Maxime Hervy

Roberto Olcese

Mohammed M Bettahar

Martine Mallet

Aurélien Renard

Libeth Maldonado

Damien Remy

Guillain Mauviel

Anthony Dufour
email: anthony.dufour@univ-lorraine.fr

Evolution of dolomite composition and reactivity during biomass gasification

Keywords: biomass, dolomite, gasification, syngas, tar, up-grading

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Renewable alternative routes for energy production appear as an important challenge to reduce the greenhouse gases emissions while meeting the growing world energy demand.

Biomass gasification is an attractive way to convert a renewable resource into several energy carriers, such as substitute natural gas, bio-hydrogen, or Fischer-Tropsch biofuel [START_REF] Bridgwater | Renewable fuels and chemicals by thermal processing of biomass[END_REF].

Nevertheless, the removal of the syngas pollutants (such as tars) is required before further valorization, and this step remains the major issue in the development of gasification processes.

Tar is formed by pyrolysis reactions combined with incomplete oxidation reactions in gasifiers [START_REF] Milne | Biomass Gasifier Tars: Their Nature, Formation and Conversion[END_REF][START_REF] Lardier | Gas and Bed Axial Composition in a Bubbling Fluidized Bed Gasifier: Results with Miscanthus and Olivine[END_REF][START_REF] Nowakowska | Kinetic Study of the Pyrolysis and Oxidation of Guaiacol[END_REF] and is the most problematic pollutant contained in the syngas [START_REF] Milne | Biomass Gasifier Tars: Their Nature, Formation and Conversion[END_REF]. Tar is composed of a wide variety of aromatic and oxygenated compounds classified in different classes depending on their properties [START_REF] Coll | Steam reforming model compounds of biomass gasification tars: conversion at different operating conditions and tendency towards coke formation[END_REF] or their thermal history (primary, secondary and tertiary tar) [START_REF] Milne | Biomass Gasifier Tars: Their Nature, Formation and Conversion[END_REF]. These compounds can condense along the process chain thus impeding syngas valorization. The concentration of tar must be drastically reduced in order to meet the standards required for syngas ranging from about 50 mg/Nm 3 (or a dew point lower than 35°C for engine [START_REF] Jenbacher | Fuel gas quality, special gases[END_REF]) to 0.1 mg/Nm 3 (for Fischer-Tropsch synthesis) [START_REF] Le Coq | Syngas treatment unit for small scale gasification-application to IC engine gas quality requirement[END_REF]. In addition, tar in syngas considerably reduces the cold gas efficiency of gasifiers. Indeed, benzene can represent 0.5% of biomass LHV in air dense fluidized bed gasifier [START_REF] Lardier | Gas and Bed Axial Composition in a Bubbling Fluidized Bed Gasifier: Results with Miscanthus and Olivine[END_REF].

For these reasons, the catalytic conversion of tar into gaseous products (CO, H2, CH4) is of tremendous importance for improving the gasification processes. The main drawbacks of the catalytic methods are the cost of the catalysts and their deactivation. This topic has been intensively studied over the past 30 years, and several reviews have reported the main conclusions [START_REF] Torres | Hot Gas Removal of Tars, Ammonia, and Hydrogen Sulfide from Biomass Gasification Gas[END_REF][START_REF] Sutton | Review of literature on catalysts for biomass gasification[END_REF][START_REF] Woolcock | A review of cleaning technologies for biomass-derived syngas[END_REF][START_REF] Valderrama Rios | Reduction of tar generated during biomass gasification: A review[END_REF][START_REF] Shen | Recent progresses in catalytic tar elimination during biomass gasification or pyrolysis-A review[END_REF][START_REF] El-Rub | Review of Catalysts for Tar Elimination in Biomass Gasification Processes[END_REF][START_REF] Anis | Tar reduction in biomass producer gas via mechanical, catalytic and thermal methods: A review[END_REF][START_REF] Asadullah | Barriers of commercial power generation using biomass gasification gas: A review[END_REF][START_REF] Bulushev | Catalysis for conversion of biomass to fuels via pyrolysis and gasification: A review[END_REF][START_REF] Dayton | A review of the literature on catalytic biomass tar destruction[END_REF][START_REF] Devi | A review of the primary measures for tar elimination in biomass gasification processes[END_REF][START_REF] Guan | Catalytic steam reforming of biomass tar: Prospects and challenges[END_REF][START_REF] Han | The reduction and control technology of tar during biomass gasification/pyrolysis: An overview[END_REF]. These studies focused on the conversion of real biomass tars, or tar surrogate molecules.

Benzene, toluene and naphthalene are often considered as surrogate tar compounds due to their significant concentration in syngas and to their stability [START_REF] Sutton | Review of literature on catalysts for biomass gasification[END_REF][START_REF] Devi | Pretreated olivine as tar removal catalyst for biomass gasifiers: investigation using naphthalene as model biomass tar[END_REF][START_REF] Jess | Mechanisms and kinetics of thermal reactions of aromatic hydrocarbons from pyrolysis of solid fuels[END_REF]. Benzene is the most important aromatic compound present in the syngas and the most stable one [START_REF] Jess | Mechanisms and kinetics of thermal reactions of aromatic hydrocarbons from pyrolysis of solid fuels[END_REF][START_REF] Kinoshita | Tar formation under different biomass gasification conditions[END_REF][START_REF] Dufour | Evolution of Aromatic Tar Composition in Relation to Methane and Ethylene from Biomass Pyrolysis-Gasification[END_REF]. For this reason it is a good surrogate of tertiary (high temperature) tar.

A large variety of catalysts were tested for tar decomposition: chars [START_REF] Shen | By-products recycling for syngas cleanup in biomass pyrolysis -An overview[END_REF][START_REF] Park | Reduction of primary tar vapor from biomass by hot char particles in fixed bed gasification[END_REF][START_REF] Feng | In-situ steam reforming of biomass tar over sawdust biochar in mild catalytic temperature[END_REF][START_REF] Hervy | Catalytic cracking of ethylbenzene as tar surrogate using pyrolysis chars from wastes[END_REF], zeolites [START_REF] Buchireddy | Biomass Gasification: Catalytic Removal of Tars over Zeolites and Nickel Supported Zeolites[END_REF][START_REF] Yao | Investigation of nickel-impregnated zeolite catalysts for hydrogen/syngas production from the catalytic reforming of waste polyethylene[END_REF], iron oxides [START_REF] Azhar Uddin | Catalytic decomposition of biomass tars with iron oxide catalysts[END_REF], noble metals [START_REF] Chen | Promoting effect of trace Pd on hydrotalcite-derived Ni/Mg/Al catalyst in oxidative steam reforming of biomass tar[END_REF][START_REF] Juutilainen | Zirconia: Selective oxidation catalyst for removal of tar and ammonia from biomass gasification gas[END_REF][START_REF] Cheah | Nickel cerium olivine catalyst for catalytic gasification of biomass[END_REF][START_REF] Constantinou | Low-temperature conversion of phenol into CO, CO2 and H2 by steam reforming over La-containing supported Rh catalysts[END_REF][START_REF] Wang | Catalyst property of Co-Fe alloy particles in the steam reforming of biomass tar and toluene[END_REF][START_REF] Li | Metal catalysts for steam reforming of tar derived from the gasification of lignocellulosic biomass[END_REF], alkaline and alkaline earth metals (AAEM) [START_REF] Raveendran | Influence of mineral matter on biomass pyrolysis characteristics[END_REF][START_REF] Klinghoffer | Influence of char composition and inorganics on catalytic activity of char from biomass gasification[END_REF], nickel catalysts [START_REF] Shen | In-situ catalytic conversion of tar using rice husk char-supported nickel-iron catalysts for biomass pyrolysis/gasification[END_REF][START_REF] Blanco | Characterization and evaluation of Ni/SiO2 catalysts for hydrogen production and tar reduction from catalytic steam pyrolysisreforming of refuse derived fuel[END_REF], clay minerals [START_REF] El-Rub | Review of Catalysts for Tar Elimination in Biomass Gasification Processes[END_REF], olivine [START_REF] Kuhn | Olivine catalysts for methane-and tar-steam reforming[END_REF][START_REF] Virginie | Characterization and reactivity in toluene reforming of a Fe/olivine catalyst designed for gas cleanup in biomass gasification[END_REF], and calcined rocks (magnesite, calcite, dolomite) [START_REF] González | Reduction of tars by dolomite cracking during two-stage gasification of olive cake[END_REF][START_REF] Constantinou | A comparative study of the steam reforming of phenol towards H2 production over natural calcite, dolomite and olivine materials[END_REF][START_REF] Tennant | Steam-pyrolysis activation of wood char for superior odorant removal[END_REF]. Carbonate rocks, such as dolomite, were identified as promising catalysts due their abundance and low-cost [START_REF] Donnot | Reactions of Dolomite with CO<Subscript>2</Subscript> at Gasification Temperatures of Wood[END_REF][START_REF] Pérez | Hot Gas Cleaning and Upgrading with a Calcined Dolomite Located Downstream a Biomass Fluidized Bed Gasifier Operating with Steam-Oxygen Mixtures[END_REF][START_REF] Corella | Biomass Gasification in Fluidized Bed: Where To Locate the Dolomite To Improve Gasification?[END_REF][START_REF] Delgado | Calcined dolomite, magnesite, and calcite for cleaning hot gas from a fluidized bed biomass gasifier with steam: life and usefulness[END_REF][START_REF] Taralas | Effects of MgO, CaO and calcined dolomites on model substance cracking and conversion of tar from biomass gasification/pyrolysis gas[END_REF].

Dolomite is a natural magnesium ore (MgCO3.CaCO3) composed of a mixture of calcium carbonate and magnesium carbonate at a concentration ratio of nearly 1:1. A tremendous number of articles highlighted that dolomite is an active catalyst for tar removal [START_REF] Pérez | Hot Gas Cleaning and Upgrading with a Calcined Dolomite Located Downstream a Biomass Fluidized Bed Gasifier Operating with Steam-Oxygen Mixtures[END_REF][START_REF] Simell | Steam Reforming of Gasification Gas Tar over Dolomite with Benzene as a Model Compound[END_REF][START_REF] Aldén | Catalytic Cracking of Naphthalene on Dolomite[END_REF][START_REF] Simell | Catalytic decomposition of gasification gas tar with benzene as the model compound[END_REF][START_REF] Simell | Tar-decomposing activity of carbonate rocks under high CO2 partial pressure[END_REF][START_REF] Gusta | Catalytic Decomposition of Biomass Tars with Dolomites[END_REF][START_REF] Sarıoğlan | Tar removal on dolomite and steam reforming catalyst: Benzene, toluene and xylene reforming[END_REF][START_REF] Zhang | Catalytic destruction of tar in biomass derived producer gas[END_REF][START_REF] De Andrés | Behaviour of dolomite, olivine and alumina as primary catalysts in air-steam gasification of sewage sludge[END_REF][START_REF] Ammendola | Dual bed reactor for the study of catalytic biomass tars conversion[END_REF][START_REF] Elliott | The effect of catalysis on wood-gasification tar composition[END_REF][START_REF] Aldén | Conversion of Tar in Pyrolysis Gas from Wood Using a Fixed Dolomite Bed[END_REF][START_REF] Sjöström | Sala Dolomite-Catalysed Conversion of Tar from Biomass Pyrolysis[END_REF][START_REF] Simell | Catalytic purification of tarry fuel gas with carbonate rocks and ferrous materials[END_REF]. A calcination step is required to increase the catalytic activity of dolomite by eliminating CO2 and transforming (CaMg(CO3)2) into active CaO and MgO [START_REF] Hu | Steam gasification of apricot stones with olivine and dolomite as downstream catalysts[END_REF]. CaO is known to be more active than MgO in tar cracking reactions and is mainly responsible for the high activity of calcined dolomite [START_REF] Clemens | The effect of calcium on the rate and products of steam gasification of char from low rank coal[END_REF].

However, a synergistic interaction between CaO and MgO plays an important role in the activity of dolomite. Indeed, catalysts based on mechanical mixtures of MgO and CaO have presented a better performance than CaO alone [START_REF] Delgado | Biomass Gasification with Steam in Fluidized Bed: Effectiveness of CaO, MgO, and CaO-MgO for Hot Raw Gas Cleaning[END_REF]. In addition, dolomite was proved to be more active than magnesite (MgCO3) and calcite (CaCO3) for tar reforming [START_REF] Delgado | Calcined dolomite, magnesite, and calcite for cleaning hot gas from a fluidized bed biomass gasifier with steam: life and usefulness[END_REF][START_REF] Simell | Catalytic purification of tarry fuel gas with carbonate rocks and ferrous materials[END_REF]. The higher activity of dolomite is mainly explained by the lower deactivation by coke deposition, as a result of the presence of MgO which inhibits the formation of stable carbonaceous materials at the surface of CaO, and which promotes the formation of less stable carbon species that are easily gasified [START_REF] Alarcón | New effects during steam gasification of naphthalene: the synergy between CaO and MgO during the catalytic reaction[END_REF].

Therefore, the deactivation of CaO active sites by coke deposition is reduced. The presence of iron in dolomites is also expected to play a role in the steam reforming of tar as well as in the water-gas shift reaction [START_REF] Gusta | Catalytic Decomposition of Biomass Tars with Dolomites[END_REF][START_REF] Simell | Catalytic purification of tarry fuel gas with carbonate rocks and ferrous materials[END_REF]. Some articles specifically studied the decomposition of benzene [START_REF] Simell | Steam Reforming of Gasification Gas Tar over Dolomite with Benzene as a Model Compound[END_REF][START_REF] Simell | Catalytic decomposition of gasification gas tar with benzene as the model compound[END_REF], toluene [START_REF] Simell | Effects of gasification gas components on tar and ammonia decomposition over hot gas cleanup catalysts[END_REF][START_REF] Taralas | Modeling the Thermal Destruction of Toluene (C7H8) as Tar-Related Species for Fuel Gas Cleanup[END_REF][START_REF] Taralas | Kinetic modelling of VOC catalytic steam pyrolysis for tar abatement phenomena in gasification/pyrolysis technologies[END_REF] and naphthalene [START_REF] Aldén | Catalytic Cracking of Naphthalene on Dolomite[END_REF][START_REF] Lammers | Catalytic Tar Removal from Biomass Producer Gas with Secondary Air[END_REF] over calcined dolomite. The tests were conducted at 700-900 °C under different atmospheres: steam, carbon dioxide or syngas mixtures. The benzene decomposition reactions mainly generated H2 and CO2, while CO was a minor product and was generated in lower amount with rising temperature. These reactions were inhibited by hydrogen which may be dissociatively adsorbed on the metal-oxide adsorption sites, thus preventing the tar access to adsorption sites [START_REF] Kobayashi | Ab initio MO study on adsorption of a hydrogen molecule onto magnesium oxide (100) surface[END_REF][START_REF] Hattori | Heterogeneous Basic Catalysis[END_REF]. The CO2 concentration in the syngas and the temperature strongly influence the activity of dolomite. Indeed, the dolomite can be carbonated (depending on the thermodynamic equilibrium of CO2/CaO/CaCO3) which results in the decrease of its catalytic activity for tar cracking [START_REF] Donnot | Reactions of Dolomite with CO<Subscript>2</Subscript> at Gasification Temperatures of Wood[END_REF][START_REF] Simell | Tar-decomposing activity of carbonate rocks under high CO2 partial pressure[END_REF]. Dolomite can be deactivated by hydration and carbonation reactions [START_REF] Quitete | Application of Brazilian dolomites and mixed oxides as catalysts in tar removal system[END_REF].

Syngas cleaning processes are classified in two types depending on their location; either in the gasifier (primary methods) or downstream of the gasifier (secondary methods) [START_REF] Brandt | High Tar Reduction in a Two-Stage Gasifier[END_REF].

Secondary methods may be slightly more efficient than primary ones but a specific catalytic cracking reactor is required thus increasing the investment and operating costs of the gasification unit. Dolomite as in-bed material was shown to be an efficient catalyst for tar cracking [START_REF] Corella | Biomass Gasification in Fluidized Bed: Where To Locate the Dolomite To Improve Gasification?[END_REF] allowing the production of a relatively clean syngas. For this reason, dolomite is often used as bed material in gasification and notably in many recent studies [START_REF] Shao | Ash Deposition in Air-Blown Gasification of Peat and Woody Biomass in a Fluidized-Bed Gasifier[END_REF][START_REF] Rapagnà | Olivine, dolomite and ceramic filters in one vessel to produce clean gas from biomass[END_REF][START_REF] Tian | Syngas production from air-steam gasification of biomass with natural catalysts[END_REF][START_REF] Jeremiáš | CO2 gasification of biomass: The effect of lime concentration in a fluidised bed[END_REF].

In fluidized bed gasifier, dolomite is in contact with biomass particles, and a modification of its surface properties is likely to occur. These modifications could change the long-term catalytic activity of "used dolomite" for tar cracking, thus modifying the syngas yield and quality.

This crucial topic has never yet been explored for dolomite whereas it has been extensively studied for olivine. Indeed, several studies showed that olivine is coated by ash materials during biomass gasification and this ash layer considerably increases the reactivity of olivine for tar cracking and WGS reactions [START_REF] Kern | Reactivity tests of the water-gas shift reaction on fresh and used fluidized bed materials from industrial DFB biomass gasifiers[END_REF][START_REF] Kirnbauer | The positive effects of bed material coating on tar reduction in a dual fluidized bed gasifier[END_REF][START_REF] Kirnbauer | Investigations on Bed Material Changes in a Dual Fluidized Bed Steam Gasification Plant in Güssing, Austria[END_REF][START_REF] Kirnbauer | The mechanism of bed material coating in dual fluidized bed biomass steam gasification plants and its impact on plant optimization[END_REF][START_REF] Kuba | Influence of bed material coatings on the water-gas-shift reaction and steam reforming of toluene as tar model compound of biomass gasification[END_REF][START_REF] Kuba | Deposit build-up and ash behavior in dual fluid bed steam gasification of logging residues in an industrial power plant[END_REF][START_REF] Kuba | Mechanism of Layer Formation on Olivine Bed Particles in Industrial-Scale Dual Fluid Bed Gasification of Wood[END_REF]. This present work reveals that the impact of ashes may be very different for dolomite than for olivine.

To the best of our knowledge, this work presents for the first time the effect of dolomite aging under significant gasification conditions (in a pilot plant) on tar conversion. Dolomites before and after gasification of biomass were characterized by complementary methods (BET, SEM-EDX, XRD, XPS, TPO, TPD). Then the reactivity of these 2 dolomites (pristine and used) was studied by a "probe" key reaction of tar conversion which is the steam reforming of benzene. Finally the mechanisms of benzene steam reforming over dolomites are discussed.

Materials & Methods

Presentation of the dolomite samples

The dolomite originating from Cantabria (Spain) was calcined at 1000°C for few hours before being used as a catalyst in a biomass gasification plant at Portalegre, Portugal (EQTEC technology) [START_REF] Coll | EQTEC GASIFIER TECHNOLOGY[END_REF]. The gasification reactor was a dense fluidized bed operated at a temperature of 750°C for 10 days using air as gasifying agent. Biomass was a mixture of wood (70%) and miscanthus (30%) and was fed in the reactor at a flow-rate of 50kg/h. The analysis of biomass is presented in supplementary material. The dolomite samples were denoted as "fresh" after calcination, and "used" after gasification. Both samples were sieved to particle sizes from 100 to 250µm.

Characterizations of the catalysts

The morphology of fresh and used dolomite materials was studied on a scanning electron microscope (SEM) JEOL model JSM-6490LV combined with energy dispersive X-ray spectroscopy (EDX) to analyse the chemical composition of the surface.

For inductively coupled plasma mass spectrometry (ICP-MS) analysis of the bulk, the samples were dissolved in lithium metaborate (LiBO2) and nitric acid (HNO3) solutions following the CNRS-SARM procedure [START_REF] Carignan | Routine analyses of trace elements in geological samples using flow injection and low pressure on-line liquid chromatography and ICP-MS: A study of geochemichal reference materials BR, DR-N, UB-N, AN-G and GH, Geostand[END_REF].

The crystalline structure of the fresh and used dolomites was analysed by X-ray diffraction (XRD) measurements. XRD analyses were performed at room temperature using a Philips Panalytical X'Pert Pro MPD diffractometer equipped with Xcelerator detector. XRD patterns were collected with a CuKα radiation source operating at 40kV and 40mA. The diffraction pattern was collected in the range 2θ = 10-80°, with a step size of 0.016° in 2θ, and a scan speed of 1.7min -1 . XPS analyses were performed using a KRATOS Axis Ultra X-ray photoelectron spectrometer (Kratos Analytical, Manchester, UK) equipped with a monochromatic Al Kα X-ray source (hν=1486.6eV) operated at 150W. Spectra were collected at normal (90°) take-off angle and the analysis area was 700×300µm. The base pressure in the analytical chamber was 10 -9 mbar during XPS measurements. Wide scans were recorded using an analyser pass energy of 160eV, while narrow scans were obtained using a pass energy of 20eV (instrumental resolution better than 0.5eV).

BET areas were determined by the one-point method using adsorbed nitrogen at 77K in a Micromeritics TriStart II instrument. The solids were previously degassed under vacuum at room temperature overnight.

Temperature Programmed Desorption (TPD) and Temperature Programmed Oxidation (TPO) were performed by heating the dolomite samples at 850°C under a 30 NmL/min flow of pure nitrogen (TPD) or air (TPO). A 2.5g dolomite sample was placed in a quartz U-reactor (internal diameter=10mm) heated by an electrical furnace at 5°C/min to 850°C. The exhaust gas was analysed online by a micro-gas chromatograph (µGC) (SRA Instrumentation R3000).

Catalytic experiments

The experiment set-up has been presented previously [START_REF] Olcese | Gas-phase hydrodeoxygenation of guaiacol over Fe/SiO2 catalyst[END_REF]. Catalytic experiments were conducted on a fixed bed quartz reactor (internal diameter 10mm) placed in an electric oven. The catalytic bed was supported by a quartz sintered plate. A thermocouple was positioned within the bed of dolomite. The inlet gas composition was controlled by mass flow controllers (Bronkhorst). Benzene (supplied by Fluka, 99.9% purity) and demineralised water were added in the gas mixture by two syringe pumps and vaporised in a vaporisation chamber, swept by nitrogen and connected through a heated line to the catalytic bed. At the outlet of the catalytic bed, permanent gases were analysed on-line with a µ-GC Varian 490 equipped with 4 modules.

The outlet was also connected to a heated sampling line and to a heated sampling loop for online analysis of benzene by a GC/FID. The operating conditions of the catalytic tests were chosen in order to obtain a contact time similar to the one used in the industrial gasification fluidized bed of EQTEC. The mass of catalyst was 3.0g and the total gas flow rate was set at 45NmL/min. These conditions were set to be consistent with the industrial fluidized bed conditions (mass of fluidized dolomite bed and total outlet syngas flow rate). The benzene inlet concentration was 0.5vol% which corresponds to the content usually measured in the syngas [START_REF] Lardier | Gas and Bed Axial Composition in a Bubbling Fluidized Bed Gasifier: Results with Miscanthus and Olivine[END_REF]. Catalytic tests were performed at 800, 850 and 900°C without applying pre-treatment to the materials. The conversion of benzene was constant over time on stream and the value presented is an average of several points (GC/FID analysis) over more than 50 minutes.

The benzene conversion (XB) was calculated according to Eq.1, where is the molar flow rate of benzene feeding the reactor, and the molar flow rate of benzene leaving the reactor.

Eq.1

The selectivity (Si) of the reaction products (i) was also calculated according to Eq.2:

Eq.2

Results and discussion

Characterization of the catalysts

Morphology of particles and chemical composition of the bulk catalysts by ICP

Table 1 compares the values of BET surface area of fresh and used dolomite samples.

For fresh dolomite, the measured values were not high enough to be accurate. Both types of catalysts exhibited low BET surface areas (< 20m²/g). However, a slight increase of the BET surface area was observed for used dolomite samples. SEM images and EDX analyses of dolomite materials are presented in Figure 1 and Table 2.

EDX analysis was averaged on various particles. Dolomite is a natural material and presents an average composition of about 17% Mg and 34% Ca. After its use as a bed material in the fluidised bed reactor, the surface of used dolomite is enriched by inorganic elements initially contained in the biomass. The surface of dolomite particles is coated by a layer of ashes and becomes smoother as shown on Figure 1. The mechanisms of coating are out of the scope of the present study and have been previously studied in details [START_REF] Lardier | Gas and Bed Axial Composition in a Bubbling Fluidized Bed Gasifier: Results with Miscanthus and Olivine[END_REF][START_REF] Kaknics | Experimental Study and Thermodynamic Modelling of High Temperature Interactions Between Molten Miscanthus Ashes and Bed Particles in Fluidized Bed Reactors[END_REF][START_REF] Michel | The mechanism of agglomeration of the refractory materials in a fluidized-bed reactor[END_REF]. The coating behaviour depends on gasification conditions and on the composition of the bed, the reactor walls and the ashes of biomass.

Si and P are not detected on fresh dolomite, but their concentrations at the surface of used dolomite are of about 2.5 and 3.7wt.% respectively (Table 2). SEM-EDX is only a first qualitative approach of dolomites chemistry. For this reason, ICP-MS, XRD, XPS and TPD/TPO analyses were performed in order to study more deeply the chemical composition of the catalysts. 1. SEM-EDX analysis of the 2 dolomites (before and after biomass gasification in a pilot scale gasifier). SEM-EDX highlights the modification of the surface of particles which are coated by a layer of ashes during biomass gasification.

Table 3 presents the bulk chemical composition of the catalysts analyzed by ICP-MS.

These results confirm that used catalysts were enriched in inorganic species during the biomass gasification. Fresh dolomite is mainly composed of CaO (50%) and MgO (38%), with some traces of iron, silicium and aluminium (presented as oxides due to the ICP analysis). After gasification, used dolomite is mainly enriched in SiO2 (4.3 wt.%, which is similar to EDX analysis) and P2O5 (0.39 wt.%, which is lower than EDX analysis). The two main species, CaO and MgO, were found to decrease mainly due to the increase of SiO2, while Al2O3 (1.02 wt.%) slightly increased in used dolomite. The loss on ignition increased for used dolomite (from 9.01 to 11.88wt.%) as a result of coke deposit and carbonates. Other species present in biomass ashes (K, Mn, etc.) are also present in higher amount in the used dolomite which is in agreement with ash composition of biomass (presented in supplementary material). Ash composition depends on biomass type and culture mode but these elements are often found as the main inorganic elements of many ligno-cellulosic biomasses. Therefore this ash coating is representative and significant for the gasification of various biomasses in air-dense fluidized bed.

XRD characterization of the catalysts

As observed in Figure 2, the diffraction peaks of fresh dolomite show the presence of periclase (MgO), lime (CaO), and portlandite (Ca(OH)2). The peaks representative of dolomite structure (CaMg(CO3)2) are not detected. This crystalline modification can be explained by the effect of the calcination treatment initially applied to this material, which is in agreement with previous studies [START_REF] Constantinou | A comparative study of the steam reforming of phenol towards H2 production over natural calcite, dolomite and olivine materials[END_REF][START_REF] Hu | Steam gasification of apricot stones with olivine and dolomite as downstream catalysts[END_REF]. The XRD spectrum of used dolomite reveals the formation of significant amounts of quartz (SiO2) and traces of corundum (Al2O3). This evolution corroborates the ICP and SEM-EDX results demonstrating the enrichment of ash-based species in dolomite particles during biomass gasification. XRD analysis also shows a relative decrease of MgO vs.

CaO in used dolomite. A small peak of calcite is detected in the used dolomite probably due to a limited carbonation at 750°C in the gasifier (around 15%vol. of CO2 at the exit of the fluidised bed).

Composition of the surface by XPS

The XPS results are presented in Table 4. Binding energy (BE) and atomic concentration (%) were ascribed to the element according to published data [START_REF] Constantinou | A comparative study of the steam reforming of phenol towards H2 production over natural calcite, dolomite and olivine materials[END_REF][START_REF] Alarcón | Catalytic cooperation at the interface of physical mixtures of CaO and MgO catalysts during steam gasification of naphthalene[END_REF][START_REF] Derycke | Surface chemical characterization of different pyrite size fractions for flotation purposes[END_REF].

For fresh dolomite, the binding energies of the C 1s signal ( respectively) much lower than the bulk oxides as analyzed by ICP. This result may be related to the highly hydroxylated surface of the catalyst.

While iron and aluminium were observed in the bulk by ICP (0.4 and 1.0%, respectively), these elements are not detected at the surface of dolomite by XPS (before and after gasification).

Their content may be beyond the detection limit of the XPS technique, or these species could be mainly located in the bulk and not at the external surface analysed by XPS.

Concerning the used dolomite, silica species are found at the surface (1.0%) which is consistent with SEM-EDX analysis. The Mg 2p and Ca 2p intensity peaks decrease after gasification: their atomic ratio dropped from 13.56 to 9.72 at.% for Ca, and from 1.86 to 1.00 at.% for Mg (Table 4). This relative decrease in Ca and Mg is consistent with the bulk analysis by ICP measurements (Table 3). TPD and TPO experiments were performed in order to complete these XPS analysis and to get a better insight on the nature of carbonaceous species present in fresh and used dolomites.

TPD and TPO characterizations

The amounts of CO2 released by fresh and used dolomite during TPD and TPO analysis are presented in Figure 4 (A) and (B) respectively.

No carbon monoxide was released during these analyses. During TPD (Figure 4A), both samples display a single CO2 peak occurring at a similar temperature (between 500 and 750°C).

The maximum peak is at 660°C and 715°C for fresh dolomite and used dolomite, respectively.

The CO2 release obtained under pure nitrogen atmosphere is attributed to the decomposition of carbonates present in both samples. It can be noted that the amount of CO2 desorbed was significantly higher for the used dolomite, which could be explained by the formation of carbonates during the gasification reactions. This is in agreement with XRD analysis (Figure 2).

During the TPO analysis, a significant CO2 peak is obtained at low temperature (460°C)

for used dolomite (Figure 4B). This peak results from the oxidation of a reactive carbon produced during gasification. In addition, a CO2 peak similar to that obtained during the TPD tests is observed for both samples. However, for used dolomite, the maximum of this peak is achieved at slightly higher temperature than during TPD (790°C vs 715°C), and the amount of CO2 released is much higher. This difference can be attributed to the oxidation of a more stable coke produced during the gasification reactions and oxidized at high temperature (≈800°C). To the best of our knowledge we present here the first TPO analysis of a dolomite used in a pilot scale gasifier during a significant time (10 days). Figure 4. (A) TPD and (B) TPO analysis of fresh and used dolomites.

It is interesting to notice that similar ratios of carbonates to total carbon [CCarbonates/Ctotal] were found between TPD/TPO (CCarbonates/Ctotal=0.13) and XPS (CCarbonates/Ctotal=0.16) for the used dolomite. This would mean that the evolution of CO2 analysed during TPD/TPO may account for the decomposition of functions (carbonates and coke deposit) which are mainly formed at the external surface of particles during biomass gasification.

Steam reforming of benzene over fresh and used dolomites

In order to "probe" the reactivity of the two dolomite samples, their catalytic activity for benzene steam reforming were compared at three temperatures (800, 850 and 900°C). The evolution of benzene conversion is presented in Figure 5.

For both dolomites, the increase in temperature increases the benzene conversion.

Indeed, the conversion of benzene is multiplied by 3.5 between 800 and 900°C. It is interesting to note that used dolomite is less active than the fresh one. The activity of used dolomite decreases by about 25% compared to the fresh sample. At 900°C the conversion of benzene obtained with fresh dolomite reaches 70% against 54% with used dolomite. To assess the effect of aging on the reaction mechanisms, the selectivity on reaction products has been determined (Table 5). Similar gaseous species were produced by fresh and used dolomites: H2, CO2, CO, and CH4. For both samples, the main products were H2 and CO2.

Traces of C2H4 were produced by steam reforming of benzene only over the fresh dolomite.

For fresh dolomite, rising temperature from 800 to 900°C results in a decrease of H2 (-3.9%) and CO2 (-2.3%) selectivities at the expense of CO (+3.7%) and CH4 (+2.5%). This means that the overall selectivity in products is not mainly controlled by the thermodynamic equilibrium of the water gas-shift reaction which promotes H2 and CO2 at lower temperatures. The increase in CO/CO2 with temperature is in agreement with results reported by Constantinou et al. in a stream rich in H2 [START_REF] Constantinou | A comparative study of the steam reforming of phenol towards H2 production over natural calcite, dolomite and olivine materials[END_REF]. At higher temperature, the cracking reaction of benzene forming CH4 seems to be promoted. The decrease in H2 selectivity with temperature may be linked to the higher selectivity in CH4. Further discussion is provided after the presentation of a potential mechanism of benzene reforming over dolomites in the next section. Concerning the used dolomite, the evolutions of CO2 and CO are more significant with rising temperature: -8.3% and +5.6%, respectively. Contrary to the fresh dolomite, the selectivity in H2 was almost constant with rising temperature. This finding confirms that the equilibrium of water gas-shift reaction does not control the global selectivity in CO2/H2/CO in this case. The steam gasification of coke present in the used dolomite (deposited during biomass gasification and as analyzed by TPO) may also contribute the higher selectivity in H2 and CO notably at 900°C. More complex phenomena occur and will be detailed in the next section. The selectivities in CH4 and in C2H4 are lower for the used dolomites. This result may be explained by a lower cracking rate of benzene over the used dolomite.

Discussion on the reaction mechanisms

In a deep study Alarcon et al. have proposed a detailed mechanism for the steam reforming of aromatics over CaO and MgO [START_REF] Alarcón | New effects during steam gasification of naphthalene: the synergy between CaO and MgO during the catalytic reaction[END_REF]. These authors have proposed, by analogy with the mechanism of hydrocarbons steam-reforming over metal supported catalysts [START_REF] York | Methane Oxyforming for Synthesis Gas Production[END_REF], that aromatic hydrocarbons reforming over CaO or MgO catalysts would take place in two main step [START_REF] Alarcón | Catalytic cooperation at the interface of physical mixtures of CaO and MgO catalysts during steam gasification of naphthalene[END_REF]: (i) the decomposition of the hydrocarbon and the water on the catalyst surface, and then (ii) the gasification of the deposited carbon with steam. The overall process could be written according to the following simplified mechanism:

ArH + CxHy- (R1) H2O + O-+ H2 (R2) CxHy-+ O- H2, CO, CO2, CH4 + 2 (R3)
where ArH is an aromatic hydrocarbon specie, is a M 2+ O 2-unit (the active site in dolomite). The deposit (CxHy-) may also decompose into smaller fragments and further form H2, CH4, and Cx [START_REF] Alarcón | New effects during steam gasification of naphthalene: the synergy between CaO and MgO during the catalytic reaction[END_REF][START_REF] Alarcón | Catalytic cooperation at the interface of physical mixtures of CaO and MgO catalysts during steam gasification of naphthalene[END_REF].

The decomposition of water molecules to hydrogen is easy over metal supported catalyst when the metal phase is a noble or transition metal [START_REF] York | Methane Oxyforming for Synthesis Gas Production[END_REF]. This is much less likely to occur over the non-reducible CaO or MgO oxides, unless they contained catalytic traces of a transition metal (e; g; iron). Contrary to the reaction (R2) proposed by a previous study, we rather think that gaseous H2 stems from the decomposition of gaseous water on the carbon fragments rather than from the direct water decomposition on the metal oxides. In other words, the water molecules are reduced by the surface carbon fragments but not by the fixed valence metal oxides.

Based on our results and on literature analysis [START_REF] Constantinou | A comparative study of the steam reforming of phenol towards H2 production over natural calcite, dolomite and olivine materials[END_REF][START_REF] Alarcón | New effects during steam gasification of naphthalene: the synergy between CaO and MgO during the catalytic reaction[END_REF][START_REF] Hattori | Heterogeneous Basic Catalysis[END_REF][START_REF] Alarcón | Catalytic cooperation at the interface of physical mixtures of CaO and MgO catalysts during steam gasification of naphthalene[END_REF][START_REF] Lavalley | Infrared spectrometric studies of the surface basicity of metal oxides and zeolites using adsorbed probe molecules[END_REF], an overall mechanism of benzene steam reforming over dolomite is proposed (Figure 6). Figure 6. Chemical mechanism of benzene steam reforming over dolomite proposed based on the experimental results of the present study and on previous works [START_REF] Constantinou | A comparative study of the steam reforming of phenol towards H2 production over natural calcite, dolomite and olivine materials[END_REF][START_REF] Alarcón | New effects during steam gasification of naphthalene: the synergy between CaO and MgO during the catalytic reaction[END_REF][START_REF] Hattori | Heterogeneous Basic Catalysis[END_REF][START_REF] Alarcón | Catalytic cooperation at the interface of physical mixtures of CaO and MgO catalysts during steam gasification of naphthalene[END_REF][START_REF] Lavalley | Infrared spectrometric studies of the surface basicity of metal oxides and zeolites using adsorbed probe molecules[END_REF].

First, a theoretical investigation of benzene adsorption over calcite and dolomite dry surfaces reported that Ca sites are the most energetically favorable for hydrocarbon adsorption on both minerals [START_REF] Rigo | Hydrocarbon Adsorption on Carbonate Mineral Surfaces: A First-Principles Study with van der Waals Interactions[END_REF]. Moreover, van der Waals interactions increase the stability for all sites but more strongly on the Ca site. The benzene molecule was found to be adsorbed in a Ca hole, the aromatic ring forming an angle of 15 to 35° with the surface. For MgO, benzene adsorption takes place on weakly acidic OH groups, rather than on the Mg sites. The rate determining step of the steam-reforming of benzene over dolomite is the adsorption of the reactant hydrocarbon molecule [START_REF] Simell | Effects of gasification gas components on tar and ammonia decomposition over hot gas cleanup catalysts[END_REF]. This can be explained by the interactions between a neutral molecule and a strongly polarized solid surface. The stable and symmetrical benzene molecule is not easily polarizable, even with the highly ionic CaO or MgO dipoles. As a result, the chemical adsorption is weak and the Van der Waals interactions may become important [START_REF] Itoh | Nanoscale Metal Oxide Particles as Chemical Reagents. Intrinsic Effects of Particle Size on Hydroxyl Content and on Reactivity and Acid/Base Properties of Ultrafine Magnesium Oxide[END_REF]. From hard and soft acids and bases principles [100,101], benzene is a soft base and Ca 2+ a hard acid, also their interaction is fundamentally expected to be very poor. This interaction should be of van der Waals type, in good accordance with the theoretical investigation reported by Rigo et al. [START_REF] Rigo | Hydrocarbon Adsorption on Carbonate Mineral Surfaces: A First-Principles Study with van der Waals Interactions[END_REF]. From these studies, it can be proposed that benzene is rather adsorbed and decomposed on the calcium oxide by interaction with the Ca ++ component (Figure 6).

Second, concerning the adsorption and role of water, MgO, CaO and other non-reducible oxides (activated at high temperatures) exhibit the presence of highly basic O 2-sites [START_REF] Lavalley | Infrared spectrometric studies of the surface basicity of metal oxides and zeolites using adsorbed probe molecules[END_REF]. The OH groups were shown to exist in the naphthalene steam-reforming reaction conditions over pure MgO or MgO+CaO mixture, but not over pure CaO [START_REF] Alarcón | New effects during steam gasification of naphthalene: the synergy between CaO and MgO during the catalytic reaction[END_REF]. On the other hand, the introduction of steam results in the disappearance of the carbonaceous matter (mainly in the form of formiate species) from the surface of pure MgO and MgO-containing mixture, while formiate species remains in the surface of CaO catalyst [START_REF] Alarcón | New effects during steam gasification of naphthalene: the synergy between CaO and MgO during the catalytic reaction[END_REF]. The authors concluded that the OH groups, stemming from MgO, could participate both to the gaseous products formation and to coke removal. Chemically, it is impossible that anionic OH groups directly participate to the oxidation of carbons belonging to C-C or C-H σ bonds. Thus, OH -cannot be active species in the benzene steam-reforming. On the contrary, H2O may be responsible for the direct oxidation of hydrocarbons (CmH(p-1)-OH) (Figure 6). Due to its chemical properties, OH -can be reactive towards the oxidation of the carbon of C=O double π bonds of oxygenate species by a nucleophilic attack. The presence of these species over alkaline oxides were evidenced by Alarcon et al. [START_REF] Alarcón | Catalytic cooperation at the interface of physical mixtures of CaO and MgO catalysts during steam gasification of naphthalene[END_REF]. These species disappear from the surface after the introduction of steam in the reactor over MgO or CaO+MgO, but not over pure CaO. This led us to conclude that the role of MgO in MgO+CaO mixture (and therefore in dolomite) is to bring the OH groups for the oxidation of oxygenate species deposited over CaO (after benzene sorption). This reaction would occur at the MgO-CaO interface or through the spillover effect of the OH groups [START_REF] Conner | Spillover in Heterogeneous Catalysis[END_REF] (Figure 6).

In an investigation on MgO [START_REF] Itoh | Nanoscale Metal Oxide Particles as Chemical Reagents. Intrinsic Effects of Particle Size on Hydroxyl Content and on Reactivity and Acid/Base Properties of Ultrafine Magnesium Oxide[END_REF] prepared from Mg(OH)2 precursor treated at various temperatures, it was shown that the surface OH concentration was similar for small or large MgO particles, but that the latter allowed more germinal OH pairs as well as OH surface island domains with weak Brönsted acidity. The reactivity of the OH groups is thus related to the morphology of MgO which, in turn, depends on the nature of the precursor and pre-treatments.

This may contribute to the difference of reactivity of MgO in MgO+CaO mechanical mixture, and dolomite.

Therefore in our proposed mechanism (Figure 6), the first step consists in the adsorption of benzene over the basic site of CaO [START_REF] Rigo | Hydrocarbon Adsorption on Carbonate Mineral Surfaces: A First-Principles Study with van der Waals Interactions[END_REF] to form an active carbon deposit (CmHp), CxHY gas species (as analyzed in our work), and H2. Then, the active carbon deposit can produce coke.

Water may either directly oxidize this active carbon deposit or may be dissociatively chemisorbed over MgO active site to form OH basic species. These OH basic species contribute to the formation of formiate and other oxygenate intermediates, then forming unidentate carbonate through OH spill-over. Formiate and carbonates have been analyzed by XPS in this present work and by infrared spectroscopy by Alarcon et al. [START_REF] Alarcón | New effects during steam gasification of naphthalene: the synergy between CaO and MgO during the catalytic reaction[END_REF][START_REF] Alarcón | Catalytic cooperation at the interface of physical mixtures of CaO and MgO catalysts during steam gasification of naphthalene[END_REF] and Constantinou et al. [START_REF] Constantinou | A comparative study of the steam reforming of phenol towards H2 production over natural calcite, dolomite and olivine materials[END_REF]. This mechanism corroborates the results of Constantinou et al. [START_REF] Constantinou | A comparative study of the steam reforming of phenol towards H2 production over natural calcite, dolomite and olivine materials[END_REF] who demonstrated that OH species are important reaction intermediates for the conversion of aromatics into Cx and H2. It has also been proved that unidentate carbonates are less stable than bicarbonates and constitute the main species formed over dolomite [START_REF] Constantinou | A comparative study of the steam reforming of phenol towards H2 production over natural calcite, dolomite and olivine materials[END_REF][START_REF] Alarcón | New effects during steam gasification of naphthalene: the synergy between CaO and MgO during the catalytic reaction[END_REF][START_REF] Philipp | On the active site of MgO/CaO mixed oxide for oxidative coupling of methane[END_REF]. The former would be the precursors in CO2 formation.

The proposed mechanism supports our experimental results. When the temperature increases, the cracking of benzene promotes the formation of CxHy gas species, especially over fresh dolomite. This reaction is less favored with the used dolomite probably due to the lower availability of CaO sites (as analyzed by XPS). The temperature increase is also associated to a decrease in CO2 and an increase in CO selectivities (Table 5). This evolution maybe explained by the cracking of benzene, as well as by the complex coupling between OH species formation and spill-over leading to CO/CO2 (from formiate and carbonate intermediate). Carbon monoxide is generated by the thermal decomposition of formiate and higher homologs. This reaction was enhanced by increasing temperature, at the expense of the carbonates and CO2 route. Indeed, at high temperature, this route (formation of OH by H2O dissociation on MgO sites, and OH spillover) maybe slower than the formiate thermal decomposition microkinetic rate. Therefore the OH source may be limited leading to a lower formation of CO2 from carbonate. This effect is even more marked for the used dolomite since MgO sites, required for OH species formation, are expected to be less available. The CO2 route is also inhibited by increasing cracking reactions with rising temperature. Further studies are needed to study more deeply the detailed chemistry of these intermediate C-O species at the temperatures of hydrocarbons steam reforming.

For used dolomite, the deposit of coke and ashes, detected in this study by various analytical tools, reduced the global availability of CaO and MgO active sites, as presented in the simplified scheme in Figure 7.

Conclusion

The evolution of dolomite surface chemistry during its use as bed material in a pilot-scale gasification reactor and its resulting activity on tar reforming have been studied for the first time.

Benzene steam reforming was selected as a model reaction in order to probe the activity of the dolomites.

BET, SEM-EDX, XRD, XPS, and TPO/TPD analyses revealed a drastic change of the surface properties of used dolomites. After being used as a bed material, used dolomite was covered by a layer rich in Si from biomass ash. In addition, coke was deposited at the used dolomite surface after gasification reactions.

The catalytic activity of used dolomite was ≈24% lower than that of fresh dolomite. This drop in catalytic activity can be attributed to the Si-rich layer covering the surface of used dolomite together with the coke deposition which decreased the availability of the active sites (CaO, MgO). In addition, SiO2 has low catalytic activity towards tar reforming reactions. The SiO2 and coke deposition could also limit the interactions between CaO and MgO, thus allowing the production of stable coke deactivating the catalyst.

This study presents a methodological approach that can be further used to study the effect of different operating conditions during biomass gasification on the activity and composition of dolomites (or other materials). Based on our experimental results and on the literature data, a new reaction mechanism of benzene steam reforming on fresh and used dolomite was proposed.

1 . 3 2. 3 2. 1 . 3 2. 2 . 3 2. 3 . 3 3. 2 . 3 3. 3 .

 133132333233 Introduction ....................................................................................................................... Materials & Methods ........................................................................................................ Presentation of the dolomite samples ........................................................................................................... Characterizations of the catalysts .................................................................................................................. Catalytic experiments ....................................................................................................................................... 3 3. Results and discussion ...................................................................................................... 3 3.1. Characterization of the catalysts .................................................................................................................... 3 3.1.1. Morphology of particles and chemical composition of the bulk catalysts by ICP ............................................. 3 3.1.2. XRD characterization of the catalysts ......................................................................................................... 3 3.1.3. Composition of the surface by XPS ............................................................................................................. 3 3.1.4. TPD and TPO characterizations ............................................................................................................... Steam reforming of benzene over fresh and used dolomites ................................................................... Discussion on the reaction mechanisms ...................................................................................................... 3 4. Conclusion ......................................................................................................................... 3

Figure 2 .

 2 Figure 2. Diffractogram of the fresh and used dolomites

Figure 3 .

 3 Figure 3. XPS spectra of carbon (C 1s) and oxygen (O 1s) species analysed on fresh dolomite and used dolomite after gasification.

Figure 5 .

 5 Figure5. Effect of temperature and dolomite aging on benzene conversion during steam reforming (C6H6=0.5 vol%, H2O=22.2 vol% in N2, 3 g dolomite,[START_REF] Constantinou | A comparative study of the steam reforming of phenol towards H2 production over natural calcite, dolomite and olivine materials[END_REF] NmL/min total flow rate).

Figure 7 .

 7 Figure 7. Deactivation of dolomites in biomass gasification reactors: the ashes formed from biomass oxidation leads to a deposit of Si (and other species present in ashes) layer over dolomite, and coke deposit also reduces the available active sites for benzene and water adsorption.

Table 1 .

 1 BET surface area of dolomite and olivine.

	Catalyst	BET (m 2 /g)
	Fresh dolomite	0.4
	Used dolomite	19.5

Table 2 .

 2 SEM-EDX analysis of fresh and used dolomite

		Fresh dolomite		Used dolomite
	Elements (wt.%)	Min	Average Max	Min	Average Max
	O	44.7	49.4	59.8	41.1	48.1	54.8
	Mg	1.6	17.0	36.2	3.6	5.3	7.5
	Ca	19.1	33.7	48.5	37.0	41.9	46.6
	P	nd	nd	nd	2.2	2.5	2.7
	Si	nd	nd	nd	3.3	3.7	4.1

Table 3 .

 3 Elemental composition of fresh and used dolomite (obtained by ICP-MS).

	Element (wt.%)	Fresh dolomite	Used dolomite
	Loss on ignition (wt.%)	9.01	11.88
	SiO2	0.53	4.35
	Al2O3	0.26	1.02
	Fe2O3	0.39	0.43
	MnO	0.02	0.14
	MgO	38.30	34.16
	CaO	50.13	47.20
	Na2O	0.02	0.04
	K2O	0.01	0.28
	TiO2	0.02	0.05
	P2O5	0.03	0.39

Table 4

 4

	Two oxygen species were observed through the O 1s binding energies at 531.17eV and
	532.80 eV. The first one corresponds to O-C or O-H bonds, related to the presence of both
	carbonaceous and hydroxyl species [95]. These 2 types of bonds unfortunately cannot be
	discriminated. The second O 1s signal at 532.80eV is generally attributed to chemisorbed water
	[95]. However, the high percentage of 40.47% is much higher than that of the carbonaceous

) highlight the presence of various carbonaceous species: graphitic or/and aliphatic hydrocarbons (C-C, C-H) at 284.6 eV, ethers (C-O) at 286.02 eV, aldehydes or carboxylic acids (C=O) at 287.89 eV, and carbonates ((CO3) 2-) at 289.40 eV. Carbonates are part of the dolomite composition and are present in higher relative amount on the surface than in the bulk (as shown by XRD) mainly explained by the exposure to air (at room temperature) of the decarbonated samples. The other carbon species than carbonates may be attributed to carbonaceous deposit [94]. species identified by the C-O bonding at 286.02 eV. Therefore the O 1s binding at 531.17eV may rather correspond to structural hydroxyls of the fresh catalyst. Its surface would be highly hydroxylated probably due to room temperature storage. The Mg 2p and Ca 2p signals correspond to surface concentrations (13.56 and 1.86%

Table 4 .

 4 XPS analysis of fresh and used dolomites

	Element	Fresh Dolomite BE (eV) at.%	Used Dolomite BE (eV) at.%
	O 1s O-C/O-H 531.17	40.47	532.16	27.59
	O 1s H2O	532.80	5.33	533.66	6.68
	C 1s C-(C,H)	284.60	23.88	284.59	23.00
	C 1s C-OR, C-	286.02	3.32	285.72	16.49
	OH, C-O
	C 1s C=O	287.89	1.25	287.14	5.75
	C 1s CO3 -2	289.40	10.33	290.24	8.74
	Ca 2p	346.80	13.56	347.75	9.72
	Mg 2p	49.40	1.86	50.35	1.03
	Si 2p	-	-	101.25	1.00
	The evolution of the oxygenated and carbonaceous species observed with XPS after gasification
	is illustrated in Figure 3. After gasification the atomic concentration of carbon at the surface of
	used dolomite significantly increases (from 38.8 to 53.9 wt.%), and the concentration of oxygen
	drops from 45.8 to 34.3 wt.%. Carbonaceous species containing ethers or carboxylic functions
	strongly increase after gasification (from 3.32 to 16.49%, and from 1.25 to 5.75%, respectively)
	probably due to oxygenated carbon deposit (Table 4). The content of hydrocarbons originally
	contained in the dolomite structure (C 1s C-(C-H)) is almost unchanged. The concentration of
	carbonates (C 1s CO3 2

-)) decreases a little after gasification. O 1s signal considerably decreases after gasification. Globally, after gasification, the surface becomes richer in C-O species and poorer in Ca-O and Mg-O.

Table 5 .

 5 Molar selectivity of the products generated by steam reforming of benzene.

		H2 (mol%)	CO2 (mol%)	CO (mol%)	CH4 (mol%)	C2H4 (mol%)
	T (°C)	800 850 900 800 850 900 800 850 900 800 850 900 800 850 900
	Fresh dolomite 66.9 65.8 63.0 25.7 25.2 23.4 1.9	3.1	5.6	3.0	4.4	5.5	0.8	0.9	0.6
	Used dolomite	65.5 66.9 66.5 30.7 27.4 22.4 2.6	3.5	8.3	1.2	2.2	2.9	0.0	0.0	0.0

Acknowledgements

The ANR agency is thanked for a part of the financial support through the project Gameco (coordinated by EDF). EDF (O. Authier) is acknowledged for having promoted this study and EQTEC for providing the dolomite materials.


Pouhet Raphaëlle
email: pouhet@insa-toulouse.fr

Cyr Martin
email: martin.cyr@insa-toulouse.fr

Bucher Raphaël
email: bucher@insa-toulouse.fr

Influence of the initial water content in flash calcined metakaolin-based geopolymer

Keywords: Geopolymer, Metakaolin, water content, compressive strength, porous network, transfer properties

This study assesses the influence of the initial amount of water in metakaolin-based geopolymer on some of its properties in the hardened state. Results suggest a linearly decreasing influence of the amount of water on the mechanical performance of the geopolymer and showed that the majority of this water was not bonded to the structure.

Moreover, it was found that whatever the amount of water initially introduced, all the samples contained the same amount of water after a drying period. The study of the porous network showed a large pore volume of 50% of the total volume which corresponds to the volume of water initially introduced. Finally, the observation of numerous macropore helped to explain the high transfer coefficients measured on the geopolymer.

Introduction

The evolution of knowledge on alkali activated materials (AAM) and, more specifically, on geopolymers (GP), tends to show that they could provide an efficient alternative to ordinary Portland cement (OPC) [START_REF] Shi | Alkali-Activated Cements and Concretes[END_REF]Provis et al., 2009a;[START_REF] Bligh | Development of geopolymer precast floor panels for the Global Change Institute at University of Queensland[END_REF]Pacheco-Torgal et al., 2014;[START_REF] Palomo | A review on alkaline activation: new analytical perspectives[END_REF]. Geopolymers are aluminosilicate materials formed by the activation of an aluminosilicate source, such as metakaolin, by a strongly basic alkaline solution. This results in the formation of amorphous materials at room temperature (Duxson et al., 2007a), showing compressive strength comparable to that of a traditional hydraulic binder (Pouhet and Cyr, 2016a). For the geopolymer the Water/Cement mass ratio is not much used in the literature, as it could be for OPC. However, without necessarily anticipating the consequences that it can have on the hardened state of the material, it is easy to see that the amount of water introduced can be very important. The purpose of this study is to anticipate the performance of a metakaolin-based geopolymer by studying the influence of the proportion of water initially introduced into the mixture.

The silica, aluminium, alkali and water brought to the geopolymer mixture by the raw materials are mentioned in the literature in the form of three molar ratios, which fix the total formulation of a geopolymer: SiO 2 /Al 2 O 3 , Na 2 O/Al 2 O 3 and H 2 O/Na 2 O. The main issue with the use of these ratios is that the variation of any one of them induces a variation of the total quantity of water in the mixture. Thus it is often difficult to distinguish the influence of just the initial amount of water on the parameters studied. Some authors use special molar ratios to define the total amount of water in the formulation: H 2 O/(SiO 2 +Al 2 O 3 ), H 2 O/R (some use [START_REF] Lizcano | Effects of Water Content and Chemical Composition on Structural Properties of Alkaline Activated Metakaolin-Based Geopolymers[END_REF][START_REF] Perera | Disposition of water in metakaolinite based geopolymers[END_REF]and Okada et al., 2009, respectively) but that implies that the other molar ratios have already been set. Other authors use a "Liquid-to-solid" or Water-to-solid" mass ratio [START_REF] Álvarez-Ayuso | Environmental, physical and structural characterisation of geopolymer matrixes synthesised from coal (co-) combustion fly ashes[END_REF][START_REF] Provis | Correlating mechanical and thermal properties of sodium silicate-fly ash geopolymers[END_REF][START_REF] Zuhua | Role of water in the synthesis of calcined kaolin-based geopolymer[END_REF]Zang et al., 2010;[START_REF] Park | What is the role of water in the geopolymerization of metakaolin?[END_REF]. However, this is not always defined in the same way due to the fact that the activation solution is liquid but contains sodium and silica. Sometimes the ratio corresponds to the mass of activating solution over the aluminosilicate mass [START_REF] Álvarez-Ayuso | Environmental, physical and structural characterisation of geopolymer matrixes synthesised from coal (co-) combustion fly ashes[END_REF]Provis et al., 2009a), sometimes it is the total mass of water over the sum of all the solid components introduced into the mix (Pouhet and Cyr, 2016a), and sometimes it is used without being defined (Zang et al., 2010;[START_REF] Zuhua | Role of water in the synthesis of calcined kaolin-based geopolymer[END_REF].

H 2 O/R 2 O, R=Na or K) or H 2 O/Al 2 O 3
The impact of the molar ratios on the hardened state of metakaolin-based geopolymer is now widely referenced in the literature regarding their mechanical performances [START_REF] Kamalloo | Modeling of compressive strength of metakaolin based geopolymers by the use of artificial neural network[END_REF][START_REF] Rowles | Chemical optimisation of the compressive strength of aluminosilicate geopolymers synthesised by sodium silicate activation of metakaolinite[END_REF]Duxson et al., 2007b), microstructures [START_REF] Duxson | Understanding the relationship between geopolymer composition, microstructure and mechanical properties[END_REF][START_REF] Barbosa | Synthesis and characterisation of materials based on inorganic polymers of alumina and silica: sodium polysialate polymers[END_REF], and porous networks [START_REF] Duxson | Understanding the relationship between geopolymer composition, microstructure and mechanical properties[END_REF][START_REF] Steins | Effect of aging and alkali activator on the porous structure of a geopolymer[END_REF] but very few studies discuss the influence of the initial amount of water introduced into the mixture on the hardened state of geopolymer, independently of the other components [START_REF] Zuhua | Role of water in the synthesis of calcined kaolin-based geopolymer[END_REF][START_REF] Okada | Water retention properties of porous geopolymers for use in cooling applications[END_REF][START_REF] Lizcano | Effects of Water Content and Chemical Composition on Structural Properties of Alkaline Activated Metakaolin-Based Geopolymers[END_REF]. Among these authors, Lizcano et al. assessed the effects of chemistry, and curing and ageing conditions on water loss kinetics, porosity, and the structure of metakaolin-based geopolymers [START_REF] Lizcano | Effects of Water Content and Chemical Composition on Structural Properties of Alkaline Activated Metakaolin-Based Geopolymers[END_REF][START_REF] Lizcano | Effects of Water Content and Chemical Composition on Structural Properties of Alkaline Activated Metakaolin-Based Geopolymers[END_REF]. Their results showed that the initial amount of water was the dominant factor affecting the density and the open porosity of geopolymers after curing and extended ageing. This study also showed that whatever the amount of water introduced initially, after 21 days in open moulds at 20 °C and around 74 % R.H., all specimens had a final water content of around 15% to 20% (for geopolymer activated by sodium). Finally, the authors demonstrated a strong correlation between the initial water content and the open porosity of the geopolymer. Okada et al. [START_REF] Okada | Water retention properties of porous geopolymers for use in cooling applications[END_REF] obtained similar results on metakaolin-based geopolymer, showing that the increase in the initial water content increased both the size and the volume of the porous network. Perera et al. [START_REF] Perera | Disposition of water in metakaolinite based geopolymers[END_REF] showed that water was present in the geopolymer in three forms: 1-"free water" (or intergranular water), which is removed between room temperature and 150 °C; 2-water removed between 150 °C and 300 °C, which would correspond to the "interstitial water"; and 3-water related to OH -groups bound into the structure of new-formed products, representing only very small amount of water measured between 300 °C and 700 °C [START_REF] Perera | Disposition of water in metakaolinite based geopolymers[END_REF]. White et al. [START_REF] White | The effects of temperature on the local structure of metakaolin-based geopolymer binder: A neutron pair distribution function investigation[END_REF] used neutron pair distribution function (PDF) to improve their understanding of the local atomic structural characteristics of geopolymer binders derived from metakaolin, specifically the nature and amount of the water associated with these materials. Their findings were consistent with those of previous studies, showing that only small amounts of water (<5% for geopolymer activated by potassium) may exist in small pores and as terminal hydroxyl groups in the Si-Al framework structure. Most of the water is lost below 200 °C, indicating that it is mainly found in large pores or hydrating the charge-balancing cations associated with the Si-Al framework.

The above mentioned studies provide some important information about the organization of the water in the hardened metakaolin-based geopolymer, but none of them addresses the consequences of this presence of water. This study therefore proposes an investigation into the influence that the initial water content in metakaolin-based geopolymer has on the hardened state, in the aim of being able to anticipate the final behaviour of the material in terms of mechanical performance and transfer properties.

Materials and methods

Geopolymer raw materials and synthesis

The source of aluminosilicate was a metakaolin obtained by flash calcination, produced in the south west of France by ARGECO Développement. The term "flash calcination" refers to the combustion process where the particles of kaolinite are transformed into metakaolin by passing near a flame for a few tenths of a second (temperature around 700 °C) [START_REF] San Nicolas | Characteristics and applications of flash metakaolins[END_REF]. Table 1 presents the mass content of oxides in the flash metakaolin, obtained by ICP-OES, using an Optima™ 7000 DV ICP-OES (PerkinElmer) equipped with a CCD sensor.

These results show very large silica content, mainly due to the presence of quartz in the raw kaolin, confirmed by XRD analyses. However, after quantification, only the amorphous part of this silica (around 29% of the metakaolin mass) was taken into account in the formulation of the geopolymer. The specific surface area of the flash metakaolin was 13 m²/g (BET). The activating solution used was an industrial sodium silicate solution (Bétol 49T, Woellner) containing 8 % Na 2 O by mass and having an SiO 2 /Na 2 O ratio of 3.3 (Table 1). In order to adjust the sodium content, pure-grade NaOH was added to this commercial solution 24 h before the geopolymer preparation. This initial solution had a high water content (65.6 wt%, Table 1), so a moderate evaporation of a certain amount of water, performed at low temperature 24 h before the preparation, was necessary for some geopolymers. The geopolymer was prepared in two steps. First, pure NaOH and water were added to the industrial waterglass solution to obtain the desired SiO 2 /Na 2 O and H 2 O/Na 2 O molar ratios.

After total dissolution of the sodium hydroxide, the solution was cooled to 20 °C for 24 h and then mixed with the metakaolin until the mixture was homogeneous.

The geopolymer mortars were mixed and made according to French standard EN 196-1 in a 5

L mixer (Automix, Controls ® ), using the activation solution instead of the mixing water. The formulations are presented in Table 2. Standard sand (EN 196-1 and ISO 679: 2009 standards) composed of well crystallized quartz and having a controlled particle size between 0 and 2 mm was used. The mortar specimens were cast in 4 x 4 x 16 cm moulds using a shock table, and pastes were cast in 7 x 4.3 x 2.2 cm plastic prisms. All specimens were cured at 20 °C in sealed bags for the first 24 hours and then demoulded and stored at 20 °C and 95 % R.H. until testing.

Test methods

The mechanical tests in flexion and compression were performed on 3 prismatic mortar specimens of dimensions 4 x 4 x 16 cm, according to EN 196-1 (Automatic mortar press, Class A -3R ® RP30/200FP). The mineralogical study was carried out by X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer, with Bragg-Brentano configuration and copper radiation (Cu Kα, λ = 1,5406Å). The acquisitions were made on powder having a maximum particle size of 80 µm, between 5° and 70° 2θ, with a step size of 0.02° and an acquisition time of 1.075 seconds per step. The total pore volume was determined in this study by measuring the porosity accessible to water via the French standard NF P18-459 (2010). Tests were carried out on at least three 7 x 4.3 x 2.2 cm prisms of geopolymer paste.

The balance accuracy for the mass measurements was 0.001g. Mercury intrusion porosimetry measurements (MIP) were performed with a Pascal 140 porosimeter coupled with a Pascal 240 (Fusion Instrument) and the samples analysed were pieces of paste of around 1 g obtained by fresh fracture at 7 days. Gas absorption/desorption analyses were performed using a Tristar 3020 apparatus (Micromeritics), with a degassing temperature of 100 °C, on a geopolymer paste piece prepared in the same way as those studied by mercury porosimetry. X-ray microtomography images were obtained with a Nanotom 180 from Phoenix / GE. The acquisition parameters used were a voltage of 80 kV and an intensity of 100 µA. The resolution obtained for this type of sample was a 5.4 micron voxel. The volumes were rebuilt using the Datos X software (Phoenix) and VG Studio Max (Volume Graphic) to obtain an image of the porosity of the material. Finally, cross-sections of 100 day old geopolymer were observed by scanning electron microscopy (SEM) after application of a focused ion beam (FIB) apparatus (FEI Helios NanoLab 600i). FIB uses Ga + ions to remove material with a very high spatial precision. In this way, cross-sections of a representative sampling area were obtained at a specific location after surface observation. For this operation, the sample was positioned in the SEM chamber, tilted at 52° and a 2 micron thick layer of platinum (Pt) was laid down for surface protection. FIB milling was then applied to etch the sample. In this study, we chose to work around these ratios and clearly dissociate the solid from the liquid. Thus only two parameters had to be fixed:

Results and discussion

Influence of water content on mechanical performance

-The geopolymer molar formulation, corresponding to the solid part initially introduced and written in the form xSiO 2 yAl 2 O 3 zNa 2 O.

-The amount of water introduced, written in the form of the water content (mass percentage) or the Water/Solid mass ratio.

The formulation of the metakaolin-based geopolymer was the subject of an earlier study, which highlighted an optimal proportion regarding the mechanical performances corresponding to the initial composition: 3.6SiO 2 1Al 2 O 3 0.9Na 2 O [START_REF] Pouhet | Formulation and durability of metakaolin-based geopolymers[END_REF].

To assess the influence of the water content on the mechanical performance, five geopolymer mortars with increasing amounts of water were tested in flexural and compressive strength.

The corresponding formulations are listed in Table 2. Given that the waterglass solution provided large quantities of water in addition to silica and sodium, one formulation, GP0. Plotting the measured strengths as a function of the proportion of water in the mixture showed the same trends in flexion and compression. It was observed that, between water proportions of 28% and 37%, increasing the amount of water decreased the mechanical performance (only between 29% and 37% in the case of the flexural strength). In view of these figures, it would seem that the mechanical performance of the geopolymers would follow a linear trend with a decrease in mechanical strength proportional to the initial amount of water, at least in the W/S domain studied. This result therefore demonstrates a different behaviour that observed for cement witch shows a non-linear decrease in performance with the addition of water (e.g. Bolomey). Furthermore , this observation is in agreement with the literature, which postulates that the introduced water is not used for the formation of hydrate as in Portland cement [START_REF] Perera | Disposition of water in metakaolinite based geopolymers[END_REF][START_REF] White | The effects of temperature on the local structure of metakaolin-based geopolymer binder: A neutron pair distribution function investigation[END_REF], and therefore does not participate in the structuration of the material. Thus the addition of water in a geopolymer formulation systematically entails a weakening of the material, probably by increasing the porosity.

Investigation on the bound water

The literature shows that, in the hardened geopolymer, the water does not appear to be bound in the network in hydrate form as it is in cement matrix [START_REF] Barbosa | Synthesis and characterisation of materials based on inorganic polymers of alumina and silica: sodium polysialate polymers[END_REF][START_REF] Lizcano | Effects of Water Content and Chemical Composition on Structural Properties of Alkaline Activated Metakaolin-Based Geopolymers[END_REF][START_REF] White | The effects of temperature on the local structure of metakaolin-based geopolymer binder: A neutron pair distribution function investigation[END_REF][START_REF] Perera | Disposition of water in metakaolinite based geopolymers[END_REF]. Thus, determining the fate of the water in the hardened geopolymer paste is indispensable for a better understanding of its influence. It was therefore decided to study the amount of water that could be removed by drying the specimens at relatively moderate temperature, in order to observe the influence on the mechanical performances. Three water contents were chosen, corresponding to W/S ratios of 0.40, 0.47 and 0.56, with three drying temperatures of 50 °C, 80 °C and 105 °C. After stabilization of the mass loss for each drying temperature, the amount of water remaining in the geopolymer was calculated. Total water contents of the geopolymers for the three W/S ratios according to the drying temperature are presented in Figure 3. The figure clearly shows that, whatever the initial water content in the geopolymer paste, this content was reduced to about 3% for all the formulations after drying at 105 °C, which corresponds to a relative loss of 90-92% of the initial water mass. The evolution of the water content in geopolymer versus the drying temperature tended to an asymptotic value above 105 °C, suggesting a slower evolution for the loss of the 10% remaining water.

In order to visualize the temperature at which the remaining water evaporates, TGA analysis was carried out on the GP0.40, coupled with mass spectroscopy. The results obtained are presented in Figure 4. According to the TGA result, evaporation of the remaining water would be a long process, as traces of water were still measured at 700 °C. This could indicate that a small proportion of the water initially introduced remained in small pores or as terminal hydroxyl groups in the structure and that this quantity of water would be the same regardless of the initial amount of water introduced, as described in the literature [START_REF] Perera | Disposition of water in metakaolinite based geopolymers[END_REF][START_REF] White | The effects of temperature on the local structure of metakaolin-based geopolymer binder: A neutron pair distribution function investigation[END_REF]. At the end, the total molar formulation calculated by considering the water "fixed" to the structure (therefore not evaporated at 105 °C) was nearly the same for the In order to confirm that the departure of this "free water" was not detrimental to the geopolymer structure, the same drying protocol was applied to GP0.40, GP0.47 and GP0.56 geopolymer mortars, which were then tested in compression. As previously, the geopolymers were kept without external exchanges for seven days and then placed in a climatic chamber until stabilization of the mass loss. Figure 5 presents the compressive strengths measured according to the drying temperature for the three W/S ratios studied. Two behaviours can be observed:

-The formulation having the least water, GP0.40, showed a loss of strength that was moderate but increased with increasing drying temperature.

-The GP0.47 and GP0.56 formulations showed similar behaviour, with very close compressive strength values regardless of the applied temperature.

Some hypotheses can be advanced to explain the behaviours observed. It was measured that about 90% of the introduced water was evaporated between 20 °C and 105 °C. The fact that the mechanical performances of the GP0.47 and GP0.56 formulations did not change between these two temperatures could validate the assumption that it was indeed "free water" and therefore not related to the structure and its stiffness. In view of the above results, it is highly probable that this was also the case for the GP0.40 formulation despite the loss of strength.

However, for this formulation having the lowest water content, the loss of strength observed could have been due to a difference in the organization of the porous network containing this "free water". In this case, the porosity could have been finer or arranged differently, so that the evaporation of the "free water" would lead to damage in the material and therefore to a decrease in mechanical performance.

Influence of the water content on the volume and organization of the porous network of geopolymers

In this study the intention was to characterize all of the water in the geopolymer. So it was firstly necessary to evaluate the size and organization of the water present in the air bubbles trapped during preparation. GP0.40 geopolymer paste prepared without any shocks or vibration was analysed by X-ray tomography to visualise the air occlusions, assess their size distributions and quantify their cumulative volume. A 3D section of the sample, having dimensions 2 x 2 x 5 mm was isolated and analysed. Figure 6a presents the 3D section,

showing the overall sample and the air occlusions, which were easily identified by their dark coloration. Using this strong contrast between the air and the matrix allowed the software to isolate this volume and to represent it separately. Figure 6b, which isolates the entrapped air fraction, shows significant numbers of spheres with diameters ranging from 40 µm up to nearly 300 µm. It was calculated that this cumulative volume represented 2% of the sample.

The distribution of the sphere sizes is illustrated by different colours: the blue corresponds to diameters smaller than 200 µm, with decreasing diameters shown in darker shades of blue, while all other colours correspond to diameters ranging from 200 µm to 300 µm. Figure 6b shows a homogeneous distribution of the bubbles in the sample with no privileged organization. However, the result does not show if there is an interconnectivity of the porosity, which would be too fine for the resolution of the device.

13

To define the porosity of the geopolymer and the influence of the initial water content, the total porosity accessible to water was measured (NF P18-459) and mercury intrusion porosimetry (MIP) was performed on geopolymer specimens having three different W/S mass ratios. Figure 7a shows the porosity accessible to water measured using the same three drying temperatures as those used for the study of the bonded water.

The information obtained as described in the previous two paragraphs showed that, for the GP0.47 and GP0.56 formulations, the temperature of 105 °C caused no loss of strength and led to final water contents close to 3%. Thus, the total porosity values measured for these formulations with a drying temperature of 105 °C was very probably close to the total porosity values of the materials, i.e. 53.0% and 56.4% respectively. Since the drying behaviour of the three formulations was similar (Figure 7a), it seems reasonable to conclude that, despite the loss of mechanical strength, the total porosity of the GP0.40 formulation can be measured by drying at 105 °C and so would be 49%. This value is in agreement with values found in the literature for similar formulations [START_REF] Boher | Experimental investigation of gas diffusion through monomodal materials. Application to geopolymers and Vycor ® glasses[END_REF]. For the three W/S mass ratios, the initial water volume introduced into the mixture was calculated, then compared to the total pore value (obtained at T drying of 105 °C). These results, presented in Figure 7b, show a very good correlation, illustrating a direct proportionality between the volume of water introduced and the final porosity of the geopolymer. So, in addition to confirming that the water in the geopolymer appears to be present mostly as "free water", these results show that the total porosity of a geopolymer can be chosen and fixed before its preparation. Since a linear relationship was also made between the initial amount of water and the mechanical performance (Figure 2 a), it is possible to say that there is a direct correlation between total porosity and mechanical strength. Furthermore , knowing that the porosity has a direct influence on the transfer properties and thus on the durability of the materials, the possibility of choosing its value in advance could be a clear advantage for the development of geopolymer.

MIP analysis was performed on GP0.40, GP0.47 and GP 0.56 to evaluate the influence of the water content on the geopolymer porous network microstructure. The relative mercury volume introduced versus the pore access diameter is presented in Figure 8a along with the curve of the relative pore volume versus diameter (Figure 8b). For the geopolymer having a W/S mass ratio of 0.40 it was calculated that 95% of the mercury was introduced for pore diameters of 7 to 20 nm (with 83% between 10 and 20 nm), which shows a homogeneous distribution of the pore access diameter. Thus, when the curve of the relative pore volume versus diameter is observed (Figure 8b), the geopolymer has a single population of pore access centred on a diameter of 13 nm. These results confirm the monomodal nature of the porous network of the geopolymer observed in the literature [START_REF] Rovnaník | Effect of curing temperature on the development of hard structure of metakaolin-based geopolymer[END_REF][START_REF] Boher | Experimental investigation of gas diffusion through monomodal materials. Application to geopolymers and Vycor ® glasses[END_REF].

These figures also show that all three formulations led to monomodal porous networks, whatever the amount of water initially introduced. The cumulative pore volume representation confirmed the increase in total volume measured with water intrusion when the initial water amount increased and its derivative revealed that the pore access diameter also increased. The median pore access diameter therefore increased from 13 nm for GP0.40 to 29 nm for GP0.47 and, finally, 36 nm for GP0.56. When the median diameter increased, the spread of the value also increased, as 90% of the mercury was introduced between 7 and 18 nm for GP0.40, between 11 and 35 nm for GP0.47, and between 16 and 42 nm for GP0.56. The notable difference in pore access diameter between the formulation having the least water and the other two could support the hypothesis made previously on the fact that GP0.40 had such a fine porosity that the evaporation of the "free water" during drying damaged the structure.

A study of nitrogen adsorption-desorption isotherms also made on geopolymer GP0.40 provided additional information. The adsorption and desorption isotherms of Figure 9a show a distinct hysteresis loop for relative partial pressure (P/P 0 ) between 0.8 and 1.0. The passage of the desorption curve below the absorption curve (until P/P 0 of 0.75) was associated with degassing problems during the analysis (degassing temperature of 100 °C). Nevertheless, the closeness of the hysteresis loop at P/P 0 > 0.40 and a type IV sorption behaviour with a hysteresis loop of type 1 on a loop of type 2, which have already been identified on metakaolin geopolymer samples by [START_REF] Steins | Effect of aging and alkali activator on the porous structure of a geopolymer[END_REF], indicate the absence of micropores and the formation of a well-defined mesoporous texture having interconnected pores with narrow necks and wide voids, called 'ink-bottle pores'. The BJH pore-size distribution (Figure 9b) confirms the monomodal nature of the porous network centred on 15 nm. Finally, in order to be able to observe the porosity of a hardened geopolymer clearly, SEM observations were made on a 100 day old GP0.40 section obtained by cutting with the focused ion beam technique. With this cutting of nanometric precision, it is possible to see inside the material without degrading the porous network, as can be seen in Figure 10. It should be noted that the black streaks observed on the SEM images are due to the ion beams used to cut the materials, and the lighter layer at the top of the cross-section corresponds to the platinum depot made to protect the cut out.

Valuable information can be obtained from these images:

-Heterogeneity is clearly observed within the geopolymer in Figure 10a and b as, in addition to the quartz grains recognisable by their well-defined shapes and their grey colour, many cavities are also observed.

-A larger zoom (Figure 10 b) and c)) reveals that these cavities seem to be formed by multiple layers contained within the matrix. The shape, orientation and size of these layers and cavities recall the structure of kaolinite or metakaolinite layers [START_REF] San Nicolas | Approche performantielle des bétons avec métakaolins obtenus par calcination flash[END_REF]. This shows that some of the introduced metakaolin was not fully dissolved by the activating solution during the geopolymerization.

-In addition to this large layered porosity, the largest zoom (Figure 10d) clearly shows the monomodal mesoporosity of the geopolymer observed during MIP. The many black holes visible in Figure 10d all measure around 10-30 nm and were found in very confirms the presence of the "ink bottle" effect observed by analysis of the results of absorption/desorption of gas.

-The observation of this monomodal mesoporosity on this scale, resembling that of a sponge, also confirms the interconnectivity of geopolymer porosity.

Consequences regarding the durability

To go further, it is interesting to discuss the direct consequences of these results for the development of geopolymers as construction materials. In the science of building materials, porosity is sometimes considered as a direct indicator of the durability of materials (Baroghel-Bouny, V; 2004). In fact, the porosity is related to the transfer properties of the material, which are responsible for the more or less rapid intrusion of compounds that are harmful to the structure, such as carbon dioxide or chlorides. In view of the large interconnected porosity of metakaolin-based geopolymers, the transfer properties had to be considered. Intrinsic air permeability measurements were performed on geopolymer mortars using the Cembureau apparatus. A very high permeability of 8456.10 -18 m² was measured after drying at 105 °C.

Tests of chloride ion diffusion by migration were also carried out on three geopolymer concrete samples, according to standard NT Build 492. Generally, for cementitious materials, the depth of penetration of chloride ions corresponds to about half of the sample at the end of the test. On geopolymer samples, the chloride ions had completely passed through the test piece at the end of the test (i.e. after 24 h at 10 V) making it impossible to calculate a chloride diffusion coefficient. Thus it seems that the diffusion of chlorides through metakaolin-based geopolymer is much faster than through known materials based on Portland cement and the transfer properties of this geopolymer appear to be unfavourable compared with those of Portland cement. The SEM images obtained (Figure 10) confirm these data by showing a large volume of void in the structure, allowing the transport of air or water. However, comparison with a cement matrix is difficult in view of the great differences in chemical composition between these two materials. A carbonation study previously performed on the same geopolymer material concluded that, although carbonation of the pore solution was extremely rapid in geopolymer, the risks related to the durability of the structure would be limited (Pouhet and Cyr, 2016b). The amount of water initially introduced into the formulation of geopolymers necessarily has a direct influence on the transfer properties of the material and must be taken into account. However, it would be hasty to draw conclusions regarding the consequences on the durability of the structure without further investigations.

Conclusion

This study has proposed an evaluation of the influence that the amount of water initially introduced has on the hardened properties a geopolymer mixture with a fixed formulation.

The conclusions drawn and the consequences that can be expected regarding the development and the durability of geopolymer are listed below.

Increasing the amount of initial water in the mixture significantly reduces the mechanical performance measured at 7 days. It seems that this decrease follows a linear trend, which implies that the final performance can be anticipated as soon as the water content is known.

However, this also implies that great caution must be taken during the fabrication of geopolymer, especially in large quantities, as a small error in the mass of water introduced may result in a significant variation in the final mechanical performance. That being said, a very wide range of applications seem possible for geopolymers, particularly in the field of construction, as they develop significant mechanical performance in just seven days with a range of water content traditionally used in construction materials.

The investigation of the bonded water during geopolymerization led to the conclusion that more than 90% of the water introduced would be "free water", which is easily removable, and therefore would not contribute to the stiffness of the material. The water present in the hardened geopolymer porosity would thus have no influence regarding the mechanical performance of the material. This water could be largely withdrawn, which could be useful for certain applications such as refractory materials. However, it should be noted that the departure of this pore water can in some cases be detrimental to the structure and lead to a decrease in strength.

The porosity studies reported here show a pore volume for the geopolymer paste that is large but in agreement with the literature. It appears that the total volume of the geopolymer pores is proportional to the volume of water initially introduced in the mixture, which is consistent with the conclusion of the bonded water study presented in this paper. The quantity of water introduced thus makes it possible to fix the final pore volume, and also the mean diameter of the mesoporosity. The transfer properties measured in this study are in agreement with the large porosity observed for the geopolymer, but further investigations on the durability must be carried out before drawing conclusions on the long term behaviour of these materials.

  Using three molar ratios (SiO 2 /Al 2 O 3 , Na 2 O/Al 2 O 3 and H 2 O/Na 2 O) to formulate the geopolymer led to a systematic variation of the water content, as illustrated in Figure 1. Increasing the SiO 2 /Al 2 O 3 ratio decreased the amount of water in the formulation. Conversely increasing the Na 2 O/Al 2 O 3 or the H 2 O/Na 2 O ratios increased the total water content.

Figure 1 .

 1 Figure 1. Examples of evolution of the proportions of the geopolymer constituents (MK + sodium silicate (solid fraction) + total water) and the Water/Solid mass ratio depending on a) SiO 2 /Al 2 O 3 and b) Na 2 O/Al 2 O 3 molar ratios, the other two molar ratios being keep constant (including H 2 O/Na 2 O).

  38, required an additional step during its preparation. The significant water contribution of the activation solution (65.5%) made it impossible to obtain formulations having a Water/Solid mass ratio lower than 0.4. Excess water was therefore removed from the waterglass solution by evaporation 24 h before the mortar preparation, by heating at 50 °C under agitation until a loss of mass equal to the required amount of water was achieved. No visual modification was observed on the remaining solution. The results of the mechanical tests (performed in compression and in bending) on the five mortars of Table 2 are presented in Figure 2.

Figure 2 .

 2 Figure 2. a) Compressive and b) Flexural strengths of geopolymer mortars at 7 days depending on the water proportion introduced in the initial mixture (Solid formulation: 3.6 SiO 2 1 Al 2 O 3 0.9 Na 2 O)

Figure 3 .

 3 Figure 3. Evolution of the total amount of water in GP0.40, GP0.47 and GP0.56 depending on the drying temperature.

  three formulations and corresponded to 3.6 SiO 2 1 Al 2 O 3 0.9 Na 2 O 1.3 H 2 O. The presence of carbon dioxide in the mass spectroscopy spectrum means that carbonation took place during the seven days of curing, which is consistent with results obtained in a previous study(Pouhet and Cyr. 2016b).

Figure 4 .

 4 Figure 4. Thermogravimetric analysis of geopolymer GP0.40 at 7 days performed at 20.0 °C/min up to 950 °C with H 2 O and CO 2 mass spectroscopy spectrum.

Figure 5 .

 5 Figure 5. Compressive strength of geopolymer mortars at 7 days performed after mass loss stabilization depending on the drying temperatures for three different Water/Solid mass ratios.

Figure 6 .

 6 Figure 6. X-ray tomography visualization of G0.40 paste showing: a) 3D section of the sample and b) the same 3D section presenting only the air occlusions.

Figure 7 .

 7 Figure 7. Total porosity accessible to water a) versus the drying temperature used and b) versus the volume of water introduced in the initial geopolymer mixture, for a drying temperature of 105 °C

Figure 8 .

 8 Figure 8. Relative mercury volume introduced and relative pore volume versus pore access diameter for geopolymer pastes G0.40, GP0.47 and GP0.56.

Figure 9 .

 9 Figure 9. a) Nitrogen adsorption-desorption isotherms and b) BJH pore-size distribution for the GP0.40 geopolymer paste.

Figure 10 .

 10 Figure 10. Electronic scanning microscope images of a cross section of 100 day old geopolymer obtained by focused ion bean a) x2000, b) x5000, c) x10000 and d) x35000

Table 1 .

 1 Mass composition of raw materials.

SiO 2 Al 2 O 3 CaO MgO Fe 2 O 3 K 2 O Na 2 O TiO 2 SO 3 H 2 O

	Flash metakaolin	68.10 24.10 0.91	0.22	3.73	0.35	0.08	1.14	0.03	-
	Waterglass solution 26.5	-	-	-	-	-	8.0	-	-	65.6

Table 2 .

 2 Mass composition of geopolymer (solid molar composition: 3.6SiO 2 1Al 2 O 3 0.9Na 2 O)

	Sample ID	Mk (g)	Silicate (g) NaOH (g)	H 2 O (g)	Sand (g)	Water in GP (wt.%)	Water/ solid
	GP0.38	450	372	37.8	-12.1 *	1350	27%	0.38
	GP0.4	450	372	37.8	5	1350	29%	0.40
	GP0.5	450	372	37.8	65.1	1350	33%	0.50
	GP0.6	450	372	37.8	125.1	1350	37%	0.60
	GP0.7	450	372	37.8	185.1	1350	41%	0.70

* negative value corresponding to the mass of water evaporated from the alkali silicate solution.

Acknowledgments

The authors are very grateful to SEAC Guiraud and the Association Nationale de la Recherche Technique (ANRT) for supporting the present research. Argeco Developpement and Woellner are also acknowledged for their material support. The quality of the assistance provided at the Raimond Castaing microscopy structure and the CIRIMAT are also acknowledged.