Marie Giroudon

Matthieu Peyre Lavigne

Cédric Patapy

Alexandra Bertron

Matthieu Peyre Lavigne

Blast-furnace slag cement and metakaolin based geopolymer as construction materials for liquid anaerobic digestion structures: Interactions and biodeterioration mechanisms

Keywords: Anaerobic digestion, slag cement, geopolymer, biodeterioration, ammonium

Introduction

Biogas production is based on the degradation of organic matter by microorganisms under controlled anaerobic conditions [START_REF] Chen | Role of biomass-derived carbon-based composite accelerants in enhanced anaerobic digestion: Focusing on biogas yield, fertilizer utilization, and density functional theory calculations[END_REF][START_REF] Huang | Mesophilic anaerobic co-digestion of aloe peel waste with dairy manure in the batch digester: Focusing on mixing ratios and digestate stability[END_REF][START_REF] Yun | Use of bio-based carbon materials for improving biogas yield and digestate stability[END_REF]. The process called anaerobic digestion consists of four consecutive degradation reactions: hydrolysis, acidogenesis, acetogenesis and methanogenesis [START_REF] Batstone | The IWA Anaerobic Digestion Model No 1 (ADM1)[END_REF][START_REF] Evans | Environmental Biotechnology -Theory and Application[END_REF][START_REF] Wang | Microbial characteristics in anaerobic digestion process of food waste for methane production-A review[END_REF] and has been evaluated as being one of the most energy-efficient and environmentally friendly technologies for bioenergy production [START_REF] Fehrenbach | Kriterien einer nachhaltigen Bioenergienutzung im globalen Ma\s sstab[END_REF][START_REF] Yun | Synergistic effects of Fe salts and composite additives on anaerobic digestion of dairy manure[END_REF][START_REF] Yun | Use of bio-based carbon materials for improving biogas yield and digestate stability[END_REF]. In the context of climate change and depletion of fossil resources, the development of anaerobic digestion structures is encouraged by the European Union, in particular through the European Directives 2001/77/EC and 2009/28/EC (2001,2009). The process leads to the production of (i) a renewable energy source, biogas, which is mainly composed of methane (CH 4 ) and carbon dioxide (CO 2 ) [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] and (ii) a digestate that can be used as a fertilizer, due to nitrogen, phosphorous and potassium content and as the bioavailability of nitrogen is greater in the digestate than in the initial organic waste [START_REF] Huang | Mesophilic anaerobic co-digestion of aloe peel waste with dairy manure in the batch digester: Focusing on mixing ratios and digestate stability[END_REF][START_REF] Weiland | Biogas production: current state and perspectives[END_REF]. Optimized productivity is achieved in mesophilic conditions (usually 35°C) [START_REF] Kothari | Different aspects of dry anaerobic digestion for bio-energy: An overview[END_REF] and at pH between 6.5 and 7.5 [START_REF] Braun | Anaerobic digestion: a multi-faceted process for energy, environmental management and rural development[END_REF][START_REF] Chandra | Methane production from lignocellulosic agricultural crop wastes: A review in context to second generation of biofuel production[END_REF][START_REF] Liu | Prediction of methane yield at optimum pH for anaerobic digestion of organic fraction of municipal solid waste[END_REF]. Although the anaerobic digestion technology is widely used, biogas production and digestate reutilization still need to be improved, in order to achieve maximum energy output. To this end, new strategies are being developed such as the co-digestion or the addition of accelerants, leading to a better degradation efficiency [START_REF] Huang | Mesophilic anaerobic co-digestion of aloe peel waste with dairy manure in the batch digester: Focusing on mixing ratios and digestate stability[END_REF][START_REF] Wang | Mesophilic anaerobic co-digestion of acorn slag waste with dairy manure in a batch digester: Focusing on mixing ratios and biobased carbon accelerants[END_REF][START_REF] Xu | Improving performance and phosphorus content of anaerobic co-digestion of dairy manure with aloe peel waste using vermiculite[END_REF][START_REF] Zhang | Low-cost composited accelerants for anaerobic digestion of dairy manure: Focusing on methane yield, digestate utilization and energy evaluation[END_REF]. Anaerobic digestion plants can be set up with different types of organic waste and the biogas recovered can be used in many ways: production of heat, electricity, and vehicle fuel, or can even be injected after purification, into the network of natural gas [START_REF] Frigon | Biomethane production from starch and lignocellulosic crops: a comparative review[END_REF][START_REF] Holm-Nielsen | The future of anaerobic digestion and biogas utilization[END_REF][START_REF] Yun | Use of bio-based carbon materials for improving biogas yield and digestate stability[END_REF].

The implementation of anaerobic digestion on an industrial scale requires airtight, isolated production structures. In this context, reinforced concrete, which is also an economical building material, is the most widely used material for the construction of digesters. Inside the digester, the cover concrete is in direct contact with the anaerobic digestion medium in the liquid phase, while polymeric liners or plastic layers are often applied to protect concrete in the gas phase [START_REF] Bachmann | 8 -Design and engineering of biogas plants[END_REF]. At every stage of the digestion, the liquid medium contains species that are particularly aggressive to concrete [START_REF] Koenig | Biogenic acid attack on concretes in biogas plants[END_REF][START_REF] Voegel | Impact biochimique des effluents agricoles et agroindustriels sur les structures/ouvrages en béton dans la filière de valorisation par méthanisation (ou digestion anaérobie)[END_REF][START_REF] Voegel | Mechanisms of cementitious material deterioration in biogas digester[END_REF][START_REF] Voegel | Biodeterioration of cementitious materials in biogas digester[END_REF], such as volatile fatty acids (VFA) (several g/L), ammonium (about 1 g/L) and dissolved CO 2 (several g/L). These species are responsible for concrete leaching (for VFA and ammonium) and/or carbonation (for dissolved CO 2 ) of the matrix (Bertron et al., 2005a;[START_REF] Magniont | A new test method to assess the bacterial deterioration of cementitious materials[END_REF][START_REF] Voegel | Mechanisms of cementitious material deterioration in biogas digester[END_REF]. Moreover, the microorganisms colonize the concrete surface as a biofilm (Voegel et al., 2019a). Biofilms allow enhance the metabolic capacity of cells [START_REF] Flemming | Biofilms: an emergent form of bacterial life[END_REF] and seem to amplify the material deterioration, in terms of kinetics and intensity of alteration [START_REF] Magniont | A new test method to assess the bacterial deterioration of cementitious materials[END_REF]. So far, few studies have investigated the fate of concrete exposed to fermenting biowaste. [START_REF] Koenig | Biogenic acid attack on concretes in biogas plants[END_REF] studied the effects of anaerobic digestion in the liquid and gas phases of an industrial scale pilot digester. After 1 ½ years in the liquid phase, the authors observed erosion of the cover concrete and a degraded depth of 12.8 mm, with significant decalcification in the outer layer, features that are typical of an attack by acids, and precipitation of calcium carbonates. Voegel et al. (2019aVoegel et al. ( , 2019b[START_REF] Voegel | Mechanisms of cementitious material deterioration in biogas digester[END_REF] identified deterioration mechanisms corresponding to a combination of decalcification and carbonation on cement paste specimens immersed in a synthetic biowaste for 4 weeks. The presence of alkaline cementitious materials slightly increased the pH [START_REF] Voegel | Biodeterioration of cementitious materials in biogas digester[END_REF]. Nevertheless, the liquid composition varies greatly over time and according to the 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][START_REF] Xu | Methane production from the anaerobic digestion of substrates from corn stover: Differences between the stem bark, stem pith, and leaves[END_REF]. Thus, depending on the type of biowaste, the composition in terms of VFA, ammonium and CO 2 varies and may impact the material differently.

Moreover, these studies considered only classic cement matrices (based on CEM I, CEM II, CEM III or calcium aluminate cement). Thus, the degradation mechanisms of materials in anaerobic digestion systems remain unknown for various liquid compositions and on innovative matrices. Furthermore, the impact of the materials on the process efficiency (biogas production) has never been studied.

The purpose of this laboratory study was to thoroughly evaluate the bio-geochemical interaction mechanisms between the anaerobic digestion medium and the materials at the local scale and their impact (i) on the quantitative and temporal production of methane and (ii) on the mineralogical and chemical alteration of the material that may compromise its long-term durability and thus the sustainability of the structure. In order to address these interface issues, the exploration strategy was to study the impact of different materials on anaerobic digestion by monitoring the liquid environment and the biogas production on the one hand, and to evaluate changes in the solid samples exposed to the medium on the other hand. Cement paste specimens made of blast-furnace slag cement (CEM III), a metakaolin-based alkali-activated geopolymer, and an ordinary Portland cement (OPC) paste (reference cement CEM I) were exposed to the liquid phase of inoculated cattle manure for more than 36 weeks. Ground Granulated Blast furnace Slag (GGBS) is a by-product of the iron industry and metakaolin is a powder obtained by calcination (600 to 800°C) and grinding of a clay mainly composed of kaolinite (AFNOR, 2012a). They may be used alone or as supplementary cementitious materials (SCM) with ordinary Portland cement (EN 206/CN, [START_REF] Afnor | NF EN 206/CN. Concrete -Specification[END_REF](AFNOR, , 2012b) ) to reduce the environmental impact of the cementitious material manufacturing by reducing the content of OPC, which is CO 2 emitter and energy-consuming [START_REF] Juenger | Supplementary cementitious materials: New sources, characterization, and performance insights[END_REF].

Moreover, CEM III cement pastes had already shown increased durability to acid attacks compared to ordinary Portland cement (CEM I) due to the addition of aluminium-rich ground granulated blastfurnace slag (Bertron et al., 2005b;[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]. The slag addition increases the chemical resistance to material leaching by decreasing the calcium content of the binder and leads to a reduced portlandite content, a lower Ca/Si ratio of C-S-H 1 -the main hydrated phase of the material -and the formation of C-A-S-H [START_REF] Elakneswaran | Hydration study of slag-blended cement based on thermodynamic considerations[END_REF] 1 Cement chemistry shorthand notations: [START_REF] Lothenbach | Supplementary cementitious materials[END_REF], with higher chemical stability than C-S-H [START_REF] Roosz | Thermodynamic properties of C-S-H, C-A-S-H and M-S-H phases: Results from direct measurements and predictive modelling[END_REF]. Furthermore, the addition of slag to cement helps to densify the paste by filling the large pores with hydration products and the resulting finer pore structure improves the transfer properties of the paste [START_REF] Perlot | Influence of cement type on transport properties and chemical degradation: application to nuclear waste storage[END_REF]. The French prescription guide T47 [START_REF] Cimbéton | Guide de prescription des ciments pour des constructions durables[END_REF] recommends slag cements for use in aggressive chemical media and they are actually used in the design of concrete for digester structures. On the other hand, geopolymers have shown encouraging behaviour when exposed to acidic abiotic [START_REF] Drugă | Interaction between wastewater microorganisms and geopolymer or cementitious materials: Biofilm characterization and deterioration characteristics of mortars[END_REF][START_REF] Duan | An investigation of the microstructure and durability of a fluidized bed fly ash-metakaolin geopolymer after heat and acid exposure[END_REF][START_REF] Singh | Geopolymer concrete: A review of some recent developments[END_REF] and biotic [START_REF] Grengg | Long-term in situ performance of geopolymer, calcium aluminate and Portland cement-based materials exposed to microbially induced acid corrosion[END_REF][START_REF] Khan | Performance Evaluation of Geopolymer against In Situ Aggressive Sewer Environment[END_REF] environments. However, the chemical and mineralogical compositions of geopolymers are very variable and some of them, such as fly ash geopolymers [START_REF] Khan | Deterioration of alkali-activated mortars exposed to natural aggressive sewer environment[END_REF], and pure white metakaolin geopolymer [START_REF] Grengg | Long-term in situ performance of geopolymer, calcium aluminate and Portland cement-based materials exposed to microbially induced acid corrosion[END_REF], showed poor resistance to microbially induced deterioration (MID). The behaviour of these matrices therefore needs to be studied in order to assess their suitability for use in an anaerobic digestion environment.

A = Al 2 O 3 , C = CaO, F = Fe 2 O 3 , H = H 2 O, M = MgO, S = SiO 2
Cement paste specimens were exposed to the fermenting biowaste in biochemical methane potential reactors with a high solid/liquid ratio in order to reproduce local chemical conditions in the vicinity of the concrete walls of the digester. The liquid and gas phases were monitored over time.

Scanning Electronic Microscopy (SEM) observations coupled with Electron Probe Micro-Analysis (EPMA), and X-Ray Diffraction (XRD) analyses were used to investigate the chemical and mineralogical changes of the specimen.

Materials and methods

The experimental protocol implemented in this study is presented in Figure 1. It consisted of a series of bioreactors where construction materials samples were immersed in inoculated cattle manure at mesophilic temperature (35°C) in order to reproduce anaerobic digestion conditions. During the 256 days experiment (5 cycles of anaerobic digestion), both gas and liquid phases were monitored (see section 2.3) while the chemical and mineralogical changes occurred in the cementitious material samples were analysed at the end of the third and fifth cycles (see section 2.5). ) and water. The metakaolin used in this study came from the calcination of a kaolin with a relatively high level of impurity and included quartz (45%), mullite (2%), calcite (1%), anatase (1%) and kaolinite (1%), as well as a considerable amount of iron oxide (3.7% of Fe 2 O 3 ) [START_REF] Pouhet | Formulation and durability of metakaolin-based geopolymers (phd)[END_REF][START_REF] San Nicolas | Characteristics and applications of flash metakaolins[END_REF].

A procedure adapted from the French standard NF EN 196-1 (AFNOR, 2016) was used to mix the paste specimens, i.e., for each cement paste, the binder and the water were mixed at low speed (140 ± 5 rpm) for 60 seconds and at high speed (285 ± 10 rpm) for 90 seconds. The pastes were cast in cylindrical moulds 75 mm high and 25 mm in diameter. Each specimen was protected from the atmosphere by a plastic coating after pouring. 24h after pouring, the CEM III and CEM I pastes were removed from their moulds, sealed in plastic bags and kept at 20°C for a 27-day endothermic cure.

MKAA pastes were cured for 7 days in their closed moulds and then stored in sealed plastic bags. At 28 days of maturation, the paste specimens were exposed to the fermenting biowaste.

Preparation of laboratory reactors and immersion of cement pastes

To start, 100 mL of microbial inoculum collected from an industrial biogas plant in Haute-Garonne (France) and cattle manure from the experimental farm of the Engineering School of Purpan (France) (the substrate) were poured into airtight biochemical methane potential (BMP) reactors [START_REF] Holliger | Towards a standardization of biomethane potential tests[END_REF]. For each kind of material, the experiment was carried out in duplicate with two bioreactors each containing a sample. In addition, two control bioreactors without material were also taken into account.

Immediately after the first feed with substrate, the paste specimens were immersed in bioreactors.

The solid/liquid ratio (cement paste surface area/total liquid volume) was approximately 85 cm 2 .L -1 , much higher than the in situ ratio in a standard industrial digester of approximately 4 cm 2 .L -1 [START_REF] Voegel | Biodeterioration of cementitious materials in biogas digester[END_REF]. This ratio was chosen high in order to reproduce and emphasize local phenomena in the vicinity of the concrete wall, where the chemical conditions are strongly influenced by the interactions between the biowaste and the cement matrix.

After the immersion of the samples, water was added to reach a volume of 800 mL and the gas phase was flushed with N 2 in order to remove O 2 . The bioreactors were maintained at 35°C in an oven for 5 cycles. At each new cycle, the liquid medium from the previous cycle was kept in the reactors, supplemented with water (to reach the initial volume of 800 mL) and small amounts of new cattle manure were incorporated (Table 1). The mass of cattle manure added increased with the cycles in order to offset a decrease in the substrate's methanogenic potential over time. Each cycle was considered complete when the gas production in the control reactors stopped, except for the last cycle which lasted until the biogas production was over in all the bioreactors (see durations in Table 1). During the fourth cycle, once the biogas production was complete, the reactors were kept at ambient temperature for 6 weeks because the laboratory closed for the summer break.

Sampling and analysis of the anaerobic digestion medium

Twice a week, the gas pressure was measured and liquid was sampled. If the gas pressure was above 0.250 bar, some gas was sampled for analysis (O 2 /N 2 , H 2 , CH 4 and CO 2 ) by gas chromatography (GC Trace 1300 Thermofisher) and the gas pressure was then brought back to ambient pressure. Samples of 5 mL of the liquid medium were collected and the pH of each sample was measured. The liquid samples were centrifuged (Eppendorf, Centrifuge 5430R) for 5 minutes (4°C, RCF 7197). In one hand, the concentrations of ionic species PO 4 3-, Na + , NH 4 + , K + , Cl -, Ca 2+ and Mg 2+ were analysed by Ion Chromatography (Dionex Thermofisher DX320). In the other hand, 1 mL of the supernatant part was sampled and mixed with 0.5 mL of an internal standard (solution of 2-ethyl-2-butyric acid at 1 g.L -1 in demineralized water and containing 5% orthophosphoric acid H 3 PO 4 ) to be analysed by gas chromatography (Varian SERIE 3900/430) in order to provide the concentrations of some VFA (acetic, propionic, isobutyric, butyric, isovaleric, and valeric acids).

Calculation method for aqueous inorganic CO 2 and gaseous ammonium contents

As the analyses carried out did not allow the aqueous inorganic CO 2 and gaseous ammonium contents to be measured, gas and liquid chromatography results, together with thermochemical equilibria associated with Henry's law, were used to calculate these missing data.

Theoretical background: Henry's Law

Henry's law states that the partial pressure of the dilute solute in a solution is directly proportional to its liquid mole fraction. The proportionality factor is called the "Henry's law constant" ( ) (Equation A).

(A)

In Equation A, , , and are the total pressure, the composition of solute in vapour gas (molar fraction of the gas phase), Henry's law constant of solute , and the molar fraction of the liquid phase, respectively.

The constants were calculated considering a temperature of 35°C, the atmospheric pressure and the pH after each gas sampling (values of the Henry's law constants from [START_REF] Batstone | The IWA Anaerobic Digestion Model No 1 (ADM1)[END_REF]).

Aqueous inorganic CO 2 content

The inorganic CO 2 content corresponds to the molar fraction of the liquid phase divided by the water concentration, 55.41 mol.L -1 (Equation B).

where (B)

Using the thermochemical equilibria (Reactions 1 and 2), and the gas composition, the total inorganic CO 2 content can be calculated by successively calculating the and concentrations.

,

(2)

where and are the thermodynamic equilibrium constants, values from [START_REF] Batstone | The IWA Anaerobic Digestion Model No 1 (ADM1)[END_REF].

= 4.73E -7 and = 5.60E -11 at 35°C.

Gaseous ammonium content

Using the thermochemical equilibria (Reaction 3) and the total ammonium concentration in the liquid phase, the and concentrations were calculated.

,

where is the thermodynamic equilibrium constant, value from [START_REF] Batstone | The IWA Anaerobic Digestion Model No 1 (ADM1)[END_REF]. = 6.16E - 10 at 35°C.

Henry's law then allowed the gaseous ammonium content to be calculated, assuming equilibrium and no limitation of liquid/gas transfer (Equation C):

where (C)

Analysis of cementitious materials

At the end of each cycle, the cylindrical cement paste specimens were removed from the bioreactors and a diamond saw was used to collect slices for solid analyses. Quarter sections were intended for SEM and EPMA analysis. They were embedded in an epoxy resin (Mecaprex Ma2+ from Presi), and dry polished using silicon carbide polishing disks (Presi) [START_REF] Bertron | Processing of electron microprobe data from the analysis of altered cementitious materials[END_REF]. Then, the polished sections were coated with carbon. Analyses by SEM (JEOL JSM-LV, 15 kV) combined with EPMA analyses (Cameca SXFive, 15 kV, 20 nA) enabled the microstructural and chemical characterization of the specimens. For EPMA, the points to be analysed were chosen to avoid residual anhydrous grains.

Chemical profiles were established from the surface in contact with the liquid phase to the core of the specimen. For each point, the following elements were analysed: Ca, Si, Al, Fe, Mg, S and P.

Element mass percentages were expressed as mass percentages of the associated oxides.

The mineralogical composition changes of the specimens were characterized by XRD (Brucker D8

Advance, Co cathode, 40 kV, 40 nA). The plane side of the slice, which was directly in contact with the liquid medium, was first analysed. Then, the sample was abraded in order to characterize the degradation in depth, and this until reaching the sound material (Bertron et al., 2005a).

XRD and EPMA analyses were performed after the third cycle. SEM observations were carried out after each cycle. The third cycle was marked by a sharp increase of methane production in these bioreactors (370 NmL of CH 4 per gram of cattle manure). During the fourth and fifth cycles, methane production in the MKAA reactors remained slightly higher than in the control bioreactors.

Results

Characteristics of the liquid phase according to the material during the anaerobic digestion

Biogas production and composition

Even if the presence of the materials delayed the biogas production, at the end of the experiment the cumulative amount of methane produced was equivalent for all the bioreactors (-4.4%, -0.6% and -7.7% relative to the control for CEM I, CEM III and MKAA, respectively).

Table 2 gives the composition of the gas phase (N 2 , CH 4 , CO 2 ) of the bioreactors at the end of each cycle in volume percent. Mean values of the two duplicate bioreactors are presented with the standard deviations. 12.3 ± 3.2 15.1 ± 0.9 62.7 ± 1.3 52.9 ± 1.2 49.9 ± 3.4 CO 2 0.1 ± 0.0 0.6 ± 0.0 15.5 ± 2.7 17.2 ± 3.4 28.0 ± 0.4

Table 2 shows that the composition of the gas phase in the control bioreactors varied little over time, with about twice as much CH 4 as CO 2 , and approached the composition of an industrial biogas (55 to 65% of CH 4 and 30 to 40% of CO 2 [START_REF] Rasi | Biogas composition and upgrading to biomethane[END_REF]). In contrast, in the bioreactors containing the materials, the proportion of N 2 decreased through the cycles since more and more biogas was produced. Moreover, the proportion CO 2 /CH 4 varied depending on materials and time. While it was almost constant in the control bioreactors (about 1/2), this ratio was very low in the bioreactors containing materials during the first cycle. It then increased (more slowly in the bioreactors containing MKAA due to the delay of the efficient biogas production) and tended to reach ½ thereafter. The alkalinity of the cementitious materials (CEM I and CEM III) impacted the pH which increased significantly in the first few days, reaching a maximum of about 9 on day 3. Thereafter, the negative effect of the cementitious materials seems to have been overcome and the pH decreased, until values similar to those of the control bioreactors were observed at day 76. At the same time, biogas production accelerated in these bioreactors.

pH and volatile fatty acids production

Singular behaviour was observed in the bioreactor containing the MKAA. As for the bioreactors containing cementitious materials, the pH first increased due to the high alkalinity of the material.

However, it reached a very high value, of 9.6, on the third day. The pH remained at this high value during the first cycle and started to decrease halfway through the second cycle, on day 70. During the third cycle, a rapid decrease of pH was observed in the MKAA bioreactors, and pH conditions approached those of production in the other bioreactors. This pH drop occurred at the same time as biogas production started.

During the fourth and fifth cycles, the pH was stable for all types of bioreactors. However, it was observed that the pH in the MKAA bioreactors remained about 0.3 higher than in the others.

Figure 4 gives the total acid concentrations in the liquid phase of the bioreactors during the experiment. Among the VFA typically metabolized by microorganisms in anaerobic digestion (acetic, propionic, butyric, isobutyric and isovaleric acids) [START_REF] Cibis | Isolation of acetic, propionic and butyric acid-forming bacteria from biogas plants[END_REF], acetic acid was predominant far ahead of the others, as in several previous studies [START_REF] Koenig | Biogenic acid attack on concretes in biogas plants[END_REF]Voegel et al., 2019b[START_REF] Voegel | Mechanisms of cementitious material deterioration in biogas digester[END_REF]. The choice was thus made to present the total acid concentration for each material.

The first cycle showed a significant increase in the VFA concentration at the start of the digestion.

Thereafter, two different types of behaviour occurred.

For the control bioreactors and the bioreactors containing CEM I and CEM III, the production reached a maximum during the second week, with concentrations of 0.35 g.L -1 , 0.26 g.L -1 and 0.29 g.L -1 respectively. This initial increase corresponded to the acidogenesis step and was followed by the slow consumption of the VFA marking the continuation of the digestion of the cattle manure.

Although the CEM I, CEM III and control bioreactors shared similar behaviour, it was observed that the materials had an impact (i) on the quantity of VFA produced, the CEM I bioreactors producing 0.09 g.L -1 less acid than the control ones, and (ii) on the kinetics of production: the decrease in the total acid concentration was slower in the bioreactors containing materials and some VFA remained at the end of the first cycle, highlighting a less efficient consumption of VFA. This induced a new, smaller increase in the VFA concentration at the start of the second cycle. Thereafter, the VFA were well consumed and their concentration increased only very little at the start of subsequent cycles.

During the first 17 days, the evolution of the VFA concentration in the MKAA bioreactors followed the same trend as in the other bioreactors. Afterwards, the VFA concentration in the MKAA bioreactors presented a different evolution: instead of decreasing, it continued to increase during the first cycle and the accumulation of VFA accelerated with the start of the second cycle. The VFA concentration reached 1.58 g.L -1 on day 103 and suddenly dropped to only 0.09 g.L -1 at the end of the third cycle (day 128). This behaviour was strongly linked to the pH since the drop in the VFA concentration occurred just after the pH decrease.

In the fourth and fifth cycles, all the bioreactors had low concentrations of VFA, showing their good consumption during the anaerobic digestion. In anaerobic digestion, ammonium is present in the substrate and is also produced by the degradation of some proteins [START_REF] Yenigün | Ammonia inhibition in anaerobic digestion: A review[END_REF], depending on the biowaste or the inoculum. In our case, it can be observed that the initial addition of inoculum and substrate induced a significant increase of the ammonium concentration while the following additions of substrate did not lead to such an increase. Thus, ammonium was provided by the inoculum and not by the biowaste. Once again, the singular behaviour of the bioreactor containing the MKAA can be noted.

In the bioreactors containing the cementitious materials and in the control bioreactors, the ammonium content reached about 750 mg.L -1 after 30 days of digestion. After this initial increase, the ammonium concentrations tended to fall slowly through cycles, probably due to the consumption of ammonium for the growth of the microbial communities [START_REF] Kuypers | The microbial nitrogen-cycling network[END_REF].

The ammonium concentrations were much lower (about 400 mg.L -1 ) in the bioreactors containing MKAA during the first cycle and dropped during the second cycle. The ammonium concentration remained around 130 mg.L -1 for the rest of the experiment. This will be investigated further in the discussion section. In the bioreactors containing the cementitious materials and in the controls, the sodium concentration remained between 100 and 200 mg.L -1 throughout the experiment. Because of its sodium activation, the MKAA released much more Na + : its concentration in the bioreactor liquid phase increased until day 60 and remained around 2000 mg.L -1 thereafter.

Release of ions in solution

The potassium concentration followed the same trend in the bioreactors containing the cementitious materials and in the control bioreactors: a first increase until day 35 and slight variations thereafter.

The potassium concentration in the control bioreactor was between 300 mg.L -1 and 400 mg.L -1 during the experiment. Bioreactors containing the cementitious materials showed higher potassium concentrations, between 300 mg.L -1 and 700 mg.L -1 in the bioreactor containing CEM I and between 250 mg.L -1 and 500 mg.L -1 in the bioreactors containing CEM III. The potassium concentration in the bioreactors containing MKAA remained between 100 and 200 mg.L -1 throughout the experiment.

Moreover, the K + concentration for the MKAA bioreactors was found to be lower than that of the control bioreactor during the whole experiment: the release of Na + could have led to an exchange of the Na + and the K + ions in the material [START_REF] O'connor | Ion exchange in the chargebalancing sites of aluminosilicate inorganic polymers[END_REF].

The phosphate concentrations followed a similar trend in the control bioreactors and in the bioreactors containing CEM I and CEM III: a first decrease from 25 ± 2 mg.L -1 to 5 ± 1 mg.L -1 and then an increase leading to stable concentrations between 8 and 15 mg.L -1 . In the bioreactors containing MKAA, the phosphate concentration was similar to that in the other bioreactors during two cycles, but rose significantly afterwards (24 ± 3 mg.L -1 ).

Magnesium is provided by the inoculum and by the leaching of the materials. In the control bioreactors and in the bioreactors containing CEM I and CEM III, the magnesium concentration started at about 3 mg.L -1 and reached 60 ± 6 mg.L -1 at the start of the second cycle. The magnesium concentration was then stable at about 36 mg.L -1 , 55 mg.L -1 and 67 mg.L -1 in the control bioreactor, the bioreactor containing CEM I and the bioreactor containing CEM III, respectively. The initial magnesium concentration (day 1) in the bioreactor containing MKAA was higher than in the others (27.4 ± 2.9 mg.L -1 ) but the following increase was less significant than in the other bioreactors. The magnesium concentration then remained between 20 and 30 mg.L -1 .

Microstructural, chemical and mineralogical changes in the material samples

Microstructural changes

Figure 6 shows the images of the samples of materials obtained by SEM in back-scattered electron (BSE) mode before immersion and after the third and the fifth cycles. The surfaces in contact with the fermenting biowaste are at the top of the images and the cores of the specimens at the bottom.

BSE images are characterized by a range of grey levels, which are proportional to the average atomic weight of the analysed volume (or density). Thus, the grey level gives information about the different phases, the heaviest elements being bright and the lightest elements being dark. The presence of a small quantity of solid (i.e. a high quantity of porosity) in an analysed area is characterized by a darker contrast.

After the immersion, the cores of the specimens showed a high density of anhydrous residual grains (lighter grey grains). The outer layer (in contact with the biowaste) showed darker grey zones, corresponding to a decrease of mass density following the exposure to the fermenting biowaste.

Most of the cracks observed were probably linked to shrinkage caused by sample preparation and to the more fragile nature of the external zones. The CEM I paste already showed a lower density zone after 3 cycles and a local dissolution was observed after the 5 th cycle (top right of the image). This dissolution may have been associated with high local concentrations of acid.

After the 3 rd cycle, the CEM III paste showed cracks, probably due to the way the sample was prepared. However, after the 5 th cycle, no dissolution was observed but a zone of lower density appeared in the external part.

In contrast, the MKAA paste did not show any sign of deterioration after 3 cycles and only a thin layer on the external part presented a lower density after 5 cycles.

Chemical and mineralogical changes

Chemical composition profiles (EPMA), microscopic observations (SEM in BSE mode) and XRD analyses were carried out to identify the structural, chemical and mineralogical changes after the 3 rd cycle for the CEM I (Figure 7) and CEM III pastes (Figure 8). Zone 2 is 400 µm thick and shows an enrichment in sulfur. The relative silica content drops in this zone whereas the aluminium content increases. The slight intensification of the ettringite peaks in the mineralogical patterns suggests that, as in the study by [START_REF] Voegel | Mechanisms of cementitious material deterioration in biogas digester[END_REF], the sulfur enrichment could come from the precipitation of secondary ettringite in this zone. This secondary ettringite precipitation is classically observed in transition zones of specimens exposed to leaching because of the diffusion of sulfates from the altered zone (Bertron et al., 2005a;[START_REF] Faucon | Long-term behaviour of cement pastes used for nuclear waste disposal: review of physico-chemical mechanisms of water degradation[END_REF].

CEM I and CEM III

The total oxide content remains stable.

Zone 3, the thickness of which is about 400 µm in the CEM I paste and 180 µm in the CEM III paste, is first marked by a decrease in the calcium and sulfur contents. The total oxides content decreases as a result of the decrease of CaO. In this zone, the SEM image shows a very low density and a heterogeneous layer. The mineralogical patterns show the precipitation of calcium carbonates, calcite and vaterite, with a decrease of the peak intensities in the initial phases. This suggests combined dissolution/precipitation phenomena.

Zone 4, about 30 µm thick, is marked by a sudden increase of the phosphorus content -from 0 to 1.25% in the CEM I paste and from 0 to 1% in the CEM III paste. This zone also shows a decrease in the CaO, SiO 2 and total oxides contents. After the third cycle, the only remaining crystallized phases are the calcium carbonates, calcite and vaterite. Mineralogical analyses of the same sample immersed for three consecutive cycles

MKAA

The chemical composition of the MKAA specimen after the third cycle is presented in Figure 9. The specimen did not seem to present degradation at any stage of the experiment, except on a thin outer layer (50 µm after the third cycle), which was darker on the SEM image: the release of alkaline ions did not impact the major oxides over more than 50 µm.

The XRD analyses after the third cycle were identical to the initial XRD analyses of the sound paste.

The crystallized phases were mainly quartz but hematite, calcite, anatase and mullite were also present. Their analyses through time did not highlight any mineralogical change between the core of the specimen and the outer zone. However, it should be noted that the mainly amorphous nature of the sound MKAA paste did not allow every mineralogical change to be identified.

Production of biogas and volatile fatty acids

Correlations of the biogas production, the VFA production and the pH of each type of bioreactor were investigated to understand the effect of the nature of each sample on the anaerobic digestion process: the presence of materials had an impact on the digestion but it was limited in time. In the short term, two types of behaviour were identified:

 In the bioreactors containing CEM I and CEM III, the VFA were not well consumed and accumulated in the liquid environment during the first cycle but biogas was still produced.

 In the bioreactors containing MKAA, the material presence had a strong impact on the liquid environment for three cycles since the VFA accumulated first and no biogas was produced during two cycles.

For the control bioreactors, the first cycle showed an acidification of the media from a pH of 8.38 up to 7.12, corresponding to VFA production (hydrolysis and acidogenesis steps) [START_REF] Han | Steel slag as accelerant in anaerobic digestion for nonhazardous treatment and digestate fertilizer utilization[END_REF]. It can be noticed that the initial value of the pH was much higher than the ones reported by [START_REF] Han | Steel slag as accelerant in anaerobic digestion for nonhazardous treatment and digestate fertilizer utilization[END_REF] and [START_REF] Yun | Use of bio-based carbon materials for improving biogas yield and digestate stability[END_REF], respectively 6.7 and 6.1. According to [START_REF] Yun | Use of bio-based carbon materials for improving biogas yield and digestate stability[END_REF], this low pH comes from the partial acid peculiarity of the cattle manure. In the current study, since the cattle manure was not fresh, the substrate acids were probably already consumed. Moreover, the inoculum used was adapted to the substrate and was rich in bicarbonates, which explains the higher pH. A maximum VFA concentration of 0.35 g.L-1 was reached and the VFA were then well consumed by the microorganisms: after the thirteenth day, the total acid concentration decreased and biogas was produced, meaning that the anaerobic digestion process was complete. This effective digestion of the substrate continued through the following cycles with a pH that remained stable between 7.3 and 7.8, which is slightly lower but coherent with the pH values of the control check sample of [START_REF] Han | Steel slag as accelerant in anaerobic digestion for nonhazardous treatment and digestate fertilizer utilization[END_REF].

The bioreactors containing CEM I and CEM III pastes underwent a pH increase in the first three days due to the alkaline nature of the materials, which released alkaline ions during the first cycle.

Afterwards, the pH decreased while the VFA concentration increased over a period of 10 days. The increase in VFA concentration was slower and less significant in these bioreactors than in the control ones (maximum VFA concentration of 0.26 g.L -1 and 0.29 g.L -1 in the bioreactors containing CEM I and CEM III respectively). From day 13, a slow consumption of the acids by the methanogenic archaea occurred, highlighted by a decrease of the VFA concentration. The lack of efficiency in the VFA production and consumption was associated with a low production of biogas during the first cycle.

During the second cycle, the bioreactors containing cementitious materials reached a pH about 7.5, which was similar to that of the control bioreactors and suitable for anaerobic digestion -especially the methanogenic microbial activities, for which the optimal pH range is between 6.5 and 8.2 [START_REF] Kothari | Different aspects of dry anaerobic digestion for bio-energy: An overview[END_REF]. During this cycle, the bioreactors containing the cementitious materials produced biogas in greater quantities than the control ones did, due to the undigested cattle manure remaining after the first cycle. From the third cycle, these bioreactors behaved similarly to the controls and the presence of materials no longer disturbed the anaerobic digestion process.

The lower VFA production of the first cycle in the bioreactors containing cementitious materials was also observed by [START_REF] Voegel | Mechanisms of cementitious material deterioration in biogas digester[END_REF]. In their study, CEM I pastes (water/cement = 0.4) were immersed in an inoculated synthetic biowaste, representative of organic domestic waste, for a single cycle of anaerobic digestion. The substrate induced significantly higher maximum acid concentrations than in the current study, about 2.90 g.L -1 (43 mmol.L -1 ) in the control bioreactors and a slightly lower concentration, about 2.85 g.L -1 (41 mmol.L -1 ), in the bioreactors containing CEM I paste. Moreover, the pH values between 7 and 8 found in the studies by Voegel et al. (2019b[START_REF] Voegel | Mechanisms of cementitious material deterioration in biogas digester[END_REF] were similar to those encountered in the current study, whereas a slightly lower pH, around 7, was observed in the liquid phase of a laboratory fermenter in a study by [START_REF] Koenig | Biogenic acid attack on concretes in biogas plants[END_REF].

In the MKAA bioreactors, the pH was very high, between 9 and 9.5, during the first two cycles. This can be explained by the strong release of alkalis that occurred during the first cycle (about 2 g/L of Na + , Table 3). These high pH values were much higher than the ones tolerated by the methanogenic archaea (optimal pH around 7 [START_REF] Chae | Selective inhibition of methanogens for the improvement of biohydrogen production in microbial electrolysis cells[END_REF]). These are the most fragile microorganisms of the anaerobic digestion process and are sensitive to pH variations [START_REF] Chae | Selective inhibition of methanogens for the improvement of biohydrogen production in microbial electrolysis cells[END_REF], unsuitable pH leading to very low biogas production. At the same time, the VFA accumulated in the medium and accentuated the environment's aggressiveness. The formation of VFA in the medium showed that the high pH did not disturb the first steps of the anaerobic digestion (hydrolysis, acidogenesis), although the optimal operating pH of these bacteria is between 5.5 and 6.5 [START_REF] Hagos | Anaerobic co-digestion process for biogas production: Progress, challenges and perspectives[END_REF][START_REF] Han | Mesophilic Acidogenesis of Food Waste-Recycling Wastewater: Effects of Hydraulic Retention Time, pH, and Temperature[END_REF][START_REF] Kim | Effects of various pretreatments for enhanced anaerobic digestion with waste activated sludge[END_REF]M. Kim et al., 2003;[START_REF] Yu | Acidogenesis of dairy wastewater at various pH levels[END_REF]. The third cycle showed a decrease of the pH, probably due to the accumulation of the VFA in the medium, since most alkalis were released during the first cycle. This decrease of pH allowed the methanogenic activities to take place, and thus the rapid consumption of the VFA, leading to a significant production of biogas. The accumulation of methane produced during the third cycle indicated that not all the methane potential was expressed earlier, due to the delay in the anaerobic digestion reactions.

According to the thermochemical equilibria (Reactions 1 and 2) associated with Henry's law at 35°C, the increase in pH during the first cycles in the bioreactors containing materials led to a reduction of the volatilization potential of CO 2 , which can explain the lower CO 2 amount in the gas phase during this period.

In all the bioreactors containing materials, when conditions suited to anaerobic digestion were well established, the process remained efficient over time. Moreover, the final total production of methane was equivalent in all the bioreactors (Figure 2), so the presence of the materials nevertheless allowed the expression of the methane potential of the entire amount of substrate. It can be assumed that the retarding effect of the MKAA on the biogas production is only expressed in the short term, as geopolymers rapidly lose their alkaline charge, which is mainly present in the liquid contained inside pores [START_REF] Khan | Corrosion investigation of fly ash based geopolymer mortar in natural sewer environment and sulphuric acid solution[END_REF].

Ammonium production

A significant difference of ammonium concentration was observed between, on the one hand, the control bioreactors and the bioreactors containing the cementitious materials and, on the other hand, the bioreactors containing MKAA. This suggests a specific behaviour of the MKAA towards ammonium, which needs to be studied more thoroughly.

Control bioreactors and bioreactors containing CEM I and CEM III

The ammonium concentrations in the CEM I, CEM III and control bioreactors increased until a maximum of about 750 mg.L -1 was reached on the 35 th day. Similar concentrations were found in Voegel et al.'s study (2016) (800 mg.L -1 ), where the presence of CEM I cement paste also did not influence the ammonium production. It could be expected that each addition of cattle manure would lead to an increase in the potential of hydrolysable nitrogen but the ammonium concentrations actually decreased during the experiment, meaning that this element was mainly brought by the inoculum. In Voegel et al. (2019b), the ammonium concentration did not increase with a new substrate addition either, since the measured concentrations were 988 mg.L -1 (54.9 mmol.L -1 ) in the first cycle and 940 mg.L -1 (52.24 mmol.L -1 ) in the second. The decrease of the ammonium concentration could be explained by several phenomena: nitrogen consumption by bacteria for growth [START_REF] Kuypers | The microbial nitrogen-cycling network[END_REF], stripping of the ammonium in the gas phase in the form of ammonia [START_REF] Limoli | Ammonia removal from raw manure digestate by means of a turbulent mixing stripping process[END_REF], and/or formation of precipitates containing ammonium (struvite, for example) [START_REF] Cerrillo | Struvite precipitation as a technology to be integrated in a manure anaerobic digestion treatment plant -removal efficiency, crystal characterization and agricultural assessment[END_REF][START_REF] Escudero | Struvite precipitation for ammonium removal from anaerobically treated effluents[END_REF].

Using the chemical oxygen demand (COD) of the CH 4 production with the biomass production yields proposed by the Anaerobic Digestion Model n° 1 (ADM1) [START_REF] Batstone | The IWA Anaerobic Digestion Model No 1 (ADM1)[END_REF], the quantity of biomass produced per cycle was estimated and its growth only required 61 mg of nitrogen, which is insufficient to justify the difference of 253 mg of nitrogen lost between the maximum concentrations (day 37) and the end of the fifth cycle.

The method used for gas chromatography did not allow identifying and measuring ammonia in the gas phase. Thus, the thermochemical equilibrium (Reaction 4) associated with Henry's law at 35°C

and atmospheric pressure, were used and it was found that the amount of volatile ammonia passing into the gas phase did not exceed 0.15 mg for the whole experiment in these bioreactors.

(4)

Moreover, thermodynamic calculations using the geochemical code PHREEQC and considering the temperature, the pH and the concentrations in the solution (N as ammonium, P as phosphate, K, Mg, Cl, Ca, Na, inorganic carbon) (Table 3) were carried out to assess the possibility of struvite precipitation during the digestion. The MINTEQ database was used together with the struvite solubility value at 35°C; pK sp struvite = -13.20 [START_REF] Bhuiyan | A Solubility and Thermodynamic Study of Struvite[END_REF]. Figure 10 shows the variation of the struvite saturation index of the liquid phase of the bioreactors containing CEM I and MKAA with time.

Precipitation conditions are met for a positive index, while a negative index corresponds to dissolution if struvite is present. The bioreactors containing CEM I were chosen to represent the bioreactors containing CEM I and CEM III and the control bioreactors. In these bioreactors, struvite precipitation was continuously possible during the experiment, with higher indices in the first cycle and values closer to 0 at the end of the experiment. The ammonium loss during the experiment could thus be explained by the precipitation of struvite.

Bioreactors containing MKAA

In comparison with the other bioreactors, the bioreactors containing MKAA reached a lower maximum ammonium concentration, of about 400 mg.L -1 , on day 30. At this stage of the process, the pH was about 9.5. According to the pKa of the acid and conjugate base NH 4 + /NH 3 , i.e. 9.25 (Reaction 4), the ammonium was thus present in the form of both ammonium and ammonia. The volatile ammonia form could have passed into the gas phase, leading to a weaker ammonium concentration measured in the liquid phase of the MKAA bioreactors. Nevertheless, this hypothesis implies that a fall in pH should lead to an increase in the ammonium concentration in the liquid medium, whereas the concentration actually dropped quickly during the second cycle, at the same time as the pH started decreasing. Moreover, the thermochemical equilibrium (Reaction 4) associated with Henry's law at 35°C and atmospheric pressure allowed the amount of ammonia in the gas phase to be calculated: this amount was very small throughout the experiment and did not exceed 0.5 mg of NH 3 , which does not explain the lower ammonium concentration found in the liquid phase of the MKAA bioreactors.

The thermodynamic calculations using the geochemical code PHREEQC allowed us to follow the variation of the Struvite Saturation Index of the liquid phase of the bioreactors containing MKAA (Figure 10). The index was positive during the first cycle, but with lower values than in the other bioreactors, while the ammonium concentration was lower in the bioreactors containing MKAA.

Moreover, whereas the acidification of the environment (from day 66) led to negative or neutral struvite saturation indices, the ammonium concentration decreased in the bioreactors containing MKAA. Thus, even if struvite can precipitate at the beginning of the experiment, the trend of the ammonium concentration in the bioreactors containing MKAA remains unexplained, and is probably the result of a combination of phenomena.

According to the literature, natural zeolites (hydrated aluminosilicates) have been successfully used in wastewater treatment to remove ammonium [START_REF] Luukkonen | Metakaolin geopolymer characterization and application for ammonium removal from model solutions and landfill leachate[END_REF][START_REF] Wang | Study on a fixed zeolite bioreactor for anaerobic digestion of ammonium-rich swine wastes[END_REF]. The ion exchange properties of the material are attributed to the charge-balancing cation (Na + or K + ) being replaced by another cation, such as K + , Ag + , NH 4 + or Pb 2+ [START_REF] O'connor | Ion exchange in the chargebalancing sites of aluminosilicate inorganic polymers[END_REF]. As synthetic zeolites, metakaolin-based alkali-activated geopolymers have been studied for ammonium removal by [START_REF] Luukkonen | Metakaolin geopolymer characterization and application for ammonium removal from model solutions and landfill leachate[END_REF]. In this study, the maximum capacity of metakaolin geopolymer to remove NH 4 + was 46% higher than that of natural zeolite, showing its potential interest in such an application. The adsorption process is influenced by the pH: for pH higher than 9.25, NH 4 + deprotonates to NH 3 (Reaction 4) which is not favourable to adsorption. Actually, the ammonium adsorption capacity seems to decrease even from pH ≥7 [START_REF] Marañón | Removal of ammonium from aqueous solutions with volcanic tuff[END_REF].

According to the former results, it is thus possible that ammonium was adsorbed on the surface of the MKAA pastes through an exchange mechanism between a cation of the sound paste and the NH 4 + cation in the liquid phase. The liquid medium analyses showed high concentrations of Na + (2000 mg.L -1 on average), this element being leached from the MKAA matrix. The drop of ammonium concentration during the second cycle might be explained by the sudden drop of pH (due to the VFA accumulation) leading to more suitable adsorption conditions. Then, although the ammonium concentration appeared to be lower in the bioreactors containing MKAA, the production of ammonium in the liquid medium due to digestion was probably similar in all the bioreactors, the difference being adsorbed by the MKAA.

Exchange phenomena between the MKAA and NH 4 +

In order to investigate the assumption concerning the adsorption of NH 4 + by the MKAA, based on the literature, an additional test was carried out to study the interaction between the MKAA and the NH 4 + cation. The test design was adapted from the experimental protocols implemented by [START_REF] Lin | Adsorption mechanisms of high-levels of ammonium onto natural and NaCl-modified zeolites[END_REF] and [START_REF] Luukkonen | Metakaolin geopolymer characterization and application for ammonium removal from model solutions and landfill leachate[END_REF].

This experiment aimed (i) to investigate the exchange phenomena between the MKAA and NH 4 + for different pH values and (ii) to understand how the ammonium cation interacted with the material. To this end, MKAA powder was immersed in an ammonium nitrate solution and the ammonium concentration was monitored for various pH values. The remaining MKAA powder was also analysed.

Experimental protocol

An 800 mg.L -1 (0.0444 mol.L -1 ) ammonium nitrate solution was prepared with demineralized water (according to the ammonium concentration found in the control bioreactors) and a cylindrical specimen of MKAA paste (75 mm high and 35 mm in diameter) was ground to 80 µm using a disc mill.

500 mL of solution was poured into a glass reactor thermostatically controlled at 35°C with magnetic stirring (Annexe A). The pH value was measured continuously and recorded every 15 seconds. After about 30 minutes (the time for the temperature of the ammonium nitrate solution to reach 35°C), 10 g of MKAA powder was poured into the reactor. After 3 hours, 1 mL of concentrated HNO 3 acid (69%)

was added in order to lower the pH. Samples of the liquid were then taken at different stages of the After the end of the main experiment, the solution was centrifuged in order to collect the MKAA powder. It was then immersed in isopropanol for 15 minutes to dry by solvent exchange [START_REF] Scrivener | A Practical Guide to Microstructural Analysis of Cementitious Materials[END_REF][START_REF] Snellings | RILEM TC-238 SCM recommendation on hydration stoppage by solvent exchange for the study of hydrate assemblages[END_REF]. After another centrifugation, the powder was collected and placed in a vacuum desiccator for 1 hour to finish drying. This powder and a control MKAA powder were analysed in several ways: thermogravimetric analysis (NETZSCH STA 449F3); 27 Al, 29 Si and 1 H nuclear magnetic resonance (NMR) (Brucker Avance 4000 III HD MAS); and infrared spectrometry (FTIR) (Perkin-Elmer Spectrum). No significant differences were observed in the 27 Al and 29 Si NMR spectra between the control MKAA powder and the powder from the experiment (called MKAA -NH 4 + powder), and these results are not presented.

Liquid analysis

The ammonium concentration [NH 4 + ] was measured by Ion Chromatography and the results from reactor 1 and the supplementary experiment are presented in Table 4. These analyses highlighted an exchange mechanism between the MKAA and the ammonium solution since the measured concentrations were different from the initial ammonium concentration of the solution (800 mg.L -1 ).

When the MKAA powder was added, the pH increased to 9.29 and the ammonium concentration decreased from 800 mg.L -1 to 437.9 mg.L -1 . The pH decrease from 9.29 to 7.06 led to a decrease of the ammonium concentration to 351.1 mg.L -1 . From then on, the ammonium concentration increased when the pH was decreased, but still remained below 800 mg.L -1 .

MKAA powder analyses

Differential Thermo Gravimetry (DTG), 1 H NMR and FTIR analyses of the control MKAA powder and the powder from the reactor 1 are presented in Figure 12 (a), (b) and (c) respectively.

Figure 12 (a) shows that, despite the drying by solvent exchange, the MKAA -NH 4 + powder contained more free water than the control powder (intense peak at 143°C). The DTG analysis differed slightly with a weak, broad peak centred on 318°C in the MKAA -NH 4 + powder, which could be correlated with the thermal decomposition of the NH 4 + cation identified in two different clays at 327°C and 330°C by [START_REF] Gautier | NH4-smectite: Characterization, hydration properties and hydro mechanical behaviour[END_REF] and [START_REF] Yariv | Study of the adsorption of organic molecules on clay minerals by differential thermal analysis[END_REF], respectively. Some calcium carbonates were identified by a bump at 670°C [START_REF] Wianglor | Effect of alkali-activated metakaolin cement on compressive strength of mortars[END_REF] in the control MKAA powder.

Figure 12 (b) shows a fine, intense peak for both specimens at 0.94 ppm, which corresponds to the SiOH group usually found between 1 and 2.2 ppm in the literature [START_REF] Hunger | Multinuclear solid-state NMR studies of acidic and non-acidic hydroxyl protons in zeolites[END_REF][START_REF] O'connor | Ion exchange in the chargebalancing sites of aluminosilicate inorganic polymers[END_REF]. All the peaks between 3.5 and 4.9 were associated with water and, more precisely for hydrated Na-form zeolites, with physisorbed water molecules [START_REF] Hunger | Multinuclear solid-state NMR studies of acidic and non-acidic hydroxyl protons in zeolites[END_REF][START_REF] O'connor | Ion exchange in the chargebalancing sites of aluminosilicate inorganic polymers[END_REF].

Finally, a peak was detected in the MKAA -NH 4 + powder only at 6.82 ppm and corresponded to the ammonium cation, whose signal appears between 6.5 and 7 ppm [START_REF] Jacobs | Interaction of ammonia with Broensted acid sites in different cages of zeolite Y as studied by proton MAS NMR[END_REF][START_REF] O'connor | Ion exchange in the chargebalancing sites of aluminosilicate inorganic polymers[END_REF].

In addition to the peaks associated with water (3500 and 1640 cm -1 ) [START_REF] Ahmed | XRD and ATR/FTIR investigations of various montmorillonite clays modified by monocationic and dicationic imidazolium ionic liquids[END_REF][START_REF] Aredes | Effect of cure temperature on the formation of metakaolinite-based geopolymer[END_REF][START_REF] Wang | Synthesis and mechanical properties of metakaolinite-based geopolymer[END_REF], the silicates (992 cm -1 ) [START_REF] O'connor | Ion exchange in the chargebalancing sites of aluminosilicate inorganic polymers[END_REF] and the carbonates (867 cm -1 ), peaks centred on 3190 cm -1 and 1400 cm -1 can be seen in Figure 12 (c) and correspond to the ammonium cation [START_REF] Mookherjee | Ammonium ion behaviour in feldspar: variable-temperature infrared and 2H NMR studies of synthetic buddingtonite, N(D,H)4AlSi3O8[END_REF].

Thus, the analyses of the materials showed that ammonium from the solution was indeed exchanged towards the MKAA.

Exchange phenomena conclusions

In light of the previous results, it can be concluded that exchange phenomena between MKAA and NH 4 + do exist. More precisely, the liquid analysis showed that the efficiency of this exchange depended on the pH, with a much greater transfer of ammonium to the MKAA when the pH was around 7. Moreover, the analyses of the MKAA powders confirmed the presence of the ammonium cation in the material, but without changing the Si-Al structure of the geopolymer. The nature of this exchange phenomenon would therefore be adsorption. Only 10 g of material was inserted in 500 mL of a 800 mg.L -1 nitrate ammonium solution, whereas the cylinders immersed in the bioreactors are about 100 g for 800 mL of solution (ammonium in similar concentration, about 750 mg.L -1 ). Even though the exchange surface is much higher in the case of a powdered material, the monolithic samples of MKAA constituted, in theory, a sufficient source of exchange that could explain the lower ammonium concentrations found in the bioreactors containing MKAA.

Deterioration of the material samples

CEM I and CEM III

The chemical composition analysed in this study can be compared with results from previous studies.

Voegel (2017) studied the biodeterioration phenomena of cementitious paste (water/cement = 0.40) in the submerged part of a digester with a laboratory test using typical food biowaste (solid/liquid ratio=224 cm 2 .L -1 ). After chemical and mineralogical tests, a zonation was established and phenomena were highlighted.

The chemical profiles were correlated with mineralogical changes and the CEM I and CEM III paste zonations were as follows:

-A sound zone at depth, corresponding to Zone 1, composed of the crystalline phases commonly found in the CEM I and CEM III matrices.

-A decrease in the CaO content and an enrichment in sulfur in Zone 2, probably correlated with the precipitation of secondary ettringite.

-Variations in the CaO content in Zone 3 and a slight decalcification, together with the precipitation of CaCO 3 .

-A peak of phosphorus and decalcification of the matrix in Zone 4

From a mineralogical point of view, the X-Ray patterns obtained in the different zones of the CEM I and CEM III pastes after the third cycle showed leaching and carbonation phenomena: the initial phases (ettringite, portlandite, brownmillerite, C 3 S and C 2 S) were dissolved and calcium carbonates (calcite and vaterite) precipitated.

the laboratory. The presence of the initial phases portlandite and ettringite after 173 days of immersion demonstrated low leaching of the materials.

After the third cycle, the same phenomena as in Voegel's study (2017) were detected for the CEM I and CEM III samples, with equivalent degraded depths between the two materials. However, it can be noted that, even with a longer exposure time in the cattle manure, the degradation of the samples was less since the altered zone seemed to stop at zone 4, where the phosphorous peak was found. This may have been due to (i) the different substrate used, which led to lower concentration of aggressive agents, and/or (ii) the lower water/cement ratio of our matrices, leading to a reduction of the porosity, influencing the transport mechanisms and reducing the specific surface area [START_REF] Koenig | Acid Resistance of Ultra High-Performance Concrete (UHPC)[END_REF].

MKAA

Even though the amorphous part of the material could not be characterized, the MKAA samples seemed to show very stable behaviour with a small apparent degraded depth. However, the MKAA released a large amount of alkalis at the start of the immersion, leading to a greater buffering capacity compared to other binders: more than 90% of the water introduced was "free water" [START_REF] Pouhet | Influence of the initial water content in flash calcined metakaolin-based geopolymer[END_REF] and contained a large amount of alkaline ions, which are easily released from the paste via the porous network. This behaviour could be linked to the observations of [START_REF] Khan | Corrosion investigation of fly ash based geopolymer mortar in natural sewer environment and sulphuric acid solution[END_REF], who studied the degradation of a fly ash (FA) based geopolymer mortar in a naturally aggressive sewer environment. A mixture of sodium hydroxide solution and sodium silicate solution was used as the activator. Although the geopolymer showed less degradation in terms of mass loss, dry bulk density, compressive strength and surface disintegration compared to a Sulfate Resistant (SR) ordinary Portland cement mortar, the geopolymer was also found to have a much greater neutralization depth with a loss of alkalinity. This shows a greater penetration of the aggressive agents and could be a concern for steel reinforced concrete. Moreover, the same geopolymer was compared to an alkali-activated slag mortar (AASm) in another study [START_REF] Khan | Deterioration of alkali-activated mortars exposed to natural aggressive sewer environment[END_REF] (same activator) and the FA geopolymer showed much higher overall matrix deterioration than the AASm.

Nevertheless, the very different chemistries of the FA-based and AAS mortars do not allow a qualitative comparison with the current study. In the study by [START_REF] Grengg | Long-term in situ performance of geopolymer, calcium aluminate and Portland cement-based materials exposed to microbially induced acid corrosion[END_REF], a metakaolin similar to the one used in this study, which was quartz-rich, rust brown iron-rich (GP-K2) with potassium silicate, was placed in a power main outlet storage basin heavily affected by MID in Austria, together with a geopolymer made with high purity white metakaolin (GP-K1), a calcium aluminate cement and a CEM I based material. Among these materials, after 18 months, the degree of surface cracking and the estimated degradation rates were the lowest for the GP-K2 but highest for the GP-K1. Thus, as pointed out by [START_REF] Grengg | Long-term in situ performance of geopolymer, calcium aluminate and Portland cement-based materials exposed to microbially induced acid corrosion[END_REF] and [START_REF] Khan | Deterioration of alkali-activated mortars exposed to natural aggressive sewer environment[END_REF], depending on the design and the type of geopolymer, its resistance to bacteriological attacks in acid medium can vary widely.

Conclusion

This study presented the exposure of three types of binder paste specimens to five consecutive anaerobic digestion cycles of cattle manure in laboratory conditions, in an 8 months experiment. A high solid/liquid ratio used for this study was chosen to exacerbate and evaluate the local interaction mechanisms between the different materials and the anaerobic digestion medium.

The presence of the alkaline materials disrupted the efficiency of the anaerobic digestion process at the local scale by slowing down the production and consumption of VFA and decreasing the production of biogas. Whereas the Portland based materials, CEM I and CEM III pastes, impacted the anaerobic digestion process for only one cycle, the alkali-activated materials (MKAA) released larger amounts of alkalis at the start of the immersion, inducing a much higher pH, the VFA accumulation, and almost no biogas production during the first two cycles. After 5 cycles, the final biogas productions were equivalent for all the bioreactors and the anaerobic digestion process worked under normal, efficient conditions. Thus, given the moderately aggressive conditions induced by the liquid phase of anaerobic digestion, all materials can therefore be considered for construction application in such environment.

In terms of deterioration pattern the CEM I and CEM III biodeterioration were expressed by a combination of dissolution of the initial phases and decalcification on the surface, and precipitation of CaCO 3 , in accordance with literature recent results. The original study of an alkali-activated geopolymer in an anaerobic digestion medium has revealed a very good behaviour of the material with very low degradations. In addition, the material was found to have interesting ammonium adsorption properties at the pH of the anaerobic digestion, significantly reducing the ammonium concentration in the liquid medium.

Figure 1 :

 1 Figure 1: Schematic representation of the experimental protocol to study the interactions between the anaerobic digestion environment and the cementitious materials 2.1 Binder materials

Figure 2

 2 Figure 2 gives the total production of CH 4 per gram of cattle manure in each type of bioreactor and

Figure 2 :

 2 Figure 2: Total production of CH 4 (NmL.g -1 of cattle manure) at the end of each digestion cycle and cumulative total production at the end of the experiment (Total)

Figure 3

 3 Figure 3 gives the evolution of the pH during the five cycles of anaerobic digestion in the liquid

Figure 3 :

 3 Figure 3: Evolution of the pH during the five cycles of anaerobic digestion in the bioreactors

Figure 4 :

 4 Figure 4: Evolution of the total acid concentration during the five cycles of anaerobic digestion in the bioreactors

Figure 5 :

 5 Figure 5: Evolution of the ammonium concentration during the anaerobic digestion of cattle manure in the bioreactors

Figure 6 :

 6 Figure 6: Observation of the paste by scanning electronic microscopy in back-scattered electron mode, before immersion and after the third and fifth cycles. Surface in contact with the fermenting biowaste is at the top of each image and the core of the specimen is at the bottom.

Figure 7 :

 7 Figure 7: (a) Chemical composition profile, according to the distance to the surface, of a CEM I paste immersed in the liquid phase in anaerobic digestion conditions (EPMA), and SEM observations of the polished sections in BSE mode. (b) Mineralogical analyses of the same sample immersed for three consecutive cycles

Figure 8 :

 8 Figure 8: (a) Chemical composition profile, according to the distance to the surface, of a CEM III paste immersed in the liquid phase in anaerobic digestion conditions (EPMA), and SEM observations of the polished sections in BSE mode. (b)

Figure 9 :

 9 Figure 9: (a) Chemical composition profile according to the distance to the surface (EPMA) of an MKAA paste immersed in the liquid phase in anaerobic digestion conditions), and SEM observations of the polished sections in BSE mode. (b) Mineralogical analyses of the same sample immersed for three consecutive cycles

Figure 10 :

 10 Figure 10: Variation with time of the Struvite Saturation Index of the liquid phase of the bioreactors containing CEM I and MKAA

  experiment to be analysed by Ion Chromatography (Thermo Electron ICS 300, CS16 column). The pH evolution and the distribution of the liquid samples in time (W1 to W5) are presented in Figure11(a). The experiment was performed in duplicate (reactor 1 and reactor 2). In order to reach another final pH, a supplementary experiment was run with the addition of 350 µL of HNO 3 only. The pH evolution and the distribution of the liquid samples S1 & S2 in time are presented in Figure 11 (b).

Figure 11 :

 11 Figure 11: pH evolution according to time and distribution in time of the liquid sampling: (a) in the two duplicate reactors for 1 mL of added HNO 3 and (b) in the solution of the supplementary experiment for 350 µL of added HNO 3

Figure 12 :

 12 Figure 12: (a) Differential thermogravimetry, (b) 1 H Nuclear Magnetic Resonance, and (c) Fourier Transform Infrared Spectroscopy analyses of the control MKAA powder and the powder from the experiment (MKAA -NH 4 + ). The main peaks are identified.

Table 1 :

 1 Approximate mass of cattle manure added per cycle and duration of each cycle

1 st cycle 2 nd cycle 3 rd cycle 4 th cycle 5 th cycle

	Temperature	35°C	35°C	35°C	35°C	Room	35°C
	Mass of cattle manure added
		6	6	8	10		13
	(g)
	Duration of cycle (weeks)	7	6.5	5	4	6	8

Table 2 :

 2 Composition of the gas phase (N 2 , CH 4 , CO 2 ) at the end of each cycle(vol.%)

% End of 1

st cycle End of 2 nd cycle End of 3 rd cycle End of 4 th cycle End of 5 th cycle

		N 2	24.5 ± 3.2	32.5 ± 3.8	31.7 ± 1.1	33.9 ± 1.4	27.9 ± 1.8
	Control	CH 4	54.5 ± 1.7	42.5 ± 1.5	42.8 ± 0.4	44.7 ± 0.4	46.8 ± 1.5
		CO 2	22.6 ± 1.6	23.0 ± 2.4	24.0 ± 0.8	19.4 ± 1.2	25.3 ± 0.1
		N 2	64.2 ± 7.1	33.7 ± 1.1	34.5 ± 5.0	31.4 ± 2.1	23.8 ± 2.6
	CEM I	CH 4	34.8 ± 6.4	48.8 ± 1.2	43.9 ± 3.1	44.0 ± 2.7	48.4 ± 1.0
		CO 2	2.6 ± 0.8	15.1 ± 2.4	20.1 ± 1.3	21.2 ± 1.8	27.6 ± 2.0
		N 2	49.2 ± 13.3	34.2 ± 6.4	32.6 ± 4.8	35.8 ± 5.7	24.7 ± 1.6
	CEM III	CH 4	45.3 ± 8.7	46.9 ± 5.8	45.4 ± 2.6	44.8 ± 3.4	49.6 ± 0.3
		CO 2	7.1 ± 4.5	17.3 ± 0.7	20.2 ± 1.9	17.6 ± 1.8	25.1 ± 1.7
		N 2	87.5 ± 2.9	81.4 ± 1.1	16.6 ± 1.2	26.7 ± 4.7	21.6 ± 4.5
	MKAA	CH 4

  Table 3 gives the Na + , K + , PO 4 3-and Mg 2+ concentrations (mg.L -1 ) in the liquid phase of the bioreactors at the start of each cycle. Mean values of the two duplicate bioreactors are presented with the standard deviations.

Table 3

 3

	: Na	+ , K	+ , PO 4 3-and Mg	2+ concentration (mg.L	-1

) in the liquid phase of the bioreactors at the start of each cycle mg.L

-1 Start of the 1 st cycle Start of the 2 nd cycle Start of the 3 rd cycle Start of the 4 th cycle Start of the 5 th cycle [Na + ]

	Control	2.2 ± 0.6	56.3 ± 3.5	32.6 ± 1.4	38.6 ± 4.2	38.3 ± 5.9
	CEM I	2.9 ± 1.3	56.7 ± 2.4	50.1 ± 7.8	55.6 ± 6.1	58.4 ± 3.5
	[Mg 2+ ]
	CEM III	2.8 ± 0.6	67.2 ± 2.5	65.3 ± 2.0	68.9 ± 4.8	65.7 ± 1.7
	MKAA	27.4 ± 2.9	45.3 ± 3.6	19.3 ± 1.6	19.9 ± 1.5	30.7 ± 2.6
	Control 117.5 ± 13.4	167.0 ± 1.7	125.6 ± 0.9	149.8 ± 120.8	121.3 ± 0.9
	CEM I	130.5 ± 13.4	195.5 ± 9.8	155.6 ± 2.2	154.4 ± 3.4	138.4 ± 2.7
	CEM III	123.5 ± 0.7	197.6 ± 0.4	145.9 ± 1.6	140.4 ± 1.9	142.6 ± 1.2
	MKAA 636.0 ± 77.4 2222.2 ± 80.6 1749.3 ± 61.2 1908.1 ± 2.2 2007.9 ± 13.1
	Control 231.0 ± 19.8 379.1 ± 11.6	292.3 ± 9.9	390.7 ± 18.8	469.5 ± 33.7
	CEM I	324.5 ± 30.4 655.6 ± 162.8 534.6 ± 16.0	681.5 ± 13.0	703.5 ± 7.3
	[K + ]
	CEM III	247.0 ± 1.4	436.4 ± 2.6	338.8 ± 3.0	425.1 ± 1.3	518.1 ± 6.5
	MKAA 168.0 ± 18.4	100.0 ± 0.1	68.5 ± 0.8	119.3 ± 13.7	166.2 ± 24.2
	Control	26.7 ± 9.0	6.2 ± 0.3	15.2 ± 0.5	11.7 ± 8.5	14.3 ± 6.6
	CEM I	25.1 ± 3.7	4.6 ± 0.9	13.9 ± 0.8	14.3 ± 2.6	16.6 ± 2.0
	[PO 4 3-]
	CEM III	22.5 ± 2.1	3.6 ± 1.1	11.9 ± 1.0	8.2 ± 5.7	16.7 ± 1.9
	MKAA	21.0 ± 3.5	4.2 ± 2.7	21.6 ± 2.0	24.6 ± 1.0	26.7 ± 1.9

Table 4 :

 4 Ammonium concentration of the liquid withdrawals according to the pH, in mg.L

	-1

Acknowledgements

The authors gratefully thank the French National Research Agency (ANR) for funding the project BIBENdOM -ANR -16 -CE22 -001 DS0602. The authors also thank Evrard Mengelle, Simon Dubos, Mansour Bounouba and Chantha Kim for the pilot's design and the analytical support of the bioreactors.

Annexe

These main findings are in accordance with the results obtained by [START_REF] Bertron | Biodeterioration of concrete in agricultural, agro-food and biogas plants: state of the art and challenges[END_REF], where the authors sum up results on CEM I and CEM III from Voegel's study (2017). In addition, a fifth zone was found in Voegel's studies, which presents almost complete decalcification and the complete dissolution of hydrates and anhydrous phases, leading to an amorphous gel of silicon and aluminium.

The values from Table 2 allowed the inorganic carbon contents (H 2 CO 3 , HCO 3 -and CO 3 2-forms) in the liquid medium to be calculated at the end of each cycle. Most of the values were between 1000 and 2000 mg.L -1 in the control bioreactors and in the bioreactors containing CEM I and CEM III. They were much higher than the values found by Voegel et al. (2019b[START_REF] Voegel | Mechanisms of cementitious material deterioration in biogas digester[END_REF] (about 140 mg.L -1 or 2.3 mmol.L -1 ) who used a different digestate that initially contained less dissolved CO 2 , but were consistent with the concentrations of between 2000 and 3000 mg.L -1 found in industrial digesters [START_REF] Mccarty | Anaerobic Waste treatment Fundamentals[END_REF].

Moreover, the inorganic carbon content being strongly dependent on the pH and the rate of CO 2 in the gas phase, higher concentrations could be encountered in the medium during the digestion cycle, when the pH is higher or when the proportion of methane is smaller and that of CO 2 is larger. These high values accounted for the carbonation of the CEM I and CEM III samples. The presence of vaterite in the external part could be explained by the presence of highly decalcified C-S-H with low Ca/Si ratio [START_REF] Morandeau | Investigation of the carbonation mechanism of CH and C-S-H in terms of kinetics, microstructure changes and moisture properties[END_REF]. However, it could also be linked to the lower pH of the external zone since the crystallization of vaterite is predominant 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]. Moreover the experiment temperature of 35°C (close to 30°C) promotes the precipitation of both calcite and vaterite [START_REF] Girou | Etude des cinétiques de précipitation des carbonates de calcium[END_REF][START_REF] Ševčík | Characterization of vaterite synthesized at various temperatures and stirring velocities without use of additives[END_REF][START_REF] Stepkowska | Calcite, vaterite and aragonite forming on cement hydration from liquid and gaseous phase[END_REF][START_REF] Wray | Precipitation of Calcite and Aragonite[END_REF]).

In the study by [START_REF] Koenig | Biogenic acid attack on concretes in biogas plants[END_REF], in addition to calcium carbonates, struvite and quartz were detected in the surfaces of samples immersed in the liquid phase in a pilot scale fermenter. In the same study, vivianite, calcium-aluminium-iron-chloride-silicates, calcium-aluminium-iron oxides, ettringite and portlandite phases were measured on the near-surface area of samples immersed in


Pierre-Etienne Martin
email: pierre-etienne.martin@u-bordeaux.fr

Jenny Benois-Pineau
email: jenny.benois-pineau@u-bordeaux.fr

Boris Mansencal
email: boris.mansencal@labri.fr

Renaud Péteri
email: renaud.peteri@univ-lr.fr

Laurent Mascarilla

Jordan Calandre
email: jordan.calandre1@univ-lr.fr

Julien Morlier
email: julien.morlier@u-bordeaux.fr

Sports Video Annotation: Detection of Strokes in Table Tennis task for MediaEval 2019

Action detection and classification is one of the main challenges in visual content analysis and mining. Sport video analysis has been a very popular research topic, due to the variety of application areas, ranging from multimedia intelligent devices with user-tailored digests, up to analysis of athletes' performances. Datasets with sport activities are available now for benchmarking of methods. A large amount of work is also devoted to the analysis of sport gestures using motion capture systems. However, body-worn sensors and markers could disturb the natural behaviour of sports players. Furthermore, motion capture devices are not always available for potential users, be it a University Faculty or a local sport team. Coming years will build upon the basic "Sports Video Annotation: Detection of Strokes in Table Tennis" task offered in 2019. The ultimate goal of this research is to produce automatic annotation tools for sport faculties, local clubs and associations to help coaches to better assess and advise athletes during training

INTRODUCTION

Action detection and classification is one of the main challenges in visual content analysis and mining [START_REF] Stoian | Fast Action Localization in Large-Scale Video Archives[END_REF]. Sport video analysis has been a very popular research topic, due to the variety of application areas, ranging from multimedia intelligent devices with usertailored digests, up to analysis of athletes' performances [START_REF] Einfalt | Activity-Conditioned Continuous Human Pose Estimation for Performance Analysis of Athletes Using the Example of Swimming[END_REF].The Sport Video Annotation project was initiated between the Faculty of Sports STAPS of the University of Bordeaux, the LaBRI -Université de Bordeaux and the MIA lab. -La Rochelle University. It is supported by the CNRS federation MIRES and the New Aquitaine Region in the framework of an "APP Recherche". The goal of this project is to develop artificial intelligence and multimedia indexing methods for the recognition of table tennis sports activities. The aim is to evaluate the performance of athletes, with a particular focus on students, in order to develop optimal training strategies. To that aim, a video corpus named TTStroke-21 was recorded with volunteered players. These data represent a large scientific interest for the Multimedia community participating in the MediaEval campaign.

Other datasets such as UCF-101 [START_REF] Soomro | UCF101: A Dataset of 101 Human Actions Classes From Videos in The Wild[END_REF], HMDB [START_REF] Kuehne | HMDB: A large video database for human motion recognition[END_REF], [START_REF] Kay | The Kinetics Human Action Video Dataset[END_REF] and AVA [START_REF] Gu | AVA: A Video Dataset of Spatio-temporally Localized Atomic Visual Actions[END_REF] are used as benchmarks for action classification methods. Others, such as the Olympic Sports dataset [START_REF] Carlos Niebles | Modeling Temporal Structure of Decomposable Motion Segments for Activity Classification[END_REF] focus on sport actions only. However none of them is dedicated to a specific sport and its associated rules. Furthermore, TTStroke-21 is annotated manually by professional players or teachers of Table Tennis, making the annotation process longer, but more temporally and qualitatively accurate. Classification methods as I3D model [START_REF] Carreira | Quo Vadis, Action Recognition? A New Model and the Kinetics Dataset[END_REF] or LTC model [START_REF] Varol | Long-Term Temporal Convolutions for Action Recognition[END_REF] performing well on UCF-101 dataset inspired the work done in [START_REF] Martin | Sport Action Recognition with Siamese Spatio-Temporal CNNs: Application to Table Tennis[END_REF] and [START_REF] Martin | Optimal choice of motion estimation methods for fine-grained action classification with 3D convolutional networks[END_REF] through a SSTCNN -Siamese Spatio Temporal Convolutional Neural Network. Here the video stream and derived computed optical flow are passed through the branches of the SSTCNN. The similarity of actions -strokes -in TTStroke-21 makes the classification task challenging and the multi-modal method seemed to improve performances. In [START_REF] Liu | Spatiotemporal Relation Networks for Video Action Recognition[END_REF], spatio temporal dependencies are learned from the video using only RGB images and scores are promising but are still below the multi-modal methods of I3D.

PARTICULAR CONDITIONS

Because TTStroke-21 is constituted of videos with identifiable players of Table Tennis, this dataset is subject to particular conditions in order to respect the personal data and privacy of the players. These Special Conditions apply to the use of Images generated in the framework of the program Sports video annotations: classification of strokes in table tennis, for the implementation of the MediaEval program. They constitute the specific usage agreement referred to in the Usage agreement for the MediaEval 2019 Research Collections, signed between the User and the University of Delft. The full and complete acceptance, without any reservation, of these Special Conditions is a mandatory prerequisite for the provision of the Images as part of the MediaEval programme. A complete reading of these conditions are necessary and engage the user, for example, to obscure the faces (blurring, black banner, etc.) before any publication and to destroy the data by October 1st 2020.

DATASET DESCRIPTION

In MediaEval 2019, we deliver a subset of TTStroke-21 data set which has been specifically recorded in a sport faculty facility using a light-weight equipment, such as GoPro cameras. It is constituted of player-centred videos recorded in natural conditions without markers or sensors, see This taxonomy was designed with professional table tennis teachers.

All videos are recorded in MP4 format.

The organisation of the delivered data is as follows:

• The provided dataset is split into two subsets: i)training set and ii)test set; • In each directory, there are several videos (in MP4 format)

and each video may contain several actions; • Each video file is accompanied with a XML file describing the actions present in the video; • For each action there are 3 attributes: the starting frame, the ending frame, and the stroke class; • In the train set XML files, all the attributes are specified but in the test set XML files, only the starting and ending frames are specified while the stroke class attribute is purposely set to an invalid value ("Unknown") and should be updated by the participants to one of the 20 valid classes. .

TASK DESCRIPTION

The Sport Video Annotation task consists in assigning a label from a given taxonomy of 20 classes of Table Tennis strokes to each action delimited by starting frame and ending frame in each test video file. Participants may submit up to four runs. For each runs, they must provide one XML file per video file, with the actions associated to the recognised stroke class. Runs may be submitted as an archive (zip or tar.gz file) with each run in a different directory. Participants should also indicate if any external data (other dataset, pretrained networks, ...) was used to compute their runs. The task is considered fully automatic. Once the video are provided to the system, results should be produced without any human intervention.

EVALUATION

In MediaEval 2019 we propose a light-weight classification task. It consists in classification of table tennis strokes which temporal borders are supplied in the XML files accompanying each video file. Hence for each test video the participants are invited to produce an xml file in which each stroke is labelled accordingly to the given taxonomy. This means that the label "unknown" has to be replaced by the label of the stroke class which participant's system has assigned. All submissions will be evaluated in terms of per-class accuracy(PCA) and of global accuracy (GA).The PCA is computed for each i-th class as:

PCA i = T P i /(N дt i ) (1)
Here T P i is the number of True Positives, i.e. correctly labelled, by the participant's system, strokes for the given i-th class,N дt i is the number of recorded strokes of the i-th class in the test dataset.

GA = T P/(N дt ) (2)
Here T P = T P i is the number of correctly labelled strokes for the whole dataset, and N дt is the number of strokes in the ground truth -the whole test set.

DISCUSSION

Participants are welcome to share their difficulties and the results even if they are negative. Better understanding of automatic classification methods are easier when all aspects of the methods are shared.

Thank you for participating at MediaEval 2019 and more specifically to our task: "Sports Video Annotation: Detection of Strokes in Table Tennis". We look forward to seeing you at the MediaEval Workshop and to discussing your results further.

Fig 1 .

 1 It comprises 20 table tennis strokesMediaEval'19, 27-29 October 2019, Sophia Antipolis, France P-e Martin et al. a. Video acquisition b. Annotation platform c. Dataset samples

Figure 1 :

 1 Figure 1: TTStroke-21

ACKNOWLEDGMENTS

We would like to thank all the players and annotators who contributed to TTStroke-21, Alain Coupet for his dedication to the project, Xavier Daverat and Chantal Durand for their help on the Particular Conditions formulation. This work was supported by Region of Nouvelle Aquitaine grant CRISP and Bordeaux Idex Initiative.