A Cuccurullo

D Gallipoli

A W Bruno

C Augarde

P Hughes

Christian La Borderie

A comparative study of the effects of particle grading and compaction effort on the strength and stiffness of earth building materials at different humidity levels

Keywords: Suction, Hypercompaction, Strength, Particle size distribution, Earth construction

INTRODUCTION

The expression "raw earth" or "unstabilised earth" indicates a building material consisting of a compacted mix of soil and water, which is put in place with the least possible transformation [START_REF] Jaquin | The strength of unstabilised rammed earth materials[END_REF]. Compared to standard engineering materials, raw earth can lead to a reduction in both carbon emissions and energy consumption not only during construction but also during service life of buildings [START_REF] Gallipoli | A geotechnical perspective of raw earth building[END_REF][START_REF] Morel | Compressive strength testing of compressed earth blocks[END_REF]. The hydrophilic nature of raw earth explains the ability of this material to regulate humidity and temperature inside dwellings, thus increasing the comfort of occupants without requiring energy-intensive air conditioning installations [START_REF] Pacheco-Torgal | Earth construction: Lessons from the past for future eco-efficient construction[END_REF]. Unfortunately, despite these advantages, the deployment of raw earth into construction practice has so far been very limited due to insufficient knowledge of important aspects of material design. For example, there is still considerable lack of information about the influence of index properties on the strength, stiffness and hygro-thermal inertia of earth materials at different humidity levels. The engineering behaviour of earth building materials is strongly influenced by pore water content, which is in turn linked to the relative humidity of the surrounding air. A decrease of ambient humidity produces an increase of capillary suction with a corresponding decrease of pore water saturation, and vice versa. Climatic conditions change significantly with geographical location and seasonal variations of ambient conditions can be significant, yet relatively few studies have investigated the impact of relative humidity on the strength and stiffness of earth building materials [START_REF] Jaquin | The strength of unstabilised rammed earth materials[END_REF][START_REF] Beckett | The effect of humidity and temperature on the compressive strength of rammed earth[END_REF][START_REF] Bui | Effect of moisture content on the mechanical characteristics of rammed earth[END_REF][START_REF] Gerard | A unified failure criterion for unstabilized rammed earth materials upon varying relative humidity conditions[END_REF][START_REF] Champiré | Impact of relative humidity on the mechanical behavior of compacted earth as a building material[END_REF][START_REF] Bruno | Mechanical behaviour of hypercompacted earth for building construction[END_REF][START_REF] Xu | Loading-unloading shear behavior of rammed earth upon varying clay content and relative humidity conditions[END_REF]. These studies have indicated that both strength and stiffness increase non-linearly as suction increases and degree of saturation reduces, levelling off at very high suctions. This behaviour is consistent with the simple capillarity model of Fisher [START_REF] Fisher | On the capillary forces in an ideal soil; correction of formulae given by WB Haines[END_REF], which predicts that the stabilising effect of a water meniscus at the contact between soil particles grows with increasing suction tending towards an asymptote. An increase of humidity also leads to more ductile behaviour while a decrease of humidity generates a relatively brittle response, especially at high clay contents. Jaquin et al. [START_REF] Jaquin | The strength of unstabilised rammed earth materials[END_REF] were among the first to investigate the interaction between earth building materials and the surrounding atmosphere. They performed unconfined compression tests on air-dried samples at different water contents showing that strength and stiffness increase as water content decreases from 10.2 % to 5.5 %. This range of water content is, however, still higher than the typical value of 1-2 % measured inside earth building materials in field conditions [START_REF] Bui | Compression behaviour of non-industrial materials in civil engineering by three scale experiments: the case of rammed earth[END_REF]. Bui et al. [START_REF] Bui | Effect of moisture content on the mechanical characteristics of rammed earth[END_REF] measured the unconfined compressive strength of earth materials with distinct particle gradings over a water content range from 11% to 1-2 %. These two limits correspond to the typical water contents of earth materials after manufacture and after field equalisation, respectively. Also in this case, the mechanical properties of the material deteriorated as water content increased and suction reduced. The study also highlighted that a small increase of moisture content from a typical field level of 1-2 % to about 4% (caused, for example, by intense rainfall or a marked change of ambient humidity) did not induce a significant drop of strength. The impact of particle grading and clay content on material performance has been mostly overlooked by past research. Current earth building guidelines recommend specific classes of soils, whose particle size distribution and clay content must fit within admissible bands, but the effect of grading and clay content on the strength and durability of the material remains unclear [START_REF] Maniatidis | A review of rammed earth construction. Innovation Project "Developing Rammed Earth for UK Housing[END_REF]. Earlier studies [START_REF] Maniatidis | A review of rammed earth construction. Innovation Project "Developing Rammed Earth for UK Housing[END_REF][START_REF] Walker | Rammed earth: design and construction guidelines[END_REF][START_REF] Delgado | The selection of soils for unstabilised earth building: A normative review[END_REF][START_REF] Ciancio | Advances on the assessment of soil suitability for rammed earth[END_REF] have suggested that soils with clay contents from as low as 5% to as high as 30% are acceptable for earth building, though there is still no consensus on clear selection criteria. Beckett and Augarde [START_REF] Beckett | The effect of humidity and temperature on the compressive strength of rammed earth[END_REF] were among the few authors who investigated the effect of clay content on the strength of earth building materials showing that, regardless of humidity and temperature, a clay content near the recommended minimum corresponds to the highest material strength. They argued that this was due to the larger water retention capacity of clays, whose small pore network can hold moisture in bulk, rather than pendular, form over a very wide suction range [START_REF] Jaquin | Unsaturated characteristics of rammed earth[END_REF]. This is also consistent with a large base of geotechnical research, which has demonstrated that finer soils exhibit higher water contents than coarser soils at any suction level [START_REF] Likos | Hysteresis of capillary stress in unsaturated granular soil[END_REF]. A relatively high clay content therefore increases the moisture content of the earth material, which undermines strength and stiffness especially in humid environments. In another study, Xu et al. [START_REF] Xu | Loading-unloading shear behavior of rammed earth upon varying clay content and relative humidity conditions[END_REF] amended a natural soil with different proportions of fine sand to produce three different earth building materials with clay contents of 35%, 26%, and 17%, respectively. They performed a series of triaxial tests on all three materials equalised at different humidity levels observing a strong dependency of the mechanical properties on both ambient humidity and clay content. Strength and stiffness decreased with an increase in relative humidity but the magnitude of this reduction depended on clay content. In contradiction with Beckett and Augarde [START_REF] Beckett | The effect of humidity and temperature on the compressive strength of rammed earth[END_REF], they observed that shear strength tended to increase with growing clay content under fixed levels of relative humidity and confining pressure. The magnitude of this increase tended to be smaller when humidity was larger due to the softening action of water. Other studies by Delinière et al. [START_REF] Delinière | Physical, mineralogical and mechanical characterization of ready-mixed clay plaster[END_REF], Kouakou and Morel [START_REF] Kouakou | Strength and elasto-plastic properties of non-industrial building materials manufactured with clay as a natural binder[END_REF] and Taylor et al. [START_REF] Taylor | Compressive strength testing of earthen plasters for straw bale wall application[END_REF] have also provided experimental evidence in agreement with the conclusions of Xu et al. [START_REF] Xu | Loading-unloading shear behavior of rammed earth upon varying clay content and relative humidity conditions[END_REF]. Finally, Hamard et al. [START_REF] Hamard | A procedure to assess the suitability of plaster to protect vernacular earthen architecture[END_REF] reported that an increase in clay content does not always result in an increase of shear strength and that there is an optimum amount of clay that corresponds to a maximum value of shear strength. Beyond this optimum, a further increase of clay content leads to large material shrinkage and a reduction of shear strength. It must, however, be noted that the study by Hamard et al. [START_REF] Hamard | A procedure to assess the suitability of plaster to protect vernacular earthen architecture[END_REF] focused on ready-mixed clay plasters rather than compacted earth. Past studies have also shown that denser earth exhibits larger values of stiffness and strength [START_REF] Bruno | Mechanical behaviour of hypercompacted earth for building construction[END_REF][START_REF] Attom | The effect of compactive energy level on some soil properties[END_REF][START_REF] Bruno | Effect of very high compaction pressures on the physical and mechanical properties of earthen materials[END_REF][START_REF] Kouakou | Strength and elasto-plastic properties of nonindustrial building materials manufactured with clay as a natural binder[END_REF][START_REF] Olivier | Le matériau terre: Essai de compactage statique pour la fabrication de briques de terre compressées[END_REF]. Bruno et al. [START_REF] Bruno | Hygro-mechanical characterisation of hypercompacted earth for sustainable construction[END_REF] found that dry density increases less than linearly with growing compaction effort whereas strength and stiffness increase more than linearly with growing dry density. The strength of highly compacted earth is generally in the range 4.2-10 MPa [START_REF] Bruno | Hygro-mechanical characterisation of hypercompacted earth for sustainable construction[END_REF], which is comparable to the strength of chemically stabilised earth materials [START_REF] Walker | Properties of some cement stabilised compressed earth blocks and mortars[END_REF][START_REF] Morel | Compressive strength testing of compressed earth blocks[END_REF][START_REF] Deboucha | A review on bricks and stabilized compressed earth blocks[END_REF][START_REF] Muntohar | Engineering characteristics of the compressed-stabilized earth brick[END_REF]. Particle grading may therefore play an important role in the design of earth building materials as it governs the ability of the soil grains to assemble into a denser structure when compacted under a given effort. This paper explores the above issues by presenting an experimental investigation of the simultaneous effects of particle grading, dry density and ambient humidity on the mechanical behaviour of unstabilised earth building materials. Unlike previous laboratory studies, which have been mostly restricted to unconfined compression tests, this paper focuses on the measurement of stiffness and strength inside a triaxial cell under variable levels of radial confinement.

MATERIALS AND METHODS

EARTH CHARACTERISATION

The base soil used in the present work has been provided by the brickwork factory Bouisset from the region of Toulouse (France). Figure 1 shows the particle size distribution of this base soil [START_REF] Cuccurullo | Influence of particle grading on the hygromechanical properties of hypercompacted earth[END_REF] together with the recommended lower and upper limits according to current guidelines for the manufacture of compressed earth bricks [START_REF]Compressed earth blocks for walls and partitions: definitions -Specifications -Test methods -Delivery acceptance conditions[END_REF][START_REF]Compressed earth blocks: Standards -Technology series[END_REF][START_REF] Mopt | Bases Para el Diseño y Construcción con Tapial[END_REF]. [START_REF]Compressed earth blocks for walls and partitions: definitions -Specifications -Test methods -Delivery acceptance conditions[END_REF]; CRATerre-EAG [START_REF]Compressed earth blocks: Standards -Technology series[END_REF] and MOPT [START_REF] Mopt | Bases Para el Diseño y Construcción con Tapial[END_REF] (after Cuccurullo et al. [START_REF] Cuccurullo | Influence of particle grading on the hygromechanical properties of hypercompacted earth[END_REF]).

The main properties of the base soil, including the Atterberg limits of the fine fraction (i.e. the fraction smaller than 0.400 mm), were determined in a previous publication [START_REF] Cuccurullo | Influence of particle grading on the hygromechanical properties of hypercompacted earth[END_REF] and are summarised in Table 1. Past studies [START_REF] Tercruso | Caractérisation des briques de terre crue de Midi-Pyrénées[END_REF] have also indicated a predominantly kaolinitic clay fraction with a limited tendency to swell/shrink upon wetting/drying, which is advantageous for earth building. Figure 2 shows the plasticity properties of the soil with reference to the Casagrande chart, which classifies this material as a low plasticity clay [START_REF] Cuccurullo | Influence of particle grading on the hygromechanical properties of hypercompacted earth[END_REF]. Figure 2 also indicates that the soil fits inside the recommended plasticity region for the manufacture of compressed earth bricks according to AFNOR [START_REF]Compressed earth blocks for walls and partitions: definitions -Specifications -Test methods -Delivery acceptance conditions[END_REF]; CRATerre-EAG [START_REF]Compressed earth blocks: Standards -Technology series[END_REF] and Houben and Guillaud [START_REF] Houben | Earth construction -A comprehensive guide[END_REF]. compressed earth bricks by AFNOR [START_REF]Compressed earth blocks for walls and partitions: definitions -Specifications -Test methods -Delivery acceptance conditions[END_REF]; CRATerre-EAG [START_REF]Compressed earth blocks: Standards -Technology series[END_REF] and Houben and Guillaud [START_REF] Houben | Earth construction -A comprehensive guide[END_REF] (from Cuccurullo et al. [START_REF] Cuccurullo | The effect of ambient humidity on the stiffness and strength of a hyper-compacted silty clay for earth building[END_REF]).

The base soil was then blended with 68% of silica sand (by overall dry mass) to obtain a second earth mix with a clay content equal to the recommended minimum [START_REF] Cuccurullo | Influence of particle grading on the hygromechanical properties of hypercompacted earth[END_REF]. Figure 3 shows the particle size distribution of the added sand, whose grading is monodispersed with particle dimensions between 0.06 and 2 mm. compressed earth bricks by AFNOR [START_REF]Compressed earth blocks for walls and partitions: definitions -Specifications -Test methods -Delivery acceptance conditions[END_REF]; CRATerre-EAG [START_REF]Compressed earth blocks: Standards -Technology series[END_REF] and MOPT [START_REF] Mopt | Bases Para el Diseño y Construcción con Tapial[END_REF] (after Cuccurullo et al. [START_REF] Cuccurullo | Influence of particle grading on the hygromechanical properties of hypercompacted earth[END_REF]).

Table 2 summarizes the composition of the two earth building materials considered in this study, i.e. earth mix 1, which is the base soil, and earth mix 2, which is the blend of base soil and silica sand. Figure 4 shows the particle size distributions of these two mixes in relation to the recommended limits for the manufacture of compressed earth bricks [START_REF]Compressed earth blocks for walls and partitions: definitions -Specifications -Test methods -Delivery acceptance conditions[END_REF][START_REF]Compressed earth blocks: Standards -Technology series[END_REF][START_REF] Mopt | Bases Para el Diseño y Construcción con Tapial[END_REF]. Inspection of Figure 4 indicates that earth mix 1 exhibits a well-graded particle distribution, which is slightly finer than the upper limit and a clay content coinciding with the maximum recommended value. Conversely, earth mix 2 exhibits a poorly-graded particle size distribution, which cuts through the admissible band with a clay content corresponding to the minimum recommended value. The particle size distribution of earth mix 2 falls entirely inside the recommended grading band while that of earth mix 1 is slightly outside. The deviation of earth mix 1 from current guidelines is, however, not significant and both mixes are assumed to be compliant with existing recommendations. compressed earth bricks by AFNOR [START_REF]Compressed earth blocks for walls and partitions: definitions -Specifications -Test methods -Delivery acceptance conditions[END_REF]; CRATerre-EAG [START_REF]Compressed earth blocks: Standards -Technology series[END_REF] and MOPT [START_REF] Mopt | Bases Para el Diseño y Construcción con Tapial[END_REF] (after Cuccurullo et al. [START_REF] Cuccurullo | Influence of particle grading on the hygromechanical properties of hypercompacted earth[END_REF]).

Material Base soil percentage [%] Added sand percentage [%] Clay content [%]

EARTH COMPACTION

Figure 5 presents the standard Proctor compaction curve of earth mix 1, which was determined by Cuccurullo et al. [START_REF] Cuccurullo | The effect of ambient humidity on the stiffness and strength of a hyper-compacted silty clay for earth building[END_REF] in compliance with the French norm NF P94-093 [START_REF] Afnor | Soils: investigation and testing-Determination of the compaction reference values of a soil type-Standard proctor test-Modified proctor test[END_REF]. For ease of interpretation, Figure 5 also shows the equisaturation lines, which converge towards the ''no porosity'' point corresponding to a water content of zero and a dry density equal to that of the soil particles. Inspection of Figure 5 indicates a maximum dry density of 1.97 g/cm 3 , which corresponds to an optimum water content of 12.4%.

Figure 6 shows instead the hypercompaction curves for earth mixes 1 and 2, which were obtained by applying a large static vertical pressure of 100 MPa [START_REF] Cuccurullo | Influence of particle grading on the hygromechanical properties of hypercompacted earth[END_REF] to the sample inside a 50 mm diameter cylindrical mould using a load-controlled Zwick/Roell Amsler HB250 press with a capacity of 250 kN. The earth was compacted by two cylindrical pistons acting at the top and bottom as this increased the uniformity of compaction stress, and hence material fabric, across the sample height compared to the case of single compression where the load is applied on only one side. This is because, during double compression, the friction between the earth and the mould creates two opposite gradients of the compaction stress extending from each sample extremity to the middle section. Conversely, during single compression, the friction generates only one gradient of compaction stress extending between the two extremities of the sample. Additional details of the hypercompaction procedure are available in Cuccurullo et al. [START_REF] Cuccurullo | Influence of particle grading on the hygromechanical properties of hypercompacted earth[END_REF] and Bruno [START_REF] Bruno | Hygro-mechanical characterisation of hypercompacted earth for sustainable construction[END_REF].

Inspection of Figure 6 shows that the finer and well graded earth mix 1 exhibits considerably higher values of dry density compared to the coarser and poorly graded earth mix 2. The optimum water content is 4.88 % for earth mix 1 and 6.50 % for earth mix 2, while the maximum dry densities are 2.31 g/cm 3 and 2.12 g/cm 3 , respectively. Comparison of Figures 5 and6 also indicates that the hypercompaction procedure results in a significantly denser material with a considerably lower value of the optimum water content compared to standard Proctor compaction.

TRIAXIAL TESTING PROGRAM

Triaxial samples of 50 mm diameter and 100 mm height were manufactured by one-dimensional static compaction under either an equivalent Proctor load (Proctor compacted samples) or a pressure of 100 MPa (hypercompacted samples).

Proctor compacted samples of earth mix 1 were fabricated by sieving the dry material through a 2 mm mesh and subsequently mixing it with the optimum water content of 12.4 % (see Figure 5). The moist soil was then statically compacted in 10 layers to attain the maximum Proctor dry density (see Figure 5), taking care to scarify the surface of each layer before adding the next one. The hypercompacted samples of earth mixes 1 and 2 were instead fabricated at their respective optimum water contents, i.e. 4.88 % and 6.50 % (see Figure 6), following the procedure described in the previous section.

Twelve samples of each of the three materials (i.e. Proctor compacted earth mix 1, hypercompacted earth mix 1 and hypercompacted earth mix 2) were manufactured and divided into four sets of three samples. One set was oven-dried for three days at a temperature of 105 °C while the other three sets were equalised inside a climatic chamber at humidity levels of 25%, 62% and 95%, respectively, and constant temperature of 25 °C. The samples in the climatic chamber were weighed every day until equalisation, which was assumed to be complete when the sample mass changed less than 0.1 % over at least one week (this took generally 15 days). After equalisation, the average mass W and volume V of each set of three samples was measured. The total suction, was also determined from the imposed values of temperature, T and relative humidity, RH according to Kelvin's law as:

= - ln( ) (1)
where R is the gas constant and Vm is the molar volume of water. At the end of each triaxial test, three earth fragments of about 50 grams were taken at the top, middle and bottom of the sample to determine the corresponding water contents according to the French norm NF P 94-050 [START_REF]Soils: investigation and testing -Granulometric description -Wet sieving method[END_REF]. The average water content w was then calculated from these three measurements for each of the three samples equalised at the same humidity level. All measurements were generally very similar, thus confirming the uniformity of moisture content across the samples. By assuming no variation of moisture content during the triaxial tests, the average values of bulk density ρb, dry density ρd, void ratio e, degree of saturation Sr and porosity n at the start of the tests were calculated from the volume V, mass W and water content w (taking a specific gravity Gs equal to the value reported in Table 1) as:

= (2) = ( ) (3)
= 1 - (4)
=

Table 3 summarises the average properties at the start of the triaxial tests for each set of three samples equalised at the same humidity level or oven-dried. Recall that pore moisture is assumed to remain unchanged during the triaxial tests and, therefore, the values of water content in Table 3 are the same at the start and the end of the triaxial tests. Due to experimental problems, reliable measurements of water content could not be obtained for the hypercompacted samples of earth mix 2 equalised at the humidity levels of 62% and 95%, which explains the gaps in Table 3. The value of total suction of the oven-dry material is also absent from Table 3 as it could not be calculated from Equation 1 due to absence of information about the ambient humidity inside the furnace. Inspection of Table 3 indicates that the equalisation of earth mix 1 at distinct humidity levels produces distinct degrees of saturation and that the difference between degrees of saturation is more marked for the hypercompacted samples than for Proctor compacted ones. This also suggests that the sensitivity of degree of saturation to ambient humidity is higher for earth building materials than standard geotechnical fills due to the higher density of the former materials compared to the latter ones. Note also that these differences of degree of saturation correspond to distinct magnitudes of inter-particle capillary bonding and, therefore, distinct levels of strength and stiffness as discussed later in the paper.

The equalised or oven-dried samples were sheared inside a triaxial cell with an axial displacement rate of 0.06 mm/min. Throughout the triaxial tests, the back-pressure line was open to the atmosphere to allow the drainage of pore air from the unsaturated samples. The flow of vapour through the backpressure line was, however, considered negligible and the sample water content was therefore assumed constant. Shearing was continued until failure, which generally took between 23 and 35 minutes depending on the test. For each humidity level, three samples were sheared under different radial stresses of 0 kPa, 300 kPa and 600 kPa, respectively, to explore the effect of earth confinement inside thick walls.

Test results were subsequently processed to determine the initial Young's modulus and the peak strength for each confining pressure and humidity level. In particular, the initial Young's modulus was measured as the slope of the stress-strain curve over the low-pressure range, i.e. up to 20% of the peak strength, where the material response is reasonably linear.

Figure 7 shows the stress-strain curves measured during the triaxial tests of the hypercompacted samples of earth mix 1 equalised at distinct humidity levels. For each humidity level, three curves are reported corresponding to distinct degrees of lateral confinement. Inspection of Figure 7 indicates that, at a given humidity level, the peak strength increases by a margin of between 30% and 50% as the radial stress increases from zero to 600 kPa, which highlights the beneficial effect of lateral confinement on material strength.

Moreover, at a given confining pressure, the peak stress increases as the ambient humidity decreases, which provides further evidence of the inverse relationship between strength and degree of saturation due to the progressive formation of capillary menisci at particle contacts during desaturation. These capillary water menisci bond earth grains together and therefore enhance the strength of the material [START_REF] Beckett | The effect of humidity and temperature on the compressive strength of rammed earth[END_REF][START_REF] Lourenço | Formation and evolution of water menisci in unsaturated granular media[END_REF]. The largest levels of strength, from 15 MPa to more than 20 MPa, were measured on dry samples which should by definition contain no capillary water at all. These samples should, therefore, be no different from water saturated ones and should thus exhibit the lowest values of strength instead of the highest ones. This apparent contradiction is due to the conventional assumption of oven-dryness as the reference material state corresponding to complete absence of pore water. In reality, even inside ovendry samples there is a residual presence of adsorbed water subjected to extremely high tensile stresses, which firmly bonds earth particles together.

Inspection of Figure 7 also indicates that the material response changes from fragile to ductile as humidity increases, with the highest level of brittleness observed on the oven-dry samples. Therefore, an increase of ambient humidity produces a reduction of shear strength while enhancing the ability of the material to undergo plastic deformation before failure. Figure 8 presents the stress-strain curves measured during the triaxial tests of the hypercompacted samples of earth mix 2 showing, once again, that peak strength increases as relative humidity decreases at all confining pressures. As in the case of earth mix 1 (Figure 7), the highest strength levels were measured on the oven-dry samples with a maximum of about 6 MPa, which is, however, significantly lower than the corresponding strength of earth mix 1. Inspection of Figure 8 also confirms that, as relative humidity grows, the behaviour changes from fragile to ductile, thus increasing the ability of the material to deform plastically before failure. The beneficial effect of lateral confinement is more marked than in the previous case, with an up to six-fold increase of strength as the radial stress grows from zero to 600 kPa at constant humidity.

The comparison of the triaxial responses of earth mix 2 (Figure 8) and earth mix 1 (Figure 7) indicates that strength and brittleness are significantly lower in the former case compared to the latter one, despite the two mixes are both hypercompacted and exhibit particle size distributions that are both admissible according to existing guidelines (Figure 4). The main difference between these two mixes consists in the dispersion of grain sizes, which corresponds to a well-graded fine material in the case of earth mix 1 and a poorly-graded coarse material in the case of earth mix 2. This diversity of grading is also reflected in the significantly different levels of dry density for the same compaction effort and water content (Figure 6). Distinct materials inside the admissible grading band of Figure 4 can therefore generate very different mechanical responses even when compacted under similar conditions. In particular, the results from this study indicate that a fine well-graded earth generates higher strength levels than a coarse poorly-graded earth under similar compaction conditions. Thus, the role of particle grading may outstrip that of average particle size, which means that coarser materials will not always generate higher strength levels. Figure 9 shows the stress-strain curves of the triaxial tests on the Proctor samples of earth mix 1 from which the same conclusions as before can be drawn regarding the qualitative effect of ambient humidity on material strength. Inspection of Figure 9 also indicates an up to three-fold increase of strength with growing radial stress from 0 to 600 kPa at constant humidity, which represents an intermediate response compared to the previous two materials. Most importantly, the strength levels measured on the Proctor compacted samples of earth mix 1 are generally higher than those recorded on the hypercompacted samples of earth mix 2 (Figure 8), despite the former samples exhibit a markedly lower density than the latter ones. An earth material of relatively low density with a well-graded particle size distribution can therefore exhibit higher strength levels that those of a considerably denser material with a poorly-graded distribution. This confirms the key role of particle grading in enhancing material strength, a role which is more significant than that of material density. This important conclusion has not found adequate space in previous studies, which have instead focused on compaction effort as the main means of improving material strength.

Finally, a comparison between test results on Proctor compacted (Figure 9) and hypercompacted (Figure 7) samples of earth mix 1 shows that strength levels are significantly higher in the latter case than in the former one. This is expected as, for a given particle size distribution, a stronger compaction effort generates a larger material density and, hence, higher strength levels. Proctor compacted samples show, however, greater ductility compared to hypercompacted ones, which is consistent with previous observations of increasing ductility with decreasing strength, especially at high humidity levels. Figure 10 summarizes the evolution of strength (i.e. the peak axial stress) with relative humidity for the three materials subjected to confining pressures of 0 kPa, 300 kPa and 600 kPa. For earth mix 1, hypercompaction increases the sensitivity of material strength to humidity variations compared to Proctor compaction. This result is consistent with previous data in Table 3 showing that hypercompacted samples, equalised at different humidity levels, exhibit greater differences of degree of saturation than Proctor compacted ones. As the degree of saturation is inversely related to the inter-particle capillary bonding, a larger variation of degree of saturation with changing humidity should correspond to a larger variation of strength. The effect of degree of saturation on the mechanical properties of each material can be synthetically described by comparing their strength and stiffness envelopes at distinct humidity levels.

The strength envelopes at constant humidity are obtained by plotting the values of peak deviator stress " measured from the three triaxial tests under different radial stresses against the corresponding values of mean stress #. These experimental data are then interpolated by the following linear equation:

" = $ % & # (7)
where the coefficients C and M are respectively the intercept and slope of the strength envelope at each humidity level. The above coefficients can also be converted indicates that an increase of ambient humidity produces a decrease of friction angle that is more marked in the hypercompacted samples than in the Proctor compacted ones. Among the hypercompacted samples, the decrease is most evident for earth mix 1 as, in this case, the value of the friction angle decreases from 56.6° to 30.8° as the humidity level increases from oven-dry conditions to 95% (Table 4). As for the Proctor compacted samples of earth mix 1, the friction angle changes relatively little with ambient humidity as shown by the approximately parallel strength envelopes of Figure 13.

Conversely, a variation of ambient humidity produces a change of cohesion that, in relative terms, is more modest for the hypercompacted samples (Tables 4 and5) than for the Proctor compacted ones (Table 6). This is graphically shown in Figures 11 and12 where the strength envelopes of the hypercompacted materials tend to converge towards a narrow area as the mean stress reduces towards zero. The trend is clearest for the hypercompacted earth mix 1, whose cohesion is approximately constant at all humidity levels (Table 4) -except for a deviation at a humidity of 95 % when a larger scatter of data is also observed.

CONCLUSIONS

The mechanical properties of compacted soils strongly depend on dry density and this emphasizes the specific nature of earth building materials, which are generally much denser than standard geotechnical fills. This paper has presented an experimental investigation of the simultaneous effects of dry density, ambient humidity and particle grading on the mechanical behaviour of three earth building materials manufactured from soils with markedly distinct particle size distributions and compacted under significantly different efforts. From this investigation, particle grading has emerged as a key parameter, whose influence on the mechanical performance of earth building materials has often been overlooked but it appears even more important than that of dry density or average particle size. This study has, for example, shown that the strength and stiffness of a Proctor compacted well-graded fine earth can be considerably higher than those of a hypercompacted poorly-graded coarse earth, despite the density of the former material is much lower than that of the latter one. This result is even more surprising if one considers that both those earths exhibit index properties (i.e. particle size distribution and plasticity characteristics) that comply with current earth building recommendations. The observed differences of strength and stiffness must therefore be entirely ascribed to disparities of particle grading. This finding should prompt further research on the optimisation of earth mixes for building applications together with a revision of current guidelines for the selection of suitable materials.

In line with previous investigations, the present study has also found that: a) material ductility grows with decreasing strength and increasing ambient humidity, b) mechanical characteristics tend to improve with growing dry density and c) growing levels of ambient humidity produce an increase of degree of saturation with a consequent deterioration of strength and stiffness. Finally, shear strength increases significantly as the confining stress grows at all humidity levels, which highlights the beneficial effect of the lateral confinement inside thick walls. Conversely, material stiffness remains generally constant as lateral confinement increases at any level of ambient humidity. The magnitude of the increase of shear strength with lateral confinement depends on both particle grading and material density. In this work, it has been found that the beneficial effect of lateral confinement becomes more evident as dry density decrease and particle grading becomes poorer.

Figure 1 :

 1 Figure 1: Particle size distribution of the base soil in relation to existing recommendations for the manufacture of compressed earth bricks by AFNOR[START_REF]Compressed earth blocks for walls and partitions: definitions -Specifications -Test methods -Delivery acceptance conditions[END_REF]; CRATerre-EAG[START_REF]Compressed earth blocks: Standards -Technology series[END_REF] and MOPT[START_REF] Mopt | Bases Para el Diseño y Construcción con Tapial[END_REF] (after Cuccurullo et al.[START_REF] Cuccurullo | Influence of particle grading on the hygromechanical properties of hypercompacted earth[END_REF]).

Figure 2 :

 2 Figure 2: Plasticity properties of the base soil in relation to existing recommendations for the manufacture of

Figure 3 :

 3 Figure 3: Particle size distribution of added sand in relation to existing recommendations for the manufacture of

Figure 4 :

 4 Figure 4: Particle size distribution of earth mixes in relation to existing recommendations for the manufacture of

Figure 5 :

 5 Figure 5: Standard Proctor compaction curve for earth mix 1 (after Cuccurullo et al. [38]).

Figure 6 :

 6 Figure 6: Hypercompaction curves, corresponding to the application of a static pressure of 100 MPa, for earth mixes 1 and 2 (after Cuccurullo et al. [32]).

Figure 7 :

 7 Figure 7: Results from triaxial tests on the hypercompacted earth mix 1 at different confining pressures and distinct humidity levels: oven-dry (a), 25 % (b), 62 % (c), 95 % (d).

Figure 8 :

 8 Figure 8: Results from triaxial tests on the hypercompacted earth mix 2 at different confining pressures and distinct humidity levels: dry (a), 25 % (b), 62 % (c), 95 % (d).

Figure 9 :

 9 Figure 9: Results from triaxial tests on the Proctor compacted earth mix 1 at different confining pressures and distinct humidity levels: dry (a), 25 % (b), 62 % (c), 95 % (d).

Figure 10 :

 10 Figure 10: Evolution of strength (i.e. peak axial stress) with relative humidity at different confining pressures: unconfined (a), 300 kPa (b) and 600 kPa (c).

Figures 11 ,

 11 Figures 11, 12 and 13 show the strength envelopes of the hypercompacted earth mix 1, hypercompacted earth mix 2 and Proctor compacted earth mix 1, respectively. Similarly, Tables 4, 5 and 6 summarise the strength parameters of the hypercompacted earth mix 1, hypercompacted earth mix 2 and Proctor compacted earth mix 1, respectively.

Figure 11 :

 11 Figure 11: Strength envelopes of hypercompacted earth mix 1 at different humidity levels.

Figure 12 :

 12 Figure 12: Strength envelopes of hypercompacted earth mix 2 at different humidity levels.

Figure 13 :

 13 Figure 13: Strength envelopes of Proctor compacted earth mix 1 at different humidity levels.

Figures 14 ,

 14 Figures 14, 15 and 16 show the stiffness envelopes of the hypercompacted earth mix 1, hypercompacted earth mix 2 and Proctor compacted earth mix 1, respectively. These envelopes are obtained by plotting the values of the initial Young's modulus 1, measured from the three triaxial tests under different confinement levels, against the corresponding values of radial stress 2 at each humidity level. Inspection of Figures 14, 15and 16 indicates that, as already observed for the strength, an increase of ambient humidity produces a decrease of stiffness. The effect of confinement on stiffness is less evident compared to strength as the Young's modulus remains relatively constant with growing radial stress at constant humidity. Only in the case of the oven-dry samples there is a clear increase of stiffness with growing radial confinement. In general, stiffness measurements present a larger scatter and a more uncertain trend compared to strength measurements, which reflects the relatively high inaccuracies associated to the determination of the initial Young's modulus.

Figure 14 :

 14 Figure 14: Stiffness envelopes of hypercompacted earth mix 1 at different humidity levels.

Figure 15 :

 15 Figure 15: Stiffness envelopes of hypercompacted earth mix 2 at different humidity levels.

Figure 16 :

 16 Figure 16: Stiffness envelopes of Proctor compacted earth mix 1 at different humidity levels.

Table 1 :

 1 Main properties of the base soil (from Cuccurullo et al.[START_REF] Cuccurullo | Influence of particle grading on the hygromechanical properties of hypercompacted earth[END_REF]).

	Particle size distribution		Atterberg limits
	Gravel content (> 2 mm, %)	0	Plastic limit wP (%)	18.7
	Sand content (≤ 2 mm, %)	31	Liquid limit wL (%)	29.0
	Silt content (≤ 63 μm, %)	35	Plasticity index IP (%)	10.3
	Clay content (≤ 2 μm, %)	34	Mineralogical composition
	Clay activity A (-)	0.30	Goethite, Muscovite, Orthose, Kaolinite,
	Specific gravity Gs (-)	2.65	Quartz, Calcite

Table 2 :

 2 Composition of the two earth materials tested in the present work (after Cuccurullo et al.[START_REF] Cuccurullo | Influence of particle grading on the hygromechanical properties of hypercompacted earth[END_REF]).

Table 3 :

 3 Average samples properties at the start of the triaxial test, i.e. after equalisation at different humidity levels or oven-drying.

	Relative humidity, RH [%]	Bulk density, ρb [g/cm 3 ]	Water content, w [%]	Dry density, ρd [g/cm 3 ]	Porosity, n [%]	Degree of saturation, Sr [%]	Total suction, ! [MPa]
			Hypercompacted earth mix 1
	Oven-dry	2.28	0	2.28	14.1	0	-
	RH = 25 %	2.31	0.68	2.29	13.4	11.7	190
	RH = 62 %	2.33	2.24	2.28	13.9	36.7	65
	RH = 95 %	2.38	4.61	2.28	14.0	74.9	7
			Hypercompacted earth mix 2
	Oven-dry	2.12	0	2.12	20.1	0	-
	RH = 25 %	2.12	0.36	2.11	20.3	3.78	190
	RH = 62 %	2.15	-	-	-	-	65
	RH = 95 %	2.13	-	-	-	-	7
			Proctor compacted earth mix 1
	Oven-dry	1.95	0	1.95	26.3	0	-
	RH = 25 %	1.99	0.88	1.97	25.6	6.76	190
	RH = 62 %	1.98	2.43	1.93	27.1	17.3	65
	RH = 95 %	2.09	4.91	1.99	24.9	39.3	7

Table 4 :

 4 Strength parameters of hypercompacted earth mix 1 at different humidity levels.

		M [-]	φ [°]	C [MPa]	c [MPa]
	OVEN-DRY	2.31	56.6	3.53	2.31
	RH = 25 %	1.74	42.3	4.20	2.20
	RH = 62 %	1.12	28.2	4.77	2.28
	RH = 95 %	1.24	30.8	3.15	1.52

Table 5 :

 5 Strength parameters of hypercompacted earth mix 2 at different humidity levels.

		M [-]	φ [°]	C [MPa]	c [MPa]
	OVEN-DRY	1.96	47.4	0.60	0.33
	RH = 25 %	1.88	45.7	0.38	0.21
	RH = 62 %	1.80	43.9	0.32	0.18
	RH = 95 %	1.70	41.4	0.34	0.17

Table 6 :

 6 Strength parameters of Proctor compacted earth mix 1 at different humidity levels.

		M [-]	φ [°]	C [MPa]	c [MPa]
	OVEN-DRY	1.72	41.9	1.94	1.01
	RH = 25 %	1.53	37.5	1.46	0.73
	RH = 62 %	1.41	34.9	1.22	0.60
	RH = 95 %	1.53	37.6	0.56	0.29
	Inspection of Figures 11-13 and Tables 4-6

ACKNOWLEDGEMENTS

The authors wish to acknowledge the support of the European Commission via the Marie Skłodowska-Curie Innovative Training Networks (ITN-ETN) project TERRE 'Training Engineers and Researchers to Rethink geotechnical Engineering for a low carbon future' (H2020-MSCA-ITN-2015-675762).


E Gürbüz

V Albin

V Lair

A Ringuedé

M Cassir

OXIDATION BEHAVIOR OF H2 AND CO PRODUCED BY H2O AND/OR CO2 REDUCTION IN MOLTEN CARBONATES: EFFECT OF GAS ENVIRONMENT AND HYDROXIDES

Keywords: Molten Carbonate, Hydroxide additives, Electrochemistry, Conductivity, H2O/CO2 reduction

Co-electrolysis of water and carbon dioxide in molten carbonates is nowadays a key issue in MCEC (molten carbonate electrolysis cell) and, reversely, for a better understanding of MCFC (molten carbonate fuel cell). The products of water and carbon dioxide reduction are the fuels that might be used in MCFC or other energy devices. The present study is dedicated to a thorough electrochemical investigation by voltammetric techniques (Cyclic Voltammetry CV or Linear Sweep Voltammetry LSV) at a Pt electrode of such fuels in Li2CO3-K2CO3 eutectic, under varied atmosphere conditions, including partial pressures of H2O, CO2, H2 and Ar at 650 °C. The conditions of formation of soluble and/or adsorbed H2 and CO are established together with qualitative thermodynamics information on the redox systems involved. C is only detected in very specific conditions. Knowing that hydroxides, produced in the conditions of MCEC and MCFC, have a significant role on the operation of such devices, an electrochemical analysis (CV and impedance measurements) of the effect of added amounts of hydroxides is developed for the first time, showing the enhancement of oxidation currents and the progressive increase in electrolyte conductivity.

Introduction

Water electrolysis and, more recently, H2O and CO2 co-electrolysis in molten carbonate electrolytes, MCEC (molten carbonate electrolysis cell), have attracted the interest of many research teams all over the world [START_REF] Lorenz | Electrolysis of molten carbonates: anodic and cathodic gasevolving reactions[END_REF][START_REF] Kaplan | Conversion of CO2 to CO by Electrolysis of Molten Lithium Carbonate[END_REF][START_REF] Frangini | A Novel Process for Solar Hydrogen Production Based on Water Electrolysis in Alkali Molten Carbonates[END_REF][START_REF] Hu | Electrochemical performance of reversible molten carbonate fuel cells[END_REF][START_REF] Hu | Performance and Durability of the Molten Carbonate Electrolysis Cell and the Reversible Molten Carbonate Fuel Cell[END_REF][START_REF] Hu | Electrode Kinetics of the Ni Porous Electrode for Hydrogen Production in a Molten Carbonate Electrolysis Cell (MCEC)[END_REF][START_REF] Hu | Electrode kinetics of the NiO porous electrode for oxygen production in the molten carbonate electrolysis cell (MCEC)[END_REF][START_REF] Chery | Overview on CO2 Valorization: Challenge of Molten Carbonates[END_REF][START_REF] Chery | Thermodynamic and experimental approach of electrochemical reduction of CO2 in molten carbonates[END_REF][START_REF] Chery | CO2 electrochemical reduction into CO or C in molten carbonates: a thermodynamic point of view[END_REF][START_REF] Chery | Mechanistic approach of the electrochemical reduction of CO2 into CO at a gold electrode in molten carbonates by cyclic voltammetry[END_REF][START_REF] Meskine | CO2 electrolysis in a reversible molten carbonate fuel cell: Online chromatographic detection of CO[END_REF][START_REF] Perez-Trujillo | A numerical and experimental comparison of a single reversible molten carbonate cell operating in fuel cell mode and electrolysis mode[END_REF][START_REF] Pérez-Trujillo | Preliminary theoretical and experimental analysis of a Molten Carbonate Fuel Cell operating in reversible mode[END_REF][START_REF] Wu | A New Technology for Efficient, High Yield Carbon Dioxide and Water Transformation to Methane by Electrolysis in Molten Salts[END_REF][START_REF] Liu | Syngas production: diverse H2/CO range by regulating carbonates electrolyte composition from CO2 /H2O via co-electrolysis in eutectic molten salts[END_REF][START_REF] Ji | The optimization of electrolyte composition for CH4 and H2 generation via CO2/H2O co-electrolysis in eutectic molten salts[END_REF][START_REF] Li | Carbon dioxide electrolysis and carbon deposition in alkaline-earth-carbonate-included molten salts electrolyzer[END_REF][START_REF] Licht | Comparison of Alternative Molten Electrolytes for Water Splitting to Generate Hydrogen Fuel[END_REF]. The complex reduction of CO2 and H2O has been addressed here and there, but there are still many major aspects to investigate thoroughly; in particular, when combining water and carbon dioxide in different ratios, including hydrogen and inert gas atmospheres. Moreover, the problem of CO2 capture and purification/valorization by electrolysis in molten carbonates is becoming a major issue [START_REF] Meléndez-Ceballos | Input on the Measurement and Comprehension of CO2 Solubility in Molten Alkali Carbonates in View of Its Valorization[END_REF][START_REF]IPCC Special report on carbon dioxide capture and storage[END_REF][START_REF] Cassir | Strategies and new developments in the field of molten carbonates and high-temperature fuel cells in the carbon cycle[END_REF][START_REF]Fuel Cell Energy, Carbon Capture with Direct Fuel Cell Carbonate Powerplants[END_REF][START_REF]Fuel cell energy announces new carbon capture project with drax power station[END_REF].

In parallel, the role of hydroxides, formed in situ or added, has been pointed out in different electrochemical devices involving molten carbonates: MCEC/MCFC (molten carbonate fuel cell), DCFC (direct carbon fuel cell) and MCFC/SOFC (solid oxide fuel cell) hybrid systems [START_REF] Gürbüz | Significance of Molten Hydroxides With or Without Molten Carbonates in High-Temperature Electrochemical Devices[END_REF][START_REF] Rosen | Molten Carbonate Fuel Cell Performance for CO2 Capture from Natural Gas Combined Cycle Flue Gas[END_REF][START_REF] Audasso | New, Dual-Anion Mechanism for Molten Carbonate Fuel Cells Working as Carbon Capture Devices[END_REF][START_REF] Arato | Kinetic modelling of molten carbonate fuel cells: Effects of cathode water and electrode materials[END_REF][START_REF] Audasso | Molten Carbonate Fuel Cell performance analysis varying cathode operating conditions for carbon capture applications[END_REF][START_REF] Evans | Electromotive Force (emf) Determination of Transport Numbers for Native and Foreign Ions in Molten Alkali Metal Carbonates[END_REF][START_REF] Nishina | Gas electrode reactions in molten carbonate media: IV. Electrode Kinetics and Mechanism of Hydrogen Oxidation in (Li+K)CO3 Eutectic[END_REF]. Thus, elucidation of the mechanisms involving the participation of hydroxides in molten carbonates is compulsory for optimizing the operation of the mentioned fuel cell/electrolyzer systems. Although some advances can be outlined on how molten hydroxides may act, in particular for MCFC application, no fundamental electrochemical study has ever been developed in the literature.

The direct analysis by voltammetric techniques of H2O reduction in molten carbonates is difficult to identify thoroughly because of its proximity to the reduction limit (due to carbonates reduction), so we focussed specifically on the re-oxidation of the hydrogen produced. CO2 reduction has been investigated in various papers. At a gold electrode, a mechanistic approach has been established, but gold can be partially passivated in carbonate media, which conduced us to use a Pt electrode where both H2O and CO2 reduction are close to the reduction limit [START_REF] Chery | Mechanistic approach of the electrochemical reduction of CO2 into CO at a gold electrode in molten carbonates by cyclic voltammetry[END_REF][START_REF] Meskine | Electrochemical investigations on CO2 reduction mechanism in molten carbonates in view of H2O/CO2 co-electrolysis[END_REF]. Due to such considerations, we analysed specifically the reoxidation process which corresponds to fuel oxidation in fuel cells.

Thus, the present study is dedicated in a first part to the systematic analysis by cyclic voltammetry or linear sweep voltammetry (LSV) of the oxidation behaviour of the products of CO2 and H2O reduction in varied gaseous atmospheres (CO2, H2O, H2) in Li2CO3-K2CO3 eutectic using a Pt working electrode as in a recent publication [START_REF] Meskine | Electrochemical investigations on CO2 reduction mechanism in molten carbonates in view of H2O/CO2 co-electrolysis[END_REF]. The second part is a new approach dedicated to the influence of the addition of hydroxides in the molten carbonate eutectic through conductivity and cyclic voltammetry measurements.

II-Experimental

The electrochemical cell, an alumina cylindrical crucible placed in an alumina reactor, was described elsewhere [START_REF] Chery | CO2 electrochemical reduction into CO or C in molten carbonates: a thermodynamic point of view[END_REF][START_REF] Meléndez-Ceballos | Influence of Cs and Rb additions in Li-K and Li-Na molten carbonates on the behaviour of a commercial MCFC nickel cathode[END_REF]. The electrolyte consists of Li2CO3 and K2CO3 (Sigma-Aldrich, purity > 98%) at the eutectic composition (62:38 mol%). Carbonate powders are dried at 120 °C for 24 h before heating at 650 °C under CO2 (purity 99.99%). The system operates at atmospheric pressure. After any change in the atmosphere, a period of at least 4 h is left before analysis to ensure equilibrium. The 3-electrode setup was depicted previously [START_REF] Cassir | Stability and Characterization of Oxygen Species in Alkali Molten Carbonate: A Thermodynamic and Electrochemical Approach[END_REF]. A Pt plate was used as the working electrode (1*1 cm 2 , AMTS, 99.95% purity). Prior to each measurement, the platinum plates were polished on SiC grinding paper P1200, P2400 and P4000 at 300 rpm and rinsed with Milli-Q purified water. An Autolab PGSTAT30 potentiostat controlled by either the GPES or NOVA software was used. Water pressure was obtained by bubbling the other gases in a temperature-controlled flask containing water.

Conductivity measurements were carried out by impedance spectroscopy with two gold planar electrodes set in parallel. The electrodes were similarly previously polished. Their dimensions are 1*1 cm 2 , 0,5 mm thickness, and the distance between them is 2 cm. Measurements were carried out using the same potentiostat. The parameters used were the following: scanning frequencies between 0.1 and 10 5 Hz and 19 points per decade. Three different signal amplitudes were used: 5 mV, 50 mV and 100 mV. For each of them, the measurement was repeated at least 3 times to ensure repeatability.

III-Results and discussion

Effect of gas environment

In order to clearly identify the re-oxidation species of either H2O or CO2 reduction, the electrochemical experiments were carried out in gas atmospheres containing varied proportions of CO2, H2O and inert gas (Ar). The reactions of interest for co-electrolysis are given below:

+ 2 → + (1)
+ 2 → + (2)
As a first step, we established a water environment (slightly diluted with Argon) and investigated the oxidation of hydrogen formed by H2O reduction in the absence of CO2. As observed in Fig. 1, the cyclic voltammogram reveals that water reduction at Pt electrode is difficult to analyze due to the lack of a well-defined wave, which is roughly the same at Au electrode. Nevertheless, two oxidation peaks appear during the reverse scan, one around -1.1

V vs Ag + /Ag and the other close to -0.5 V vs Ag + /Ag. These two peaks can be attributed to the oxidation of hydrogen proceeding from H2O reduction, consistently with studies on H2 oxidation in the literature, either at Au or Pt electrodes [START_REF] Nishina | Gas electrode reactions in molten carbonate media: IV. Electrode Kinetics and Mechanism of Hydrogen Oxidation in (Li+K)CO3 Eutectic[END_REF][START_REF] Cassir | Study of cerium species in molten Li2CO3-Na2CO3 in the conditions used in molten carbonate fuel cells. Part II: potentiometric & voltammetric behavior[END_REF]40]. Indeed, no other reactant is present, and the cathodic reverse potential (-1.8 V vs Ag + /Ag) is too low to consider a significant reduction of carbonates at a Pt electrode [START_REF] Meskine | Electrochemical investigations on CO2 reduction mechanism in molten carbonates in view of H2O/CO2 co-electrolysis[END_REF]. Besides, no C re-oxidation peak is observed, as well as no solid deposit can be identified at the electrode surface. Scan rate 100 mV s -1 . Reverse potential at -1.8 V and -2 V vs Ag + /Ag.

An example of carbon deposition, that tends to occur in oxobasic conditions but can happen in any case provided the reverse reduction potentials are negative enough, is given in Figure 2. At a reverse potential of -2 V vs Ag + /Ag, one can observe the intersection of the forward and reverse scan indicating a solid deposit, with an intense peak visible at around -1.1 V vs Ag + /Ag and a smaller one at -1.0 V vs Ag + /Ag, which does not appear when the cathodic reverse potential is lower (red curve). This peak is attributed to the oxidation of C, in agreement with a previous study [START_REF] Ijije | Electro-deposition and re-oxidation of carbon in carbonate-containing molten salts[END_REF]. The doublet could be due to two different carbon oxidation phenomena at the Pt polycrystalline surface. At a reverse potential of -1.8 V vs Ag + /Ag, the two peaks corresponding to C oxidation disappeared and only those relative to CO oxidation are visible (roughly here at -0.9 and -0.4 V and vs Ag + /Ag). In previous studies in similar conditions at both Au or Pt electrodes, we have attributed such oxidation reactions to COsol/CO2sol (both species dissolved) and COads/CO2ads (both adsorbed), respectively [START_REF] Chery | Mechanistic approach of the electrochemical reduction of CO2 into CO at a gold electrode in molten carbonates by cyclic voltammetry[END_REF][START_REF] Meskine | Electrochemical investigations on CO2 reduction mechanism in molten carbonates in view of H2O/CO2 co-electrolysis[END_REF]. These systems are also visible when the reverse potential is -2 V vs Ag + /Ag.

Besides, CO2 reduction mechanism was also discussed in a previous article [START_REF] Meskine | Electrochemical investigations on CO2 reduction mechanism in molten carbonates in view of H2O/CO2 co-electrolysis[END_REF]. It was found that the mechanism in Li-K at 650 °C over Pt is as follows: )

E (V) E i + ↔ (3) 2 ↔ + (4)
As dissolved species are considered for CO2 reduction/ CO oxidation, it is also important to discuss speciation. It has been shown that both CO2 and CO can form intermediates in molten carbonates [START_REF] Carof | Carbon species solvated in molten carbonate electrolyser cell from first-principles simulations[END_REF][START_REF] Corradini | Carbon dioxide transport in molten calcium carbonate occurs through an oxo-Grotthuss mechanism via a pyrocarbonate anion[END_REF].

+ ↔ (5)
+ ↔ (6)
The existence of the carbonite ion has also been suggested [START_REF] Claes | Solubility of CO2 in the molten Na2CO3-K2CO3 (42 mol %) eutectic mixture at 800°C[END_REF].

+ ↔ + (7)
Even though CO2 can be in a pyrocarbonate form around 20 % of the time, the ion only has a lifetime of some ps according to a recent molecular dynamics study [START_REF] Carof | Carbon species solvated in molten carbonate electrolyser cell from first-principles simulations[END_REF]. Oxalates were found to be stable though reaction 6 did not spontaneously happen. Finally, a recent Raman investigation in Li-Na eutectic also showed CO4 2-and HCO4 -as active species under wet gas inlet [START_REF] Zhang | Identification of Active Surface Species in Molten Carbonates Using in situ Raman Spectroscopy[END_REF]. As for the speciation of water itself, it will be discussed later in the hydroxide part.

Coming back to the oxidation peaks of Fig. 1, ascribed to hydrogen, a complementary study by Linear Sweep Voltammetry under hydrogen, as shown in Fig. 3 was conducted to thoroughly confirm this hypothesis. As can be seen, two peaks appear at around -1 and -0.5

V vs Ag + /Ag (the second complex one being composed of three overlapped peaks), when only the oxidation sweep is investigated. When this same LSV experiment is conducted in the absence of hydrogen, no peaks are visible, and the same LSV can also be obtained over gold (not shown here), proving the oxidation phenomena to be unrelated to Pt. To better understand such phenomena, the scan rate was varied between 100 mV/s and 700 mV/s (in the case of the complex second peak, only the most intense component was analyzed).

Figure 4 below reveals the evolution of both peak intensities and potentials with respect to scan rate.

- Both peak potentials present a slight variation with scan rate, indicating quasi-rapid phenomena. The first peak intensity presents a linear dependence on the square root of the scan rate (R 2 = 0.994), whereas the second peak intensity varies linearly with scan rate (R 2 = 0.995). Thus the first peak corresponds to a diffusion-limited process while the second is adsorption-controlled. Previous studies only assigned a peak around -1.1 V (over Au or Pt)

to the oxidation of hydrogen, which would represent the first peak observed here [START_REF] Nishina | Gas electrode reactions in molten carbonate media: IV. Electrode Kinetics and Mechanism of Hydrogen Oxidation in (Li+K)CO3 Eutectic[END_REF]40].

We observe two peaks related to this phenomenon, and the variation with scan rate seems to indicate that the first peak is oxidation of soluble H2, and the second adsorbed H2. The first oxidation peak, representing dissolved hydrogen, involves (as any electrochemical phenomenon) a very weak hydrogen intermediate adsorption, while the second one a strong adsorption of hydrogen requiring a higher oxidation potential (additional energy). In fact, we can generalize to the three components around -0.5 V vs Ag + /Ag: they are due to hydrogen adsorbed at three different hydrogen adsorption sites at the Pt polycrystalline surface.

From the obtained intensities, it appears that soluble H2 is hard to visualize without reducing water first, in other words having H2 directly formed in the vicinity of the electrode. Figure 5, where this peak is more intense, also highlights this point. Finally, a study under water and different amounts of CO2 was conducted to visualize and separate CO and H2 peaks. As soon as little amount of CO2 is added, 2 peaks around -1 V vs Ag + /Ag instead of one can be distinguished, a peak at a potential a little lower than -1 V vs Ag + /Ag appearing. This peak has been reported in previous literature as oxidation of soluble CO [START_REF] Meskine | Electrochemical investigations on CO2 reduction mechanism in molten carbonates in view of H2O/CO2 co-electrolysis[END_REF].

undoubtedly the oxidation of H2 that was previously seen. As soon as 5 mol% CO2 is added, the peak seemingly shifts toward anodic potentials. This could either be simply due to the CO2 pressure effect on potential, or the adsorbed CO oxidation also taking place. It is possible that both species compete for adsorption on the electrode surface.

As oxidation peaks found around -1 V vs Ag + /Ag attributed to C, H2 and CO are very close, studies become more complex and it is of high importance to carefully analyze the gas compositions, oxoacidity, potential domain in order to identify the suitable reactions. That is why studies in the literature focused either on CO2 reduction (and subsequent C or CO oxidation) [START_REF] Meskine | Electrochemical investigations on CO2 reduction mechanism in molten carbonates in view of H2O/CO2 co-electrolysis[END_REF][START_REF] Ijije | Electro-deposition and re-oxidation of carbon in carbonate-containing molten salts[END_REF] or H2 oxidation [START_REF] Nishina | Gas electrode reactions in molten carbonate media: IV. Electrode Kinetics and Mechanism of Hydrogen Oxidation in (Li+K)CO3 Eutectic[END_REF], and it has been mentioned that cyclic voltammograms featuring oxidation of CO and oxidation of H2 are qualitatively identical [40]. To gain better understanding of the reactions and their potentials, a potential-oxoacidity diagram can be considered. This type of diagram has been previously reported in the literature [START_REF] Chery | CO2 electrochemical reduction into CO or C in molten carbonates: a thermodynamic point of view[END_REF]. In our case, the calculations were done at 650 °C for the Li-K (62 :38 mol%) eutectic, which were the conditions used for electrochemical study, based on data from NIST-JANAF [START_REF] Chase | NIST-JANAF Thermochemical Tables. 4th ed[END_REF]. The resulting diagram is given in Figure 6. In oxoacidic conditions, it is possible to reduce CO2 into CO and H2O into H2 at relatively low potentials. It is not possible to predict which species reduces first, as the potentials depend on the pressure of CO and H2, that are most of the time unknown. However, both reactions should occur at potentials close to each other, barring kinetic issues. This indeed highlights the intriguing possibility of CO2 and H2O co-electrolysis into syngas. Oxoacidic conditions are more favorable thermodynamically, but in oxobasic conditions, both reactions are still possible, even though depending on the pressure of CO, reduction into carbon, which is undesirable in our case, could be more dominant.

At standard MCFC anode (MCEC cathode) conditions, 64 mol% H2/16 mol% CO2/20 mol% H2O, water should thermodynamically reduce at -0.69 V vs Li2O/O2 and at this condition CO2 will reduce beforehand unless the CO pressure is higher than 2.5 x 10 -1 bar, which is unlikely considering no CO is fed.

Effect of the addition of hydroxides

Previous studies on the H2 oxidation have led to three main mechanisms suggested by Ang and Sammels [START_REF] Ang | Influence of Electrolyte Composition on Electrode Kinetics in the Molten Carbonate Fuel Cell[END_REF], Jewulski and Suski [START_REF] Jewulski | Model of the isotropic anode in the molten carbonate fuel cell[END_REF], and White and Twardoch [START_REF] White | The electrochemical behaviour of solutions of molten ternary alkali carbonate mixture equilibrated with carbon dioxide-water mixtures at 460°C[END_REF][START_REF] White | The Behavior of Water in Molten Salts[END_REF]. The first two mechanisms consider hydroxides as an intermediate species, while the last one suggests bicarbonates. Nishina et al. studied the H2 oxidation in the Li-K eutectic at 650° C, at various metal electrodes, using cyclic voltammetry, electrochemical impedance spectroscopy and chronocoulometry [START_REF] Nishina | Gas electrode reactions in molten carbonate media: IV. Electrode Kinetics and Mechanism of Hydrogen Oxidation in (Li+K)CO3 Eutectic[END_REF]. They compared their findings to the three pathways proposed in the literature, only to conclude that the H2 oxidation mechanism in their case was better described by the hydroxide mechanism suggested by Jewulski and Suski, which is:

↔ 2 (8) 2 { + ↔ + + } (9) 2 + ↔ + (10)
Applying the principle of microscopic reversibility, the water reduction mechanism in molten carbonates would be the exact opposite of the hydrogen oxidation one [START_REF] Frangini | A Novel Process for Solar Hydrogen Production Based on Water Electrolysis in Alkali Molten Carbonates[END_REF].

Another reason why hydroxides are an important species is that they are the associated oxobase of water, just as carbonates are the oxobase of CO2. The autodissociation equilibrium is given in Reaction 11.

2 ↔ + (11)
In such case, they are likely to be present in the melt under water-containing atmospheres, as shown in the literature [START_REF] Frangini | A Novel Process for Solar Hydrogen Production Based on Water Electrolysis in Alkali Molten Carbonates[END_REF][START_REF] Rosen | Molten Carbonate Fuel Cell Performance for CO2 Capture from Natural Gas Combined Cycle Flue Gas[END_REF][START_REF] Audasso | New, Dual-Anion Mechanism for Molten Carbonate Fuel Cells Working as Carbon Capture Devices[END_REF]. Indeed, even though it is generally accepted that water can form various dissolved species in molten carbonates, such as hydroxides and bicarbonates, it appears that the dominant species at 650 °C are hydroxides. Thermodynamical calculations relative to sodium hydroxide reaction into sodium bicarbonate have been carried out in the literature [START_REF] Frangini | A Novel Process for Solar Hydrogen Production Based on Water Electrolysis in Alkali Molten Carbonates[END_REF].

+ ↔ (12)
They revealed through equilibrium concentration calculations that hydroxides are the dominant species above 550 °C, consistently with the proposed hydrogen oxidation mechanism above. Specifically, at 650 °C, the ratio [NaHCO3]/[NaOH] was found to be 0.02 at a pressure of CO2 equal to 0.5 atm.

Finally, it is known that molten hydroxides present high conductivity [START_REF] Zecevic | Direct Carbon Fuel Cell With Molten Hydroxide Electrolyte[END_REF], so the effect of hydroxides was investigated both in terms of electrochemical reactivity and conductivity by adding different mol% of LiOH. As seen in Figure 7, cyclic voltammetries carried out in the same conditions for various hydroxide contents under water reveal that currents are enhanced overall. Water reduction should indeed be favored with hydroxides present in the melt. In order to make sure the hydroxide ion is electroactive in our working conditions, an experiment was carried out under CO2 and without water, shown in Figure 8. It is expected that hydroxides react according to the the following acid-base reaction: Both oxidation peaks split into two: the peaks of hydrogen oxidation appear. The first appearing peak, around -1.1 V vs Ag + /Ag, indeed depends on the square root of scan rate, and in these conditions carbon is not observed. This H2 can only be obtained by reduction of hydroxides, proving they are electroactive in these conditions. Of course, water is to be provided for the hydroxides to remain stable, or Reaction 13 will occur in the presence of CO2.

2 + ↔ + (13) -2.0 -1.5 -1.
Finally, a possible influence of hydroxide addition on the conductivity was studied by EIS. It was found that the gas atmosphere had no significant influence on the conductivity. Its evolution with added hydroxide is reported in Figure 9. The conductivity indeed increases with added LiOH content, thanks to the high mobility and low size of the lithium as well as hydroxide ions. The variation is even sharper for higher percentages. The Li-K eutectic without hydroxide features a conductivity of 1.3 S.cm -1 , while the highest value is obtained for Li-K + 70 mol% LiOH, around 3.5 S.cm -1 , which is close to conductivity values in pure LiOH at this temperature, extrapolated from the literature [START_REF] Claes | Electrical Conductivity and Specific Mass of the Molten LiOH-LiNO3, NaOH-NaNO3, and KOH-KNO3 Mixtures[END_REF]. Already at a lower value of 10 mol% of LiOH, the conductivity reaches 1.45 S.cm -1 . Arrhenius law was also verified for each melt composition to check variation of the conductivity with the temperature. An example is given in Figure 10. From the Arrhenius law, activation energies were calculated, and are reported in Figure 11. Activation energies can be considered stable between 0 mol% and 30 mol% added LiOH, which is the zone of interest, at values of around 0.25 to 0.30 eV, which are consistent with previous literature on molten carbonates [START_REF] Lair | Theoretical predictions vs. experimental measurements of the electrical conductivity of molten Li2CO3-K2CO3 modified by additives[END_REF]. At higher percentages, an increase can be seen, with a value of 0.52 eV reached for 70 mol% LiOH. The melt is then composed mainly of hydroxides instead of carbonates. Two zones could be distinguished: at lower LiOH amounts, there is a conductivity increase with constant activation energy, while at higher percentages of added LiOH, both conductivity and activation energy steeply increase.

The first zone could be due to the effect of the lithium ion addition, not changing the overall conduction mechanism, and an effect of the bigger hydroxide ion could be seen in the second zone, where activation energy is much greater.

It is indeed best to limit addition of hydroxides to a reasonable amount, less than 30 mol%, to not deeply alter the base melt of eutectic Li-K (62:38 mol%) while enhancing conductivity (up to 1.72 S.cm -1 for 30 mol% added LiOH) and water reduction. This paves the way for a direct application of a mixed carbonate/hydroxide electrolyte in the MCFC/MCEC system: not only would it be helpful both in fuel cell and electrolysis mode due to increased conductivity, but specifically in MCEC mode water reduction would be favored.

IV-Conclusion

According to our electrochemical investigation at a Pt working electrode, including CV and LSV, H2 and/or CO are produced through H2O and/or CO2 reduction in molten Li2CO3-K2CO3 at 650 °C in atmospheres containing different proportions of CO2, H2O, H2 and Ar.

Both fuels are detected in a soluble or adsorbed form: this is confirmed by the linear evolution of the oxidation peaks with respect to v 1/2 or v, respectively for diffusion-or adsorption-controlled processes. C is only detected species when the cathodic potential is very negative provoking the reduction of carbonate ions into C. We also confirmed that the oxidation of CO occurs in two steps under CO2-rich atmospheres. In water-rich atmospheres, both oxidation of soluble H2 and CO are overlapped for low ratios of CO2; at 30% of CO2,

Fig. 1 -Fig. 2 -

 12 Fig. 1-Cyclic voltammogram at a Pt electrode in Li2CO3-K2CO3 under 70 mol% H2O/30 mol% Ar at 650 °C. Scan rate 100 mV s -1 .

Fig. 4 -

 4 Fig.[START_REF] Hu | Electrochemical performance of reversible molten carbonate fuel cells[END_REF]-Evolution of both oxidation peak potentials and currents with scan rate, with a the first (around -1 V vs Ag + /Ag) and b the second peak (around -0.5 V vs Ag + /Ag).

Fig. 5 -

 5 Fig. 5-Cyclic voltammogram reverse scans on a Pt electrode at 100 mV/s under 70 mol% H2O and various amounts of CO2.

Fig. 6 -

 6 Fig. 6-Potential-oxoacidity diagram calculated for Li-K (62 :38 mol%) at 650 °C.

Fig. 7 -

 7 Fig. 7-Cyclic voltammograms on a Pt electrode at 100 mV/s under 70 mol% H2O/15 mol% CO2.

Fig. 8 -

 8 Fig. 8-Cyclic voltammogram on a Pt electrode at 100 mV/s under 100 mol% CO2 right after 40 mol% LiOH addition.

Fig. 9 -

 9 Fig. 9-Evolution of conductivity with added LiOH molar percentage at 650 °C.

1 )Fig. 10 -

 110 Fig. 10-Example of an Arrhenius plot for 50 mol% of added LiOH.

Fig. 11 -

 11 Fig. 11-Evolution of activation energies with added LiOH molar percentage.

  only CO oxidation can be observed. Under hydrogen-rich atmospheres without CO2, scanning directly towards anodic potentials allows to detect a low-intensity oxidation peak due to soluble H2 and a high-intensity oxidation peak due to adsorbed H2; the latest is decomposed into several peaks with close-by potentials, indicating that hydrogen is adsorbed on different adsorption sites on the polycrystalline working Pt electrode. In water-rich atmospheres, addition of hydroxides enhances the oxidation peaks due to H2, showing that the presence of hydroxides favors H2O reduction. Even under CO2 atmosphere, hydroxides react with CO2 to produce water: H2 and CO peaks are overlapped. The increase in the conductivity of Li2CO3-K2CO3 with progressive addition of hydroxides is clearly evidenced by EIS: the activation energy deduced from Arrhenius plots is constant up to 30% of added LiOH. Thus there is a significant effect of hydroxides both on the electrochemical oxidation reaction and on the conductivity of the electrolyte. This confirms the role of hydroxides produced in situ in MCFC operation conditions and opens the possibility of using a molten carbonate eutectic containing hydroxides in the MCFC/MCEC system, which presents the advantage of relying on a mature MCFC technology and the combination of CO2 capture with energy generation. The direct use of molten hydroxide melts in fuel cell/electrolyzer applications, scarcely mentioned in the literature, would require further studies and scale-up experiments.

Acknowledgments

This work was supported by the French programs PLANEX ANR-11-EQPX-0-01 and ANR MCEC 17-CE05-0025-01.