Julien Aupoil

Jean-Baptiste Champenois

Jean -Baptiste D'espinose De Lacaillerie

Arnaud Poulesquen

Interplay between silicate and hydroxide ions during geopolymerization

Keywords: silicate, hydroxide, Hammet, alkali, metakaolin, dissolution, condensation, 2

Two set of activating solutions were prepared with increasing sodium hydroxide content, either containing or not silicates. Their alkalinities, here defined as the ability of solutions to resist changes in pH, were determined and compared by measuring Hammet acidity functions that can be assimilated to extended pH values. Such Hammet functions in sodium silicate solutions are reported for the first time. The impact of both the alkalinity and the initial Hammett function on the reactivity of metakaolin (MK)-based pastes was assessed using Isothermal Conduction micro-Calorimetry (ICC). It was concluded that the reactivity of MK mixed with sodium hydroxide solution related directly to the Hammett function values whereas in sodium silicate mixes, the alkalinity value was a more pertinent parameter. A mechanism was deduced to clarify the role of hydroxide ions during the geopolymerization, highlighting at the same time the role of silicate species as hydroxide reservoir to nurture the dissolution process.

Introduction

Geopolymers refer to alumino-silicate binders obtained by reaction of a powdered aluminosilicate source, such as dehydroxylated kaolin (metakaolin, MK), with an alkali hydroxide or alkali silicate solution, as described by Davidovits [START_REF] Davidovits | GEOPOLYMERS -Inorganic polymeric new materials[END_REF] . Schematically, the geopolymerization process can be described by three simplified steps [START_REF] Duxson | Geopolymer technology: the current state of the art[END_REF][START_REF] Provis | The role of mathematical modelling and gel chemistry in advancing geopolymer technology[END_REF][START_REF] Fernandez-Jimenez | Microstructure development of alkali-activated fly ash cement: a descriptive model[END_REF][START_REF] Khale | Mechanism of geopolymerization and factors influencing its development: a review[END_REF] . First, MK dissolution in the activating solution provides aluminate and silicate species to the reacting medium. These species then rearrange in solution, to finally polycondense, yielding the 3D network of the hardened geopolymer.

Carrying out a detailed mechanistic study of these three steps is a hard task. They occur concurrently and furthermore, no realistic reaction equations can be written to describe any of these steps. However, since most of the expected reaction steps are exothermic, the use of Isothermal Conduction Calorimetry (ICC) was previously reported [START_REF] Buchwald | Reaction progress of alkaline-activated metakaolin-ground granulated blast furnace slag blends[END_REF][START_REF] Granizo | Alkaline activation of metakaolin -An isothermal conduction calorimetry study[END_REF][START_REF] Yao | Geopolymerization process of alkali-metakaolinite characterized by isothermal calorimetry[END_REF][START_REF] Zhang | Quantitative kinetic and structural analysis of geopolymers. Part 1. The activation of metakaolin with sodium hydroxide[END_REF][START_REF] Zhang | Quantitative kinetic and structural analysis of geopolymers. Part 2. Thermodynamics of sodium silicate activation of metakaolin[END_REF][START_REF] Sun | Isothermal calorimetry and in-situ XRD study of the NaOH activated fly ash, metakaolin and slag[END_REF] as being an efficient tool for studying the whole geopolymerization process. Indeed, heat flow and heat profiles recorded during geopolymerization provide qualitative and quantitative information. As an example, the global geopolymerization extent at a given time has been calculated as the ratio of the cumulated heat Q(t) release to the theoretical total heat at completion, Q max [START_REF] Buchwald | Reaction progress of alkaline-activated metakaolin-ground granulated blast furnace slag blends[END_REF][START_REF] Zhang | Quantitative kinetic and structural analysis of geopolymers. Part 1. The activation of metakaolin with sodium hydroxide[END_REF][START_REF] Zhang | Quantitative kinetic and structural analysis of geopolymers. Part 2. Thermodynamics of sodium silicate activation of metakaolin[END_REF] . Hence, in alkali hydroxide activating solutions free from silicates, Zhang et al. [START_REF] Zhang | Quantitative kinetic and structural analysis of geopolymers. Part 1. The activation of metakaolin with sodium hydroxide[END_REF] reported an increase of the aforementioned geopolymerization extent with increasing hydroxide ions content.

The relationship between activating solution composition and geopolymerization has been the subject of an extensive body of literature. Indeed, such a topic is a key issue for the use of geopolymers and their industrial development. For examples, Rahier et al. [START_REF] Rahier | Low-temperature synthesized aluminosilicate glasses .3. Influence of the composition of the silicate solution on production, structure and properties[END_REF] reported that the activating solution modulus, defined as the SiO 2 /Na 2 O molar ratio, and the amount of water, defined as the H 2 O/Na 2 O molar ratio, are driving parameters to tune geopolymers stoichiometry and consistency respectively. At a Na/Al molar ratio equal to one, the optimal geopolymer stoichiometry can thus be reached by choosing the appropriate SiO 2 /Na 2 O modulus of the activating solution. Later, Duxson et al. [START_REF] Duxson | Understanding the relationship between geopolymer composition, microstructure and mechanical properties[END_REF] observed an increase in the proportion of unreacted metakaolin and a decrease in the geopolymers density when increasing the silicate content of activating solutions as a consequence of the resulting simultaneous increase of the SiO 2 /Al 2 O 3 and SiO 2 /Na 2 O molar ratios. Such a result was further rationalized by using Density Functional Theory (DFT)-based Coarse-Grain Monte-Carlo simulations [START_REF] White | Molecular mechanisms responsible for the structural changes occurring during geopolymerization: Multiscale simulation[END_REF] . Similarly, Zhang et al. [START_REF] Zhang | Quantitative kinetic and structural analysis of geopolymers. Part 2. Thermodynamics of sodium silicate activation of metakaolin[END_REF] reported that an increase in the activating solution modulus, defined as the SiO 2 /Na 2 O molar ratio, leads to a decrease in the geopolymerization extent, evaluated by isothermal calorimetry.

The use of [START_REF] Edward | IONIZATION OF ORGANIC COMPOUNDS .II. THIOACETAMIDE IN AQUEOUS SODIUM HYDROXIDE[END_REF] Si Nuclear Magnetic Resonance (NMR) clearly exhibited that silicates connectivity in activating solutions increases when increasing the modulus of the solutions [START_REF] Engelhardt | Si NMR-Spectroscopy of Silicate Solutions .II. Dependence of Structure of Silicate Anions in Water Solutions from Na:Si Ratio[END_REF][START_REF] Svensson | Polysilicate Equilibria in Concentrated Sodium Silicate Solutions[END_REF][START_REF] Kinrade | Silicon-29 NMR Studies of Aqueous Silicate Solutions .1. Chemical Shifts and Equilibria[END_REF] . Duxson et al. [START_REF] Duxson | Understanding the relationship between geopolymer composition, microstructure and mechanical properties[END_REF] postulated that the higher the silicates connectivity, the lower the silicates species lability.

According to these authors, highly connected silicate species would thus hardly rearrange and densify before gelation, leading to the formation of a gel of lesser density and thiner pore size distribution, around metakaolin grains.

In most of the aforementioned studies, the activating solutions compositions are described in terms of silicate content, SiO 2 /Na 2 O and H 2 O/Na 2 O molar ratios. The influence of the free hydroxide ions content on the geopolymerization reaction is thus not explicit. Although it had already been mentioned by Xu and Van Deventer [START_REF] Xu | The geopolymerisation of alumino-silicate minerals[END_REF] and Zhang et al [START_REF] Zhang | Quantitative kinetic and structural analysis of geopolymers. Part 1. The activation of metakaolin with sodium hydroxide[END_REF] , to the best of our knowledge, only one scientific paper has been dedicated to that topic. Indeed, by increasing the alkali hydroxide content of activating solutions and using fly ash as an aluminosilicate source, Phair and Van Deventer [START_REF] Phair | Effect of the silicate activator pH on the microstructural characteristics of waste-based geopolymers[END_REF] reported that a higher activating solution pH leads to a higher polycondensation extent. However, pH reported values seem to be approximate as they are given only in full pH units [START_REF] Phair | Effect of the silicate activator pH on the microstructural characteristics of waste-based geopolymers[END_REF] . In any case, these three studies [START_REF] Zhang | Quantitative kinetic and structural analysis of geopolymers. Part 1. The activation of metakaolin with sodium hydroxide[END_REF][START_REF] Xu | The geopolymerisation of alumino-silicate minerals[END_REF][START_REF] Phair | Effect of the silicate activator pH on the microstructural characteristics of waste-based geopolymers[END_REF] clearly highlight the importance of hydroxide ions in the geopolymerization process.

An effect of solution pH on aluminosilicates dissolution was indeed evidenced by Xu and Van

Deventer [START_REF] Xu | The geopolymerisation of alumino-silicate minerals[END_REF] when studying in diluted suspensions the dissolution of 15 aluminosilicates minerals.

They concluded that the extent of dissolution increases for increasing alkali hydroxide solution concentrations. Complementarily, Duxson et al. [START_REF] Duxson | Geopolymer technology: the current state of the art[END_REF] (and references therein) and Granizo et al. [START_REF] Granizo | Effect of temperature and alkaline concentration on metakaolin leaching kinetics[END_REF] amongst others also pinpointed that the dissolution kinetics of aluminosilicate sources was highly dependent on the initial pH of the solution.

However, all the previously mentioned experiments [START_REF] Duxson | Geopolymer technology: the current state of the art[END_REF][START_REF] Xu | The geopolymerisation of alumino-silicate minerals[END_REF][START_REF] Granizo | Effect of temperature and alkaline concentration on metakaolin leaching kinetics[END_REF] were carried out using solutions free from silicates and using high liquid to solid ratio. As such, the role of silicate species and their interplay with hydroxide ions on the dissolution step during geopolymerization have never been investigated. This literature gap may have resulted from the difficulty to assess by standard pHmetry pH values of solutions containing extremely high amounts of alkali metal ions.

In consequence, the present study firstly aims at quantifying the hydroxide content and the alkalinity of two sets of activating solutions. Two sets of solutions of increasing sodium hydroxide content were considered: the first one with a fixed silicate content and the second one devoid of silicate species. Each time, Hammet acidity functions of studied solutions were measured instead of pH, as described in the theoretical part. The corresponding alkalinity [START_REF] Drever | The Geochemistry of Natural Water[END_REF] , defined here as the ability of a solution to resist to changes in pH, was evaluated for the two sets of solutions. Then, the geopolymers reactivity observed by mixing metakaolin into these solutions was evaluated by using Isothermal Conduction Calorimetry (ICC). Finally, these new data led us to discuss the role of silicate species and their interplay with hydroxide ions during geopolymerization.

Theoretical basis.

In activating solutions used for geopolymers elaboration, molality of alkali metal ions can range between 1 mol / kg of water to 15 mol / kg of water. The use of a glass electrode sensitive to oxonium ions H 3 O + to assess pH values is then questionable in such media. First, a high sodium concentration in basic solutions often leads to an alkaline error, since the alkali ion concentration can be typically orders of magnitudes larger than the oxonium ions one. As an example, the alkali ion concentration is 10 14 times higher than the oxonium one in a sodium silicate solution containing 10 mol / kg of sodium ions at pH ≈ 13. Then, water activity in the studied solution can be far below the value of 0.9 expected in a 3 mol / kg potassium chloride electrode filling solution. As a consequence, the ionic flow from the electrode to the studied medium could be strongly impacted, leading to false measurements. Finally, in the specific case of silicatecontaining activating solutions, silicate species can interact or even deteriorate the glass electrode surface, leading to additional errors.

Consequently, in this work, the measurement of Hammet acidity functions [START_REF] Flexser | The Determination of Ionization by Ultraviolet Spectrophotometry: Its Validity and its Application to the Measurement of the Strength of Very Weak Bases[END_REF][23][24] was chosen instead of standard pH-metry to quantify the amount of hydroxide ions in activating solutions.

Such a function represents the hydroxide ions ability to deprotonate a weak acid, noted SH in reaction equation 1, added in small amounts in the aqueous medium. It is obviously equivalent to pH in dilute systems. Upon introduction in the studied medium, again in small amounts, the weak acid SH plays the role of an acid-base indicator whose ionization ratio is measured quantitatively thanks to the intensity shift of its UV-visible spectrum induced by its deprotonation . This concept was initially introduced to measure acidity of extremely acidic solution and later adapted to basic solutions, either in aqueous alkali hydroxide solutions or in non-aqueous solvents [START_REF] Schwarzenbach | Über die Alkalinität starker Lösungen der Alkalihydroxyde[END_REF][START_REF] Paul | H 0 AND RELATED INDICATOR ACIDITY FUNCTIONS[END_REF][27][START_REF] Rochester | Correlation of Acidity Functions with Equilibria of p-Nitroaniline in Aqueous Sodium Hydroxide Solution[END_REF][START_REF] Edward | IONIZATION OF ORGANIC COMPOUNDS .II. THIOACETAMIDE IN AQUEOUS SODIUM HYDROXIDE[END_REF][30] .

degrees between reactants and products, according to the model used by Edward [START_REF] Edward | IONIZATION OF ORGANIC COMPOUNDS .II. THIOACETAMIDE IN AQUEOUS SODIUM HYDROXIDE[END_REF] . The corresponding equilibrium constant K is then written as:

(Equation 1)

with a i the activity of the species i.

As reaction equation 1 represents an acid-base reaction between the two couples SH/S -and OH -/H 2 O, the constant K can also be written as the ratio of the associated ionization constants: K a for the acid ionization constant of SH and K w for the one of water (tabulated by Bandura and Lvov [START_REF] Bandura | The ionization constant of water over wide ranges of temperature and density[END_REF] and taken as 10 -14 at 25°C),

Hammett 23 defined the acidity function H -according to equation 2. Rearranging equation 1 also provides equation 3, another definition of the acidity function:

(Equation 2) (Equation 3)

With:

pK x the cologarithm of constant K x ,
the ionization ratio,

[i] the concentration of species i in mol / kg of water, and γ i its activity coefficient defined such as

a i = γ i .[i]/[i]° where [i]° is the standard state concentration of species i conventionally set equal to 1 mol / kg.
The experimental determination of activity coefficients being complex, equation 3 is mainly of theoretical interest, except in the rare cases when good approximations of activity coefficients 24,[START_REF] Edward | IONIZATION OF ORGANIC COMPOUNDS .II. THIOACETAMIDE IN AQUEOUS SODIUM HYDROXIDE[END_REF] are possible. Nevertheless, equation 3 clearly states that the function H -takes into account not only the hydroxide ions activity but also their environment through the parameter n and the variables a H2O and γ SH /γ S-, respectively related to the ions hydration, the free water content and the ionic force of the solution. In consequence, the acidity function is an appropriate tool to represent the ability of hydroxide ions to react in a solution.

If both S -and SH forms absorb UV-visible light at different wavelengths, the [S-]/[SH]

ionization ratio can be measured by using spectrocolorimetry. From Beer-Lambert's law, the ionization ratio can be obtained from absorbance measurements for a constant indicator concentration according to (eq. 4).

(Equation 4)

With

A SH the absorbance of a solution where the indicator is fully protonated, A S-the absorbance of a solution where the indicator is fully deprotonated, A the absorbance of any basic solution with intermediate concentration.

A set of three solutions is thus needed to determine an ionization ratio.

3. Experimental section.

Activating solutions elaboration

Sodium hydroxide solutions were elaborated with an increasing molality ranging from 3.17 .10 1 and2. metakaolin is also given.

Geopolymers preparation

Prior to the cure and any measurements, geopolymer pastes were prepared by mixing 32.05 g of metakaolin (MK, Metakaolin Argical M1000 from Imerys, characteristics in Table 3) with the appropriate weight of activating solution to obtain a constant (initial water)/MK weight ratio of 0.78. Elaborated geopolymers formulations are reported in Tables 1 and2.

Acidity function measurements

Thiazole Yellow G (TYG, also known as Titan Yellow, Sigma-Aldrich) was used in the present study as the weak acid UV-visible-sensitive indicator, chosen to be compatible with the expected acidity function range of the studied solutions. Absorptions of activating solutions containing TYG were measured in the spectral range 300 to 600 nm, with a resolution of 1 nm, in 10-mm quartz cells at 25°C. Absorbance spectra were recorded on a dual-beam spectrophotometer (Genesys 10S UV-Visible, ThermoFischer Scientific), with a xenon flash lamp. Indicator-free activating solutions were used as backgrounds so that only the indicator would contribute to the recorded spectra. For the whole set of solutions, absorbance of indicator-free solutions was checked to be close to zero for wavelength ranging from 400 to 500 nm.

The absorbance at 480 nm has been checked to be proportional to TYG concentration up to approximately 5.10 -4 mol / kg in sodium hydroxide solution, defining the range of validity of the Beer-Lambert law (details are available in supplementary information). Accordingly, the working concentration of TYG in all activating solutions was set to 5.10 -5 mol / kg. In sodium hydroxide solutions, absorption bands centered at 405 and 475 nm have been observed respectively for the SH form and S -form, with an isobestic point at 433 nm. In sodium silicate solution with increasing amounts of sodium hydroxide, absorption bands were centered at 390 and 480 nm, with an isobestic point at 438 nm. Previous observations from Allain and Xue 33 in sodium hydroxide solutions support the existence of a simple acid-base equilibrium for TYG as described in Reaction Equation 1. Allain and Xue also reported that TYG has a good chemical resistance to hydroxide ions, and a large dynamic spectral range. In addition, the similar position of the S - form absorption band in both media corroborates the absence of any drastic conformation modification, at least for the deprotonated form. This meant that silicates did not interact to any significant level with the indicator and that the latter was only involved in a simple acid-base equilibrium. Acidity functions were thus calculated from absorbance at 480 nm.

The pK a of TYG has been measured in sodium hydroxide solution following Safavi and Abdollahi's procedure [START_REF] Safavi | Optical sensor for high pH values[END_REF] . The pK a value of 12.92 ± 0.01 at 25°C measured in this work is consistent with the 12.92 value reported by Safavi and Abdollahi. TYG was thus considered as a suitable weak acid indicator for acidity function measurements in study alkali silicate activating solutions.

Finally, the working range of TYG was established (see Sup. Inf.) to be comprised between 11.9 and 13.9, corresponding to an H -range equal to approximately pK a ± 1, which is consistent with the dynamic spectral range found by Safavi and Abdollahi [START_REF] Safavi | Optical sensor for high pH values[END_REF] and Allain and Xue 33 .

Water activity measurements

Water activity was measured in all considered activating solutions in presence of 5.10 -5 mol/L of TYG. Measurements were done at 25 ± 2°C with a Hygropalm HP23-AW-A analyzer equipped with a HC2-AW water activity probe (Rotronic) and calibrated in temperature and humidity with Rotronic certified humidity standards at 50% RH and 80% RH. After an equilibration time of 5 min, water activity was measured with a precision of ± 0.008. Water activity measurement relies on Equilibrium Relative Humidity measurement (ERH, in %), when the atmosphere in the sample holder is at equilibrium with the solution (a H2O = ERH/100).

29 Si Nuclear Magnetic Resonance.

Si NMR measurements were performed on silicate activating solutions to investigate the silicate connectivity. Spectra were recorded at a Larmor frequency of 99.36 MHz (at 25 ± 2°C) in zirconia rotors using a Bruker Avance spectrometer and a 7-mm commercial Bruker MAS probe but without spinning. 1600 transients were acquired using a single 90° pulse of 6.4 µs and a recycle time of 10 s. The recycle time was verified to be long enough by increasing it ten folds and confirming that intensities did not vary. Spectra were referenced externally to tetramethylsilane (TMS). The proportions of the different types of silicon centers were obtained by the integrated intensities of their resonances using the software Dmfit developed by D.

Massiot et al. [START_REF] Massiot | Modelling one-and two-dimensional solid-state NMR spectra[END_REF] . The different silicon centers were designated according to Engelhardt's nomenclature [START_REF] Engelhardt | Si NMR-Spectroscopy of Silicate Solutions .II. Dependence of Structure of Silicate Anions in Water Solutions from Na:Si Ratio[END_REF] . Each center is designated as Q since silicon atoms are quadri-coordinated to oxygen. A superscript Q n 0 ≤ n ≤ 4 indicates the number of silo o bonds i-O-Si), without considering the protonation degree of non-bridging oxygens. For example Q 0 designates a single silicate, Q 1 a silicate with one neighbor silicate (end-chain or in a dimer) and so on. When present, subscript c indicates that silicates are part of a three-membered ring. These Q 2

resonances are detected at slightly different frequencies compared to the ones of Q 2 groups in chains or larger rings due to their more constraint geometry.

Isothermal Conduction Calorimetry

Approximately 5 g of each elaborated geopolymer has been then introduced in a sealed ampoule to assess its reactivity by using Isothermal Conduction Calorimetry at 25°C on a TAM Air microcalorimeter. Water was used as the reference, to compensate for possible external temperature disturbances. Due to the external mixing procedure, a parasitic heat flow signal associated to the introduction of sample interfered with the initial reaction signal. The equilibration time of this interference amounted to about 1.5 h, measured on an inert sample (available in supplementary information). Normalized heat flow release during geopolymerization, expressed in mW/g of paste, was recorded as a function of time during 90 h.

Cumulative heat releases were obtained by integrating heat flow profiles after the end of the introduction peak here defined as when the heat flow drops to a value of 8 mW / g to minimize the contribution of the introduction peak.

Results

Acidity function H -and alkalinity evaluation.

Acidity function values of were measured for the first time in sodium silicate solutions and compared to the ones of pure sodium hydroxide solutions. Results are plotted as a function of added sodium hydroxide molality in Figure 1. In a sodium hydroxide solution free from silicate, an addition of approximately 0.97 mol / kg sodium hydroxide is needed to raise the H -value from 12.00 to 14.01 (value from Bowden 30 ).

Within this concentration range and in the specific case of pure sodium hydroxide solutions, H -is fairly equivalent to theoretical pH calculated by pK w + log [OH -] (with a mean deviation of 0.03) and our measured values were consistent with previously reported data [START_REF] Edward | IONIZATION OF ORGANIC COMPOUNDS .II. THIOACETAMIDE IN AQUEOUS SODIUM HYDROXIDE[END_REF] . Above 1 mol / kg, H - values are out of the working range of TYG, and cannot be measured with this indicator. For sodium hydroxide addition higher than 1 mol / kg, the deviation of the H -function from the linearity is illustrated by plotting data from the literature [START_REF] Edward | IONIZATION OF ORGANIC COMPOUNDS .II. THIOACETAMIDE IN AQUEOUS SODIUM HYDROXIDE[END_REF]30,[START_REF] Rochester | Correlation of Reaction Rates with Acidity Functions in Strongly Basic Media .Part 2. Reaction of 2,4-Dinitroaniline With Aqueous Sodium Hydroxide[END_REF] . Such a deviation is mainly due to a sharp decrease in water activity as mentioned by Edward [START_REF] Edward | IONIZATION OF ORGANIC COMPOUNDS .II. THIOACETAMIDE IN AQUEOUS SODIUM HYDROXIDE[END_REF] , which was measured for reference and reported in Tables 1 and2.

In a silicate-containing solution, an addition of approximately 4.0 mol / kg sodium hydroxide was needed to raise the H -value from 12.19 to 13.59. For higher sodium hydroxide additions, the resulting H -function values were higher than the 13.9 threshold value of TYG working range.

This set of data remarkably highlights the alkalinity difference between sodium hydroxide and sodium silicate solutions. Alkalinity is here defined as the ability of a solution to resist changes in pH in a given pH range. Using this working definition, the alkalinity of both sets of studied solutions was calculated as the amount of hydroxide ions that has to be added to the solution to raise the H -value by one unit, within a H -range from 12 to 14. For H -values ranging from 12 to 14, the alkalinity values were found to be 0.48 mole added hydroxide per H -unit for a silicatefree solution and 2.86 for a solution containing 5.80 mol/kg of silicate,. Within the same acidity function range, the alkalinity of the silicate-containing solution is thus more than 5 times higher than the one of the silicate-free solution.

Silicate connectivity

Liquid state [START_REF] Edward | IONIZATION OF ORGANIC COMPOUNDS .II. THIOACETAMIDE IN AQUEOUS SODIUM HYDROXIDE[END_REF] Si NMR spectra of some sodium silicate solutions investigated herein are reported on [START_REF] Edward | IONIZATION OF ORGANIC COMPOUNDS .II. THIOACETAMIDE IN AQUEOUS SODIUM HYDROXIDE[END_REF] Si resonances are known to shift to higher frequencies (higher deshielding) when increasing the alkali hydroxide content.

This well-known fact is due both to the deprotonation of silanol groups (Si-OH) and to the formation of ion pairs (Si-O -+ Na) according to Kinrade and Swaddle [START_REF] Kinrade | Silicon-29 NMR Studies of Aqueous Silicate Solutions .1. Chemical Shifts and Equilibria[END_REF] . As expected, when the alkali hydroxide content is increased, the peak intensities decreased for highly connected silicates and increases for poorly connected silicates, reflecting the general decondensation of silica oligomers with pH. Poorly resolved Q 2 and Q 3 c contributions were decomposed by fitting spectra with gausso-lorentzian lineshapes using the freeware Dmfit [START_REF] Massiot | Modelling one-and two-dimensional solid-state NMR spectra[END_REF] . The average connectivity of Si centers was calculated from the spectral decomposition of [START_REF] Edward | IONIZATION OF ORGANIC COMPOUNDS .II. THIOACETAMIDE IN AQUEOUS SODIUM HYDROXIDE[END_REF] Si NMR measurements:

with Q n the proportion of each silicate population (%). On Figure 3, the average silicate connectivity is plotted as a function of acidity function values in silicate solutions. In the working range of TYG (dash lines), the average connectivity decreases for increasing H -values. For sodium hydroxide additions from 2.54 to 6.54 mol/kg and corresponding H -values ranging from 12.19 to 13.59 respectively, the silicate connectivity is divided by almost 2. It demonstrates the decondensation of silicates species when sodium hydroxide is added to solutions, as already reported by many authors. [START_REF] Engelhardt | Si NMR-Spectroscopy of Silicate Solutions .II. Dependence of Structure of Silicate Anions in Water Solutions from Na:Si Ratio[END_REF][START_REF] Svensson | Polysilicate Equilibria in Concentrated Sodium Silicate Solutions[END_REF][START_REF] Kinrade | Silicon-29 NMR Studies of Aqueous Silicate Solutions .1. Chemical Shifts and Equilibria[END_REF] Decondensation consists in the hydrolysis of siloxo bonds Si-O-Si. Such an effect, highlighted by Svensson et 20 al. [START_REF] Svensson | Polysilicate Equilibria in Concentrated Sodium Silicate Solutions[END_REF] , can be seen as the transfer of hydroxide ions from the solution onto silicates in the form of Si-OH groups. This leads to the consumption of initially added hydroxide ions, as illustrated by Reaction Equation 2:

i--i a H - i-H --i (Reaction Equation 2) Consequently, it was apparent that the low H -values in silicate solutions (<14) despite their high sodium hydroxide contents, up to 7.5 mol/kg, were due to the presence of silicate species in solutions and to the associated reaction described above. The presence of silicate was thus responsible for the alkalinity measured in silicate-containing solutions.

Isothermal Conduction Calorimetry measurements

Metakaolin-based pastes were then prepared with the solutions under investigation. For all the studied compositions, cumulative heat profiles could be described as a succession of two stages 6- 11 (Figure 4). The sharp heat release at the early beginning of the reaction can be ascribed mainly to the metakaolin dissolution. But this contribution often overlapped second stage of the 21 geopolymerization, namely the alumino-silicate polycondensation. After a variable duration, the heat release stabilized as the reaction slowed down. The heat value at the plateau (Figure 4 and5)

could then be assumed to be approximately proportional to the extent of geopolymerization.

Discussion

The final heat value was plotted as a function of total sodium ions content. In the same figure, the initial H -values of the activating solutions were plotted on the right axis (Figure 5).

. For both sets of experiments, i. e. with or without silicate in the activating solution, increasing the total sodium ions molality by adding sodium hydroxide up to 9.66 mol / kg led to an increase in the heat of geopolymerization. Above this 9.66 mol / kg value, the heat released during geopolymerization remained constant. It has to be noted that this molality value corresponded to a Na/Al ratio equal to 1, described in literature [START_REF] Rahier | Low-temperature synthesized aluminosilicate glasses .1. Low-temperature reaction stoichiometry and structure of a model compound[END_REF] as the optimal geopolymer stoichiometry due to the charge balance between Na + ions and AlO [START_REF] Sefcik | Thermochemistry of aqueous silicate solution precursors to ceramics[END_REF] . Besides, silicates pK a 's decrease with their connectivity [START_REF] Sefcik | Thermochemistry of aqueous silicate solution precursors to ceramics[END_REF] . In consequence, the level of silicate deprotonation should thus be significant in the present activating solutions. The release of hydroxide ions rather than water molecules would thus be favored.

Conclusion

Comparing initial H -values at identical Na content in sodium hydroxide and sodium silicate allowed assessing for the first time the role of activating solution alkalinity in the geopolymerization process, where the alkalinity is defined as the ability of activating solutions to resist to H -changes. As a consequence, the role of silicate species in the geopolymerization process is indirectly highlighted, since the presence of silicate species is responsible for activating solutions alkalinity.

The basicity of geopolymer activating solutions was investigated by UV-Visible spectroscopy using Thiazole Yellow G as a weakly acidic indicator to calculate their Hammett acidity function H -. Sodium silicate and sodium hydroxide solutions alkalinity has thus been measured by using this methodology. The present results on sodium hydroxide solutions were consistent with previous results from Safavi [START_REF] Safavi | Optical sensor for high pH values[END_REF] , validating the implemented methodology. For the first time to our knowledge, this technique was applied to alkali silicate solutions. Quantitative data on the basicity of geopolymer activating solutions were obtained, without using pH-metry. Thiazole Yellow G was an appropriate indicator to measure H -functions comprised in the range 11.9 -13.9, which is ideally suitable for concentrated alkali silicate activating solutions.

The variation of acidity function was found low in silicate-based solutions when compared to silicate-free solutions, for similar sodium hydroxide additions. This alkalinity difference, defined as the ability of studied solutions to resist changes in pH, is due to the condensation and decondensation of silicate species in the medium, as evidenced by using liquid state [START_REF] Edward | IONIZATION OF ORGANIC COMPOUNDS .II. THIOACETAMIDE IN AQUEOUS SODIUM HYDROXIDE[END_REF] Si NMR.

The reactivity of metakaolin-based geopolymers elaborated with previously investigated activating solutions was studied by using Isothermal Conduction Calorimetry. Taking into account initial H -values and corresponding alkalinity of activating solutions allow to precise the respective role of hydroxide ions and silicate species in the geopolymerization process. After metakaolin dissolution initiation b h dro ide ions the "h dro ide reservoir" on silicates can gradually release hydroxide ions into the solution during the subsequent condensation of alumino-silicate species. Those freshly released hydroxide ions would then be able to dissolve at their turn additional amounts of metakaolin. This would generate a chain reaction and a selfsustained circular mechanism. This high alkalinity explain why silicate solutions of low initial H - values lead to a similar extent of geopolymerisation compared to a silicate-free solution presenting drastically higher H -values. The fact that silicate solutions allow the reaction to take place at lower hydroxide concentrations is probably crucial in directing the reaction path towards geopolymers rather than zeolites.

Experimental work has to be enriched to further validate this proposition, especially by measuring acidity functions during the course of geopolymerization. It must be understood however that only discontinuous measurements would be possible with this technique, after extracting the solution from the geopolymer paste at different times, since translucent media are required for absorption based methods.

Conflicts of interest

There are no conflicts to declare

Figure 1 .

 1 Figure 1. Comparison of H -acidity function scales of pure sodium hydroxide () or sodium

Figure 2 .

 2 Several types of silicate species were present. The most deshielded are Q 0 silicates (δ = -71.45 ppm in NaS1) and then Q 1 (δ = -79.45 ppm in NaS1), Q 2 c (δ = -81.36 ppm in NaS1) Q 2 (δ = -87.47 ppm in NaS1), Q 3 c (δ = -89.55 ppm in NaS1), Q 3 (i = -95.52 ppm in NaS1). A Q 4 (δ = 18 -105.4 ppm) resonance was only observed in NaS1. For indication, the

Figure 2 .

 2 Figure 2. Liquid state 29 Si NMR spectra of some sodium silicate solutions investigated herein with varying sodium hydroxide contents. Peaks are named after Engelhardt notation 15 .

Figure 3 .

 3 Figure 3. Evolution of the average connectivity of silicate oligomers in sodium silicate activating

Figure 4 .

 4 Figure 4. Total heat release of geopolymer pastes prepared from a) sodium hydroxide solutions and b) sodium silicate solutions, with varying NaOH content, measured by ICC.

Figure 5 .

 5 Figure 5. Comparison of the final heat released at 90 h of MK-based geopolymers measured by

  -2 to 1.40 .10 1 mol / kg by dissolving sodium hydroxide pellets (AR grade, VWR) in ultrapure water (18.2 MΩ . A set of alkali silicate solutions was then elaborated b diluting in water a commercial alkali silicate solution (Betol ® 52T, Wöllner: 30.2% w/w SiO 2 , 14.7% w/w Na 2 O and 55.1% w/w H 2 O) and by adding sodium hydroxide pellets in order to reach increasing sodium

ions molality, ranging from 5.46 to 1.40 .10 1 mol/kg. All solutions were stirred for at least 4 h and cooled down to 25°C prior to any use. Solutions compositions are reported in Tables

Table 1 .

 1 Composition

		Solutions		Geopolymers
	[NaOH] (mol/kg)	a H2O (± 0.008)	H -	Na/Al
	Na1 3.17 .10 -2 1.000 12.51 ± 0.06 3.28 .10 -3 Na2 1.00 .10 -1 1.000 12.99 ± 0.06 1.04 .10 -2 Na3 3.17 .10 -1 -13.42 ± 0.08 3.28 .10 -2 Na4 1.00 0.987 14.01 a 1.04 .10 -1 Na5 2.00 0.956 14.42 a 2.07 .10 -1 Na6 3.01 0.917 14.72 a 3.11 .10 -1 Na7 5.00 0.812 15.19 a 5.18 .10 -1 Na8 7.00 0.692 15.62 a 7.25 .10 -1 Na9 9.66 1.00 0.520 16.06 a Na10 1.20 .10 1 -1.24 16.47 b Na11 1.40 .10 1 0.275 16.87 b 1.45	733 232 73.2 23.2 11.6 7.73 4.64 3.32 2.41 1.94 1.66

in molality, water activity and acidity function values of sodium hydroxide solutions together with the molar ratios of the corresponding geopolymers prepared with metakaolin. Acidity functions were taken from literature when indicated ( a Rochester 32 , b Schwarzenbach 25 ).

Table 2 .

 2 Molality, molar ratio, water activity and acidity function values of sodium silicate solutions. When indicated, the composition of the corresponding geopolymers prepared with

Table 3 .

 3 Physical and chemical characterization of Metakaolin by XRF, laser granulometry and N 2 adsorption-desorption (with BET method) as provided by the supplier.

			Solutions			Geopolymers
	[NaOH] added (mol/kg)	[Na] total (mol/kg)	[Si] (mol/kg)	a H2O (± 0.008)	H -	Na/Al
	NaS1 0	5.46	5.79 2.12	0.965	-	0.57	6.36
	NaS2 5.26 .10 -1	6.01	5.80 1.93	0.952 10.82 ± 1.98 0.62	5.79
	NaS3 1.54	7.01	5.80 1.65	0.928 11.64 ± 0.35 0.73	4.97
	NaS4 2.54	8.02	5.79 1.45	0.903 12.19 ± 0.14 0.83	4.34
	NaS5 3.53	9.00	5.80 1.29	-	12.52 ± 0.10	-	-
	NaS6 4.20	9.68	5.80 1.20	0.837 12.74 ± 0.10 1.00	3.60
	NaS7 5.02	10.5	5.80 1.10	-	13.10 ± 0.11	-	-
	NaS8 5.52	11.0	5.80 1.05	0.767 13.25 ± 0.12 1.14	3.16
	NaS9 6.54	12.0	5.80 9.66 .10 -1 0.711 13.59 ± 0.20 1.24	2.68
	NaS10 7.54	13.0	5.79 8.91 .10 -1 0.649 14.12 ± 0.53 1.34	2.32
	NaS11 8.54	14.0	5.80 8.27 .10 -1	-	-	-	-

  4-units. At this point, the initial H -values of activating solutions amount to 12.7 and 16.1 respectively in silicate-containing and silicate-free activating solution. At this 9.66 mol / kg sodium molality, (Na/Al = 1), the H -value in the silicate-containing solution was thus more than 3 units lower than in the silicate-free one. Despite this huge difference, and for the same total sodium ions concentration, geopolymerization extents were rather close in both systems.While, it was predictable that the composition of the reactants with respects of the stoichiometry of the final geopolymer product (Na/Al = 1) piloted the possibility or not to reach completion, and that this explains the dependence on the sodium content of the activating solution, it remained surprising at first glance that the silicate containing activating solutions, despite their much lower initial H -values, led to large geopolymerization extent. Indeed, Xu and Van Deventer[START_REF] Xu | The geopolymerisation of alumino-silicate minerals[END_REF] have demonstrated among others that increasing the pH value of the dissolution media enhances alumino-silicate minerals dissolution and one might expect consequently a much lower extent of reaction for metakaolin in silicate solutions than in pure NaOH ones. The benefits in adding silicate species in activating solutions to enhance the geopolymerization process have been previously mainly attributed to their role on condensation reactions, since silicate species are available from the beginning of geopolymerization according to Duxson et al.[START_REF] Duxson | Understanding the relationship between geopolymer composition, microstructure and mechanical properties[END_REF] . Moreover, Phair and Van Deventer[START_REF] Phair | Effect of the silicate activator pH on the microstructural characteristics of waste-based geopolymers[END_REF] have shown that increasing the alkali content leads to less condensed and more labile silicate species. A more porous and less dense geopolymer would thus be yielded at lower modulus SiO 2 /Na 2 O[START_REF] Duxson | Understanding the relationship between geopolymer composition, microstructure and mechanical properties[END_REF][START_REF] Phair | Effect of the silicate activator pH on the microstructural characteristics of waste-based geopolymers[END_REF] .However, the data presented here suggest another important role for the silicate species. When using a silicate-free solution as the activating solution, hydroxide ions are consumed by metakaolin dissolution, as described by Reaction Equation 3 as already reported by Xu and Van Deventer[START_REF] Xu | The geopolymerisation of alumino-silicate minerals[END_REF] . Due to the low alkalinity of sodium hydroxide solutions, meaning, as defined in the introduction, that they do not oppose pH changes, the pH necessarily drops during reaction and, to insure that all metakaolin is dissolved, a high initial H -value was necessary.At the opposite, when a silicate-containing solution was used to activate metakaolin and for a same Na/Al ratio (typically 1), the H -initial values were lower but a similar level of geopolymerization was reached. Obviously, the lower acidity function H -values were compensated by the strong alkalinity resulting from the presence of silicate species. While hydroxide ions are also consumed by the metakaolin dissolution which releases aluminate and silicate species in solution, following Equation 3, the hydroxide are removed from solution but the concomitant release of silicate and aluminate species in solution favors condensation of preexisting alumino-silicate species, thus releasing further hydroxide ions or water molecules in solution (Reaction Equations 4 and 5). Those freshly released hydroxide ions are then available to react in turn at the metakaolin surface, releasing more and more silicate and aluminate species which can feed the condensation. As a result, a chain reaction can be established thanks to this circular mechanism that nurtures the dissolution process. This phenomenon is enhanced in silicate-containing solutions due to their alkalinity. In other terms, the initial silicate in solution act as a reservoir of hydroxide compensating the effect of the H -values which are initially lower than in silicate-free solutions but remain probabbly somewhat constant during the dissolution.It is understood, as shown in Reaction Equation5, that condensation can also possibly release water molecules. Nevertheless, it has been shown that Si(OH) 2 O 2 2-and Si(OH) 3 O -are the predominant forms of silicate monomers in activating solutions for geopolymers with respective pK a 's of 12.6 and 15.7

	i--i Al metakaolin	H -	i-H metakaolin	-	-	i Al a (Reaction Equation 3)
	i-H a	-	-	i Al a	i--i Al a	H -	(Reaction Equation 4)
	i-H a	H -i Al a	i--i Al a H	(Reaction Equation 5)


T De Echave

M Tribet

S Gin

C Jégou

Influence of iron on the alteration of the SON68 nuclear glass in the Callovo-Oxfordian Groundwater

Keywords: SON68, glass alteration, iron, corrosion

On the French disposal concept, the high level radioactive waste, contained in a glassy structure, is intended for a deep geological disposal. It is then important to understand glass alteration, which depends on the near-field materials and the environmental conditions. The carbon steel overpack corrosion (used in the French multi-barrier concept for the disposal of high level radioactive waste) could influence glass alteration by the release of iron in the solution and the formation of iron corrosion products. To better understand these interactions, different experiments were performed using a SON68 glass (non-radioactive surrogate of the R7T7 glass)

separated by 80 µm from a pristine iron foil. A pre-corroded iron foil and a PTFE foil, replacing the pristine iron foil, were also used. This study focus then on the influence of a pristine iron and a pre-corroded iron foil on glass alteration, and the influence of iron on a confined (inside de crack) and diluted (face exposed to the homogeneous solution) environment. Depending on the previously described conditions, some variations were observed: glass alteration increased in the presence of iron, as already observed in the literature. It was also higher in the diluted environment than in the confined environment, indicating that the gel protective properties were different in both environments. Pre-corrosion of an iron foil seemed to have no influence on glass alteration in a confined environment: glass alteration was the same even if an iron corrosion layer was formed at the surface of the pristine iron foil.

Introduction

In France, the confinement of radionuclides in a borosilicate glass matrix is the chosen method for disposal of high level radioactive waste. This solution is based on a multi-barrier concept : glass incorporating in its structure the high level radioactive waste (Fission products and minor actinides) is poured in a stainless steel canister which will be contained in a carbon steel overpack of approximately 55 mm thick and intended for a deep geological disposal in the Callovo-Oxfordian (COx) claystone [START_REF] References | Dossier 2005 : Argile, Évaluation de la faisabilité du stockage géologique en formation argileuse[END_REF]. Once the glass canister is disposed in the geological repository, alteration is expected to occur mainly when water will be in direct contact with the glass. This study focuses on this stage, which it is expected to have a greater impact on glass on the long term.

It has been reported that water will be in contact with the system after several hundreds to several thousand years, corroding the carbon steel overpack and altering the borosilicate glass. It has become a major scientific issue in radioactive waste management to understand the long-term evolution of glasses. It is then necessary to understand the interactions between glass iron and the solution to assess and model the performance of this glass.

At first, the environment in the deep geological disposal will be initially aerated. A first stage of corrosion of the materials will then take place in an aerated and dry environment. However, it has been predicted that the duration of this stage will last less than 100 years: oxygen will be actively consumed by corrosion of the materials and probably microbial activities and an anoxic environment will be insured [START_REF] De Windt | Modeling of oxygen gas diffusion and consumption during the oxic transient in a disposal cell of radioactive waste[END_REF]. When water will be in contact with the system, the corrosion of the steel overpack in a reductive environment could lead to the precipitation of iron oxides and iron carbonates such as magnetite, siderite, chukanovite and a considerable amount of iron release in the solution [START_REF] Azoulay | Study of ferrous corrosion products on iron archaeological objects by electron backscattered diffraction (EBSD)[END_REF][START_REF] Neff | Corrosion of iron archaeological artefacts in soil: characterisation of the corrosion system[END_REF][START_REF] Saheb | Iron corrosion in an anoxic soil: Comparison between thermodynamic modelling and ferrous archaeological artefacts characterised along with the local in situ geochemical conditions[END_REF].

Over the last decades, many studies have been performed to better understand glass alteration and the fundamental mechanisms that control glass dissolution in pure water have been identified [START_REF] Frugier | SON68 nuclear glass dissolution kinetics: Current state of knowledge and basis of the new GRAAL model[END_REF]. At first, glass dissolution starts with hydration and interdiffusion, which induces an ion exchange between protons in the solution and alkali in the glass. This process creates a hydrated glass layer that dissolves by hydrolysis of the silicate network. Then, the hydrolysis rate decreases due to the reach of silicon saturation in the solution alongside the formation of a gel layer and the formation of secondary phases at the surface of the gel [START_REF] Gin | Nuclear Glass Durability: New Insight into Alteration Layer Properties[END_REF][START_REF] Gin | Atom-Probe Tomography, TEM and ToF-SIMS study of borosilicate glass alteration rim: A multiscale approach to investigating ratelimiting mechanisms[END_REF][START_REF] Grambow | Nuclear Waste glass dissolution: Mechanism, model and application in[END_REF][START_REF] Van Iseghem | GLAMOR -OR HOW WE ACHIEVED A COMMON UNDERSTANDING ON THE DECREASE OF GLASS DISSOLUTION KINETICS[END_REF]. However, these processes depend on several factors such as the water composition, water renewal, the near-field materials and the host rock. When water will be in direct contact with the glass, it will be at equilibrium with the near-field environment. It will be rich in cations and anions and its composition could impact glass alteration. It has been showed that in the hydrolysis process, cations in the solution react with the glass surface area creating Si-O-M sites (M represents the cations), weakening the Si-O bonds, which increases the hydrolysis rate an thus the alteration rate (Jollivet et al., 2012b). Moreover, once the gel formed, magnesium contained in the solution induces the formation Mg-silicates at the surface of the gel, which consumes silicon from the glass and increases glass alteration [START_REF] De Echave | Effect of clayey groundwater on the dissolution rate of SON68 simulated nuclear waste glass at 70[END_REF][START_REF] Fleury | SON68 glass dissolution driven by magnesium silicate precipitation[END_REF]Jollivet et al., 2012a;[START_REF] Neeway | Dissolution mechanism of the SON68 reference nuclear waste glass: New data in dynamic system in silica saturation conditions[END_REF]. Most of the magnesium precipitates in secondary phases and once all consumed, the alteration rate can reach a value close to the one observed in deionized water [START_REF] Rolland | Effect of Groundwater on the Dissolution Rate of the Simulated Nuclear Waste Glass SON68 under Gamma Irradiation[END_REF]. Furthermore, near-field materials, such as the claystone and the carbon steel overpack, seem to alter the mechanisms already stablished. Indeed, iron and its corrosion products have been seen to enhance glass alteration. A first approach explaining this increase is the sorption of silicon on the corrosion products, such as magnetite, siderite and goethite [START_REF] Godon | SON68 Glass Alteration Enhanced by Magnetite[END_REF][START_REF] Jordan | Sorption of silicates on goethite, hematite, and magnetite: Experiments and modelling[END_REF][START_REF] Mayant | Surface site density, silicic acid retention and transport properties of compacted magnetite powder[END_REF]Michelin et al., 2013a). This phenomenon is limited over time by the saturation of the corrosion products surface sites [START_REF] Philippini | Sorption of silicon on magnetite and other corrosion products of iron[END_REF]. A second approach is the precipitation of Fe-silicates at the surface of the gel [START_REF] De Combarieu | Glass-iron-clay interactions in a radioactive waste geological disposal: An integrated laboratory-scale experiment[END_REF][START_REF] Dillmann | Effect of natural and synthetic iron corrosion products on silicate glass alteration processes[END_REF][START_REF] Godon | SON68 Glass Alteration Enhanced by Magnetite[END_REF][START_REF] Mcvay | Effect of Iron on Waste-Glass Leaching[END_REF].

However, there is only few data in the literature concerning the nature of these secondary phases. Moreover, a third approach has also been considered: an iron incorporation into the gel layer. It has been suggested that iron could incorporate into the gel structure but most of the studies have shown that it incorporates into the gel porosity by precipitation of amorphous oxydrohydroxides or Fe-silicates [START_REF] Dillmann | Effect of natural and synthetic iron corrosion products on silicate glass alteration processes[END_REF]Michelin et al., 2013a;Michelin et al., 2013b;[START_REF] Rull | Comparative micro-Raman study of the Nakhla and Vaca Muerta meteorites[END_REF].

Nevertheless, the current literature reveals some gaps on those mechanisms and need to be confirmed to better predict the long term behaviour of nuclear glass.

The aim of this study is to provide further information about glass alteration in the presence of metallic iron as a model system. The leaching experiment was then carried-out using a SON68 glass (non-radioactive surrogate of the R7T7 glass, the industrial French glass for high-level radioactive waste) separated by 80 µm from an iron foil (pristine or pre-corroded) in the Callovo-Oxfordian groundwater in an anoxic environment. This study allows to compare the influence of iron on glass alteration in a confined environment, inside the crack, and in a diluted environment, face exposed to the homogeneous solution. Furthermore, information about the effect of iron precorrosion on glass alteration is provided.

Materials and methods

Experimental Setup

Glass sample

The SON68 glass was prepared by fusion of a mixture of oxides at 1150 ºC for 3 hours. Its composition is given in Table 1. The melted glass was then introduced into graphite crucibles of 2.5x2.5 cm 2 section and annealed for 2 hours at 520 ºC. A glass density of 2.75 ± 0.03 g.cm -3 was obtained by hydrostatic weighing. The glass ingots were cut with a diamond saw into coupons of 2.5x2.5x0.2 cm 3 . Each face of each coupon was polished to a glass surface roughness less than 1 µm. The polished coupons were then cut in half with a dicing saw. All coupons were ultrasonically cleaned in acetone and then in absolute ethanol. The geometric surface area of each glass coupon was 6.8 ± 0.2 cm 2 .

Leaching experiments

The experimental setup was composed of model crack samples leached at 90ºC in individual cells in the synthetic Callovo-Oxfordian clay-based groundwater solution (called then COx groundwater). The leaching solution was prepared from the COx groundwater composition at equilibrium with the clay rock calculated by the BRGM (French Geological Survey) [START_REF] Gaucher | A robust model for pore-water chemistry of clayrock[END_REF]. Its chemical composition is presented in Table 2. This solution was obtained by a mixture of different salts at different concentrations in deionized water. The pH was adjusted to a value of 6.36 at room temperature by CO 2 bubbling when all salts were dissolved. Then it was deaerated by CO 2 bubbling with 3000 ppm for one hour to eliminate the oxygen dissolved in the solution.

Three series of model crack samples were made. They consisted of one SON68 glass monolith, measured 2.5x1.2x0.1 cm 3 and one metallic iron foil of 2.5x1.2x0.0125 cm 3 (99.99% purity, Goodfellow) (Figure 1). Two of the model cracks contained a pure iron foil and one sample contained a pre-corroded iron foil replacing the pure iron foil. The pre-corroded iron foil was obtained by introducing a pure iron foil in the COx groundwater solution at 90 ºC. After 30 days, the pre-corroded iron foil was extracted and used to prepare the glass-pre-corroded iron sample. Once the model crack was assembled, the sample constituted by a glass coupon-pre-corroded iron foil was introduced in the same solution where the iron foil was pre-corroded and it was then leached for 14 days.

The model crack is presented on Figure 1 and its preparation is detailed as follow.

The two materials were separated by two PTFE (PolyTetraFluoroEthylene) threads of a diameter of 80 µm. A small amount of high temperature silicone instant gasket was introduced between both materials to ensure that they will stick together. This spacing favors the interaction between both materials and was defined as the confined environment. The external sides open to the solution were defined as the diluted environment. In each case, the system contained a model crack sample introduced in a stainless-steel vessel sustained in a Teflon basket and leached in 30 mL of the COx solution. All these leaching experiments were performed under anoxic conditions, in a glovebox under nitrogen atmosphere (P(O 2 ) < 10 ppm). Samples were altered in the cells for 14 days and 105 days when the pristine iron foil was used, and for 14 days when the pre-corroded iron foil was used. A blank experiment was also carried out for 14 days where iron was replaced by a PTFE foil (inert material for glass alteration). At the end of each experiment, samples were extracted in the glovebox, dried at room temperature for 3 days and then embedded together with a very fluid epoxy resin.

Analytical techniques

Leachate analysis

In this case solution analysis allowed to obtain a global approach of glass alteration in the presence of iron. The pH of the leachate was measured at the end of each experiment at room temperature with an uncertainty measurement of ± 0.1. The leachate was analyzed by an ICP-AES Jobin Yvon JY66 system after acidification with ultra-pure grade HNO 3 at the beginning and the end of the experiment.

Concentrations of Si, B, Li, Ca, Na, Fe and Mg were determined. The analytical uncertainty was ± 3%. Chlorides and sulfates were also analyzed with a Metrosep A Supp 16 ionic chromatography device to follow closely the concentrations of these elements. The analytical uncertainty was ± 10%.

Solid characterization

The pristine iron foil was observed both directly and at the edge after embedding. All embedded samples were polished to a micron roughness and the surface of the polished cross sections were coated with carbon for observations and chemical analysis. Samples were first characterized by Scanning Electron Microscopy (SEM) coupled with an energy dispersive X-ray spectrometer device (EDS) with an environmental scanning electron microscope on a FEI Quanta 200 ESEM FEG microscope (partial pressure of 90 Pa, high tension between 3 and 15 kV) at the Marcoule Institute for Separative Chemistry (France).

The pre-corroded iron foil as the polished cross section were also analyzed by micro-Raman spectroscopy using a Horiba Jobin-Yvon Xplora plus µ-Raman spectrometer coupled with a microscope. The excitation laser used was of 532 nm (green) focused on a 0.7x0.7 µm 2 spot and collected through an x100 objective. The laser power used was of 1.4 mW, low enough to avoid any iron oxidation [START_REF] Neff | Contribution of archaeological analogs to the estimation of average corrosion rates and long term corrosion mechanisms of low carbon steel in soil[END_REF].

Moreover, in order to study samples at a nanometer scale, a thin foil with a thickness of approximately 100 nm was prepared by a Focused-Ion Beam (FIB) technique with a dual-beam SEM/FIB microscope (LYRA GM TESCAN) at Tescan Analytics (Fuveau, France). Transmission Electron Microscopy (TEM) coupled with an energy dispersive X-ray Spectrometer Device (EDS) analysis were performed with a JEOL 2010F microscope at CP2M (Marseille, France) on the thin foil.

Results

Solution analysis

The solution was analyzed at the beginning and the end of the pre-corrosion of the iron foil and the model crack samples experiments (Table 3). These results represent only the bulk solution composition and do not give information about possible interactions in the confined environment between the glass and the iron foil nor exactly about the alteration in the diluted environment.

After pre-corrosion of the iron foil, it was observed that the silicon concentration in the COx groundwater decreased as the iron concentration increased. However, Mg, Ca and Sr concentrations remained around the same value as the initial solution. It is important to highlight that the pH value does not change significantly at the end of the iron pre-corrosion. When the cells were opened, it was possible to observe some precipitates in the solution. The iron foil aspect changed, which is in good agreement with the Fe release observed from the solution analysis indicating that iron corrosion occurred.

Concerning the model cracks experiments, when glass and iron were both present in the system, B, Li and Na concentrations increased. Since these three elements are mobile and soluble elements from the glass and are used as glass alteration tracers, these results confirmed that glass was altered significantly under these conditions. Some variations were also observed in Si, Ca and Fe concentrations as an increase of pH values. The pH increase (around 0.8 pH unit) was the same when a Teflon foil, replacing the iron foil, was used. However, the pH value was higher when the iron foil was first pre-corroded (difference of 1 pH unit). On the blank experiment, which corresponds to a glass coupon altered in COx groundwater without an iron source, the concentration of these tracers elements were under the detection limit (<0.5 mg.L -1 for boron and lithium concentrations). This indicates that glass is more altered when there is an iron source in the system. It was also possible to observe on the blank experiment an increase on the silicon concentration contrary to the systems containing an iron source. Moreover, the pH value is the same as the one observed in the sample containing a pristine iron foil, the pH is then controlled by glass alteration. A small amount of iron was observed on the blank experiment which could be associated with an impurity in the stainless-steel vessel. Moreover, it can be highlighted that magnesium concentrations had no significant variations, even if pH values were able to favor Mg silicates precipitation under these conditions (Jollivet et al., 2012a).

Solid characterizations

Pre-corrosion of the Iron foil

SEM observations of the surface of the pre-corroded iron foil displayed 3 different phases (defined as cubes, sticks and sheets) and a corroded surface layer (Figure 2). EDS analyses showed that the layer composed by the corroded iron surface contained mainly Fe, Si and O. The cubes contained a mixture of Fe, O and Ca, the sticks contained mainly Ca and O and the sheets contained mostly Si and Fe. EDS analysis on the polished cross section of the pre-corroded iron foil confirmed the surface analysis. Moreover, global observations of the cross section showed that the corrosion layer appears to be cracked (Figure 2). This layer is inhomogeneous all along the iron foil. Using different images, a corrosion thickness of approximately 16 ± 3 µm was determined. The pristine iron foil had an initial thickness of about 125 ± 3 µm. After pre-corrosion, this value varied from 80 µm to 120 µm depending on the observed area.

Raman Spectroscopy was also performed at different spots at the surface of the iron foil. It was possible to distinguish two different spectra: one from the sticks and another from the corroded surface layer (Figure 2). It was not possible to use this technique to identify the sheets and cubes structures since it was difficult to differentiate them using the microscope. The corroded surface layer Raman spectrum was identified as siderite. It was possible to obtain its spectra since it is stable under the laser power. The main bands matches with published siderite bands [START_REF] Hanesch | Raman spectroscopy of iron oxides and (oxy)hydroxides at low laser power and possible applications in environmental magnetic studies[END_REF] : the 184, 287 and 731 cm -1 bands and the more intensive at 1085 cm -1 , concerning the C-O bond. Concerning phases named as sticks, the Raman spectrum obtained was identified as aragonite, a calcium carbonate with an orthorhombic crystal system. The aragonite Raman spectrum is complex, and it has been published that it has 30 different bands with different intensities [START_REF] De La Pierre | The Raman spectrum of CaCO3 polymorphs calcite and aragonite: A combined experimental and computational study[END_REF]. The Raman spectrum presented in here shows 13 of these bands: at 111, 178,246,258,700,703,715,852,1462 and 1575 cm -1 , two intensive bands at 151 and 204 cm -1 and the most intensive band at 1080 cm -1 , concerning the C-O band.

Glass alteration in the presence of iron

The glass coupons were altered in the presence of a pre-corroded iron foil for 14 days and in the presence of a pristine iron foil for 14 and 105 days in the same conditions. Results at the end of each experiment are presented as followed. A blank experiment, replacing iron by a Teflon foil was also performed. In this last case, SEM results are not presented in this paper since the alteration layer was too thin to be observed using this technique (glass alteration thickness <300 nm). This result is in good agreement with the absence of glass tracer elements in the solution over the detection limit. It can thus be clearly assumed that the glass alteration is higher when glass is altered in the presence of iron.

Impact of pre-corrosion of the iron foil

When the pre-corroded iron foil was used, it was possible to observe by SEM analysis an alteration layer on the glass phase and two different corrosion layers (corrosion products + secondary phases) on the iron foil. Some secondary phases were also observed (Figure 3).

The alteration layer was highlighted using backscattering electrons due to its depletion in alkaline and alkaline earth elements. It was observed that at the edges, the glass alteration was larger. This could be due to a higher renewal of the solution at the edges increasing then glass alteration [START_REF] Chomat | Effect of leaching-driven flow on the alteration kinetics of an ideal crack in SON68 glass[END_REF]. These observations are in good agreement with results presented by Dillmann et al. [START_REF] Dillmann | Effect of natural and synthetic iron corrosion products on silicate glass alteration processes[END_REF]. In the confined environment, at the center of the sample, it was possible to measure the thickness of the glass alteration layer using different images. All measurements were made using only SEM images assuming an isovolumetric glass alteration. All results are summarized in Table 4. For the glasspre-corroded iron foil sample, an average value of approximately 1.3 ± 0.4 µm was obtained. However, in the diluted environment, the average value obtained was of approximately 2.3 ± 0.3 µm. These different values could suggest that, in the confined environment, silica concentration in the solution was higher than in the diluted environment.

Moreover, EDS results showed that the glass alteration was depleted in alkali such as Na and enriched with Fe. Moreover, some rare earth precipitates enriched in La, Nd were also observed all along the alteration layer. At the surface of the alteration layer, some secondary phases rich with Fe and Si were also observed.

On the iron phase, it was possible to notice by SEM analysis two different corrosion layers. (Figure 3). The layer next to the pristine iron foil was identified as the corroded layer formed during the pre-corrosion of the iron foil. It was rich with Ca and Fe and some traces of Si. The second layer, closer to the glass, was defined as secondary phases which were also rich with Si, Ca and Fe. These phases, containing elements from the solution and the glass, were also formed at the surface of the glass. Furthermore, the iron surface exposed to the diluted environment was more corroded than the one exposed to the glass. Some precipitates, rich with Ca or Si and Fe, also reached the glass assuming that there could be a higher consumption of silicon on this area. However, no difference was observed on glass alteration thickness when secondary phases reached the glass.

Using a pristine iron foil, after 14 days of leaching, SEM analysis showed similar results than the glass-pre-corroded iron sample (Figure 3): an alteration layer on the glass and the formation of precipitates on the iron phase. However, only one corrosion layer was observed on the iron foil. Under these conditions, using different images, the thickness of the alteration layer in the confined environment at the centre of the sample was 1.3 ± 0.3 µm. This value is the same as the one observed when the pre-corroded iron foil was used. This could indicate that the pre-corrosion of the iron foil did not have an influence on glass alteration.

Moreover, the alteration layer in the diluted environment was of approximately 3.6 ± 0.5 µm, higher than the one formed when the pre-corroded iron foil was used. As seen in the previous case, when secondary phases formed at the surface of the iron foil reached the glass, no difference on glass alteration thickness was observed. In conclusion, under these conditions, there was no significant differences highlighted whether the iron source was pre-corroded or not.

Impact of leaching duration

Characterizations at the nanoscale on the altered glass of the 14 days glass-pristine iron foil sample were also performed. TEM observations showed that iron is present in all the alteration layer. However, its quantity decreased from the outer layer to the pristine glass (Figure 4). EDS analysis also highlighted a thin layer enriched with Mg in the gel, indicating a possible incorporation of Mg in the alteration layer or the formation of Mg-Si secondary phases. Moreover, the alteration layer was composed of two distinct areas. There was a first dense layer, next to the pristine glass, depleted in Na, Si and Al and enriched with Fe. Both its morphology and its composition suggest that this is the gel zone. There is also a second zone, containing sticks and sheets, rich with Si and Fe. This could be suggested as the secondary phases area which is formed at the surface of the gel.

Concerning the secondary phases, the sticks were rich with Fe and Si with some traces of Mg and with a Fe/Si ratio of 1.76. The sheets were also rich with Si and Fe but with some traces of Na, Mg, Ca, Mo and a Fe/Si ratio of 1.1. This could indicate the formation of two different Fe-Si secondary phases.

After 105 days of leaching, SEM analysis on the glass-pristine iron foil sample allowed to identify, in the confined environment, a glass alteration thickness at the centre of the sample of 2.2 ± 0.4 µm, thicker than the 14 days glass-pristine iron foil sample (Figure 3 and Table 4). This means that even if a passivating layer was formed, the alteration of the glass increased versus time. In the diluted environment the thickness of the glass alteration layer was of approximately 8.0 ± 0.7 µm, twice higher than the 14 days sample, also confirming that glass alteration increased versus time.

EDS results showed that there is also an important amount of Fe and Mg incorporated in the alteration layer. As seen in the 14 days leaching experiment, the iron incorporation decreased from secondary phases to the pristine glass. In the 105 days sample, the Mg incorporation is all along the alteration layer. No concentration at the surface of the gel was observed, as seen in the 14 days sample.

Under these conditions, the quantity and size of secondary phases were also higher than the 14 days sample: in the diluted environment as in the confined environment.

Secondary phases were rich with Si and Fe, and some others rich with Ca. However, using only SEM images it was not possible to observe in the 105 days sample the two different morphologies of secondary phases observed by TEM analysis on the 14 days sample. Moreover, some rare earth precipitates were formed in the alteration layer. After 105 leaching days, these precipitates are larger than those observed in the 14 days sample.

These results displayed that the glass alteration as the iron corrosion increased versus time. Moreover, it appeared that the size and amount of the precipitates formed under these conditions also increased versus time without changing its morphology.

Discussion

In this study, it was observed that glass alteration increases in the presence of an iron source. Comparing results after 14 days of leaching using a pristine iron foil or a pre-corroded iron foil, the glass alteration thickness was of approximately 1.3 µm in the confined environment for both experiments. However, in the diluted environment, the alteration thickness was different, of approximately 2.3 µm when a pre-corroded iron foil was used, and of approximately 3.6 µm when the pristine iron was used.

Moreover, comparing results using only a pristine iron foil, it was observed that glass alteration increased versus time. From 14 days of leaching to 105 days, in the confined environment, the increase of the glass alteration was from 1.3 µm to 2.2 µm and, in the diluted environment, the increase of the alteration layer was from 3.6 µm to 8 µm. This increase of glass alteration in the presence of an iron source has already been reported in the literature and different mechanisms have been proposed to explain it [START_REF] Godon | SON68 Glass Alteration Enhanced by Magnetite[END_REF]Michelin et al., 2013a) : precipitation of Fe-Si secondary phases, incorporation of iron in the gel and sorption of silicon on iron corrosion products.

Moreover, the use of a synthetic COx solution could also induce the precipitation of Mg-Si secondary phases, increasing also glass alteration [START_REF] Debure | Borosilicate glass alteration driven by magnesium carbonates[END_REF][START_REF] Debure | Dolomite effect on borosilicate glass alteration[END_REF][START_REF] Fleury | SON68 glass dissolution driven by magnesium silicate precipitation[END_REF]Jollivet et al., 2012a). In the presence of corrosion products, it has been proposed that when glass is altered, a thicker glass alteration layer is formed than when pure iron is used [START_REF] Rébiscoul | Glass-Iron-Clay interactions in a radioactive waste geological disposal: a multiscale approach[END_REF].

However, under the experimental conditions presented in the present study, the use of a pre-corroded iron foil instead of a pristine iron foil gave no significant difference.

One may wonder the reason for these similarities. Under the present experimental conditions, an iron incorporation in the gel has been observed, but how does this incorporation take place in these systems? Furthermore, if iron is involved in the precipitation of Fe-Si secondary phases, what are these secondary phases? These different points will be discussed as followed.

Iron corrosion and its impact on glass alteration

Under the present experimental conditions in an anoxic environment, the main corrosion product obtained was siderite. This is in good agreement with data from the literature under similar experimental conditions [START_REF] Neff | Corrosion of iron archaeological artefacts in soil: characterisation of the corrosion system[END_REF][START_REF] Romaine | Electrochemical synthesis and characterization of corrosion products on carbon steel under argillite layers in carbonated media at 80 degrees C[END_REF][START_REF] Saheb | Iron corrosion in an anoxic soil: Comparison between thermodynamic modelling and ferrous archaeological artefacts characterised along with the local in situ geochemical conditions[END_REF]. No other iron oxides or iron carbonates were characterized in our system. However, in similar conditions, chukanovite could also precipitate [START_REF] Chaou | The French SON68 Glass Vapor Hydration under Different Atmospheres[END_REF][START_REF] Michelin | Archeological slag from Glinet: An example of silicate glass altered in an anoxic ironrich environment[END_REF][START_REF] Odorowski | Effect of metallic iron on the oxidative dissolution of UO2 doped with a radioactive alpha emitter in synthetic Callovian-Oxfordian groundwater[END_REF][START_REF] Saheb | Iron corrosion in an anoxic soil: Comparison between thermodynamic modelling and ferrous archaeological artefacts characterised along with the local in situ geochemical conditions[END_REF], but it has not been observed in this study. It is important to highlight that under the experimental conditions presented in this paper, no calcite was observed as presented in previous studies [START_REF] Dillmann | Effect of natural and synthetic iron corrosion products on silicate glass alteration processes[END_REF][START_REF] Schlegel | Corrosion of metal iron in contact with anoxic clay at 90 °C: Characterization of the corrosion products after two years of interaction[END_REF][START_REF] Schlegel | Alteration of nuclear glass in contact with iron and claystone at 90 °C under anoxic conditions: Characterization of the alteration products after two years of interaction[END_REF]. Instead, aragonite was identified as the main calcium carbonate, with a orthorhombic crystal system [START_REF] De La Pierre | The Raman spectrum of CaCO3 polymorphs calcite and aragonite: A combined experimental and computational study[END_REF]. Calcium carbonates can precipitate in different crystalline structures such as calcite, aragonite, vaterite.

Aragonite can be formed at temperature superior to 50 ºC and seems to be more stable than others calcium carbonates (Wray and Daniels, 1957) which is in agreement with the present experimental conditions (90ºC).

The pre-corrosion of an iron foil for 30 days led to a partial corrosion of the iron foil: the pristine iron foil was covered by iron corrosion products all along its surface.

Previous studies have shown that when only iron corrosion products are used, such as magnetite or siderite, the increase of glass alteration is less significant than when metallic iron is used (Michelin et al., 2013a;[START_REF] Rébiscoul | Glass-Iron-Clay interactions in a radioactive waste geological disposal: a multiscale approach[END_REF]. Therefore, according to the literature, in the confined environment, when the glass-pre-corroded iron model crack was used, glass alteration should have been controlled only by the iron corrosion products and it should have been lower than in the glass-pristine iron foil model crack. However, the thickness of the glass alteration layer was the same in both cases (approximately 1.3 µm). It seemed that, in the systems presented here, iron availability in the solution was the same in both cases. Since the iron corrosion layer appeared to be cracked and inhomogeneous all along the surface of the pristine glass, no protective layer at the surface of the iron foil was formed, inducing a direct contact between the pristine iron and the solution during all the leaching duration: pristine iron controlled then the iron release in the solution in both experiments.

In the literature, it has also been reported that when iron corrosion products such as magnetite or siderite are used, sorption of silicon could take place with a limited impact [START_REF] Godon | SON68 Glass Alteration Enhanced by Magnetite[END_REF]Michelin et al., 2013a;[START_REF] Neill | Various effects of magnetite on international simple glass (ISG) dissolution: implications for the long-term durability of nuclear glasses[END_REF][START_REF] Philippini | Sorption of silicon on magnetite and other corrosion products of iron[END_REF]. This could have been observed in the glass-pre-corroded iron sample. However, it was not possible to observe any silicon sorption from the solid characterizations used in this study. This mechanism was certainly negligible compared to other mechanisms taking place at the time.

Furthermore, a difference between the thickness of the alteration layer in the diluted environment was observed: the alteration thickness was greater when a pristine iron foil was used than when a pre-corroded iron foil was used (2.3 µm compared to 3.5 µm after 14 leaching days). However, it can be noticed that before preparing the glass-pre-corroded iron model crack, the iron foil was first introduced for 30 days in the COx solution: an equilibrium between the solution and the iron foil could have taken place. After 14 days of leaching, the pH bulk of the glass-pre-corroded iron sample was 8.1, contrary to the glass-pristine iron model crack sample, where the pH bulk was first 7.1. In the literature, it has been shown that glass alteration depends on the pH value [START_REF] Gin | SON 68 nuclear glass alteration kinetics between pH 7 and pH 11.5[END_REF]: when the pH value decreased, the passivating properties of the gel also decreased, increasing then glass alteration. The difference on the thickness of the glass alteration layer could thus be related to the pH value: in the diluted environment it can be assumed that a more passivating glass alteration layer was formed when the pre-corroded iron foil was used (pH = 8.1) than when a pristine iron foil was used (pH = 7.1).

Glass alteration versus time

The glass alteration thickness of each sample was measured from different SEM images. If the glass was altered at its initial rate in the COx groundwater, the thickness of the altered glass can be calculated using the forward dissolution rate determined by Jollivet et al. of r 0 = 1.7 g.m -2 .d -1 (Jollivet et al., 2012b). With such an alteration rate, after 14 days of leaching, these calculations lead to a glass alteration layer of approximately 9 µm, and after 105 days of leaching, of approximately 66 µm.

Even when iron was present in the system and led to an increase of glass alteration, theses alteration thicknesses were not reached. These results showed thus that glass alteration reached at least the drop alteration rate regime and are in good agreement with the formation of a protective gel layer.

In all samples, the thickness of the alteration layer was different in the confined environment and in the diluted environment. These observations could be related to the local solution characteristics. In the confined environment, the local transport of water could be controlled by diffusion and the solution was poorly renewed [START_REF] Chomat | Effect of leaching-driven flow on the alteration kinetics of an ideal crack in SON68 glass[END_REF][START_REF] Dillmann | Effect of natural and synthetic iron corrosion products on silicate glass alteration processes[END_REF]. In the diluted environment, no saturation of silica concentration was observed in the bulk solution, as it can be qualitatively observed from solution analysis (Table 3). Since the passivating properties of the gel depends on local physical-chemical conditions, it can then be though that there was a higher silica concentration in the solution at the centre of the crack, enabling the formation of a more passivating gel layer than in the diluted environment [START_REF] Pèlegrin | Structural evolution of glass surface during alteration: Application to nuclear waste glasses[END_REF][START_REF] Rebiscoul | Protective properties and dissolution ability of the gel formed during nuclear glass alteration[END_REF].

Furthermore, comparing results on the glass alteration thickness after 14 days and 105 days of leaching, it is worth noting that the thickness of the glass alteration layer increased versus time in both environment (Figure 5). Even though, in this study, it was possible to compare only two different leaching times, it is possible to highlight that the glass alteration rate also decreased versus time: after 91 days of leaching, in both environments the glass alteration layer was twice times thicker. This could suggest that, even with an unlimited iron source, a protective gel was formed decreasing glass alteration over time.

The passivating properties of the gel could be affected by the incorporation of iron or magnesium in the gel layer [START_REF] Rebiscoul | Morphological evolution of alteration layers formed during nuclear glass alteration: new evidence of a gel as a diffusive barrier[END_REF][START_REF] Rull | Comparative micro-Raman study of the Nakhla and Vaca Muerta meteorites[END_REF]. This incorporation could be explained as followed: precipitation of nanometric secondary phases in the gel pores or the incorporation of these two elements in the glass alteration layer as charge compensator for AlO 4 -and ZrO 6 2-, such as in the case of Ca [START_REF] Chave | Glass-water interphase reactivity with calcium rich solutions[END_REF][START_REF] Mercado-Depierre | Antagonist effects of calcium on borosilicate glass alteration[END_REF][START_REF] Thien | The dual effect of Mg on the long-term alteration rate of AVM nuclear waste glasses[END_REF]. In a previous study, similar to the one presented in this paper, where two SON68 monoliths separated by an iron source where altered in the COx groundwater, it was observed that the iron incorporation is due of an incorporation of Fe-Si precipitates in the gel pores, in the form of greenalite [START_REF] Dillmann | Effect of natural and synthetic iron corrosion products on silicate glass alteration processes[END_REF]. Moreover, in another study, using a simple glass and with FeCl 2 constant additions in the solution, it has been proposed that there could be both incorporation into the gel structure, near the pristine glass, and precipitation of secondary phases in the gel pores, near secondary phases [START_REF] Aréna | Impact of Fe, Mg and Ca elements on glass alteration: Interconnected processes[END_REF]. If there was an incorporation of Fe-silicates in the gel pores, an important amount of these secondary phases could clog these pores increasing the passivating properties of the gel [START_REF] Jollivet | Investigation of gel porosity clogging during glass leaching[END_REF]Michelin et al., 2013b). In this study, it was possible to see that the amount of iron decreased from secondary phases to the pristine glass: from approximately 15 mol.% in secondary phases to 9 mol.% in the gel and 0.7 mol.% in the pristine glass which is in good agreement with the theoretical SON68 composition (Fe = 0.8 mol.% in the glass). Iron must have been mostly provided by the solution but it could also have been provided by the glass. Nevertheless, with the analytical techniques used in this study, it was not possible to identify if there was an incorporation of iron into the gel structure or if there was a precipitation of secondary phases into the gel pores.

However, using TEM analysis results, a regular decrease of iron in the glass alteration layer lead to consider an iron diffusion into the alteration layer instead of a precipitation of iron secondary phases.

It is not yet certain how iron incorporates into the gel, but it can be confirmed by the important amount of Fe-Si secondary phases formed at the surface of the gel, that its precipitation could be the main source of the increase of glass alteration in the presence of an iron source.

Precipitation of secondary phases

Previous studies have shown that glass alteration processes are mainly associated with the precipitation of secondary phases [START_REF] Burger | Impact of iron on nuclear glass alteration in geological repository conditions: A multiscale approach[END_REF][START_REF] Fleury | SON68 glass dissolution driven by magnesium silicate precipitation[END_REF][START_REF] Inagaki | Effects of redox condition on waste glass corrosion in the presence of magnetite[END_REF]Jollivet et al., 2012a;Michelin et al., 2013b). The precipitation of Fe-silicates or Mg-silicates increases silicon consumption, decreasing the gel passivating properties. It sustains glass dissolution and it can then be considered as a major driving force behind glass alteration.

Fe-silicates

This phenomenon has been verified in the present systems: Fe-silicates were formed in every sample containing an iron source. These secondary phases were located mainly at the surface of the gel or near the iron corrosion products. On a recent study with a simplified glass, it has been suggested that the Fe-silicates formed are phyllosilicates belonging to the smectite group [START_REF] Arena | Impact of iron and magnesium on glass alteration: Characterization of the secondary phases and determination of their solubility constants[END_REF][START_REF] Burger | Impact of iron on nuclear glass alteration in geological repository conditions: A multiscale approach[END_REF][START_REF] Schlegel | Alteration of nuclear glass in contact with iron and claystone at 90 °C under anoxic conditions: Characterization of the alteration products after two years of interaction[END_REF]. Moreover, another recent study using SON68 glass model cracks with an iron source incorporated in the aperture of the model crack, has

shown that phyllosilicates, such as nontronite and greenalite, are formed at the surface of the gel, or in the gel pores [START_REF] Dillmann | Effect of natural and synthetic iron corrosion products on silicate glass alteration processes[END_REF]. Under the experimental conditions presented here, from TEM-EDS analysis, it was not possible to identify these phases. However, two different structures of Fe-silicates at the surface of the gel were observed: one containing a Fe/Si ratio of 1.1 with a small concentration of Na and traces of Mg, Ca and another containing a Fe/Si ratio of 1.76 with traces of only Na, Mg. The different Fe/Si ratios and morphologies suggest thus that, under the present experimental conditions, two different Fe-silicates precipitated.

Mg-silicates

In previous studies, when there is a magnesium source, precipitation of Mg-silicates has also been observed [START_REF] Debure | Borosilicate glass alteration driven by magnesium carbonates[END_REF][START_REF] Fleury | SON68 glass dissolution driven by magnesium silicate precipitation[END_REF]Jollivet et al., 2012a). There is a pH threshold value above which its precipitation is possible: at 90ºC it has been proposed to be at pH = 6.5 (Jollivet et al., 2012a). The pH values, obtained under the experimental conditions presented in this paper, were above the pH threshold value, favouring then Mg-silicate precipitation. Results on solution analysis showed no significant consumption of magnesium leading to the assumption that there was no significant Mg-silicate precipitation. However, EDS analysis highlighted a magnesium incorporated in the glass alteration layer. For the glass-iron foil 14 days sample, TEM-EDS analysis showed a small amount of Mg incorporation and it was concentrated at the surface of the gel. However, on the glass-iron foil 105 days sample, an incorporation of Mg all along the alteration layer was highlighted by SEM analysis. This could indicate that Mg diffused slowly versus time from the solution into the gel. In both cases, the amount of Mg incorporated into the alteration layer was under 2 mol.%, indicating that there was a small consumption of Mg and explaining why no significant differences were observed on Mg concentration in the solution (Table 3). It is important to highlight that, under the present experimental conditions, the precipitation of Fe-silicates is then highly favoured than the precipitation of Mg-silicates.

Conclusion

The aim of the present study was to investigate the SON68 glass alteration behaviour, in a synthetic groundwater, in the presence of an iron source under anoxic conditions. The influence of the iron source was studied using pure iron and a precorroded iron foil, and two different environments were also compared: a confined (inside de crack) and diluted (face exposed to the homogeneous solution)

environment. All results are consistent between them.

Pre-corrosion of an iron foil led to a partial iron corrosion: a crack and inhomogeneous corrosion layer was formed at the surface of the pristine iron. Iron corrosion led mainly to the precipitation of iron carbonates and calcium carbonates, where siderite and aragonite were identified. The pre-corrosion of the iron foil seemed to have no significant influence on glass alteration in the confined environment: there was the same iron availability in the solution in both systems (precorroded iron foil or pristine iron foil, 14 days).

In all conditions (iron pre-corrosion or not, alteration duration), a difference between both environments was also highlighted. In the diluted environment, glass alteration was higher than in the confined environment. It seemed that a more protective gel was formed in the confined environment than in the diluted environment indicating that glass alteration depended on local physico-chemical conditions. Moreover, it is worth noting that, in both environments, the glass alteration rate decreased versus time: the passivating properties of the gel increased then versus time.

This study confirms that glass alteration is higher in the presence of an iron source than without any iron source in both environments. The main processes observed in this study were the incorporation of iron in the gel formed by glass alteration and the precipitation of Fe-Si secondary phases both on the surface of the gel and the iron foil. The iron concentration in the gel diminished from secondary phases to the pristine glass indicating that iron was mainly provided by the solution. Secondary phases were observed in all experiments containing an iron source, and they were mainly composed with iron and silicon. Two different Fe-Si phases were differentiated in terms of morphology and Si/Fe ratio but with the techniques proposed in this study, it was not possible to identify them.

Further experiment of the measurement of the iron ratio Fe(II)/Fe(III) must be performed along all the alteration layer in order to provide more information about the iron incorporation. This information could point out if iron was provided by the glass (incorporated as Fe(III)) or by the iron foil (Fe(II) in anoxic conditions). Moreover, in order to provide more data for modelling programs and better understand the longterm behaviour of nuclear glass under disposal conditions, it is necessary to acquire information about the nature and crystallinity of the secondary phases formed, containing both Si and Fe, which are involved as the main mechanism responsible for the increase of glass alteration in the presence of an iron source. These results allow to have a first approach about the alteration of a SON68 glass separated by 80 µm from an iron source. More experiments will be carried out with a α-doped SON68 glass (α mean range in water of 40 µm) to better understand the influence of an α irradiation on the alteration of a nuclear glass under environmental conditions.

Table and figure captions

Table 1: Theoretical SON68 glass compositions in oxide weight percent.

Table 2: Theoretical chemical composition of the Callovo-Oxfordian clayey groundwater (mmol.L -1 ) and pH measured at 20 °C.

Table 3: pH values and results by ICP-AES of the solutions composition before (initial solution) and after the experiments (mg.L -1 ). GFeI stands for "glass-iron interaction", Bk for the glass-Teflon blank experiment, PreCor for pre-corroded iron foil.

Table 4: Thickness of the glass alteration layer in the confined and in the diluted environment of all samples. GFeI stands for "Glass-iron interaction", Bk for the blank experiment, GFeI-PreCor for "Glass-pre-corroded iron foil".

Figure 1 :

 1 Figure 1: Experimental setup: introduction of a model crack in a stainless-steel vessel

Figure 2 :

 2 Figure 2: SEM images on the surface of the pre-corroded iron foil and on a polished

Figure 3 :

 3 Figure 3: SEM observations on the model crack experiments using a pristine iron foil

Figure 4 :

 4 Figure 4: TEM image and EDS analysis on the alteration layer of the SON68 glass in

Figure 5 :

 5 Figure 5: Increase of the thickness of the glass alteration layer in the confined (centre

Figure 5

 5 Figure 5

Table 1

 1

	Oxides	SiO 2	B 2 O 3 Na 2 O Al 2 O 3 CaO	ZnO	Li 2 O Fe 2 O 3 P 2 O 5 NiO
	SON68	45.6	14.06	9.89	4.93	4.05	2.51	1.99	2.92	0.28 0.74
	Oxides Cr 2 O 3	ZrO 2	Cs 2 O SrO	Y 2 O 3 MoO 3 MnO 2 CoO Ag 2 O CdO
	SON68	0.51	2.66	1.42	0.33	0.2	1.71	0.72	0.12	0.03 0.03
	Oxides Sb 2 O

3 SnO 2 TeO 2 BaO La 2 O 3 CeO 2 Pr 2 O 3 Nd 2 O 3

	Table 4
		Sample	Confined environment	Diluted environment
		GFeI-PreCor 14 days		1.3 ± 0.4 µm			2.3 ± 0.3 µm
		GFeI 14 days		1.3 ± 0.3 µm			3.6 ± 0.5 µm
	SON68	0.01 105 days 0.02 GFeI	0.23	0.6 2.2 ± 0.4 µm 0.9	0.93	0.44 8.0 ± 0.7 µm 2.13
		Bk 14 days		< 300 nm			< 300 nm

Acknowledgements

The authors are grateful to the CEA for their financial support. We would like to thank also Tescan Analytics (Fuveau, France) for the FIB cross section, and Martiane CABIE of CP2M (Marseille, France) for the TEM analysis.

Technologies in the Materials and Nuclear Industries Xii. Amer Ceramic Soc, Westerville, Vienna, J.D., Ryan, J.V., Gin, S., Inagaki, Y., 2013. Current