnel cross section constructed by using TBM is shown in Fig. 1 [2–4]. The lining is made of reinforced pre-cast concrete segments (tubbing), which are set into place by TBM during the building process [1]. The annular gap between the rock mass and tubbing lining are filled with pea gravels [5], in Austria according to the guideline concrete segmental lining systems [6]. According to this guideline pea gravel is defined as a single-size gravel that is washed, well rounded and has grain diameters between 4 and 16 mm. Concerning the grain size, the guideline clearly recommends to use material in the fraction 8– 11 mm. Minimum requirements according to ÖNORM EN 12620 are e.g. the percentage of oversized grain to the next larger screen has to be smaller than 10, the percentage of undersized grain to the next smaller screen has to be smaller than 10 and 95% has to be rounded grain. In tunnel applications the pea gravel material will interact with locally occurring ground (and/or surface) water. The percolating solutions may leach in particular carbonates from the applied backfill material and transfer the dissolved calcium ions (Ca2+) into the main drainage of the tunnel (see Fig. 1). This leaching of Ca2+ from the backfill material may cause durability issues of the used building material and carbonate scaling in the drainage. Although large amounts of backfill materials are required for

1.2. Tunnel sustainability related to backfill leaching and scaling The sustainability of a tunnel building is related to the composition, property and durability of the materials used for construction. Thus, in case of TBM the leaching of backfill material between the rock mass and the tubbing lining should be minimized. Limestone and dolostone or other carbonate containing rocks can be intensively leached by dissolution of carbonate minerals, in particular by acidic solutions and/or soft waters, at rather high dissolution rates compared to siliceous solids [7–11]. This may cause destabilization within the annular gap which has to be treated for example with injections. Moreover it can affect longtime stability of the tunnel structure. Additionally any Ca2+ ions from e.g. the surrounding rock or the drainage system act as a source for carbonate sinter formation in combination with increased pH from the concrete lining (scaling, e.g. Rinder, Dietzel [12]). For both reasons in the guideline ‘concrete segmental lining systems’ [6] the CaCO3 content of pea gravels is limited to 10 wt% for backfill purpose. However, calcite/dolomite rich gravel deposits above this limit may be highly abundant at certain building sites and in their vicinity. Thus, the use of such carbonate containing material as a backfill material can have a large economic and ecological advantage, versus providing material within the carbonate concentration limit from distant deposits, if its non-critical durability and scaling behavior is verified. These pre-requirements are recommended to be verified by reliable data and predictions from experiments and modelling approaches, respectively. Thus, the present study focuses on developing an experimental approach to survey on-line the dissolution reactions of pea gravel as typically applied backfill material. The obtained dissolution rates of Ca2+ for carbonates from pea gravels are discussed for assessing material durability and scaling issues for three different naturally occurring water types.

Groundwater

Excavation line

Pre-cast concrete segmental lining (tubbing) Gravel backfill (annular gap)

Innerlining abutment (reinforced) Waterproof concrete slab Drainage drilling 1m

Pre-cast invert segment Tunnel main drainage

Fig. 1. Schematic cross section of the Koralmtunnel constructed by using TBM. Pea gravel is blown in the annular gap to fill the cavities between genuine rock mass and tubbing lining. Carbonates can be dissolved from the gravel backfill material by percolating ground and/or surface water. These solutions contain dissolved compounds such as Ca2+ from gravel leaching and are partly discharging into the main drainage of the tunnel.

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1.3. Reaction mechanisms and kinetics of carbonate dissolution and formation The dissolution of carbonates from gravel and potentially induced clogging of drainage systems by the neoformation of carbonates are both caused by the ability of a given solution to either dissolve or precipitate e.g. the most abundant carbonate mineral in natural and man-made surroundings calcite (CaCO3). The ability of a solution to precipitate or dissolve calcite is defined by the term



 h i h i IAP ¼ Ca2þ  CO2 ¼ cCa  Ca2þ  cCO3  CO2 3 3

ð1Þ

where IAP is the ion activity (rounded brackets) product, [Ca2+] and 2+ [CO2 and CO2 ions, 3 ] are the concentrations of aqueous Ca 3 respectively, and cCa and cCO3 denote the activity coefficients of these two ions. The saturation state

X ¼ IAP=KC

ð2Þ

is calculated by dividing the IAP by KC, the thermodynamic solubility constant of calcite. If O = 1 the solution is in thermodynamic equilibrium. If not, the solution may either dissolve (O < 1) or precipitate calcite (O > 1). The actual saturation state of calcite is influenced by e.g. temperature and ionic strength, aquocomplex formation, pH in respect to dissolved inorganic carbon (DIC) species distribution and total concentration of calcium and DIC. In most ground waters the saturation state and dissolution kinetics strongly depend on CO2 partial pressure [13,14]. Analogous equations are valid for dolomite (CaMg(CO3)2). If building materials are subjected to leaching, the durability of the building material depends on the specific dissolution rate of e.g. Ca2+ for calcite, dolomite or other minerals. For instance the dissolution of calcite in the absence of any reaction inhibiting or promoting agent is governed by three reaction mechanisms:

CaCO3 þ Hþ ¼ Ca2þ þ HCO3

ð3Þ

CaCO3 þ H2 CO3 ¼ Ca2þ þ 2HCO3

ð4Þ

CaCO3 þ H2 O ¼ Ca2þ þ HCO3 þ OH

ð5Þ

and the dissolution rate of calcite for Ca2+ is defined as


     RCa ¼ k1  Hþ þ k2  HCO3  þ k3  ðH2 OÞ  k4  Ca2þ  ðHCO3 Þ ð6Þ where the first three terms describe the forward reactions and the last term describes the backward reaction [9]. k1 to k4 are rate

constants. The dissolution behavior of calcite (and other carbonate minerals) is pH dependent: At low pH (about <3.5) RCa value depends mainly on the first term of Eq. (6) that denotes transport of H+ to the solid surface. At 4.5 < pH < 3.5 RCa depends on all three forward reactions. In neutral to alkaline environments saturation with respect to calcite is approaching, where individual O values depend on the water composition, and the backward term becomes important [9,14].

2. Methods 2.1. Experimental setup The developed testing device is based on two reaction chambers made of two 6L acrylic glass vessels and caps (Fig. 2). Both caps are identical and show seven openings: Three openings are designed for the installation of probes. Four openings allow the exchange of solution or gas e.g. input of CO2, primary solution, input solution from chamber I or output solution from chamber II. Both chambers are sealed against the outer atmosphere by a gasket between cap and vessel. Chamber I was used to prepare the input solution for chamber II. For that a temperature probe, a probe for electric conductivity and a pH probe were installed. Specific conductivity (SpC) was measured to survey the input solution. The pH probe was used to set pH to a constant value. In the present case pH 4.5 was adjusted. The experiments were carried out at constant pH (pHstat) considering the input solution for chamber II by bubbling of CO2 gas into chamber I. As soon as a pH of 4.50 was achieved the CO2-flow was cut off automatically by a magnetic valve. As pH rose to a limit of 4.55 the magnetic valve opened again to allow inflow of CO2. The control was done using a commercial aquasystem unit (Profilux 3.1). The pHstat 4.5 solution was pumped by a peristaltic pump (Ismatec ISM834C) using flexible tubes (PharMed B.P.T., color code yellow blue, ID 1.52 mm) at flow rates of 9.1, 3.8 and 1.9 ± 0.1 L d1 into chamber II. In chamber II the chemical reaction between the input solution from chamber I and the solid under consideration (in this case gravel samples with differing calcite content) takes place. Therefore, a meshed PP cylinder (open spaces 5  2 mm in diameter) was designed to be filled with the gravel and placed in 5L of distilled water. The pH, SpC and the temperature of the solution were monitored continuously. As a function of the experimental reaction time the gravel reacts in chamber II with the continuously provided input solution from chamber I. The reacted solution in chamber II is provided at the identical pump rate as it is removed for

Fig. 2. Schematic view of the experimental setup for leaching of gravels by CO2 loaded water.

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R. Fuchs et al. / Construction and Building Materials 223 (2019) 254–264

sampling (output solution). The reacted solution is sampled on a regular basis and the efflux is collected in a canister that is sealed against evaporation (Fig. 2). 2.2. Analytical procedures The mineralogical composition of the gravels was analyzed by X-ray diffraction (XRD) using a PANalytical X’Pert Pro X-ray diffractometer at a voltage of 40 kV and a current of 40 mA. Semiquantitative evaluation was done using software HighScorePlus 3.0d and database PDF2. The chemical analysis of the gravels was done quantitatively by X-ray fluorescence (XRF) using a PW2404 X-Ray spectrometer at wavelength dispersive measuring mode and subsequently values in wt% were obtained by the Super Q and ProTrace from PANalytical. A TIC analyzer (Shimadzu Solid Sample Module SSM-5000A) was used to measure the total inorganic carbon content. The average difference between the two replicate measurements was 0.04 wt%. The gravel samples were washed with distilled water before being subjected to the experimental solution. All experimental solutions were characterized by in-situ measurement of temperature, pH and specific conductivity (SpC). The pH was analyzed with two GHL pH probes for Profilux and a Profilux 3.1 aquasystem with an analytical error of ±0.03. SpC was measured with two WTW TetraCon 325 and two Schott handylab LF 12 data collectors. Dissolved ions were analyzed by ion chromatography (IC) and inductively coupled plasma optical emission spectroscopy (ICP-OES). IC analysis was carried out with a Dionex ICS-3000. Cations were analyzed using columns CG16 and subsequently CSS16. As an eluent 36 mM methane sulfonic acid were used. Suppressor was a cation self-regenerating suppressor and injection volume 25 ml. Anions were analyzed by IonPac AG19 and AS19 columns. Error of measurements was <5% for Na+, K+, Ca2+, Mg2+, and Cl. Analyses of total dissolved Al and SiO2 was carried out by PerkinElmer Optima 8300 Optical Emission spectrometer with an analytical error <10% in respect to standard NIST 1640a. 2.3. Hydrochemical modelling of experimental solutions The hydrochemical modelling of the experimental solutions was conducted using the PhreeqC program code [15] and the database phreeqc.dat. Modelled parameters for all solutions are activity coefficients, ion activities, aquocomplex concentrations, saturation indices (SI) in respect to calcite and dolomite, and the internal partial pressure pCO2 in atm. Alkalinity was modelled by ion charge balance and checked by comparing calculated to measured SpC to be 10 mS cm1.

Flow rates in the tunnel were scaled into reactor flow rates by comparing the pore water per meter tunnel section with the amount of water percolating through the tunnel. A given flow rate at the outlet of a tunnel section (i.e. 10 L s1) can be transformed in amount of water entering a tunnel section of 1 m length that is percolating through 32 m2 representing the area of the tunnel contact area with the pea gravel filling given an inner diameter 10 m. The above flow rate results in a water flux of roughly 350 L of water per meter tunnel section thus requiring two days for the pore water to be exchanged completely. This flow rate can then be directly translated into an exchange rate per unit of time that can be entered into the PHREEC code considering relationship (6) for reaction kinetics. Again, the calculated flow rates are close to the flow rates that were employed in the experiments (between 0.3 and 1.8 pore volumes per day). Steady state calcium release was calculated for three selected naturally occurring groundwater examples (see chemical composition in Table 4) as migrating pore water. After correcting for the inflowing calcium concentrations a net release of calcium per time unit (e.g. g day1) can be estimated. Based on the amount of calcite present in the scaled reactor (8 kg) an age for complete dissolution can be obtained at a given flow. For solution 2 (‘‘silicate”) an uncertainty analysis was undertaken, where the chemical composition of the inlet solution was varied systematically by changing buffer capacity to the pore water via removing or adding of Ca(OH)2 to test how small changes in calculated saturation index would affect the calculated age of the tunnel section. This removal or addition of buffer capacity resulted in a new equilibrium pH and new saturation index. Ten different inlet solution compositions at flow rates from 0.5 to 25 L s1 were tested in this way. This allowed to plot the estimated durability of the calcite in pea gravel as a function of calcite saturation index.

3. Results 3.1. Mineralogical and chemical composition of the gravels Five gravel samples were used for analysis and subsequent testing. The used material is composed of a mixture of dolomite and/or calcite bearing gravel and gravel composed of siliceous rock fragments. The mineralogical compositions of all samples and respective grain size distributions are given in Table 1. The five different gravel samples had a mineralogical composition mostly comprising of quartz (qtz) and feldspar (fsp) at different calcite (cc) levels. About 24 wt% of dolomite (dol) was present only in sample G47%cc. No carbonate was detected in sample G0%cc, which acts as a siliceous reference material. Small amounts of hornblende (hbl), mica (m) or chlorite (chl) were analyzed in the samples

2.4. Hydrochemical modelling of a tunnel The following assumptions were made for scaling the tunnel section down to a simple flow through reactor. According to Fig. 1 a 10 m inner diameter tunnel section was assumed to be covered with 10 cm pea gravel with a porosity of 25 vol%. Per meter tunnel section this translates into 3.17 m3 of tunnel filling material of which around 0.8 m3 are water filled pores. Given a specific surface area of 1.9 cm2 g1 and a density of about 2.7 g cm3 of the solid pea gravel, a total reactive surface area of ca 12.106 cm2 are in contact with 800 L pore water. Given the total pea gravel weight of 6 tons in that section around 8.1 kg of calcite are present per L of pore water. This ratio (8.1 kg L1) is on the same order of magnitude as the one used during the lab experiments (1 kg L1). In a similar way the exposed reactive surface area in the scaled model (15000 cm2 L1) are almost within a log unit of those used in the flow through experiments (600–1200 cm2 L1).

Table 1 Mineralogical composition of the gravels. The calcite and dolomite content was calculated by measured inorganic carbon and Mg concentration of the samples (see text for details). Sieve sizes in [mm]. Calculated total surface area of the gravels under the assumption that grains are fully rounded [m2/kg]. mineral

G0%cc

G2%cc

G5%cc

G9%cc

G47%cc

quartz plagioclase calcite dolomite K-feldspar chlorite muscovite hornblende

+++ +(+) 0 – + + + +

+++ + 2.0 + – – +

+++ + 4.5 – + + – –

+++ +(+) 9.3 – + + + +

+++ + 47 24 – – – –

sieve size surface area

4/8 0.4

4/8 0.4

8/12.5 0.2

8/16 0.2

8/16 0.2

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G0%cc, G2%cc, G5%cc and G9%cc. From the MgO concentration of sample G47%cc the respective dolomite content was calculated as G47%cc did not contain any other magnesium bearing minerals. The carbon incorporated in dolomite was subtracted from carbon concentrations obtained by TIC analyses. The remaining carbon was used to calculate the calcite content of G47%cc (47 wt% of cc). In case of samples. G0%cc – G9%cc the inorganic carbon concentrations were directly used to calculate the respective calcite content. Gravels G0%cc – G47%cc were named according to their carbonate content. For clarity reasons G47%cc is also referred to as G47%cc+24%do or G24%do when dolomite dissolution is discussed (Table 1) 3.2. Leaching behavior In the following the experimental approach of leaching sample G9%cc is exemplarily presented (Fig. 3). For a given flow rate the gravel material was leached until a steady state with respect to the chemical composition of the reacting solution (output solution from chamber II) was reached (see Fig. 2). Thus steady state means

conditions, where liberation of components by dissolution of the solids and inflow of CO2 rich solution from chamber I resulted in constant composition of the solution in chamber II. The overall content of dissolved ionic components was measured in-situ by SpC. This parameter was used to monitor steady state conditions and – besides pH – on-going leaching during the experiment. The leaching experiment of sample G9%cc lasted about five days to achieve steady state conditions at 9.1 L d1. Subsequently the flow rate was changed from 9.1 to 3.8 L d1. The second leaching period was monitored for seven days until it reached steady state conditions. As soon as the final steady state at a flow rate of 1.9 L d1 was reached, the experiment was stopped, and solid and solution were separated for analyses. The measured values indicated that in all experiments both pH and SpC were constant in chamber I (solvent chamber = input solution from chamber II), as intended. The SpC in chamber II (reaction chamber) followed the calcium concentration trend measured in the output solution from chamber II as calcium was the most abundant ion in the measured solution (Fig. 3C; Table 2). The SiO2 concentrations displayed a similar trend, but less pronounced due to the silica

Fig. 3. Chemical evolution of the reacting solution at different flow rates for a typical gravel experiment (G9%cc). A: Electric conductivity. B: pH. C: Calcium concentration. D: Silica concentration. Black arrow: steady state condition. Dashed line: change of flow rate.

Table 2 Element concentrations at steady state conditions. Concentrations are given in mg L1. Flow rates (f) are given in L d1. SpC denotes the specific conductivity in mS cm1. sample

f

Na

K

Ca

Mg

SiO2

Al

HCO3

Cl

SpC

G0%cc G2%cc G2%cc G2%cc G5%cc G5%cc G5%cc G9%cc G9%cc G9%cc G47%cc+24%do G47%cc+24%do G47%cc+24%do

9.14 9.14 3.76 1.86 9.14 3.76 1.86 9.14 3.76 1.86 9.14 3.76 1.86

0.09 0.05 0.06 0.25 0.03 0.02 0.03 0.19 0.10 0.08 0.08 0.03 0.03

0.34 0.20 0.35 0.52 0.18 0.36 0.54 0.35 0.49 0.81 0.23 0.37 0.69

0.50 4.52 6.03 7.00 5.78 7.70 8.78 8.28 11.2 14.1 11.8 14.6 13.6

0.07 0.16 0.17 0.19 0.10 0.11 0.12 0.36 0.31 0.29 0.58 0.70 0.77

0.4 0.3 0.4 0.5 0.2 0.2 0.3 0.4 0.4 0.4 0.1 0.1 0.2

b.d.L. b.d.L. 0.02 0.03 b.d.L. b.d.L. b.d.L. 0.02 0.03 0.03 0.03 0.03 0.04

4.1 15 19 22 18 23 27 28 36 44 39 44 46

0.17 0.17 0.32 0.51 0.17 0.34 0.50 0.17 0.39 0.68 0.19 0.39 0.68

6 25 32 39 30 40 46 51 67 84 67 76 78

b.d.L.: below detection limit.

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concentrations being close to the detection limit (0.5 mg L1, Fig. 3D). In all experiments the concentrations of dissolved components were <1 mg L1 except the most abundant ions Ca2+ and HCO 3 (see values in Table 2). Fig. 4 shows a comparison of the Ca2+ concentrations measured in the output solution during leaching of the gravel material at the different flow rates (see individual values at steady state in Table 2). Ca2+ concentrations at steady state increased from 0.94, 4.51, 5.78, 8.27 to 11.8 mg L1 with increasing order of calcite content of the gravel samples (0, 2, 5, 9 and 47 wt% of cc) at a given flow rate of 9.1 L d1 (Fig. 4A). Liberation of Ca2+ ions during the leaching of sample G0%cc was most probably derived from the dissolution of feldspar and/or hornblende as G0%cc did not contain calcite. This assumption was supported by the comparatively high silica concentration during leaching of sample G0%cc. The concentrations of dissolved silica decreased in the order of the experiments using gravels G0%cc, G9%cc, G5%cc, G2%cc and G47%cc (Table 2). The pH did not strictly follow the calcium concentrations. The pH values ranged from 5.8 to 5.3 for experiment G0%cc (no calcite present), from 6.7 to 7.2 for G5%cc and G9%cc and from 7.7 to 8 during leaching of gravel samples G2%cc and G47%cc. After changing the flow rate from 9.1 to 3.8 L d1 the calcium concentration in the experimental solution increased towards reaching a new steady state. The experiments leaching samples G9%cc and G47%cc additionally showed a second time dependent leaching curve with an initial increase in calcium concentration and a slight decrease towards the new steady state (Fig. 4B). After steady state condition was reached, the experiments using samples G2%cc, G5%cc, G9%cc and G47%cc were continued at a slower flow rate of 1.9 L d1. Accordingly, the calcium concentration increased in the output solution of the reaction chamber II (Fig. 4C).

During the leaching of sample G47%cc+24%do (representing a carbonate based molar Mg/Ca ratio of 0.22 in the solid) the molar Mg/Ca ratio of the solution at steady state conditions is about 0.085. Indicating that dolomite was less dissolved than calcite. 4. Discussion 4.1. Dissolution mechanisms and kinetics of the gravel material The dissolution of Ca2+ from pea gravels were studied at conditions where the experimental solution was permanently renewed at a given flow rate. The constant concentrations of dissolved Ca2+ ions at steady state conditions can be used to calculate dissolution rates of calcium ions from the gravels according to the equation

RCa

gr

¼ ½Ca out  f  m1

ð7Þ 1

[16], where [Ca]out is the calcium concentration (mmol L ) of the output solution of reaction chamber II at steady state condition, f the flow rate (in L d1), and m (in kg) is the amount of the used gravel. The RCa_gr value is given in mmol kg1 d1. The RCa_gr changes as a function of flow rate (Fig. 5) and represents mostly dissolution rates for calcite (or carbonates for G47%cc+24%do) except for G0%cc (dissolution of silicates). The RCa_gr values were found to be influenced by both the amount of calcite (or carbonate for G47%cc+24%do) in the solid and the flow rate. The lower the flow rate, the less is the influence of chemical composition of the gravels on the RCa_gr rates (Fig. 5). The dissolution rate of Ca2+ for calcite (or dolomite) from the gravel can be normalized to the estimated surface area of the respective pea gravel by the expression

Fig. 4. Evolution of the calcium concentration in the output solution as function of experimental time. A: Flow rate (f) is 9.1 L d1 for all experiments. B: f = 3.8 L d 1. C: f = 1.9 L d1. If only a single flow rate is given, the experimental time is normalized to the respective change of flow rate (A – D). D: Calcium and magnesium concentration during testing of gravel G47%cc+24%do. (values are given mmol L1 to display the stoichiometry of carbonate dissolution).

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main source of Ca2+ is verified to be calcite. Interestingly this behavior is also valid for G47%cc+24%do, which indicates less impact of dolomite vs. calcite dissolution on Ca leaching (Mg/Ca solid: 0.22; solution: 0.085). In the case of G47%cc+24%do the dissolution rates of Ca2+ for dolomite from the gravel, RMg_dol (analogous to equations above), were found to be about three times lower than the Ca2+ dissolution rates from calcite (Table 3). Leaching of gravel G0%cc results in a molar Ca:Si ratio in the solution of 1.8, where the calcium is assumed to originate from the dissolution of plagioclase and hornblende (see Table 2). However aluminum concentrations are below detection limit due to incongruent dissolution behavior of alumosilicates at near neutral pH conditions. 4.2. Durability of calcite and dolomite in the gravel material

Fig. 5. Dissolution rates of Ca2+ from the gravels calculated from calcium concentrations measured in the output solution from chamber II at steady state conditions as a function of flow rate (f).

RCa

gr

¼ ½Ca out  f  S1 tot

ð8Þ

by using the surface area of the used gravels (Stot; Table 2), which is estimated by grain size distribution and amount of material used for leaching. RCa_gr was then normalized to the specific calcite content of the samples to yield RCa_cc (RCa_cc = [Ca].outf.S1 cc ; Scc: surface of calcite obtained by multiplying Stot and the proportion of calcite on total solid (Table 1)). As the silica concentration indicates insignificant dissolution of Ca bearing silicates and as no other calcium source than calcite is available, nearly all dissolved calcium ions are reasonably assumed to be originated from the dissolution of calcite from gravels G2%cc to G9%cc. Since distilled water was used for the experiments, all initial carbonate found in the experiments were provided by the bubbling of CO2 during solution preparation in the solution chamber I. Uptake of CO2 results in H2CO3 formation. If calcite is dissolved by the attack of carbonic acid it reacts according to the overall reaction (4) (HCO is the dominating DIC species at 3 10.3 > pH > 6.4). Ca2+ vs. HCO 3 concentrations of the solution during leaching of gravels G2%cc, G5%cc and G9%cc accordingly results in a regression line with a slope close to 0.5 (Table 2; Fig. 6). Thus, the

The herein applied chemical conditions (pH 4.5 of the input solution) were quite aggressive in respect to common tunneling surroundings. However, using this approach the investigation of gravel leaching behavior were possible in a reasonable experimental time span. At the given experimental conditions the amount of dissolved calcite from one ton of the studied gravels over the period of a year was calculated from the obtained dissolution rates considering steady state conditions at different flow rates for dissolution (representing different saturation states; Fig. 7). The relationship of the dissolution of calcite or dolomite (from the gravel) between solution chemistry (pH, PCO2) and flow rate is directly referred to the dependency of dissolution rates on calcite saturation state in case of leaching gravels G2%cc to G47%cc (Fig. 7B). Far from equilibrium calcite dissolution occurs faster (valid for less calcite, faster flow rate and lower pH values) than in solutions that are closer to equilibrium between solution and calcite (valid for higher calcite content, slower flow rates and higher pH values). Close to equilibrium dissolution rates decrease strongly (see also [17]). From the individual dissolution rates, the overall reaction times that are required to dissolve all calcite within one gravel sample can be estimated by considering a (i) linear (time for constant dissolution rate = t) or (ii) a simplified non-linear dissolution approach by changing dissolution rates in time increments of 1 year by using the regression lines given in Fig. 7C (t’). This differential approach was applied until the calcite content became <0.01 wt% and yielded obviously larger time spans to dissolve the calcite from the gravel compared to the linear approach because both RCa_gr and RCa values depend on the calcite content (Fig. 7C, Table 3). Since sample G47%cc+24%do contains dolomite as well as calcite the dissolution rates of the respective dolomite were also considered. However, at the tested CO2 rich conditions all calcite will be dissolved within the service time of a tunnel building between 50 and 100 years. Dolomite is more resistant to dissolution than calcite. At the lowest flow rate and within service time (50 years) G24%do can be expected not to lose more than 10 wt% of gravel by dolomite dissolution using the linear (constant dissolution rate) approach (t = 144 years; Table 3). It is worth noting that the grain sizes of the tested gravels were different (see Table 1). Smaller grain sizes lead to a faster dissolution as the kinetics correlates with the reactive surface. 4.3. General durability of calcite vs dolomite

Fig. 6. Ca2+ as a function of HCO 3 concentration in output solutions from reaction chamber II. The slope of 0.5 of the regression line represents the dissolution of mainly calcite according to reaction (4).

Our experimental data was conducted at rather extreme acidic conditions by using a CO2 bearing solutions at pCO2 of 0.08 atm. In Fig. 8, dissolution rates of Ca2+ from our pea gravel experiments are compared to results from pure carbonate water systems. The dissolution rates of Ca2+ for calcite and dolomite in the present study are plotted versus the reported dissolution rates from literature, where calcite and dolomite crystals were separately used for experiments.

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Table 3 Dissolution rates of Ca2+ from the gravels (RCa_gr in mmol kg1d1), saturation states (log(1-X)) for calcite and dolomite, as well as partial pressure of CO2 (pCO2 in atm) at steady state conditions for the three given flow rates (f in L d1). The surface normalized dissolution rate of Ca2+ from calcite, RCa_cc, is given in mol cm2 s1. Analogous values are given in respect to dolomite for gravel G** 24%do. Dissolution rates were calculated for linear (t) and non-linear dissolution behavior (t’) and given in years. calcite

f

RCa_gr

RCa_cc

Xcc

pCO*2

t

t’

G0%cc G2%cc G2%cc G2%cc G5%cc G5%cc G5%cc G9%cc G9%cc G9%cc G47%cc G47%cc G47%cc

9.14 9.14 3.76 1.86 9.14 3.76 1.86 9.14 3.76 1.86 9.14 3.76 1.86

7.45E02 6.25E01 3.39E01 1.95E01 8.45E01 4.63E01 2.61E01 1.14E00 6.38E01 3.96E01 1.58E00 7.59E01 3.86E01

no calcite 9.59E11 5.20E11 2.99E11 9.84E11 5.39E11 3.04E11 7.60E11 4.24E11 2.63E11 1.95E11 9.34E12 4.71E12

2.34E06 9.55E03 4.27E02 1.45E01 4.17E03 9.55E03 2.69E02 7.94E03 1.95E02 4.57E02 1.45E01 2.00E01 2.88E01

2.09E02 5.50E04 2.75E04 1.26E04 2.45E03 2.45E03 1.29E03 4.17E03 3.72E03 2.95E03 6.17E04 6.92E04 5.13E04

– 1 2 4 2 4 6 3 5 9 12 25 50

– 1 2 3 2 4 8 4 8 14 15 30 54

dolomite G** 24%do G** 24%do G** 24%do

f 9.14 3.76 1.86

RMg_gr 1.29E04 6.28E05 3.45E05

RMg_dol 8.12E13 3.96E13 2.17E13

X*dol 2.29E03 4.37E03 1.00E02

pCO*2 6.17E04 6.92E04 5.13E04

trea 39 79 144

trea’ n.c. n.c. n.c.

Individual RCa_cc data sets show a strong pH dependence. The pH dependence of the RCa_cc values is also valid by considering all plotted data, but the overall dissolution kinetics – besides pH – mainly depends on calcite saturation state (Fig. 7B). The dissolution rates of Ca2+ for dolomite from the gravels (assuming RMg_dol = RCa_dol) in our experiments are about 3 times lower than for calcite (Table 2), which is general in accordance with literature data, where even lower dissolution rates were reported (e.g. Appelo and Postma [14], Chou, Garrels [10], Busenberg and Plummer [19] and Pokrovsky and Schott [20]; Fig. 8). In nature elevated dolomite versus calcite dissolution can be caused by distinct exposal of dolomite versus calcite surfaces to the reacting solution as also suggested by Appelo and Postma [14] for the weathering behavior of typically more friable dolostone versus limestone. Schulz [21] experimented on mixtures of quartz and carbonate particles – similar to this study – and found dolomite to be dissolved up to nearly equal rate as calcite. He found indications that dissolution reactions of dolomite-calcite-quartz mixtures differ from pure dolomite-quartz mixtures (without calcite). In the latter mixtures dolomite dissolved incongruently and thus faster as secondary precipitation of calcite is induced. In accordance with results from calcite-dolomite-quartz mixtures all experimental solutions in the present study are undersaturated with respect to calcite. In general, as shown in Fig. 8 dolomite and in particular dolomite/calcite mixtures are expected to be kinetically more resistant than solely calcite at neutral to moderate alkaline pH. However in highly alkaline environments (pH > 10) created by interaction with concrete, the use of dolostone can result in pronounced chemical attack by incongruent dissolution via the formation e.g. of brucite Mg(OH)2) and calcite [22–25]. 4.4. Implications for durability of carbonate containing gravels in tunneling

Fig. 7. Dissolution rate RCa_cc of Ca2+ for calcite in mmol m2 d1 as a function of A: flow rate, B: saturation degree of calcite (O), C: proportion of dissolved calcite in respect to the initial calcite content in the gravel per year [wt% cc a1]. cc: initial calcite content of the different gravels used for the experiments in wt%. r: f = 9.1 L d 1, Rcc = 149 cc0.736 (regression line), R2 = 0.992; : f = 3.8 L d 1, Rcc = 85.8 cc0,777, R2 = 0.984; : f = 1.3 L d 1, Rcc = 52.6 cc0.816, R2 = 0.968.

Typical environmental conditions for tunneling in the aspects of interaction of backfill material with aqueous solutions are mostly referred to the individual composition of local groundwater. Different saturation state conditions of these solutions in respect to calcite (or dolomite) are based on groundwater evolution and in particular to the uptake of CO2 and the composition of the host rocks within the aquifer. Typical ranges of groundwater composition occurring in natural aquifers are given by solutions ‘‘Silicate”, ‘‘Carbonate” and ‘‘Sulfate” in Table 4, which denote groundwater originated from the following host rocks: siliceous rock, limestone,

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Fig. 8. Dissolution rates of Ca2+ for calcite (crosses and filled symbols) and dolomite (open symbols) as a function of pH. Data from the present study are given by red symbols. The three regions 1–3 are referred to the dominance of the forward versus backward reaction kinetics during carbonate dissolution according to equation (6) (see text for discussion). See above-mentioned references for further information. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 4 Sampled groundwater (1-WW-GW and 2-KA-GW from Rinder et al. 2013, RR_GW05 from Mittermayr et al. 2013). Temperature and concentrations are given in °C, mmol L1 and atm (pCO2) respectively. groundwater

description

T

pH

K

Na

Ca

Mg

Cl

SO4

DIC

Xcc

SIcc

pCO2

1-WW-GW 2-KA-GW 3-RR_GW05

‘‘Carbonate” ‘‘Silicate” ‘‘Sulfate”

13.0 12.6 6.8

7.33 7.13 7.81

0.51 0.04 0.2

5.34 0.48 0.4

1.18 0.43 32.6

0.93 0.21 9.4

0.98 0.02 0.4

1.08 0.08 8.9

6.89 1.96 2.27

0.76 0.07 0.60

0.11 1.17 0.22

1.86 2.20 2.79

and gypsum-rich dolostone, respectively (see 1-WW-GW and 2KA-GW from Rinder, Dietzel [12], and RR_GW05 from Mittermayr, Baldermann [26]). A modelling approach of the dissolution behavior is obtained by using relationship (6) and respecting dissolution rate constants form the literature (see chapter 1.3). At first, the modelling approach was tested by comparing the obtained data to the experimental results (Fig. 9). The experimental and modelling data fit well for the three shown flow rates and the dissolution behavior of experiment G9%cc, which verifies the use of the developed modelling approach for given task. An overview about the obtained impact of the flow rate of the pore water on the calculated years for complete dissolution of calcite considering gravel with a calcite content of 9.25 wt% cc and the

‘‘silicate” solution is given in Fig. 10. The lower the flow rate is the stronger the impact on the time required for complete dissolution. In the given case a flow rate of 0.5 L s1 seems to be close to a lower limit of flow rates, where dissolution of calcite is small for the use as filling material (>600 years are necessary to dissolve the calcite in this sample). In Fig. 11 the calculated years for complete calcite dissolution considering gravel with a calcite content of 9.25 wt% cc at flow rates 0.5, 2, 4 and 8 L s1 of the ‘‘silicate” solution (see Table 4) is shown as a function of SIcc values. Highest durability of the gravel is clearly obtained for the lowest flow rates of the ‘‘silicate” solution (see also Fig. 10). However, SIcc values close to 0

Fig. 9. Modelling versus experimental data of Ca concentrations of the solution in reaction chamber II are in good agreement to each other, exemplarily shown for exp. G9%cc; flow rates: 1.86, 3.76 and 9.14 L d1.

Fig.10. Calculated years for complete calcite dissolution considering gravel with a calcite content of 9.25 wt% cc and distinct flow rates of the ‘‘silicate” solution (see Table 4), where the modelling approach according to equation (6) is used.

R. Fuchs et al. / Construction and Building Materials 223 (2019) 254–264

Fig. 11. Calculated years for complete calcite dissolution considering gravel with a calcite content of 9.25 wt% cc, different calcite saturation states (SIcc) and flow rates (f from 0.5, 2 , 4 to 8 L s1) of the ‘‘silicate” solution (see Table 4). The modelling approach is according to equation (6). The silicate solution was adjusted by changing alkalinity through the remove and addition of Ca(OH)2, thus representing different SI functions (see chapter 1.3 for details). The highest durability in 9.25 wt% cc is obtained for lowest flow rates of the ‘‘silicate” solution in accordance to Fig. 10. SIcc values close to 0 (chemical equilibrium) result in significantly higher durability.

(chemical equilibrium) also result in significant higher durability of the pea gravel. A high SIcc always translates to increased scaling potential, both in carbonate waters and at low flow rates. In this case, old tunnels have a higher risk of scaling than tunnels with a waterproofing system that prevents contact of groundwater and the concrete lining [27]. Fig. 11 can be used to approximately estimate the years required for complete calcite dissolution considering gravel with 10 wt% calcite (if all other minerals are quartz or equally stable minerals) and a low salinity solution (fresh water).

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tively estimated considering low salinity water (fresh water) at different flow rates and calcite saturation states. This contribution highlights that the application of limestone and/or dolostone as backfill material can only be reliably assessed by considering the local hydrological and hydrochemical conditions. If groundwater loaded with Ca2+ from gravel leaching is discharging to the tunnel drainage, an increase of the capacity for the formation of carbonate scaling has to be expected. Thus, the use of carbonate containing gravel material has to be evaluated for each individual case, considering the environmental boundary conditions of groundwater in its chemical and flow behavior and the mineralogical and chemical composition of the gravel and host rock. Levels of risks could be approached for real tunnel situations in Austria, where the potential of carbonate sinter formation of the locally occurring drainage solution is increasing by groundwater being exposed to carbonate minerals before entering the tunnel drainage. This is in particular relevant for groundwater from non-carbonate but silicate mineral dominated aquifers. In contrast, elevated calcite and in particular dolomite content in backfill material can be appropriate under given conditions; for instance if groundwater from lime- or dolostone aquifer is entering the tunnel building. Therefore a strict and constant limit of carbonate mineral content for backfill material in tunnel buildings is not suitable as the dissolution kinetics is governed by the local environmental conditions. Locally available carbonate bearing rock fragments can be used for backfilling in tunnel buildings by considering the locally occurring groundwater without creating a potential threat for the tunnel structure and without increasing sinter formation in the drainage system. Declaration of Competing Interest None.

5. Summary and conclusions

Acknowledgements

The main motivation of the present study was to assess if carbonate mineral bearing rocks can be used for backfilling tunnel buildings as (partial) dissolution of carbonates could lead to (i) damage of the tunnel structure and (ii) increasing sinter formation in the drainage systems. Thus leaching of the backfill material pea gravel used for tunneling may cause both technical and economic risks. For instance, Ca2+ leaching can foster the formation of calcium carbonate in the tunnel drainage system which may require maintenance action and generating high costs. Herein an experimental approach was developed, where different pea gravels were subjected to be leached by a CO2 loaded water. A combined flow through reactor unit was successfully designed, where the chemical compositions of the experimental solutions were monitored at different flow rates. The measured dissolution rates of Ca2+ for calcite (and dolomite) from the used pea gravel generally matched literature data from pure carbonate water systems. Release of calcium from silicates can be neglected versus carbonate dissolution. Experimental data of the leaching behavior of gravel material for tunneling show that dissolution rates of Ca2+ for carbonates (i) depend strongly on local (ground) water composition, (ii) increase with increasing flow rate, and (iii) are significantly lower for dolomite in comparison to calcite. Accordingly the evaluation of leaching of Ca2+ ions from backfill material for tunneling is controlled by saturation state conditions with respect to e.g. calcite, calcite/dolomite content of the used material, and discharge/flow rate of the groundwater through the gravel material. A diagram is given where the years for complete calcite dissolution from a gravel with about 10 wt% of calcite can be approxima-

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