Marie Giroudon
email: marie.giroudon@insa-toulouse.fr

Matthieu Peyre Lavigne

Cédric Patapy

Alexandra Bertron

Matthieu Peyre Lavigne

Laboratory assessment of the contribution of aggressive to concrete chemical compounds to the degradation of Portland cement-based materials during anaerobic digestion

Keywords: cement, anaerobic digestion, leaching, acetic acid, ammonium nitrate, sodium bicarbonate

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Introduction

Anaerobic digestion is a biological process allowing the degradation of organic matter into biogas and digestate by the action of microorganisms. The biogas, mainly composed of methane (CH4) and carbon dioxide (CO2) [START_REF] Lastella | Anaerobic digestion of semi-solid organic waste: biogas production and its purification[END_REF][START_REF] Lesteur | Alternative methods for determining anaerobic biodegradability: A review[END_REF][START_REF] Weiland | Biogas production: current state and perspectives[END_REF], can be used in many ways, the most common being cogeneration (75 % of current French installations [START_REF] Atee | Statistiques filières biogaz -Juillet 2018[END_REF]), which consists of the combined production of heat and electricity. However, France is now turning towards injection into the natural gas network, which allows higher yields [START_REF] Infometha | Etat des lieux de la méthanisation en Europe, InfoMetha[END_REF]. As one of the most environmentally friendly technologies for bioenergy production [START_REF] Fehrenbach | Kriterien einer nachhaltigen Bioenergienutzung im globalen Ma\s sstab[END_REF][START_REF] Yun | Use of bio-based carbon materials for improving biogas yield and digestate stability[END_REF], biogas production is supported by the European Union and more than 300 anaerobic digestion plants are being built each year in Europe [START_REF]European Biogas Association, Statistical Report of the European Biogas Association Abriged version -2018[END_REF].

Most anaerobic digestion structures are built with concrete because this material is economic, air-and watertight and thermally insulating. The digesters contain two parts: (i) the lower zone (liquid/solid phase) where the successive degradation reactions of hydrolysis, acidogenesis, acetogenesis and methanogenesis [START_REF] Cole | Methane from Biomass: A Systems Approach[END_REF][START_REF] Evans | Environmental Biotechnology -Theory and Application[END_REF] degrade the biowaste into smaller molecules and (ii) the upper zone containing the biogas produced, and where the walls are generally protected by polymeric liners [START_REF] Nathalie Bachmann | 8 -Design and engineering of biogas plants[END_REF]. In the liquid phase, the digestion of biowaste generates complex and multicomponent environments and deterioration of cementitious materials has been observed in pilot plants and in laboratory tests, in the relatively short term [START_REF] Bertron | Biodeterioration of concrete in agricultural, agro-food and biogas plants: state of the art and challenges[END_REF][START_REF] Giroudon | Blast-furnace slag cement and metakaolin based geopolymer as construction materials for liquid anaerobic digestion structures: Interactions and biodeterioration mechanisms[END_REF][START_REF] Koenig | Biogenic acid attack on concretes in biogas plants[END_REF][START_REF] Voegel | Cementitious materials in biogas systems: Biodeterioration mechanisms and kinetics in CEM I and CAC based materials[END_REF][START_REF] Voegel | Mechanisms of cementitious material deterioration in biogas digester[END_REF]. This aggressiveness towards concrete is mainly due to the presence of three chemical compounds, associated with the presence of microorganisms in the form of biofilm: volatile fatty acids (VFA), ammonium cation and dissolved CO2 are considered to be very aggressive to concrete and their concentrations can vary greatly with time and according to the inoculum and substrate [START_REF] Fisgativa | Characterizing the variability of food waste quality: A need for efficient valorisation through anaerobic digestion[END_REF][START_REF] Li | Comparison of anaerobic digestion characteristics and kinetics of four livestock manures with different substrate concentrations[END_REF].

Studies considering the deterioration of cement matrices in fermenting biowaste [START_REF] Giroudon | Blast-furnace slag cement and metakaolin based geopolymer as construction materials for liquid anaerobic digestion structures: Interactions and biodeterioration mechanisms[END_REF][START_REF] Koenig | Biogenic acid attack on concretes in biogas plants[END_REF][START_REF] Voegel | Cementitious materials in biogas systems: Biodeterioration mechanisms and kinetics in CEM I and CAC based materials[END_REF][START_REF] Voegel | Mechanisms of cementitious material deterioration in biogas digester[END_REF] have shown that, for different biowaste/inoculum pairs -and therefore different liquid medium compositions -the intensity and the alteration kinetics are variable, and the degradation is not always a result of the predominant effect of the same metabolite. Although the deterioration induced by each chemical metabolites has been investigated in the literature [START_REF] Bertron | Durabilité des matériaux cimentaires soumis aux acides organiques : cas particulier des effluents d'élevage[END_REF][START_REF] Bertron | Degradation of cement pastes by organic acids[END_REF][START_REF] Carde | Leaching of both calcium hydroxide and C-S-H from cement paste: Modeling the mechanical behavior[END_REF][START_REF] Escadeillas | La durabilité des bétons vis-à-vis des environnements chimiquement agressifs[END_REF][START_REF] Gallé | Effect of an accelerated chemical degradation on water permeability and pore structure of cement based materials[END_REF][START_REF] Lea | The action of ammonium salts on concrete[END_REF], experiments were not necessarily carried out with parameters corresponding to those encountered in the liquid medium of anaerobic digestion (e.g. data is available for very high concentrated ammonium nitrate solutions and atmospheric carbonation). Finally, the individual effects of each aggressive component, and especially their combined effects in the degradation of concrete by the complete medium, are poorly understood.

In this context, this study aims to understand the mechanisms and intensity of alteration associated with the different components of biowaste and their contributions to the total deterioration. It is thus a question of better understanding the deterioration mechanisms in anaerobic digestion structures and reviewing the normative context.

Materials and methods

Cement paste specimens made of ordinary Portland cement (OPC) were immersed in six chemically aggressive solutions for 16 weeks. The pH, the mass losses and the concentration of elements in solution were monitored over time. Scanning electron microscopy observations coupled with electron probe micro-analysis, and X-Ray diffraction analyses were used to characterize the chemical and mineralogical changes occurring in the samples.

OPC paste

There are no precise recommendations on the choice of cement for anaerobic digestion facilities in the normative texts. In agricultural construction in general, other documents issue recommendations to cement such as CEM II or CEM III [START_REF] Afnor | Concrete -Definition and classification of chemically aggressive environments -Recommendations for concrete mix design[END_REF][START_REF] Afnor | Concrete -Specification, performance, production and conformity -National addition to the standard NF EN 206[END_REF][START_REF] Cimbéton | Guide de prescription des ciments pour des constructions durables. Cas des bétons coulés en place[END_REF][START_REF] Cimbéton | Syndicat National du Béton Prêt à l'Emploi[END_REF]. The choice of cements varies according to the country, without a single, consensual practice being implemented.

Therefore, CEM I-based matrix was used in order to carry out a generic study on the comparative mechanisms and kinetics of attack of different chemical compounds. It should be noted that binders substituted with supplementary cementing materials, containing less portlandite, are more sensitive to carbonation [START_REF] Borges | Carbonation of CH and C-S-H in composite cement pastes containing high amounts of BFS[END_REF][START_REF] Ngala | Effects of carbonation on pore structure and diffusional properties of hydrated cement pastes[END_REF][START_REF] Šavija | Carbonation of cement paste: Understanding, challenges, and opportunities[END_REF] whereas OPC are more sensitive to leaching in acidic environments [START_REF] Koenig | Biogenic acid attack on concretes in biogas plants[END_REF][START_REF] Gruyaert | Investigation of the influence of blastfurnace slag on the resistance of concrete against organic acid or sulphate attack by means of accelerated degradation tests[END_REF][START_REF] Oueslati | The effect of SCMs and curing time on resistance of mortars subjected to organic acids[END_REF][START_REF] Bertron | Attack of cement pastes exposed to organic acids in manure[END_REF]. This work can be used as a basis for the study of all Portland cement-based matrices such as materials based on CEM II, CEM III, CEM IV and CEM V cements, the use of which is recommended for agricultural constructions [START_REF] Afnor | Concrete -Definition and classification of chemically aggressive environments -Recommendations for concrete mix design[END_REF][START_REF] Afnor | Concrete -Specification, performance, production and conformity -National addition to the standard NF EN 206[END_REF][START_REF] Cimbéton | Guide de prescription des ciments pour des constructions durables. Cas des bétons coulés en place[END_REF][START_REF] Cimbéton | Syndicat National du Béton Prêt à l'Emploi[END_REF]. CEM I 52.5R (OPC) pastes were made with a water/cement ratio of 0.30 and mixed according to a procedure adapted from the French Standard NF EN 196-1 [START_REF] Afnor | Methods of testing cement -Part 1: Determination of strength[END_REF] without adding sand: low speed (140 ± 5 rpm) for 60 seconds and high speed (285 ± 10 rpm) for 90 seconds. This low water/cement ratio was chosen to minimize the porosity and the penetration of aggressive agents into the material. The specimens were cast in cylindrical plastic moulds 70 mm high and 35 mm in diameter, which were closed with plastic caps. After a 28-day endothermic cure and just before the experiment, each cylinder was sawn at half height in order to obtain two cylinders about 35 mm high and 35 mm in diameter. The water porosity of the OPC paste was measured according to the NF P18-459 standard [START_REF] Afnor | Concrete -Testing hardened concrete -Testing porosity and density[END_REF] and was 32.0 %.

Synthetized aggressive solutions

Three single compound-based solutions were prepared: an acetic acid solution representing attack by VFAs (called AA solution), an ammonium nitrate solution representing the attack by ammonium cation (called AN solution) and a sodium bicarbonate solution representing the attack of dissolved CO2 (called SB solution). An acetic acid solution was chosen to represent the VFA attack since all VFAs (carboxylic acids with less than 5 carbon atoms) have equivalent effects on the cement matrix because their chemical characteristics are similar (pKa and high solubility of their calcium salts) [START_REF] Bertron | Attack of cement pastes exposed to organic acids in manure[END_REF][START_REF] Bertron | Attack of Cementitious Materials by Organic Acids in Agricultural and Agrofood Effluents[END_REF]. The acid concentration was set to 300 mmol.L -1 , slightly higher than the value of 280 mmol.L -1 used by Bertron et al. [START_REF] Bertron | Attack of cement pastes exposed to organic acids in manure[END_REF] and representative of the higher ranges of VFA concentration found in agricultural effluents [START_REF] Larreur-Cayol | Degradation of cement-based materials by various organic acids in agro-industrial waste-waters[END_REF]. For a functional digester, the VFA concentrations can vary from 40 to 250 mmol.L -1 [START_REF] Hill | Long chain volatile fatty acid relationships in anaerobic digestion of swine waste[END_REF][START_REF] Parawira | Volatile fatty acid production during anaerobic mesophilic digestion of solid potato waste[END_REF][START_REF] Viéitez | Biogasification of solid wastes by two-phase anaerobic fermentation[END_REF] and these concentrations can exceed 500 mmol.L -1 in the case of acidosis [START_REF] Voegel | Microbially induced degradation of cement-based materials in biogas production industrial process[END_REF]. The concentration of the ammonium nitrate solution was chosen according to a previous study of anaerobic digestion in a laboratory set-up [START_REF] Giroudon | Blast-furnace slag cement and metakaolin based geopolymer as construction materials for liquid anaerobic digestion structures: Interactions and biodeterioration mechanisms[END_REF], where the authors found an ammonium concentration of about 800 mg.L -1 and also correspond to the values conventionally encountered in industrial digesters since the concentrations of ammonium ions range between a few hundred mg.L -1 and several g.L -1 [START_REF] Karthikeyan | Bio-energy recovery from high-solid organic substrates by dry anaerobic bio-conversion processes: a review[END_REF][START_REF] Yenigün | Ammonia inhibition in anaerobic digestion: A review[END_REF]. The sodium bicarbonate concentration was chosen in order to represent a gas phase with 50 % of CO2 (calculated using Henry's law at 35 °C, at atmospheric pressure and for a pH of 7.5), which is slightly higher than the proportion of CO2 in optimal condition for biogas production [START_REF] Deublein | Biogas from Waste and Renewable Resources: An Introduction[END_REF][START_REF] Rasi | Biogas composition and upgrading to biomethane[END_REF][START_REF] Rasi | Trace compounds of biogas from different biogas production plants[END_REF] but which is a coherent portion with non-optimal conditions for biogas production. Thus, conditions in the high range of aggressiveness were chosen to accentuate the degradation. The solutions were prepared with demineralized water as follows:

• AA solution was a 300 mmol.L -1 acetic acid solution (glacial acetic acid 99.8-100.5 %, AnalaR® NORMAPUR -VWR) • AN solution was a 44.4 mmol.L -1 NH4NO3 solution (industrial ammonium nitrates, 99.0 % minimum of NH4NO3, Orange Label Ammonium Nitrate, MAXAM Tan) • SB solution was a 243 mmol.L -1 NaHCO3 solution (food grade sodium bicarbonate, 99.0-100.5 % NaHCO3, NOVACARB) Furthermore, solutions combining several chemical compounds together in the same concentrations as above were also studied in order to highlight the contribution of each chemical compound in terms of degradation and aggressiveness mechanisms and to identify combined mechanisms. These solutions were as follows:

• AA+AN solution was a 300 mmol.L -1 acetic acid and 44.4 mmol.L -1 NH4NO3 solution • AA+SB solution was a 300 mmol.L -1 acetic acid and 243 mmol.L -1 NaHCO3 solution • AA+SB+AN solution was a 300 mmol.L -1 acetic acid, 243 mmol.L -1 NaHCO3 and 44.4 mmol.L -1 NH4NO3 solution

As the volume of buffer solution to be added was significant (especially in the case of the AA solution), it was chosen not to buffer the solutions, in order to avoid addition of chemical compounds that could possibly interact with the OPC paste.

Test methods

For each type of solution, four samples were added to 2.5 L which corresponds to a liquid/solid ratio of about 1/17 and a surface area/volume ratio of about 97 cm 2 .L -1 . These values were chosen in accordance with previous studies [START_REF] Bertron | Attack of cement pastes exposed to organic acids in manure[END_REF][START_REF] Larreur-Cayol | Degradation of cement-based materials by various organic acids in agro-industrial waste-waters[END_REF][START_REF] Bertron | Accelerated tests of hardened cement pastes alteration by organic acids: analysis of the pH effect[END_REF] to allow comparison. The solutions were kept at 20 °C in closed plastic containers. The pH was measured regularly once the solution was homogenized (daily at the start of the experiment and several times a week thereafter). As soon as the pH of one of the solution exceeded 8.6 (upper limit of anaerobic digestion conditions), a few mL of each solution was sampled for further analyses and all containers were emptied to receive fresh solution. The experiments lasted 16 weeks (112 days). On average, the solutions were renewed every 2.5 (± 1.4) days.

Two of the four samples from the container were dedicated to mass change monitoring and the other two to the microstructural analyses of the materials.

The compositions of the liquid samples were analysed by inductively coupled plasma/optical emission spectroscopy (ICP/OES) (Optima 7000DV ICP/OES Perkin Elmer) (Si, Na, K, Ca, Al, Fe) and high performance ion chromatography HPIC (Dionex ICS-300) (SO4 2-, NH4 + ).

Mass variations were monitored over time: the samples were removed from the solutions, the excess liquid was quickly removed with absorbent paper and, after 5 minutes in the open air, the samples were weighed. After 4 weeks, 10 weeks, and at the end of the experiment, the upper edge of one sample was carefully sawn with a diamond saw and then split in half. One part was used for X-Ray diffraction analysis (XRD) (Brucker D8 Advance, Cu anti-cathode, 40 kV, 40 nA) and the other part was embedded in epoxy resin (Mecaprex Ma2+, Presi) and dry polished using silicon carbide polishing disks (Presi) (polishing disc references and grain sizes: ESCIL, P800-22 μm, P1200-15 μm and P4000-5 μm).

The flat polished sections were then coated with carbon and analysed by Electron Probe Micro-Analysis (EPMA) (Cameca SXFAA+SBe, 15 kV, 20 nA). The chemical composition changes were characterized using chemical profiles, from the surface to the core according to the distance to the surface. Each plotted graph contains values from two experimental profiles in the same specimen. Particular care was taken to analyse hydrated paste and not residual anhydrous grains. The following elements were taken into account in the selection of microprobe: Ca, Si, Al, Fe, S, Mg, Na and K.

To assess mineralogical changes, the plane side of the slice was first analysed, and the sample was then successively abraded and analysed in order to characterize deeper zones, until the sound paste was reached [START_REF] Bertron | Accelerated tests of hardened cement pastes alteration by organic acids: analysis of the pH effect[END_REF].

Results

pH evolution

Figure 1 shows the evolution of the pH of the solutions over time. Each drop in pH corresponds to a renewal of the solutions.

The pH ranges differ quite significantly depending on the nature of the aggressive solution (Figure 1):

-AA and AA+AN solutions have the lowest pH, with fairly strong variations of between 2.4 and 4; -the AA+SB+AN solution has an intermediate pH range with slight variations (between 4.9 and 5.5); -the AA+SB solution is the solution showing the smallest variations in pH, since the values are between 5.3 and 5.5, which reflects a strong buffering capacity of the solution; -unlike the other solutions, the AN solution exhibits strong variations in pH, between 5.6 and 9, reflecting the low buffering capacity of this solution; -the SB solution has the highest initial pH and the pH variations are between 8.2 and 9.

Total leaching of chemical elements in the solutions

Table 1 brings together the cumulative mass of elements leached in solution, per sample, at the end of the experiment. The cumulated concentrations and leaching kinetics are given as supplementary data (Appendix A). It was not possible to precisely quantify the variations of sodium concentration in SB, AA+SB and AA+SB+AN solutions since the concentrations of sodium were originally very high in solution, and the variations in the measured concentrations were not large enough to be significant.

Solutions containing acetic acid and no sodium bicarbonate, i.e. AA and AA+AN, were the most aggressive in terms of leaching, with slightly lower amounts measured for the AA+AN solution compared to the AA one: in both solutions, greater amounts of calcium (mean value of the two samples of about 419 ± 5.1 mmol), silicon (17 ± 1.6 mmol) and sulfates (2 ± 0.3 mmol) were found. Moreover, these solutions were the only ones to generate aluminium (9 ± 0.1 mmol) and iron (3 ± 0.3 mmol) leaching, which might be related to their low pH. The decreasing order of ion release in both solutions was as follows:

Ca 2+ > Si 4+ > Al 3+ > K + > Fe 3+ > SO4 2-.
Solutions of ammonium nitrate AN, and acetic acid containing bicarbonate with or without ammonium nitrate, AA+SB and AA+SB+AN, were less aggressive and showed intermediate leaching values for calcium (163 ± 25 mmol), silicon (5 ± 1.2 mmol), sulfates (1 ± 0.1 mmol), and potassium (1 ± 1.2 mmol). These solutions did not lead to iron and aluminium leaching within the duration of the experiment. The decreasing order of ion release in both solutions was as follows: Ca 2+ > Si 4+ > SO4 2-and K + .

Sodium bicarbonate solution SB was the least aggressive and only small amounts of potassium (4.1 mmol) and silicon (1.2 mmol) were released. There is therefore a relative release of the ions as follows: K + > Si 4+ .

Macroscopic observations of specimens

Figure 2 shows the macroscopic aspects of the samples at the end of the immersion experiments. Samples immersed in solutions containing acetic acid (AA, AA+SB and AA+SB+AN) showed an orange colour because the intense decalcification of the cement matrix allowed the expression of the iron oxide colour. For the AA, AA+SB and AA+SB+AN solutions, a dense crack network was observed when these samples were taken out of the solutions and left to dry on a bench. This cracking was related to chemical shrinkage as a result of calcium being leached from the sample in large quantities. Figure 2 shows that the coloured zonation is a function of the distance to the exposed surface. Finally, the colour is preserved for the cement paste exposed to the sodium bicarbonate (SB) solution, whereas the sample immersed in the ammonium nitrate (AN) solution shows a slight orange colouring, without cracking.

Microstructural observations

Figure 3 shows the SEM images of the OPC samples after 4 weeks or 10 weeks of immersion in the aggressive solutions and at the end of the experiment (16 weeks). Since the samples immersed in the SB solution did not show any significant changes, the SEM images are only shown for 10 weeks and 16 weeks. Moreover, it should be noted that, on the same sample, the difference in density between the degraded zone and the sound core was sometimes so great that the two zones could not be correctly observed with the same contrast. Some of the images presented are therefore an assembly of two images with different contrasts. From the macroscopic and microscopic observations, three types of microstructural patterns can be observed:

• In the presence of acetic acid, the samples show an outer degraded zone (hereafter called the degraded zone) of very low density, yellowish to brownish in colour (Figure 2), friable and subject to desiccation shrinkage for the samples removed from the bins and dried before observation (very porous zone) (see Figure 2, samples AA, AA+SB, AA+AN and AA+SB+AN). The cracking observed on wet samples is related to the chemical shrinkage associated with the departure of the ions. This area has a completely different structure from the sound paste: the anhydrous grains are no longer visible and it has very low mechanical strength, as pointed out by several authors in studies on acetic, lactic or succinic acids [START_REF] Oueslati | The effect of SCMs and curing time on resistance of mortars subjected to organic acids[END_REF][START_REF] Bertron | Attack of cement pastes exposed to organic acids in manure[END_REF][START_REF] Pavlík | Corrosion of hardened cement paste by acetic and nitric acids part II: Formation and chemical composition of the corrosion products layer[END_REF]. Below the degraded zone, a thinner area appears, slightly darker than the sound core, and therefore slightly less dense. In this area, anhydrous cement grains can still be observed. The limits between the different zones are clear and the fronts are parallel to the surface. The degraded zone tends to detach from the rest of the sample when dry. • Samples exposed to the AN solution show an outer zone of lower density, with a nonrectilinear limit between the sound core and the zone of lower density. The attack by AN being much less aggressive than that of acetic acid, the diffusive phenomena might take a higher part in the deterioration pattern compared to acid attack that creates a sharp front of cementitious matrix dissolution. The heterogeneous microstructure of the cement matrix in terms of pores, air voids and microcracks provides preferential routes for the ingress of the AN solution, leading to this irregular deterioration front. • For samples exposed to the SB solution, no change is clearly visible on the SEM images, whether after 4, 10 or 16 weeks. However, an intensification of the calcium signal could be seen on the surface on the EDS maps (not shown here).

On samples exposed to both acetic acid and sodium bicarbonate (AA+SB and AA+SB+AN), there is a thin cracked outer layer with a slightly higher density than that of the degraded part just below.

Mass variations and degraded layer depths

Figure 4 shows the evolution of the mass variations of the OPC paste in the different solutions and Figure 5 the degraded layer depths, obtained from the SEM observations presented in Figure 3. After 112 days of immersion, the OPC paste is seen to have undergone a significant mass loss of about 33 % for the AA and AA+AN solutions, with a slightly more severe attack for the AA+AN solution. Conversely, a mass gain of 2.1 % is observed when the OPC paste is immersed in the bicarbonate sodium solution.

The other solutions have an intermediate aggressiveness. The lowest mass losses (6.1 %) are observed for the AN solution (ammonium nitrate only) whereas the immersion in the AA+SB and AA+SB+AN solutions leads to similar higher mass losses of about 11.5 % at the end of the experiment. Based on these results, it is observed that ammonium nitrate is much less aggressive to the cement matrix than acetic acid is, and sodium bicarbonate appears to provide a protective effect when it is present in solution with acetic and/or ammonium nitrate. In mixtures, the additional effect of ammonium nitrate in terms of aggressiveness appears almost negligible. It can be noted that the mass variation classification is unsurprisingly consistent with the pH of the solutions (Figure 1): the AA and AA+AN solutions have identical pH values and mass losses and the same is true for the AA+SB and AA+SB+AN solutions. The AN and SB solutions are those exhibiting the highest pH and the lowest mass losses.

Beyond the aggressiveness of the metabolites, there is therefore also an effect of acidity on degradation.

In all the solutions, the mass loss kinetics are higher in the first weeks, and tend to slow over time. This could be due to the slowing down of the penetration of aggressive agents into the cement matrix because of the greater depth of paste to cross (diffusion). It can also be noted that, in view of the cylindrical geometry of the samples, a smaller and smaller volume of paste is degraded as the aggressive agents penetrate.

Except for the AN solution, for which it is difficult to precisely identify the different areas of degradation and their evolution (see Table 1), there is a clear increase in the thickness of the degraded area over time, with a similar evolution for the AA and AA+AN solutions and for the AA+SB and AA+SB+AN solutions. This evolution is correlated with the mass losses shown in Figure 4, and with the pH. The aggressive trend shown by the mass losses is confirmed since, at the end of the experiment, the degraded depths (measured with SEM images) reached thicknesses of 0 µm, 1800 µm, 2070 µm, 2330 µm, 5000 µm and 6900 µm respectively for the SB, AN, AA+SB, AA+SB+AN, AA and AA+AN solutions. It is interesting to note that the deteriorated layer thicknesses of cement pastes exposed to solutions containing AN, AA + AN and AN, are relatively larger than those observed in other solutions when their relative aggressiveness in terms of mass losses is considered. Zone 3 shows an amorphization of the paste, with a predominantly amorphous outer zone more than 4 mm thick. It consists of an amorphous aluminium-silicate skeleton containing iron: in zone 3.a, the aluminium, iron and silicon contents show a relative increase due to the significant decrease in the calcium content and there is then a gradual decrease in aluminium and iron in zone 3.b. The outer surface is made of silica gel only.

AN solution

Beyond 3100 µm (zone 1), the paste seems unaltered, with a chemical composition corresponding to a sound OPC paste. The EPMA chemical analysis shows a very gradual decalcification from 3100 µm depth (start of zone 2) with no sharp drop in the calcium content, reflecting a less aggressive attack than for the AA solution. Zone 2 is characterized by the dissolution of portlandite and an enrichment in sulfur content, associated with the intensification of the ettringite peaks. Calcite precipitation is detected. From 2500 µm, the SiO2, Al2O3 and Fe2O3 contents increase whereas the sulfur content decreases. The SO3 content reaches 0 % in zone 3 and the SiO2 content and the total oxides content decrease from 1150 µm onwards. The paste chemical composition is stable over the last 100 µm, where only the brownmillerite phase and some ettringite remain from the initial composition of the OPC paste, and calcite is the main phase (zone 3).

SB solution

The chemical composition of the OPC paste immersed in SB solution changed very little after 16 weeks of immersion. Only a thin external layer about 50 µm thick (zone 2) shows changes, with a decrease in the total oxides content (from 80 % to 55 %) as well as a decrease in each oxide content except for calcium oxide, which remains around 50 %. The stable calcium content is might be due to the phenomena of dissolution and precipitation of calcium carbonates, since the only detected phases of the outer layer are carbonation products: mainly aragonite, but also calcite and vaterite

AA+AN solution

The specimen in the AA+AN solutions shows the greatest degraded depth, significantly greater than in AA, contrary to what the mass loss suggested. The chemical changes are similar to those of the specimen in the AA solution but with a greater degraded depth. Decalcification occurs in zone 2: it starts from 8100 µm depth and is complete at 7000 µm depth. This decalcification is less sharp than in AA solution and extends over more than a millimetre, as for the immersion in AN solution, while the drop in the total oxide content occurs suddenly at a depth of about 7000 µm. As for other solutions containing acetic acid, a sulfur enrichment associated with the intensification of the ettringite X-ray diffraction peaks is observed in zone 2, just before this element is completely leached out. The total oxides content drops from 80 % to 40 % between 7200 and 6800 µm. Then, zone 3 is made of Al2O3, Fe2O3 and SiO2 up to 1000 µm deep but only SiO2 remains on the surface.

AA+SB solution

The EPMA analysis of the OPC paste immersed in AA+SB solution shows a chemical composition similar to that resulting from the acetic acid attack, with a smaller degraded depth than for the AA solution.

Zone 2 shows a sudden drop in calcium oxide as well as total oxides and sulfur enrichment. It is marked by the dissolution of portlandite and the intensification of the ettringite peaks. Calcium carbonates, calcite and vaterite, are detected. There is no longer any SO3 in zone 3 but the paste is not completely decalcified, which could be linked to the precipitation of calcium-rich phases in this zone. The mineralogical analysis shows the dissolution of the initial phases of the paste in favour of calcium carbonate, calcite and vaterite, for which intensity of the peaks increases. In zone 4, the cement matrix is decalcified, with a total oxide content of less than 40 %. In this solution, compared with the AA solution, the aluminium and iron oxides are better preserved in the outer layer, resulting in a smaller relative increase in the silica content. As for the AA solution, the outer zone (zone 4) is completely amorphous and the surface is composed of a Si-Fe-Al gel. The increase in the total oxide content is observed in the outer 400 µm due to the increase in the Fe2O3 content.

AA+SB+AN solution

The EPMA chemical analyses of the OPC paste immersed in AA+SB+AN solution show a progressive decalcification (as for AN solution) from 3500 to 2100 µm (zones 2 and 3) and a sharper decalcification (as for AA solution) from 2000 µm to 1750 µm. Zone 2 shows an enrichment in the SO3 content associated with more intense ettringite peaks in the XRD analysis. Zone 3 is marked by the stabilization of the calcium content, probably due to the precipitation of calcium carbonates (main phases detected by DRX analyses). This induces a high total oxides content, an effect that is much more significant in this solution, in the presence of AN, than for the AA+SB solution. Zone 4 shows the decrease of the SiO2 and CaO contents, associated with the drop of the total oxides content. The external layer (zone 5) is made of an Si-Al gel. The external layer of this specimen is mainly amorphous but weak peaks of brownmillerite are detected.

Discussion

Phenomenology

According to the mass losses and the degraded depths identified in the EPMA analyses, the solutions studied in these conditions can be classified in increasing order of aggressiveness (considering the modified depth): SB solution < AN solution < AA+SB solution < AA+SB+AN solution < AA solution < AA+AN.

Degradation by acetic acid alone

The degradation mechanisms of the OPC paste immersed in AA solution were those classically encountered in acid attacks for acids whose calcium salts are soluble in water [START_REF] Bertron | Attack of Cementitious Materials by Organic Acids in Agricultural and Agrofood Effluents[END_REF]:

• A progressive dissolution of all the initial phases of the cement paste • Significant mass losses • A degraded area almost totally decalcified and made of aluminium, silicon and iron. This completely amorphous structure was similar to that of a silica gel and / or a silico-aluminous gel.

Moreover, as in the study by Bertron et al. [START_REF] Bertron | Accelerated tests of hardened cement pastes alteration by organic acids: analysis of the pH effect[END_REF], a slight sulfur enrichment was observed just in the transition zone between the amorphous outer zone and the sound zone, corresponding to the intensification of the ettringite peaks.

Calcium and sulfates were the most leached elements (Table 1), and were not found in the amorphous outer layer (Figure 6).

As in the study by Bertron et al. [START_REF] Bertron | Accelerated tests of hardened cement pastes alteration by organic acids: analysis of the pH effect[END_REF], the altered zone had weaker mechanical strength but was not dissolved and could have slowed down the ingress of aggressive agents into the core of the sample.

Degradation by ammonium nitrate alone

According to the literature [START_REF] Escadeillas | La durabilité des bétons vis-à-vis des environnements chimiquement agressifs[END_REF][START_REF] Escadeillas | Ammonium Nitrate Attack on Cementitious Materials[END_REF], ammonium salts are aggressive for the cement matrix and react according to an exchange mechanisms 2 NH4 + → Ca 2+ , leading to the leaching and decalcification of the cementitious matrix [START_REF] Lea | The action of ammonium salts on concrete[END_REF], the degradation being governed by diffusion [START_REF] Gallé | Effect of an accelerated chemical degradation on water permeability and pore structure of cement based materials[END_REF]. Highly concentrated ammonium nitrate solutions (6 mol.L -1 or more) are thus used to provide accelerated tests of pure water leaching [START_REF] Carde | Leaching of both calcium hydroxide and C-S-H from cement paste: Modeling the mechanical behavior[END_REF][START_REF] Carde | Use of ammonium nitrate solution to simulate and accelerate the leaching of cement pastes due to deionized water[END_REF][START_REF] Carde | Effect of the leaching of calcium hydroxide from cement paste on mechanical and physical properties[END_REF][START_REF] Perlot | The adaptation of an experimental protocol to the durability of cement engineered barriers for nuclear waste storage[END_REF]. Both pure water and ammonium nitrate solution lead to the total dissolution of portlandite and the decalcification of C-S-H, and to a sulfur enrichment in the degraded area, the difference being that the kinetics of the degradation in the ammonium nitrate solution are 60 to 300 times higher than in deionized water [START_REF] Carde | Use of ammonium nitrate solution to simulate and accelerate the leaching of cement pastes due to deionized water[END_REF][START_REF] Gérard | Simplified modelling of calcium leaching of concrete in various environments[END_REF][START_REF] Heukamp | Mechanical properties of calcium-leached cement pastes: Triaxial stress states and the influence of the pore pressures[END_REF][START_REF] Wan | Experimental and modelling research of the accelerated calcium leaching of cement paste in ammonium nitrate solution[END_REF]. The ammonium nitrate solution attack creates an external degraded zone [START_REF] Carde | Leaching of both calcium hydroxide and C-S-H from cement paste: Modeling the mechanical behavior[END_REF][START_REF] Carde | Effect of the leaching of calcium hydroxide from cement paste on mechanical and physical properties[END_REF][START_REF] Perlot | The adaptation of an experimental protocol to the durability of cement engineered barriers for nuclear waste storage[END_REF] and induces a significant increase in the porosity [START_REF] Gallé | Effect of an accelerated chemical degradation on water permeability and pore structure of cement based materials[END_REF][START_REF] Carde | Effect of the leaching of calcium hydroxide from cement paste on mechanical and physical properties[END_REF]. Moreover, the C/S of the C-S-H decreases and the porosity and the diffusivity increase when the NH4NO3 concentration in the immersion solution is increased [START_REF] Kurumisawa | Effects of calcium leaching on diffusion properties of hardened and altered cement pastes[END_REF][START_REF] Kurumisawa | Deteriorated hardened cement paste structure analyzed by XPS and 29Si NMR techniques[END_REF].

In this study, a low concentration of ammonium nitrate solution was used (4.44 mmol.L -1 ) in order to reproduce the anaerobic digestion conditions. In this context, the ammonium nitrate attack resulted in the progressive decalcification of the paste with the dissolution of the Ca-bearing phases (CH and C-S-H) and led to surface carbonation. The carbonation might be due to the presence of dissolved CO2 (from the air) in solution. It probably slowed down the progression of the attack by clogging the porosity [START_REF] Baroghel-Bouny | La durabilité des armatures et du béton d'enrobage[END_REF][START_REF] Shah | Changes in microstructure characteristics of cement paste on carbonation[END_REF]. As for AA solution, sulfates and calcium were the most leached elements but they were in lower concentrations. However, the amount leached out was lower than for the AA solution, which is consistent with the smaller degraded depths and mass losses of these samples, as well as the decalcification -which was only progressive in the degraded zone, whereas it was total over the entire area for the AA solution. Unlike during the acid attack, the iron remained in the outer layer.

Degradation by the sodium bicarbonate alone

After 16 weeks, the OPC paste immersed in sodium bicarbonate solution remained intact except in a thin outer layer composed only of carbonation products: calcite, vaterite and aragonite. These samples were the only ones to show mass gains, linked to the carbonation layer deposited on the surface. Above a pH of 8, when the pH increases, the equilibrium of bicarbonate to carbonate ions is shifted towards carbonate (pKa1 (H2CO3/HCO3 -) = 6.30; pKa2 (HCO3 -/CO3 2-) = 10.33). In the sodium bicarbonate solution, the pH was between 8 and 9 during the experiment (Figure 1) and was certainly higher in contact with the OPC paste. Thus, the carbonate ions in contact with Ca(OH)2 led to calcium carbonates precipitation. Among the elements analysed, only silicon and potassium were leached in small amount.

It can be noted that the amount of leached potassium was lower than in the other solutions, whereas in the other solutions this element was leached in similar quantities, whatever the solution aggressiveness. Thus, it seems that the sodium bicarbonate solution limits the leaching by clogging the porosity with calcium carbonates [START_REF] Baroghel-Bouny | La durabilité des armatures et du béton d'enrobage[END_REF][START_REF] Shah | Changes in microstructure characteristics of cement paste on carbonation[END_REF]. Whereas calcite is known to be the most stable polymorph of calcium carbonates under ambient atmospheric conditions [START_REF] Knez | Stabilization of PCC dispersions prepared directly in the mother-liquid after synthesis through the carbonation of (hydrated) lime[END_REF], the external layer of the paste was mainly made of aragonite (metastable phase) and also contained vaterite (unstable phase) in small amounts (solubility product at 25°C: log𝐾 𝑠𝑝 = -8.48, -8.34 and -7.91; molar volume: 35 cm 3 .mol -1 , 34 cm 3 .mol -1 and 38 cm 3 .mol -1 for calcite, aragonite and vaterite, respectively [START_REF] Morandeau | Carbonatation atmosphérique des systèmes cimentaires à faible teneur en portlandite[END_REF][START_REF] Sawada | The mechanisms of crystallization and transformation of calcium carbonates[END_REF]). According to several authors [START_REF] Cailleau | Influence des ions étrangers et de la matière organique sur la cristallisation des carbonates de calcium[END_REF][START_REF] Fil | Contribution à l'étude des eaux géothermales du sud tunisien : étude des mécanismes et de la prévention des phénomènes d'entartrage[END_REF][START_REF] Zidoune | Contribution à la connaissance des mécanismes d'entartrage par diverses méthodes électrochimiques[END_REF], the Na + ion has no effect on the polymorph formed. However, pure aragonite phases can be synthesized by adding sodium carbonate Na2CO3 to a Ca(OH)2 solution in controlled conditions [START_REF] Jimoh | Synthesis of precipitated calcium carbonate: a review[END_REF][START_REF] Kitamura | Controlling factors and mechanism of reactive crystallization of calcium carbonate polymorphs from calcium hydroxide suspensions[END_REF]. According to Kitamura et al. [START_REF] Kitamura | Controlling factors and mechanism of reactive crystallization of calcium carbonate polymorphs from calcium hydroxide suspensions[END_REF], the aragonite content increases when the addition rate of sodium carbonate decreases, and for a high addition rate, calcite preferably precipitates. The authors specify that, if the concentration of calcium ions is at or near the solubility of Ca(OH)2, the low local supersaturation of carbonate ion is advantageous for the crystallization of aragonite. This phenomenon could explain the presence of aragonite in the external layer of the paste. Furthermore, the presence of vaterite may be related to the lower pH of the external layer, since, at ambient temperature, vaterite precipitates for pH<10 [START_REF] Oral | Influence of pH on morphology, size and polymorph of room temperature synthesized calcium carbonate particles[END_REF][START_REF] Tai | Polymorphism of CaCO3, precipitated in a constant-composition environment[END_REF].

The formation of calcium carbonates, reflecting the carbonation phenomenon, is known to be a major risk of corrosion in reinforced concrete, by depassivation of reinforcements [START_REF] Chalhoub | Effect of Cathode-Anode distance and electrical resistivity on macrocell corrosion currents and cathodic response in cases of chloride induced corrosion in reinforced concrete structures[END_REF]. Moreover, the carbonation phenomenon could be even more detrimental for concrete with blended cement: the use of supplementary materials leads to a decrease in the portlandite content for the benefit of C-S-H, whereas the carbonation of C-S-H results in significant shrinkage, loss of cohesion, increase in the number of pores and increased porosity [START_REF] Li | Microstructural changes in alkali-activated slag mortars induced by accelerated carbonation[END_REF][START_REF] Nedeljković | Effect of natural carbonation on the pore structure and elastic modulus of the alkali-activated fly ash and slag pastes[END_REF][START_REF] Puertas | Carbonation process of alkali-activated slag mortars[END_REF][START_REF] Sanjuán | Effect of curing time on granulated blast-furnace slag cement mortars carbonation[END_REF].

Degradation by the combined solutions Acetic acid combined with the other aggressive compounds

The degradation mechanisms of the cement paste immersed in the AA+AN, AA+SB and AA+AN+SB solutions can be explained by the combination of the degradation phenomena caused by the aggressive agents in solution but the intensity and kinetics of the attack are mainly related to the AA attack. The structural degradations and the chemical composition profile of these cement pastes are similar to those of the cement paste immersed in the AA solution, although differences can be noted: acetic acid leads to the leaching of all the elements, significant mass losses and degraded depths.

Ammonium nitrate combined with the other aggressive compounds

In the AA+AN and AA+SB+AN solutions, the presence of ammonium nitrate increased the degraded depths and intensified the degradations, but in a moderate way with a relative increase in mass loss of 4% and of 24 % in degraded depth for the AA+AN solution compared to the AA solution. In the AA+AN solution, a sudden decalcification was observed as for an acid attack, combined with a more gradual decalcification in depth, probably linked to the presence of ammonium nitrate. The quantities of elements released in this solution were similar to those in AA solution, except for sodium and sulfates: it appears that the presence of ammonium nitrate accentuates the sodium leaching and the global degradation, while limiting the sulfates leaching.

For the AA+SB+AN solution, despite the periodic renewals of the solution, which is the most influential factor for the aggressiveness of an ammonium nitrate solution of given concentration [START_REF] Poyet | Accelerated leaching of cementitious materials using ammonium nitrate (6M): influence of test conditions[END_REF], the presence of ammonium nitrate in such concentration did not seem to significantly increase the degradation. If we compare the degradation of the cement paste immersed in the AA+SB solution and that immersed in the AA+SB+AN solution, it can be observed that the presence of ammonium nitrate increased the mass loss of the sample only slightly: in the presence of ammonium nitrate, some elements were more leached (calcium, potassium and silicon) and it was again observed that the leaching of sulfates was limited. The degraded depth was still greater for the AA+SB+AN solution, and is linked to the greater leaching of calcium and silicon, major compounds of the cement matrix.

No exchange mechanism 2 NH4 + → Ca 2+ was highlighted since the NH4 + concentration was measured and remained constant at about 800 mg.L -1 throughout the experiment.

Sodium bicarbonate combined with the other aggressive compounds

In the AA+SB solution, the presence of sodium bicarbonate seems to protect the cement matrix since the amount of leached elements is much smaller and iron and aluminium remain on the surface. The effect of the SB solution is probably associated, at least partially, with an effect on the pH of the AA+SB solution (initial pH of 5.3), significantly greater than that of the AA solution alone (initial pH of 2.6). The mass loss is also lower than for the AA solution but is still significant. Calcium carbonates (calcite and vaterite) were not found on the surface but in depth (zone 3) where the conditions were favourable to precipitation while more of the material was completely decalcified in the surface. Once again, the presence of vaterite in the external carbonated zone was probably linked to a pH<10.

In the AA+AN+SB solution, the lower intensity of the degradation compared to AA+AN solution confirms the protective effect of the sodium bicarbonate. Smaller degraded area and mass loss are observed. The presence of sodium bicarbonate prevents the leaching of aluminium and iron, elements leached only in the presence of acetic acid and in the absence of sodium bicarbonate. In addition, the presence of sodium bicarbonate greatly reduces the amounts of calcium, sulfates and silicon leached out. In this solution, carbonation occurs in depth (zone 3), just before the transition zone containing the sulfur enrichment (zone 2). Its effect on the chemical composition is more significant than in the case of the AA+SB solution. In addition, the presence of sodium bicarbonate in the AA+SB+AN solution induces an initial pH of the solution which is significantly higher (4.9) than for the AA+AN solution (2.4).

Comparison with real anaerobic digestion liquid media

The previous results can be compared with the study by Giroudon et al. [START_REF] Giroudon | Blast-furnace slag cement and metakaolin based geopolymer as construction materials for liquid anaerobic digestion structures: Interactions and biodeterioration mechanisms[END_REF] where OPC pastes (water/cement = 0.30) were immersed in the liquid media of anaerobic digestion in bioreactors. In the liquid containing the OPC pastes, the maximum concentrations of aggressive agents encountered were 0.26 g.L -1 (4.3 mmol.L -1 if only acetic acid is considered) of VFA, 770 mg.L -1 of NH4 + and about 2000 mg.L -1 of inorganic carbon content (or about 20 % of CO2 in the gas phase). Thus, the materials used are comparable with the present study but the concentration ranges are significantly (VFA and CO2) or slightly (NH4 + ) higher than the ranges encountered in the study in a real environment. This could be expressed by a higher intensity of degradation.

Figure 7 summarizes the main chemical and mineralogical changes in the OPC pastes exposed to (i) the synthetic AA+SB+AN solution for 16 weeks and (ii) a real anaerobic digestion medium for 17.5 weeks [START_REF] Giroudon | Blast-furnace slag cement and metakaolin based geopolymer as construction materials for liquid anaerobic digestion structures: Interactions and biodeterioration mechanisms[END_REF], together with exposure conditions and degraded depths. In both studies, the total degradation was expressed by the dissolution of the initial phases (zones 4 and 5), the decalcification of the cementitious matrix associated with a sulfur enrichment and the precipitation of secondary ettringite (zone 2). The presence of dissolved CO2 led to the carbonation of the paste, with the precipitation of calcite and vaterite (zones 3 and 4).

However, the degradation intensity (altered depth, decalcification, loss of density, mass loss, etc.) was much lower in the real environment since the degraded depth in the AA+SB+AN solution (about 4000 µm from zone 2 to the surface) was about 5 times that in the bioreactors (about 800 µm from zone 2 to the surface), mostly due to the high concentration of acetic acid in the synthetic solutions, and could correspond to real conditions in an industrial digester. Thus, no significant mass loss was detected in the experiment of Giroudon et al. [START_REF] Giroudon | Blast-furnace slag cement and metakaolin based geopolymer as construction materials for liquid anaerobic digestion structures: Interactions and biodeterioration mechanisms[END_REF], neither was any external amorphous structure. An external amorphous Si-Al gel was, however, identified in studies by Voegel et al. [START_REF] Voegel | Cementitious materials in biogas systems: Biodeterioration mechanisms and kinetics in CEM I and CAC based materials[END_REF][START_REF] Voegel | Mechanisms of cementitious material deterioration in biogas digester[END_REF], where the authors studied the biodeterioration phenomena of cementitious pastes (water/cement = 0.40) in the submerged part of a digester using typical food biowaste. Thus, the degradation intensity is strongly dependent on the evolution of the medium, the type of substrates used and, especially, how the associated process is run. For example, with a rapidly hydrolysable substrate fed continuously using a constant charge, there will be no accumulation of VFA and therefore no pH drop.

In addition, an external phosphorus enrichment has been observed in real media [START_REF] Bertron | Biodeterioration of concrete in agricultural, agro-food and biogas plants: state of the art and challenges[END_REF][START_REF] Giroudon | Blast-furnace slag cement and metakaolin based geopolymer as construction materials for liquid anaerobic digestion structures: Interactions and biodeterioration mechanisms[END_REF][START_REF] Voegel | Cementitious materials in biogas systems: Biodeterioration mechanisms and kinetics in CEM I and CAC based materials[END_REF][START_REF] Voegel | Mechanisms of cementitious material deterioration in biogas digester[END_REF], where this element was probably brought by the substrate [START_REF] Voegel | Mechanisms of cementitious material deterioration in biogas digester[END_REF].

Normative context

In order to quantify the chemical aggressiveness of an environment toward concrete and to design durable structures, the European standard EN 206/CN [START_REF] Afnor | Concrete -Specification, performance, production and conformity -National addition to the standard NF EN 206[END_REF] and the French information document FD P 18-011 [START_REF] Afnor | Concrete -Definition and classification of chemically aggressive environments -Recommendations for concrete mix design[END_REF] classify environments chemically using three classes of increasing aggressiveness: XA1, XA2 and XA3. For this purpose, the concentration and presence of some aggressive agents are taken into account: the aggressive carbon dioxide concentration (aggressive CO2), the sulfate concentration (SO4 2-), the magnesium ion concentration (Mg 2+ ), the ammonium ion concentration (NH4 + ), the pH, and the water hardness.

According to this study, some inconsistencies can be noted. Whereas both AN and AA solutions are considered to be highly aggressive ([NH4 + ]>>100 mg.L -1 and pH<<4 for AN and AA solutions respectively), it appears that the AN solution was much less aggressive than the AA solution. Moreover, the AA+SB solution is classified XA2 because of its initial pH (5.32), whereas this solution led to a higher aggressiveness than the AN solution (XA3).

Thus, it might be relevant to consider not only the pH but also the concentration of organic acids in solution, since the pH is not suitable to describe their aggressiveness [START_REF] Larreur-Cayol | Degradation of cement-based materials by various organic acids in agro-industrial waste-waters[END_REF][START_REF] Bertron | Understanding interactions between cementitious materials and microorganisms: a key to sustainable and safe concrete structures in various contexts[END_REF]. In addition, recommended cements (such as blast-furnace slag cements) should also be tested.

Conclusion

This study assessed the contribution of different aggressive agents to the global degradation of OPC pastes during anaerobic digestion. In the current experimental conditions:

• Despite the different deterioration mechanisms specific to each aggressive agent, the pH plays an important role in the aggressiveness of the solutions studied. • Immersion in an acetic acid solution led to sharp and total decalcification of the cementitious matrix, with large mass loss and degraded depth. In mixed solutions, acetic acid had a preponderant effect, which led to the total decalcification of the degraded zone, with the appearance of a well-marked structural, mineralogical and chemical zonation. • The attack by the ammonium nitrate solution expressed itself by a progressive decalcification, with significant mass loss and degraded depth. However, in mixed solution, ammonium nitrate has a negligible effect, even if the concentration used (800 mg.L -1 ) is much higher than the maximum concentration range considered by standards EN 206 and FD P18-011 for chemically aggressive environments (XA3 class: 60 to 100 mg.L -1 of NH4 + ion in solution) [START_REF] Afnor | Concrete -Definition and classification of chemically aggressive environments -Recommendations for concrete mix design[END_REF][START_REF] Afnor | Concrete -Specification, performance, production and conformity -National addition to the standard NF EN 206[END_REF]. • By clogging the porosity through carbonation and increasing the pH of the solution though its strong buffering capacity, the sodium bicarbonate solution provides a protective effect when combined with other aggressive agents. However, its effect is not preponderant since the pastes remain highly degraded when acetic acid is present. The significant carbonation in these environments induces a risk of corrosion for reinforced concrete structures. • While maintaining qualitatively comparable deterioration characteristics, the high concentrations used for the synthetic chemical solutions led to an intensification of the degradation compared to the laboratory environment. • The normative context does not allow accurate quantification of a solution's aggressiveness for OPC based material, especially in the presence of organic acids. • Modelling work allowing the pH to be adjusted while various concentrations are being studied would bring new knowledge about the action of each aggressive metabolite on the concrete attack. Finally, this could also be useful for other structures that house complex biological processes, such as wastewater treatment plants that involve anaerobic digestion, among other treatments [START_REF] Gallert | Bacterial Metabolism in Wastewater Treatment Systems[END_REF].

Figure 1 :

 1 Figure 1: pH evolution of the solutions according to time of immersion (AA: acetic acid, AN: ammonium nitrate, SB: sodium bicarbonate)

Figure 2 :

 2 Figure 2: (A) Visual appearance of the samples' surfaces at the end of the experiment and (B) visual appearance of the slices of the samples AA+AN and AA+SB+AN at the end of the experiment (AA: acetic acid, AN: ammonium nitrate, SB: sodium bicarbonate)

Figure 3 :

 3 Figure 3: SEM images of OPC paste samples immersed in the different aggressive solutions after 4 weeks, 10 weeks and 16 weeks (AA: acetic acid, AN: ammonium nitrate, SB: sodium bicarbonate). Approximate magnification: x25. Some of the images are an assembly of two images with different contrasts.

Figure 4 :Figure 5 :

 45 Figure 4: Mass losses of the OPC paste according to the time of immersion in the aggressive solutions (AA: acetic acid, AN: ammonium nitrate, SB: sodium bicarbonate)

Figure 6 :

 6 Figure 6: Schematic representations of chemical composition of oxides and mineralogical composition (analysed by EPMA and XRD, respectively) of the OPC pastes after 16 weeks of immersion in the aggressive chemical solutions (AA: acetic acid, AN: ammonium nitrate, SB: sodium bicarbonate) -Ett: ettringite; Ca: calcite; Va: vaterite; Br: brownmillerite; Am: mainly amorphous pattern -Red bold characters = intensification of the XRD signal in comparison with the deeper zone; Parentheses = significantly lower intensity of the XRD signal in comparison with the main phase 3.6.1 AA solution Zone 1 (the sound zone) contains typical phases of a Portland paste (portlandite, ettringite, brownmillerite, C3A1 , C3S and C2S). The EPMA chemical analysis shows a thick degraded zone of 6400 µm (zone 2 + zone 3) with a total and sharp decalcification of the paste occurring in zone 2. From a mineralogical point of view, this zone shows the dissolution of the portlandite and the precipitation of calcite. A slight enrichment in ettringite may be associated with the increase of the sulfur content. Zone 3 shows an amorphization of the paste, with a predominantly amorphous outer zone more than 4 mm thick. It consists of an amorphous aluminium-silicate skeleton containing iron: in zone 3.a, the aluminium, iron and silicon contents show a relative increase due to the significant decrease in the calcium content and there is then a gradual decrease in aluminium and iron in zone 3.b. The outer surface is made of silica gel only.

Figure 7 :

 7 Figure 7: Schematic representations of chemical composition of oxides and mineralogical variations (analysed by EPMA and XRD, respectively) of OPC pastes exposed to (i) a synthetic solution of acetic acid, sodium bicarbonate and ammonium nitrate

Table 1 :

 1 Cumulative leached ions per cement paste sample in the solutions during the experiment (AA: acetic acid, AN: ammonium nitrate, SB: sodium bicarbonate). *concentrations cannot be provided with sufficient precision, see text.

			Cumulative leached ion from the specimens (mmol)
		Calcium	Silicon	Aluminium	Sulfate	Iron	Potassium Sodium
	AA solution	423 ± 46	15.6 ± 1.7	9.1 ± 1	2.2 ± 0.2	3.5 ± 0.4	7.1 ± 0.8	2.9 ± 0.3
	AN solution	166 ± 18	5.1 ± 0.6	0.0 ± 0	0.5 ± 0.1 0.0 ± 0	5.3 ± 0.6	2.1 ± 0.2
	SB solution	0.0 ± 0	1.2 ± 0.1	0.0 ± 0	0.0 ± 0	0.0 ± 0	4.1 ± 0.4	-*
	AA+AN solution	415 ± 46	17.8 ± 2	9.0 ± 1	0.8 ± 0.1	3.1± 0.3	7.8 ± 0.9	7.8 ± 0.9
	AA+SB solution	137 ± 15	3.2 ± 0.3	0.0 ± 0	1.7 ± 0.2	0.0	5.0 ± 0.6	-*
	AA+SB+AN solution	185 ± 20	5.4 ± 0.6	0.0 ± 0	0.6 ± 0.1	0.0	6.9 ± 0.8	-*

Cement chemistry shorthand notations: A = Al2O3, C = CaO, F = Fe2O3, H = H2O, M = MgO, S = SiO2

Acknowledgements

The authors wish to thank the French National Research Agency (ANR) for its financial support in the project BIBENdOM -ANR -16 -CE22 -001 DS0602. The authors also thank Paul Schokmel, Tony Pons and Maud Schiettekatte for their help with the experiment and the analytical support.

Supplementary data


Aurélie Vande Put
email: aurelie.vandeput@ensiacet.fr

Charlotte Dupressoire

Carole Thouron
email: carole.tbouron@ensiacet.fr

Philippe Emile
email: philippe.emile@airbus.com

Raphaëlle Peraldi
email: raphaelle.peraldi@airbus.com

Benjamin Dod
email: benjamin.b.dod@airbus.com

Daniel Monceau
email: daniel.monceau@toulouse-inp.fr

Aurélie Vande Put

High-Temperature Oxidation Behavior of Ti6242S Ti-based Alloy

Keywords: High-temperature oxidation, Titanium-based alloy, Oxygen dissolution

The aircraft industry is always looking for improved efficiency through higher inservice engine temperatures and lighter structures. Titanium-based alloys are good candidates for such applications because of their high specific strength. However, when exposed to high-temperature oxidizing environments, a large amount of dissolved oxygen can be found in such alloys beneath the growing oxide scale, possibly leading to embrittlement. Consequently, evaluating the oxidation resistance of these alloys is essential. With this aim, long-term oxidation tests were carried out on Ti6242S alloy between 500 and 650 °C to study the effect of temperature, surface preparation and microstructure on oxide scale and oxygen dissolution. While increasing the temperature from 560 to 625 °C led to accelerated oxidation kinetics, surface preparation had no noticeable effect on mass variations and oxygen diffusion profiles. Regarding microstructure, when comparing Ti6242S samples having similar α-phase fraction but very different microstructures (fineness and morphology), there wasn't any significant effect found on mass change and oxygen diffusion after 1 kh at 650 °C.

Introduction

As economic and environmental requirements become increasingly strict, the aeronautic industry is determined to lighten aircrafts by introducing more Ti-based alloys because of their high specific mechanical properties and their good compatibility with carbon fiber-reinforced polymers. However, under operating conditions, some parts made of Ti-based alloys are exposed to high temperatures (up to 650 °C) and important oxidation phenomena can appear. The oxidation of these alloys involves the formation of a TiO 2 oxide scale and the dissolution of large amounts of oxygen within the material. For instance, pure titanium can dissolve up to 33 at.% and 8 at.% oxygen in α and β phases, respectively, at high temperatures [1]. This oxygen diffusion can greatly affect t he d uctility [2][3][START_REF] Casadebaigt | MATEC Web of Conferences, The 14th World Conference on Titanium (Ti 2019)[END_REF][START_REF] Put | MATEC Web of Conferences, The 14th World Conference on Titanium (Ti 2019)[END_REF], a nd t herefore t he d urability o f Ti-based alloys, limiting the maximum temperature at which they can be used in long-term applications. The present study focuses on the influence of surface preparation and microstructure on the oxidation behavior of Ti6242S.

Champin et al. studied the oxidation behavior for 80 days at 550 and 600 °C of 685 alloy (Ti-6Al-5Zr-0.5Mo-0.25Si) exhibiting three surface states: rolled, grit blasted and ground using P600 SiC paper. They observed that reducing surface roughness led to a decrease in mass gain [START_REF] Champin | [END_REF]. In the specific case of additively manufactured alloys, the large surface area resulting from this fabrication process leads to faster oxidation kinetics. This large surface area can be reduced through grinding, as shown by Casadebaigt et al. who oxidized TA6V (Ti-6Al-4V) alloys obtained by laser and electron beam melting in the 500-600 °C temperature range [7]. Shotpeening was also used by various researchers to improve the properties of commercially pure Ti [8][9][10] and Ti-based alloys [11,12]. After oxidation for only 1 h at 700 °C, Wen et al. noticed a slight increase in O, N and C contents in the near surface of commercially pure Ti after shot-peening followed by light grinding (aimed at removing 3-6 µm) [8]. After a much longer time (1800 h) at the same temperature in laboratory air, Thomas et al. noticed a large increase in O quantity under the surface of an IMI834 alloy (Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si-0.06C) subjected to shot-peening before oxidation [11]. Authors attributed this increase in O diffusion to the introduction of mechanical twins and pointed out that O diffusion was accompanied by silicide precipitation along the mechanical twin boundaries and in areas with high dislocation density. On the other hand, other researchers reported a decrease in mass variation of shot-peened samples oxidized at 700 °C, due to the formation of a N-rich layer at the oxide/alloy interface [9]. However, the operating temperatures of Ti-based alloys rarely, if ever, reach 700 °C, even for alloys designed for the highest operating temperatures, such as Ti6242S (Ti-6Al-2Sn-4Zr-2Mo-Si), which is known to exhibit a good oxidation resistance [START_REF] Put | MATEC Web of Conferences, The 14th World Conference on Titanium (Ti 2019)[END_REF][START_REF] Calvert | Evaluation of titanium alloys after high temperature air exposure[END_REF][START_REF] Berthaud | [END_REF][15] and is the material of this study. Excessively high temperatures can modify oxidation regimes, as shown in Ti6242 and Ti6242S alloys by Calvert and Kosaka and Gaddam et al., respectively [START_REF] Calvert | Evaluation of titanium alloys after high temperature air exposure[END_REF]15]. This can also lead to modified contents of alloying elements between phases, as found in the TA6V alloy [16,17].

Regarding microstructure, many works reported its influence on the oxidation behavior of Ti-based alloys [START_REF] Calvert | Evaluation of titanium alloys after high temperature air exposure[END_REF][18][19][20][21]. Many researchers consider that grain boundaries and interphases favor oxygen diffusion [START_REF] Calvert | Evaluation of titanium alloys after high temperature air exposure[END_REF]18,20]. Rosa d etermined that oxygen diffuses faster in the β phase than in the α phase [22]. Based on their work on IMI834 alloy, Sai Srinadh et al. reported that, after being oxidized at 750 and 800 °C in laboratory air, a duplex microstructure leads to higher mass variations than a lamellar one [18]. Nevertheless, they obtained very similar mass changes for oxidations at 600 °C and 650 °C. At 760 °C, Pitt et al. did not notice a significant difference in mass changes after the oxidation of two globular TA6V with two different grain sizes [20]. These authors claim that the different grain sizes of their alloys greatly affected the oxygen ingress in the metal. Nevertheless, the hardness profiles they obtained were similar apart from an offset. The hardness profiles evidently show that both alloys had similar oxygen diffusion coefficients. Then, the offset could be due to the fact that the two studied alloys (small and large grains) had different nominal levels of oxygen (0.16 and 0.08 wt.%, respectively), or because of the way the hardness was normalized. This leads to the conclusion that grain size has little or no effect on oxygen diffusion. The same conclusions were drawn by Casadebaigt et al. on a TA6V alloy made by additive manufacturing [17]. For these alloys manufactured using electron and laser beam melting, authors reported that a hot isostatic pressure (HIP) treatment did not have an effect on mass variations nor on oxygen diffusion coefficients, despite an important coarsening of the microstructure resulting from the HIP. Finally, Sugahara et al. oxidized TA6V samples exhibiting a globular microstructure on the one hand and a Widmanstätten microstructure on the other for 48 h between 500 and 600 °C. They reported similar mass changes for both sets of samples even though their grain sizes were very different [START_REF] Sugahara | The Effect of Widmanstatten and Equiaxed Microstructures of Ti-6Al-4V on the Oxidation Rate and Creep Behavior[END_REF].

These studies on the influence of surface and microstructure were often conducted at temperatures that are higher and for durations that are shorter than those found in real applications. Long-term oxidations carried out at lower temperatures are necessary in order to investigate the effect of surface preparation and microstructure that are closer to industrial applications. A microstructure comparison should also be performed on materials with similar volume fractions of α and β phases, to study the influence of microstructure morphology on oxygen diffusion. In this work, Ti6242S Ti-based alloy was oxidized between 500 and 650 °C for durations of up to 10 kh. Both the oxide scale and the oxygen dissolution were characterized by X-ray diffraction (XRD), Raman and fluorescence spectroscopies, scanning electron microscopy (SEM) and Vickers microhardness to determine the influence of these two parameters on the oxidation behavior. were carried out on a Ti6242S alloy exhibiting an α/β globular microstructure (Fig. 1a) resulting from a hot rolling below β transus followed by a duplex annealing, as per AMS4919 (899 °C/30 min air cooling + 788 °C/15 min air cooling). The effect of microstructure was studied using a forged Ti6242S that underwent a solution heat treatment of 1h45 at 985 °C followed by a blast air cooling and an aging at 593 °C for 9 h, leading to a duplex microstructure (Fig. 1b). Composition was measured by energy-dispersive X-ray spectroscopy (EDS) at the CIRIMAT laboratory and instrumental gas analysis (IGA) by Evans Analytical Group SAS (EAG, Tournefeuille, France) for heavy elements and light elements, respectively. Glow discharge mass spectrometry (GDMS) was also realized by EAG SAS on the rolled Ti6242S alloy, to determine its Si content. Compositions are given in Table 1, and microstructures are shown in Fig. 1.

Experimental Procedures

Materials

To study microstructure, three heat treatments were performed on a forged Ti6242S alloy to obtain additional microstructures such as globular, fine and coarse lamellar. Blocks of 15 × 15 × 40 mm 3 were cut from the initial block and treated. Samples were machined from these blocks as thin plates of approximately 12 × 12 × 2 mm 3 , whereas those from the initial microstructure measured 20 × 20 × 2 mm 3 . Details on heat treatments and obtained microstructures are shown in Figs. 1c-e and 2, respectively. α-phase fractions were determined by image analysis using Aphelion 4.3.1 software, with an accuracy of 5%. About ten images with a representative surface of the microstructure were used (0.30 mm 2 and 0.0081 mm 2 for the globular and lamellar microstructures, respectively).

Oxidation tests

The effect of surface preparation on long-term oxidation was studied using parallelepipedic specimens (19.7 × 19.7 × 3.97 mm 3 ) positioned on ceramic bricks in Nabertherm N60/85HA and N120/85HA furnaces. The majority of the specimens were ground using P240 SiC paper. Others received no surface preparation, and a few were either cut from plates after super plastic forming (SPF) or ground using P80 SiC paper or mirror polished (using 0.5 µm Al 2 O 3 particles). Oxidation tests were performed under circulating laboratory air for up to 10 kh at 500 and 560 °C, and for up to 997 h at 625 °C, with a few intermediate removals to monitor specimen mass changes. A Mettler Toledo XP56, with a mass resolution of 0.1 µg, was used for mass measurements before, during and after oxidation. Each recorded mass change was the average of two weighings.

The microstructure influence was analyzed on parallelepipedic (15 × 10 × 1 mm 3 ) specimens ground using P240 SiC paper. The oxidation test ran at 650 °C for 1 kh in flowing synthetic air + 5% H 2 O with a flux of 95 mL min -1 . Weighings were performed before, during and after oxidation, three times per specimens, with a Sartorius Genius (ME215-P) balance, with an accuracy of 20 µg.

Characterization

To characterize oxide scale phases, XRD was performed using a copper anti-cathode (λ = 1.54056 Å). XRD analyses were done using a BRUKER D8-2 and a 3000TT-Seiffert apparatus with a low incidence of 2° to 8°, a Bragg angle between 20° and 80° (in 2θ), with a step of 0.04 or 0.05° and an acquisition time of 6 to 10 s per step. To further complete oxide identification, a Labram HR 800 spectrometer from Horiba Yvon Jobin, equipped with a confocal microscope, was used for Raman and fluorescence spectroscopy with a laser characterized by a 532 nm wavelength. Raman spectroscopy was performed to detect anatase and rutile TiO 2 , while the presence of alumina and its allotropic forms was identified by fluorescence spectroscopy. Observations of sample surfaces and cross sections after heat treatments and oxidations were performed by SEM with a LEO435VP microscope using the backscattered electron (BSE) mode. Oxygen enrichment in the material was characterized by Vickers microhardness profiles obtained with a Buehler OmniMet 2100 Series microhardness tester and using a 50 g diamond indent.

Results and Discussion

Effect of Surface Preparation

Mass variations measured during and after oxidation for up to 10 kh at 500 °C and 560 °C and up to 997 h at 625 °C on rolled Ti6242S exhibiting various surface preparations are reported in Fig. 3. Oxidation kinetics were parabolic. No particular trend can be deduced from these data concerning the effect of surface preparation. Very similar mass variations were measured after 10 kh at 500 °C for a ground surface and a SPF surface state. Among the five surface preparations tested at 560 °C and 625 °C, small differences in mass variations were noticed after 10 kh and 997 h, respectively. However, they are not significant and the ranking observed between surface states is not the same for both temperatures. Finally, duplicate specimens with a SPF surface state or ground with P240 SiC paper were oxidized at 625 °C (Fig. 3b). Close mass variations were obtained for a given surface preparation, highlighting the good reproducibility of the results. Parabolic oxidation constants, k p , were calculated using Eq. 1, based on mass varia- tions from 100 h to 997 or 10 kh, i.e., after the transient stage:

with Δm S the mass variation, t the time and C a constant. Figure 4a displays the equations corresponding to a linear law applied to the mass variation versus the square root of oxidation time, for samples ground using P240 SiC paper. These equations, used to determine the parabolic oxidation constants, reveal that the C constant of Eq. 1 is negligible in the present study, i.e., for long oxidation exposures. Parabolic oxidation constants at 500 °C are equal to 2.0 × 10 -9 and 2.3 × 10 -9 mg 2 cm -4 s -1 for P240 and SPF surfaces, respectively. This is in fair agreement with the study of Gaddam et al. who estimated a parabolic constant value of 9.34 × 10 -9 mg 2 cm -4 s -1 for Ti6242S oxidized at 500 °C in ambient air but for a much shorter time (500 h) [15]. Parabolic oxidation constants range from 1.7 × 10 -8 to 2.1 × 10 -8 mg 2 cm -4 s -1 at 560 °C, while they range from 3.6 × 10 -7 to 6.9 × 10 -7 mg 2 cm -4 s -1 at 625 °C. Both intervals surround the parabolic oxidation constant determined by Gaddam et al. for Ti6242S oxidized at 593 °C for 500 h (namely 7.84 × 10 -8 mg 2 cm -4 s -1 ) [15]. Finally, k p values obtained at 560 °C are higher but close to the one determined by Berthaud et al. on a Ti6242S oxidized 10 kh at 560 °C (namely 8.7 × 10 -9 mg 2 cm -4 s -1 ) after a grinding using P600 SiC paper [START_REF] Berthaud | [END_REF]. At 500, 560 and 625 °C, the recent review from Vaché et al. gave parabolic constant values of 2.3 × 10 -9 , 3.6 × 10 -8 and 4.6 × 10 -7 mg 2 cm -4 s -1 , respectively, from a database that includes varying surface states [24]. These values are also in good agreement with those found in the present study. All these data are placed on an Arrhenius plot in Fig. 4b for an easier comparison.

XRD analyses, Raman and fluorescence spectroscopies as well as SEM observations were performed on ground samples (P240 SiC paper) after oxidation at 500, 560 and 625 °C. While anatase TiO 2 and rutile TiO 2 were present at each temperature, α-alumina was only detected at 560 °C and 625 °C. Oxide scales formed during oxidation at 560 °C and 625 °C are shown in Fig. 5. The mass variations and the oxide scale thickness are much higher after 997 h at 625 °C than after 10 kh at 560 °C. From 560 °C, a 65-degree increase led to a significant increase in the oxidation kinetics of the rolled Ti6242S alloy. Taking into account a mean parabolic oxidation constant equal to 2.65 × 10 -8 mg 2 cm -4 s -1 and 7.45 × 10 -7 mg 2 cm -4 s -1 at 560 °C and 625 °C, respectively, the oxidation kinetics was multiplied by a factor 28 due to the 65 °C temperature increase. Regarding the microstructure of the alloy, there is no noticeable difference between the two samples. However, after being oxidized at 625 °C, Ti6242S exhibits many cracks underneath the oxide/alloy interface (Fig. 5b). This embrittlement could come from the metallographic preparation and/or from a larger oxygen diffusion during oxidation. To further characterize this, microhardness measurements were made on various specimens, from the oxide/alloy interface toward the bulk. Figure 6 reports the normalized Vickers microhardness profiles of Ti6242S after different surface preparations and an oxidation at 560 °C for 10 kh. (Data were normalized because of confidentiality.) These profiles do not reveal any major difference. It can be reminded that among the samples oxidized at 560 °C, only small mass variation differences were found. Therefore, it seems fair to conclude that, between 500 °C and 625 °C, surface preparation has little effect on the long-term oxidation of a rolled Ti6242S alloy.

Effect of Initial Microstructure

Because oxygen solubility in α-Ti and β-Ti is very different, the effect of alloy microstructure on the oxidation behavior was studied with similar volume fractions of α-Ti phase. This was done in order to focus on the influence of microstructure morphology. In this work, α-phase proportion was the same in the four microstructures (between 84 and 89% with a measurement accuracy of 5%). The effect of initial microstructure on oxidation kinetics at 650 °C for 1 kh is shown in Fig. 7. Mass changes follow the same trend in all microstructures studied here. From Eq. 1 and based on the mass variations between 100 and 1000 h, parabolic oxidation constants were found equal to 5.3 × 10 -7 , 5.9 × 10 -7 , 4.9 × 10 -7 and 5.0 × 10 -7 mg 2 cm -4 s -1 for the duplex, globular, fine lamellar and coarse lamellar microstructures, respectively. These values show that, at 650 °C, microstructure morphology (Fig. 1) does not have a significant effect on mass variation. The oxidation kinetics at 650 °C are similar to those previously obtained at 625 °C when studying the influence of surface preparation (Fig. 4b). For a given oxidation temperature under different oxidizing atmospheres (ambient air, synthetic air, circulating laboratory air), Casadebaigt et al. noticed a discrepancy among the various oxidation kinetics values found in the literature for TA6V alloy [7]. In the present study, the tests at the highest temperature (650 °C) were performed under a low velocity synthetic air + 5 vol% H 2 O flow whereas the tests performed at lower temperatures (including 625 °C) were carried out in a circulating laboratory air flow. The fact that the gas velocity applied to samples tested under synthetic air + 5 vol% H 2 O was low could have limited the oxidation kinetics at 650 °C. This could explain the similar mass gains found at 650 °C and 625 °C. Another explanation could be the fact that the composition of Ti6242S varies between the two series of experiments (Table 1). Indeed, the nominal oxygen content is much higher in the forged alloy than in the rolled one. Using a 1D finite differences numerical model, Ciszak et al. studied Ti-based alloys and the influence of nominal oxygen content on their life span under oxidizing conditions [25]. Considering that the oxygen-affected zone (OAZ) corresponds to the metal thickness with an oxygen content superior or equal to 0.5 at.%, and that the component reaches its life span when the OAZ thickness amounts to about 10% of its overall thickness, they found that an increase in nominal oxygen content is associated with a decrease in life span. This means that the higher the nominal oxygen content, the less dissolved oxygen quantity is needed to reach the oxygen solubility limit of the alloy. It should be reminded that the oxygen content in the forged Ti6242S alloy oxidized at 650 °C is higher than that of the rolled Ti6242S alloy tested at 650 °C. Assuming that the oxygen solubility limit does not vary significantly if the temperature is changed from 625 to 650 °C, the greater oxygen content of the forged alloy should result in a lesser quantity of diffused oxygen within the alloy during the oxidation at 650 °C. This may be a second reason that could explain the similar oxidation kinetics found in both experiments. Figure 8a shows a cross section of the oxide scale present on the alloy with a duplex microstructure. Observations revealed a porous inner layer of 2.1 µm with needles above it. XRD analyses, associated with Raman and fluorescence spectroscopies, indicated that the oxide scale was composed of anatase, rutile and α-alumina. It was also noticed that the metal was brittle, as cracks were visible directly under the oxide/ alloy interface. These cracks could have emerged from the metallographic preparation because of the alloy embrittlement due to oxygen dissolution. Microhardness profiles are shown in Fig. 8b to characterize the oxygen diffusion inside the microstructures. Despite a certain variation in Vickers microhardness for a given profile, it is reasonable to say that, near the surface, Vickers microhardness is the same for all microstructures. Affected depths were also similar after 1 kh at 650 °C.

These results are in contradiction with the study by Satko et al. who also produced their microstructures from the same thick sheets of mill-annealed Ti6242S alloy [21]. After an oxidation of 420 h at 650 °C, they noticed a gap of about 0.25 mg cm -2 between the mass variations of the five microstructures while the dispersion difference was only of about 0.1 mg cm -2 between the mass variations obtained in the present study after 1000 h at 650 °C. While efforts were made in the present work to produce microstructures exhibiting similar α-phase contents, Satko et al. oxidized microstructures with different α-phase contents. This could explain the difference between the two studies regarding the influence of microstructure.

From the present results, it can therefore be concluded that for samples with similar α-phase contents and oxidized for up to 1 kh at 650 °C, the microstructure morphology, with varying grain shapes and interface densities, does not have an influence on mass variation nor on oxygen diffusion in Ti6242S. Because a lower temperature should favor diffusion at the interfaces over intragranular diffusion, a similar experiment should be conducted at 500 °C to confirm or infirm this conclusion. This would, however, require a very long experiment. In addition to bringing interesting results on the effect of surface preparation and microstructure on the oxidation of Ti6242S, these data are important for lifetime predictions. Oxidation kinetics data corresponding to the temperature range studied here and for such exposure durations are rare. In future works, these data could be used to predict lifetimes, as was done by Vaché et al. on Ti64 and Ti6242S Ti-based alloys [24].

Conclusions

Long-term oxidations were carried out between 500 and 650 °C on two Ti6242S alloys, one rolled and one forged, for durations of up to 10 kh. Oxide scales were characterized by XRD, Raman and fluorescence spectroscopies and SEM, while oxygen diffusion within the alloy was studied with Vickers microhardness profiles.

Based on the results obtained after oxidations for 10 kh at 500 and 560 °C and for 997 h at 625 °C on rolled Ti6242S with various surface preparations, and based on the oxidation results for forged Ti6242S with different microstructures, the following conclusions can be drawn. First, the surface preparation had little or no effect on the long-term oxidation behavior in the 500-625 °C temperature interval. Second, samples exhibiting very different microstructures but similar α-phase fractions did not show any significant difference in terms of mass variation and hardness profile after 1 kh at 650 °C.

TwoFig. 1

 1 Fig. 1 Ti6242S alloy microstructures (SEM images, BSE mode): a hot rolled Ti6242S, b-e forged Ti6242S: b duplex, c globular, d fine lamellar, e coarse lamellar

Fig. 2

 2 Fig. 2 Heat treatments applied for additional microstructures: a globular, b fine lamellar, c coarse lamellar

Fig. 3 Fig. 4 a

 34 Fig. 3 Mass changes during long-term oxidations in circulating laboratory air of hot rolled Ti6242S alloy a for one sample per surface preparation and temperature and b for all tested specimens at 625 °C (including duplicate specimens with a SPF and P240 ground surface state)

Fig. 5

 5 Fig. 5 SEM images (BSE mode) of cross sections of hot rolled Ti6242S, ground using P240 SiC paper, after an oxidation in circulating laboratory air a for 10 kh at 560 °C and b for 997 h at 625 °C

Fig. 6

 6 Fig. 6 Normalized Vickers microhardness profiles on rolled Ti6242S alloys with different surface preparations and oxidized in circulating laboratory air for 10 kh at 560 °C

Fig. 7

 7 Fig. 7 Mass changes of forged Ti6242S microstructures after 1 kh at 650 °C under flowing synthetic air + 5% H 2 O

Fig. 8

 8 Fig. 8 After an oxidation of 1 kh at 650 °C under flowing synthetic air + 5% H 2 O: a SEM image (BSE mode) of a cross section of the forged and duplex Ti6242S sample, b Vickers microhardness profiles obtained in the forged Ti6242S samples with various microstructures

Table 1

 1 Composition of Ti6242S alloys (*ppm)

	Element	Ti	Al	Sn	Zr	Mo	Si	O*	N*	C*	S*	H*
	Rolled
	wt.%	Bal	6.3	2.0	3.7	1.6	750*	850	13	19	< 5	30
	at.%	Bal	11.0	0.8	1.9	0.8	0.1	2500	44	75	< 7	1400
	Forged
	wt.%	Bal	6.0	1.8	4.0	2.1	0.1	1200	< 5	26	8.2	44
	at.%	Bal	10.5	0.7	2.1	1.0	0.2	3550	< 17	103	12	2062

Acknowledgements

This study on the effect of microstructure was supported by the French National Research Agency through the project ANR DUSTI in partnership with Airbus, Airbus Group Innovations, Aubert&Duval, Liebherr Toulouse Aerospace, the Institut Pprime, the Institut Jean Lamour and the CIRIMAT Laboratory. The contributions of Moukrane Dehmas through fruitful discussions are gratefully acknowledged.


Combes-Joret Monique
email: monique.combes@univ-reims.fr

Les comportements organisationnels positifs : entre tradition et innovation

Keywords: management bienveillant, organisation capacitante, ESAT, inclusion, handicap

étendu (1500 mots

(stress, burn-out, brown-out, bore-out …) 1 , certains ESAT en font un support de développement des habiletés et de restauration de l'image de soi des personnes en situation de handicap accompagnées, contribuant à leur émancipation et à leur bien être psychologique. [START_REF] Baret | Les établissements et services d'aide par le travail (ESAT) parviennent-ils à concilier objectifs économiques et missions médico-sociales ? Une proposition de matrice stratégique[END_REF][START_REF] Cret | Contre la thèse de la marchandisation du social : l'exemple des travailleurs handicapés en établissements et services d'aide par le travail (ESAT)[END_REF], alors même qu'ils ont toute leur place au sein des études consacrées aux comportements positifs dans les organisations [START_REF] Caza | An introduction to positive organizational scholarship[END_REF] et notamment celles portant sur les organisations capacitantes (Arnoud, Falzone, Salais, Sen, Vero et Zimmerman). En reprenant les termes de Vogt (2017, p. 333), les ESAT ont de ce point de vue un rôle à jouer : « Businesses can play a role in promoting this vision of justice within their own organizations and in the societies in which they operate.

Problématique

In order to fulfill that role, the ends of business and good management must be expanded to include the goal of expanding human capabilities and the common good ».

Méthodologie : Nous avons choisi comme stratégie de recherche la méthode des cas ((Hlady Rispal, 2002, 2015) pour comprendre la complexité des phénomènes qui se déroulent dans les établissements médico-sociaux, dans toute leur diversité et modes d'expressions contextualisés (Yin, 1984). Sur chacun des 10 ESAT retenus, nous avons mené en mode principal des focus groups avec les acteurs concernés par notre problématique, en commençant par les travailleurs Le travail en ESAT ne permet pas de soigner ou de supprimer le handicap, mais d'accéder à une vie meilleure, telle que les personnes en situation de handicap ont des raisons de valoriser (Se, 2001(Se, , 2004)). Le fait d'avoir « un vrai boulot » en ESAT concoure notamment au rétablissement des personnes vivant avec des troubles psychiques ou du comportement, et contribue au développement et à la restauration de l'image de soi pour tous les types de handicap (mental, psychique, sensoriel, moteur). Au-delà de cela, les 10 E.S.A.T. étudiés témoignent de modes de management exemplaires et congruents avec un travail émancipateur et visant au bien-être psychologique des personnes accompagnées. A l'E.S.A.T. l'ENVOL, que ce soit pour les moniteurs(trices) d'atelier, l'équipe médico-sociale, les chefs de service ou la direction, la production ne se fait pas au détriment du bien-être des personnes, mais en respectant leur état de santé, leurs besoins spécifiques et leur temporalité.

Les modes de management bienveillants respectant la temporalité des sujets

Les 5 principes d'une organisation capacitante en action

Les 10 E.S.A.T. participant au projet de recherche ont su s'affranchir de leurs contraintes économiques et innover dans le management du travail et dans l'accompagnement des personnes handicapées, quelles que soit les déficiences des personnes accueillies (intellectuelles, sensorielles, troubles psychiques, troubles du spectre autistique, handicap rare ou polyhandicap).

Les règles et principes fondateurs de ces E.S.A.T. relèvent d'une organisation capacitante. Ils présentent certains traits de l'idéaltype brossé par [START_REF] Véro | A la recherche de l'organisation capacitante : quelle part de liberté dans le travail salarié ? L'Harmattan[END_REF], rappelés ci-dessous :

-Le pluralisme (reconnaissance de la diversité des finalités du travail) ;

  : Questionnant les transformations du travail dans les Entreprises de l'Economie Sociale et Solidaire (ESS) depuis plusieurs années et engagée dans une rechercheaction partenariale (Combes-Joret, 2020-a), nous avons repéré et analysé des ESAT créatifs, pionniers dans leurs pratiques et novateurs dans leurs organisations. Ils sont qualifiés dans l'étude de Silent Leaders, leaders silencieux (Combes-Joret, 2020-b) en référence à la sociologie des absences de De Sousa Santos (2011) s'intéressant à tout ce qui est « l'ignoré », « le résiduel », « l'inférieur », « le local », « l'improductif ». Les ESAT au même titre que d'autres établissements médico-sociaux, sont invisibles socialement et relativement absents des programmes de recherche académiques

  handicapés, mais aussi les moniteurs et monitrices d'atelier, les membres des équipes médicosociales et, le cas échéant, avec les chefs de services. Nous avons parallèlement menés systématiquement des entretiens semi-directifs avec les directeur.trice.s et des observations non participantes des ateliers et autres espaces de travail. Nous avons enfin recueilli et analyser de nombreux documents internes et d'archive. Le travail empirique s'est déroulé sur 2 ans (2018-2019) et nous a permis d'interviewer 210 personnes. Tous les focus groups et entretiens ont été 1 Benoit, D. & Méric, J. (2017). Éditorial. Communication & management, vol. 14(2), 2-3. doi:10.3917/comma.142.0002 enregistrés et retranscrits dans leur intégralité pour les deux cas pilotes ESAT L'ENVOL et ESAT LES ANTES. Le tableau 1, en annexe, présente les 10 ESAT étudiés. Principaux résultats : Trois principaux résultats ressortent de nos études de cas et viennent nourrir les recherches sur les comportements organisationnels positifs et les organisations vertueuses. 1. Le travail ne permet pas de soigner mais d'accéder à une vie meilleure Sans nier les difficultés et l'illusion de normalité que peut procurer un travail dans un E.S.A.T., le travail pour les personnes en situation de handicap, c'est ce qui relie aux autres et au monde.

  Les E.S.A.T. étudiés ont développé des modes de G.R.H. innovants en réponse à leurs problématiques spécifiques (taille, implantation rurale ou urbaine, publics accueillis) et singuliers : recrutement de moniteurs(trices) sur leur capacité d'accompagnement plus que sur leurs compétences techniques (AVENIR), développement d'une double compétence chez les personnes en situation de handicap (théâtre et ménage) LES ATELIERS DU CAILLY, dispositif de reconnaissance des acquis de l'expérience, mais aussi des organisations très diverses entre ateliers « classiques » et hors les murs (mise à disposition individuelle et collective (ENVOL, LES ANTES, ADAPT …) et franchise sociale (MESSIDOR).

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  La participation (avec la création d'espace d'échange et d'information remplissant trois fonctions démocratiques selon Sen (2001) ; -Le développement (au-delà du développement professionnel (compétences, carrière) vise le développement personnel) ; -La justice (égal accès aux ressources et aux capacité d'agir, quelles que soient leurs ressources ou compétences individuelles) ; -La responsabilité de toute l'entreprise (direction et ligne managériale) dans le développement des capacités d'agir de tous ses membres. Conclusion principale : C'est dans leur capacité à remplir leur double mission, fournir un travail qui ait du sens et un accompagnement médico-social pour des personnes en situation de handicap ne pouvant travailler en milieu dit ordinaire que les 10 E.S.A.T. de notre étude (et bien d'autres encore) sont des « Silent Leaders » et des organisations vertueuses et invisibles socialement. Face aux questions d'émancipation du travail et de Qualité de Vie au Travail qui se posent dans nombre d'organisations privées comme publiques, les E.S.A.T. que nous avons étudiés sont parvenus à créer des environnements capacitants (au sens de Sen, 2004) propices au développement des personnes accueillies, à la restauration de leur estime de soi et de leur insertion socioprofessionnelle. Et si le milieu de travail protégé, fort de son histoire et de ces avancées, constituait un espace pour repenser le travail dans notre société ? Si l'exemplarité des E.S.A.T. résidait dans leur capacité à proposer un travail autonome au sens fort de Gorz (1988) : un travail choisi, épanouissant, dégagé des contingences économiques et marchandes, permettant d'accéder à une vie meilleure ? Limites et voies de recherche futures : Notre communication s'inscrit dans un travail de recherche en cours. La saison 1 (2017-2020) nous a conduit à réaliser les 10 études de cas, la saison 2 (2020-2023) va nous permettre de finaliser l'analyse de la masse de données recueillies et de poursuivre nos réflexions sur la transformation des organisations de l'ESS.