B Planel

C A Davy

P Adler

G Hauss

M Bertin

V Cantarel

D Lambertin

Water permeability of geopolymers emulsified with oil submitted to Cement and Concrete Research

Keywords: Permeability, organic liquids, alkali-activated cement, pore structure, X Ray 1 micro-computed tomography

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

identifies the preferential water flow pathways, and quantifies the water permeability of hardened GEOIL mortars using a typical PC pore water. Two alkali-activated GP cements are used, either made of 100% metakaolin (MK) or of 80%MK-20% slag, for oil proportions of 0, 20, 40 or 60%vol.

For the identification of the preferential water flow pathways, two water flow scenarii are investigated numerically. Either water flows through the oil emulsion (Scenario 1) or water flows through the GP cement mortar pores (Scenario 2). The 3D structure of the oil emulsion is quantified by X Ray micro-computed tomography (X Ray micro-CT), and the pore structure of GP cement mortars is identified by image analysis. Fluid pathways are predicted for both scenarii by 3D numerical simulations. Secondly, water permeability experiments are conducted on GEOIL mortars. Water permeability K water of highly alkaline PC water is deduced over several weeks. The results validate Scenario 2; for 20 or 40%vol oil, water flows preferentially through the GP cement mortar pores (at a permeability below 10 -18 m 2 ), instead of through the percolating oil emulsion.

Finally, experiments show that K water for the GP cement mortar made of 80%MK-20%GGBFS does not remain stable over time when added with 20%vol oil. Comparatively, for the 100%MK GP cement mortar with 20%vol oil, a stable K water is recorded for 40 days.

Whatever the oil percentage, the initial K water value is identical and smaller than without oil. A dedicated experiment shows that the oil additives (surfactants) are very probably responsible for this behaviour. At 60%vol oil, water and organic matter flow progressively out of the GEOIL composite.

1 Introduction

Industrial and scientific context

In recent years, the application range of alkali-activated cements (AAC) and geopolymer (GP) cements [START_REF] Provis | Introduction to geopolymers. Structure, processing, properties and industrial applications[END_REF]2,[START_REF] Davidovits | Geopolymers[END_REF][START_REF] Duxson | Geopolymer technology: the current state of the art[END_REF][START_REF] Provis | Advances in understanding alkali-activated materials[END_REF][START_REF] Borges | Performance of blended metakaolin/blasfurnace slag alkali-activated mortars[END_REF][START_REF] Wang | Factors affecting the strength of alkaliactivated slag[END_REF][START_REF] Ding | Preparation and characterization of glassceramic foams from blast furnace slag and waste glass[END_REF] has significantly widened. In particular, GP cements and their blends with ground granulated blast furnace slag (GGBFS or slag) display an excellent ability to integrate significant amounts of organic liquids as stable emulsions.

To the best of our knowledge, the earliest research on the integration of organics into geopolymers are due to Tavor et al. [START_REF] Tavor | Recycling of industrial wastewater by its immobilization in geopolymer cement[END_REF] for recycling industrial wastewater; later research include Glad et al. [START_REF] Glad | Polymer adhesion to geopolymer via silane coupling agent additives[END_REF] and Marchon et al. [START_REF] Marchon | Molecular design of combshaped polycarboxylate dispersants for environmentally friendly concrete[END_REF].

When considering the integration of significant amounts of organic liquids (i.e. more than 10%vol), the first patent is by Seo et al. [START_REF] Seo | Porous geopolymer materials[END_REF] for the manufacturing of highly porous GP (i.e. GP foams) [START_REF] Medpelli | Geopolymer with hierarchically meso-/macroporous structures from reactive emulsion templating[END_REF], followed by Lambertin et al. [START_REF] Seo | Porous geopolymer materials[END_REF][START_REF] Cantarel | Solidification/stabilisation of liquid oil waste in metakaolin-based geopolymer[END_REF][START_REF] Cantarel | Etude de la synthèse de composites liquides organiques/géopolymère en vue du conditionnement de déchets nucléaires[END_REF][START_REF] Lambertin | Géopolymère à matériau à changement de phase organique, procédés de préparation et utilisations[END_REF][START_REF] Cantarel | Geopolymer assembly by emulsion templating: Emulsion stability and hardening mechanisms[END_REF][START_REF] Davy | 3d structure of oil droplets in hardened geopolymer emulsions[END_REF] for encapsulating organic liquids (OL) in the context of radioactive waste management. In practice, the OL, generally an oil, is emulsified in the fresh GP paste. With adequate formulation parameters, the emulsion remains stable during cement setting, and homogeneous GP-oil composites (GEOIL) are formed. Such incorporation may also be achieved for other types of OL, e.g. highly fluid organic solvents [START_REF] El-Naggar | Performance of geopolymers for direct immobilization of solvent extraction liquids: Metakaolin/LIX-84 formulations[END_REF][START_REF] Revathi | Evaluation of the role of Cetyltrimethylammoniumbromide (CTAB) and Acetylenicglycol (AG) Admixture on fly ash based geopolymer[END_REF]. This approach does not work with ordinary Portland cements [START_REF] Montgomery | Treatment of organic-contaminated industrial wastes using cement-based stabilization/solidification -i-microstructural analysis of cement-organic interactions[END_REF][START_REF] Trussel | A review of solidification/stabilization interferences[END_REF], although it is useful in a number of industrial applications, e.g. to design thermal insulating composite foams using phase-change materials (PCM) [START_REF] Lambertin | Géopolymère à matériau à changement de phase organique, procédés de préparation et utilisations[END_REF], filtration or catalysis supports [START_REF] Bai | High strength metakaolin-based geopolymer foams with variable macroporous structure[END_REF][START_REF] Bai | Porosity, mechanical and insulating properties of geopolymer foams using vegetable oil as the stabilizing agent[END_REF][START_REF] Medpelli | Geopolymer with hierarchically meso-/macroporous structures from reactive emulsion templating[END_REF][START_REF] Barbosa | Preparation of mesoporous geopolymer using metakaolin and rice husk ash as synthesis precursors and its use as potential adsorbent to remove organic dye from aqueous solutions[END_REF][START_REF] Medpelli | Iron oxide-modified nanoporous geopolymers for arsenic removal from ground water[END_REF][START_REF] Cilla | Geopolymer foams obtained by the saponification/peroxide/ gelcasting combined route using different soap foam precursors[END_REF][START_REF] Glad | Highly porous geopolymers through templating and surface interactions[END_REF][START_REF] Petlitckaia | Design of lightweight metakaolin based geopolymer foamed with hydrogen peroxide[END_REF] or as encapsulating media for radioactive OL waste [START_REF] Cantarel | Solidification/stabilisation of liquid oil waste in metakaolin-based geopolymer[END_REF][START_REF] Cantarel | Etude de la synthèse de composites liquides organiques/géopolymère en vue du conditionnement de déchets nucléaires[END_REF][START_REF] Cantarel | Geopolymer assembly by emulsion templating: Emulsion stability and hardening mechanisms[END_REF][START_REF] Davy | 3d structure of oil droplets in hardened geopolymer emulsions[END_REF].

In situ, e.g. in a nuclear waste storage facility or on a building wall, GEOIL composites, more likely as mortars than as pastes, will be in contact with more traditional Portland concretes. This may be the case for GEOIL thermal insulating foams in contact 1 with Portland-based concrete building walls. Alternately, in the context of nuclear waste storage (e.g. in France), high performance concretes (HPC) are widely used as containers, overpacks and structural elements in the engineered barrier of the storage site, while GEOIL mortars may serve for radioactive OL waste encapsulation. In both contexts, interactions of GEOIL mortars with the highly alkaline porewater of Portland concrete is expected. Because this porewater is not in equibrium with GP pore water, their interactions may induce chemical reactions (e.g. dissolution and re-precipitation of swelling phases), degrade GEOIL mortars and limit their durability.

Durability of AAC and GP cements. Since the discussion of GP long-term durability by J. Davidovits [START_REF] Davidovits | Geopolymers[END_REF], general performance and durability of GP have been investigated in a number of publications [START_REF] Bai | Porosity, mechanical and insulating properties of geopolymer foams using vegetable oil as the stabilizing agent[END_REF][START_REF] Ismail | Influence of fly ash on the water and chloride permeability of alkali-activated slag mortars and concretes[END_REF][START_REF] Hassan | Use of geopolymer concrete for a cleaner and sustainable environment -A review of mechanical properties and microstructure[END_REF][START_REF] Albitar | Durability evaluation of geopolymer and conventional concretes[END_REF][START_REF] Blyth | Impact of activator chemistry on permeability of alkali-activated slags[END_REF][START_REF] Shi | Strength, pore structure and permeability of alkali-activated slag mortars[END_REF]. A significant amount of research describes durability as the resistance to acid or sulfate-rich environments [START_REF] Hassan | Use of geopolymer concrete for a cleaner and sustainable environment -A review of mechanical properties and microstructure[END_REF][START_REF] Albitar | Durability evaluation of geopolymer and conventional concretes[END_REF]. For a number of authors, durability is equivalent to low mass and ionic transport [START_REF] Ismail | Influence of fly ash on the water and chloride permeability of alkali-activated slag mortars and concretes[END_REF][START_REF] Albitar | Durability evaluation of geopolymer and conventional concretes[END_REF][START_REF] Blyth | Impact of activator chemistry on permeability of alkali-activated slags[END_REF][START_REF] Baroghel-Bouny | Conception des bétons pour une durée de vie donnée des ouvrages-Maitrise de la durabilit vis à vis de la corrosion des armatures et de l'alcali-réaction-Etat de l'art et guide pour la mise en oeuvre d'une approche performantielle et prédictive sur la base d'indicateurs de durabilité[END_REF].

Mass and ionic transport is related to GP pore network. It is generally acknowledged that most of GP pores are open (i.e. percolating). With values ranging between 20-45% for metakaolin-based GP [START_REF] Benavent | Effect of composition and aging on the porous structure of metakaolin-based geopolymers[END_REF], this GP porosity is significantly larger than for ordinary Portland cements (OPC). GP pastes are considered meso-and macro-porous, with a pore space organization allowing ink bottle effects, as in OPC [START_REF] Diamond | Mercury porosimetry: an inappropriate method for the measurement of pore size distributions in cement-based materials[END_REF][START_REF] Nguyen | Porous arrangement and transport properties of geopolymers[END_REF]. However, this feature does not imply that GP have high fluid transport ability, since only the percolating pore structure controls this property. For instance, Blyth et al. [START_REF] Blyth | Impact of activator chemistry on permeability of alkali-activated slags[END_REF] use the beam-bending technique to derive liquid permeability for different alkali-activated slags. Results are correlated with nitrogen sorption and mercury intrusion porosimetry to determine the pore size of the percolating pore space controlling fluid mass transport.

For hydroxide-activated slag, liquid permeability is on the order of 1-4×10 -19 m 2 . This permeability is associated to a percolating pore size of 50 nm (capillary pores) at 7 days, i.e. the mesopore space controls fluid mass transport. For silicate-activated slag, water permeability is on average of 10 -22 m 2 , i.e. three orders of magnitude smaller than for hydroxide-activated slag. The percolating pore size, associated to the low permeability of silicate-activated slag, is located in the mesopore range, with a value around 3 nm (i.e. in the GP gel pores). For Type III OPC paste (with W/C=0.5) after 7 days curing, water permeability is of 6×10 -21 m 2 , i.e. smaller than hydroxide-activated slag but one order of magnitude bigger than for silicate-activated slag. The authors conclude that silicateactivated slags are able to resist the ingress of ions and molecules at least as well as OPC-based systems, i.e. that their durability is equivalent or greater.

Aims and scopes

To the best of our knowledge, for hardened GEOIL composites, no research has been undertaken yet on their fluid transport properties, and in particular mass transport (associated with permeability in the Darcy's sense). The aim of this contribution is to investigate the water permeability K water of GEOIL mortars, in presence of highly alkaline water that is typical of Portland concrete pore water. It is assumed that this water will impinge on the GEOIL composites.

The study is divided into two main parts. First, the two most plausible scenarii for water flow are analyzed numerically, and secondly, K water is characterized experimentally, in order to determine what is the actual water flow scenario (Figure 1).

For the numerical prediction of water permeability, with Scenario 1, water is assumed to flow only through the percolating oil emulsion. The latter is characterized in 3D by X Ray micro-computed tomography (X Ray micro-CT), and data are used directly for numerical computations. With Scenario 2, water is assumed to flow only through the GP cement mortar (and not through the oil). The pore structure of the GP cement mortar is characterized by indirect means (Mercury Intrusion Porosimetry -MIP, and nitrogen sorption-desorption isotherms). Coupled to 1D modelling of permeability (using the analytical Katz-Thompson equation), these indirect measurements do not yield similar values to those obtained experimentally. Instead, 3D numerical simulation is performed by using the negative space of the oil emulsion characterized by X Ray micro-CT, and, to obtain more accurate results, by assuming a local mortar permeability value, taken from the experiments on the GP mortar alone.

In the experimental campaign, the actual water flow scenario is determined. An original water permeability set-up is used, which avoids carbonation of the highly alkaline water during the long duration experiments (7 to 40 days). The experiment provides water permeability K water in Darcy's sense. For performance assessment, K water is also considered as an accelerated durability indicator, compared to generally longer chloride diffusion experiments [START_REF] Baroghel-Bouny | Conception des bétons pour une durée de vie donnée des ouvrages-Maitrise de la durabilit vis à vis de la corrosion des armatures et de l'alcali-réaction-Etat de l'art et guide pour la mise en oeuvre d'une approche performantielle et prédictive sur la base d'indicateurs de durabilité[END_REF].

Complementarily, the effect of changing the GP cement composition and the oil proportion on K water are investigated experimentally. GEOIL mortars are made with two distinct cement matrices, either based on 100% metakaolin (MK) or by blending 80%wt MK and 20%wt slag. The former has proven its efficiency in encapsulating oil [START_REF] Cantarel | Solidification/stabilisation of liquid oil waste in metakaolin-based geopolymer[END_REF][START_REF] Cantarel | Etude de la synthèse de composites liquides organiques/géopolymère en vue du conditionnement de déchets nucléaires[END_REF][START_REF] Lambertin | Géopolymère à matériau à changement de phase organique, procédés de préparation et utilisations[END_REF][START_REF] Cantarel | Geopolymer assembly by emulsion templating: Emulsion stability and hardening mechanisms[END_REF][START_REF] Davy | 3d structure of oil droplets in hardened geopolymer emulsions[END_REF]; the latter is chosen in an attempt to reduce the percolating pore space and further limit GEOIL permeability, and hence, increase the expected durability [START_REF] Blyth | Impact of activator chemistry on permeability of alkali-activated slags[END_REF]. Both cements are mixed with a varying oil content, from 0 to 60%vol. The effect of oil traces in the GP mortar is quantified, in order to determine the lower limit in oil content, above which water permeability is decreased by the presence of oil.

Methods

Materials and sample preparation

Preparation of GP mortars. The 100%MK GP cement is made with a stoichiometry of 1 Al 2 O 3 1 Na 2 O 3.8 SiO 2 12 H 2 O, following [START_REF] Cantarel | Solidification/stabilisation of liquid oil waste in metakaolin-based geopolymer[END_REF][START_REF] Cantarel | Geopolymer assembly by emulsion templating: Emulsion stability and hardening mechanisms[END_REF]. It uses metakaolin (MK) purchased under the brand name Argical M1000 (Imerys, France), sodium silicate solution (Betol T M 39T) obtained from Woellner (Germany), sodium hydroxide NaOH (purity > 99%) from VWR International (USA), and deionized water. For the 80%wt MK-20%wt slag cement, ground granulated blast furnace slag (GGBFS) from Ecocem (France) is used. Cement mass proportions (Table 2) are obtained using the X Ray fluorescence results in Table 1.

For all formulations, the alkaline activating solution is prepared by mixing NaOH and water with Betol39T, sealed to prevent water evaporation and left to cool to ambient temperature before being used.

To achieve GP mortars instead of pastes, the raw solid powders P = (MK or MK+GGBFS) are mixed with pure silica sand from Le Beausset (Lafarge France), with a constant sandto-powder mass ratio (S/P) of 1. REF-MK refers to the mortar made with 100%MK and REF-MKS is for the mortar made of 80%wt MK-20%wt slag (Table 2). Solids are homogenized and progressively incorporated into the sodium silicate solution in a high-speed mechanical stirrer, equipped with an anchor-shaped blade and operated at 800 rpm, until homogeneous.

Preparation of GEOIL composites. The fresh GP mortar (either REF-MK or REF-MKS

) is mixed with 20, 40 or 60% total volume of non reactive industrial Spirax 80W90 oil (Shell, USA). Complete mixing proportions are given in Table 2. GEOIL20-MK (respectively GEOIL40-MK or GEOIL60-MK) refers to a GEOIL composite mixing REF-MK mortar with 20%vol oil (respectively with 40 or 60%vol oil). GEOIL20-MKS is made of REF-MKS mortar mixed with 20%vol oil. The mixing protocol is different from that presented in [START_REF] Cantarel | Solidification/stabilisation of liquid oil waste in metakaolin-based geopolymer[END_REF], where the oil was incorporated in the alkaline activating solution. Here, the fresh GP mortar is made, and only then is the oil added.

Shell Spirax is a model oil for radioactive oil waste conditioning [START_REF] Cantarel | Solidification/stabilisation of liquid oil waste in metakaolin-based geopolymer[END_REF][START_REF] Cantarel | Etude de la synthèse de composites liquides organiques/géopolymère en vue du conditionnement de déchets nucléaires[END_REF][START_REF] Cantarel | Geopolymer assembly by emulsion templating: Emulsion stability and hardening mechanisms[END_REF]. This oil is a synthetic base oil with less than 4%wt additives. According to the oil data sheet, these additives are Bis(nonylphenyl)amine (CAS Number 36878-20-3) for less than 2%wt and Alkyl phosphate (CAS Number 36878-20-3) also for less than 2%wt. The oil FTIR spectrum is provided in Fig. 2. It is typical of a long chain linear and ramified alkane organic polymer [START_REF] Coates | Interpretation of Infrared Spectra, A Practical Approach[END_REF], with both methylene (-CH 2 or =CH 2 ) and methyl (-CH 3 ) ends. The oil additives are not in sufficient amount to be visible and characterized by FTIR.

The oil viscosity has an average value of 2.5 Pa.sec (average of 3 measurements) evaluated on a TA instruments AR-G2 rheometer regulated at 25 o C (operated with a gap of 800 µm and a shear rate of 40 s -1 ).

The oil interfacial tension is recorded with a Tracker pendant drop tensiometer from Teclis-Itconcept. Shell Spirax oil has an interfacial tension of 1.24 mN/m when in contact with the alkaline sodium silicate solution, and an interfacial tension of 11.2 mN/m when in contact with water (average of at least 3 measurements).

Preparation of a GEOIL composite with oil traces (REF-MKO

). The effect of the presence of oil traces, either as a few oil droplets and/or hydrophilic oil additives, is investigated on a dedicated sample. To this purpose, a GEOIL sample is made by mixing and decanting the equivalent of 20%vol oil in the cooled alkaline activating solution for 30 minutes. After this duration, the oil above the solution is removed, and the alkaline solution, free from the oil mass, is mixed with the homogenized solids (MK and sand) to provide a fresh GP mortar named REF-MKO, see Table 2 with label ( * ) .

Curing protocol. Each fresh mortar paste is poured into a 37 mm diameter plastic mould up to a height H ranging between 18 and 22 mm, and cured in hermetic bags at 20 o C for 28 days. After this period, it is assumed that geopolymerization is sufficiently advanced not to modify anymore the pore network responsible for fluid transport. Complementarily, fresh mortar paste is poured into 4×4×16 cm 3 moulds for uniaxial compressive strength measurement after 7 or 30 days curing in hermetic bags at 20 o C.

For X Ray micro-computed tomography, after 7 days curing (in a hermetic bag at 20 o C), two GEOIL mortars with 20 or 40% oil are cut with a diamond saw to the size of a cuboid of 4-5 mm thickness and width, and 1-2 cm height. Each sample is placed in the X Ray apparatus without further preparation.

3D X Ray micro-computed tomography (micro-CT)

3D X Ray micro-CT is performed on two samples of GEOIL20-MK and GEOIL40-MK, with a voxel size of 1 µm. As much as possible, the observation zone is chosen in areas away from the biggest millimetric silica sand grains, which are neither in the oil nor in the porous GP cement mortar. The procedure provides data for volumes of 0.557-0.958 mm 3 (Table 4). These are assumed representative of the macroscopic scale of GEOIL mortars.

Further details on the imaging and analysis procedures are given in Section 5.

Nitrogen sorption and Mercury Intrusion Porosimetry (MIP)

Due to the expected small pore sizes, which should lie in the range of 5-15 nm and 2 µm

Permeability prediction

The binary images obtained from 3D X Ray micro-CT provide a 3D spatial repartition of both the oil emulsion and of the GP mortar, which is the negative space of the oil. Scenario 1. Let assume that fluid flows through the oil (Figure 1 bottom left). The 3D percolating oil emulsion is deduced from the images by using the approach described in [START_REF] Thovert | Computerized characterization of the geometry of real porous media: their discretization, analysis and interpretation[END_REF]. In such instance, the fluid permeability tensor through the 3D percolating oil emulsion is calculated by the numerical resolution of the Stokes equation. This can be done in a number of ways [START_REF] Davy | Three scale analysis of permeability of a natural shale[END_REF], namely using a classical computational fluid dynamic (CFD) technique applied to cubic voxels [START_REF] Adler | Flow in simulated porous media[END_REF], a lattice Boltzmann model [START_REF] Pazdniakou | Dynamic permeability of porous media by the lattice boltzmann method[END_REF] or the finite-volume technique applied to tetrahedra [START_REF] Khamforoush | Permeability and percolation of anisotropic three-dimensional fracture networks[END_REF]. Another method was used here in order to save computer time [START_REF] Arns | Fast laplace solver approach to pore-scale permeability[END_REF]. It consists of a Laplace approximation for the solution of the Stokes equation on the basis of an extension of Poiseuille's law to complex microstructures, using Euclidean distance maps. The permeability of each pore voxel is a function of the distance between the voxel and the closest solid surface. Then, one has to solve a Darcy equation with a variable permeability over the sample as was done by [START_REF] Henriette | The effective permeability of heterogeneous porous media[END_REF]. Scenario 2. Alternately, concrete pore water flow may occur through the GP mortar rather than through the oil (Figure 1 bottom right). In this case, water flows through the negative space determined by 3D X Ray micro-CT, and 3D numerical computations can be performed, provided that a local permeability field for the GP mortar cement is known.

In a first order approach, a constant value K water (GP mortar) is used for this permeability field. Its choice will be discussed in the Results section. The macroscopic permeability is then deduced by solving the Darcy equation [START_REF] Henriette | The effective permeability of heterogeneous porous media[END_REF] as before. For both scenarii, numerical simulations provide the permeability tensor (in m 2 ), ex-pressed as

K =       K xx K xy K xz K yx K yy K yz K zx K zy K zz      
where -→ x , -→ y and -→ z denote the three sample axis. In practice, the diagonal terms (K xx , K yy , K zz ) of this matrix (which represents the permeability tensor) correspond to fluid permeability along each sample axis; they are significantly different if the sample is anisotropic.

The off-diagonal terms correspond to fluid flow couplings between the axis.

Water permeability experiment

For fluid injection, synthetic CEMV pore water is made according to Andra's technical note [START_REF] Bourbon | Formulation des matériaux et des solutions du projet structurant sur les nouvelles matrices de conditionnement[END_REF] 1 by mixing 2.666g NaOH, 7.48g KOH and 0.074 g Ca(OH) 2 in 1L deionized water. The pH of the solution is checked after mixing at an average value of 13.3 +/-0.1 with a Mettler-Toledo (USA) pH-meter.

The test set-up (Figure 3) is derived from a typical triaxial cell experiment for fluid permeability measurement [START_REF] Davy | Permeability of macrocracked argillite under confinement: gas and water testing[END_REF][START_REF] Chen | Water retention and gas relative permeability of two industrial concretes[END_REF]. Its operating details are provided in the Appendix.

Its originality lies in the small argon flux applied above the alkaline water reservoir, which avoids water carbonation.

By assuming a unidirectional flow of a Newtonian fluid through the sample, and by neglecting gravity effects [START_REF] Davy | Permeability of macrocracked argillite under confinement: gas and water testing[END_REF], Darcy's law provides water permeability K as

K = µHQ v A∆P ( 1
)
where H is sample height (in m), A is its surface area (in m 2 ), µ is fluid dynamic viscosity in Pa.s at the operating temperature. Here at 20 o C, for water, µ is equal to 1

1 Andra is the French National agency in charge of radioactive waste management.

mPa.s. All permeability data are given after water percolates through the sample, i.e. after water has reached the downstream sample side. Each experiment is continued for several days (or weeks) after percolation.

3 Results and discussion

Mechanical performance of GEOIL mortars

The uniaxial compressive strength f c of GEOIL mortars is given in Fig. 4 ), but at 40% oil, the GEOIL20-MKS mortar performs significantly better than the GEOIL20-MK, with averages of 1.4MPa (for GEOIL20-MK) and 10 MPa (for GEOIL20-MKS). At 60%MK however, none of the GP cements performs sufficiently well to provide mechanical strength to GEOIL (f c is on the order of 1 MPa or less).

It is concluded that the MK-slag GP cement helps retaining mechanical strength at higher oil percentage than 100%MK GP cement. Let us now analyze whether this better mechanical strength is correlated with pore structure and fluid transport, or not.

3.2 Characterization of potential water pathways through GEOIL

Quantification of the oil emulsion by direct imaging

This technique is applied to GEOIL composites based on pure MK (no slag substitution), see Figure 5. To a first approximation, it is assumed that similar emulsions, and hence, similar results, would be obtained with slag-substituted GP cement mortar.

Since the voxel size is of 1 µm, no oil droplet smaller than 2 µm (i.e. two voxels) is quantified. Due to the capillary threshold barrier between water and oil [START_REF] Stegemeier | Mechanisms of Entrapment and Mobilization of Oil in Porous Media[END_REF][START_REF] Chatzis | Magnitude and detailed structure of residual oil saturation[END_REF][START_REF] Romano | Strong influence of geometrical heterogeneity on drainage in porous media[END_REF][START_REF] Datta | Mobilization of a trapped non-wetting fluid from a three-dimensional porous medium[END_REF],

all droplets smaller than 2 µm are more difficult to drain from the GP mortar than those larger than this value. It is inferred that, should water choose to percolate through the oil emulsion, it will go more easily through percolating droplets larger than 2 µm.

After filtering and thresholding (Figure 5a-d), the 3D representations of the oil emulsion for GEOIL20-MK (at 20% oil) and GEOIL40-MK (at 40% oil) are given in Figure 5e-f. For GEOIL20-MK, the observation volume comprises part of a millimetric non porous silica sand grain. It is assumed that despite this, the sample remains representative of the oil emulsion inside the GEOIL20-MK composite.

The percolating oil proportion above 2µm (expressed in % of total sample volume) is of 6.70% for GEOIL20-MK (for an imposed 20%vol proportion) and 23.54% for GEOIL40-MK (for an imposed 40%vol), see Table 4. This means that a significant proportion of the oil emulsion is made of droplets larger than 2µm.

For the oil proportion observed by micro-CT, the diameter d 50 taken at 50 % of the oil droplet size distribution is equal to 12 µm for GEOIL20-MK mortar and 30 µm for GEOIL40-MK mortar (Fig. 6)). This means that half the fluid pathways through the oil are equal or bigger to this size (in volume), i.e. more than half the observed oil volume is made of droplets or pathways equal to or bigger than 12 µm (GEOIL20-MK) or 30 µm (GEOIL40-MK). This is significantly bigger, by three orders of magnitude, than the pore pathways through the GP cement (see Sub-section 3.2.2). Similarly to nitrogen sorption-desorption, MIP provides data on pore entry sizes rather than actual pores [START_REF] Diamond | Mercury porosimetry: an inappropriate method for the measurement of pore size distributions in cement-based materials[END_REF], but for sizes usually bigger than 3-4 nm, and up to the mm. Results

Numerical investigation of water preferential pathways

In this part, water permeability is predicted numerically, depending whether water flows preferentially through the oil (Scenario 1) or through the GP cement (Scenario 2), see Fig. 1. Should water percolate through the oil, it has to overcome a threshold capillary pressure at the oil/water interface, which is also calculated.

Prediction of capillary threshold

The capillary threshold barrier between oil and water is determined using the pressure difference at their interface as [START_REF] Stegemeier | Mechanisms of Entrapment and Mobilization of Oil in Porous Media[END_REF][START_REF] Chatzis | Magnitude and detailed structure of residual oil saturation[END_REF][START_REF] Romano | Strong influence of geometrical heterogeneity on drainage in porous media[END_REF][START_REF] Datta | Mobilization of a trapped non-wetting fluid from a three-dimensional porous medium[END_REF][START_REF] Washburn | The dynamics of capillary flow[END_REF]]

P c = P oil -P water = γ oil-water cosθ d (2)
where γ oil-water is the surface tension at the oil/water interface, θ is the contact angle and d is the diameter of the pore considered, located at the interface between oil and water. Let us assume that θ=120 o (non wetting fluid). With an interfacial surface tension between the Shell Spirax oil and water of 11.20 ±0.15 mN/m, and a main pore diameter of 12 µm (GEOIL20-MK), Eq. ( 2) provides a capillary threshold value of 467 Pa. This value is even lower (at 187 Pa) for GEOIL40-MK. Changing the contact angle only leads to smaller capillary pressures.

Therefore, when applying a pressure gradient of 0.5 MPa with the water permeability set-up, should the capillary threshold pressure control the water flow through the system, water will flow preferentially through the oil emulsion, whatever the oil percentage in the GEOIL mortar.

Prediction of water permeability

Preliminary calculation of GP mortar permeability. When predicting permeability through the GEOIL composite, its bulk permeability K mortar is needed.

Using a classical 1D Katz-Thompson model [START_REF] Katz | Quantitative prediction of permeability in porous rock[END_REF][START_REF] Katz | Prediction of rock electrical conductivity from mercury injection measurements[END_REF], permeability K is deduced from the indirect pore structure measurements presented above, as:

K = φ d 2 c 226 τ (3)
where τ is the tortuosity of the pore system; it is taken equal to 1 in order to provide a higher bound to K mortar . φ is the sample porosity, given by MIP, and d c is a critical pore diameter, taken as the peak pore size also given by MIP and correlated with nitrogen Prediction of water permeability through GEOIL composites. Once the capillary threshold is overcome, water may flow through the oil system until reaching a steady-state, characterized by permeability. For permeability prediction, let us assume that water flow may occur either (Scenario 1) through the 3D percolating oil emulsion characterized by X Ray micro-CT, or (Scenario 2) through the GP cement. In Scenario 2, 3D numerical simulations are performed by using the negative space of the oil emulsion characterized by X Ray micro-CT, and by neglecting the contribution of the biggest millimetric sand aggregates to hinder water flow.

With the numerical tools presented in Section 2.4, whatever the flow paths considered, permeability tensors display very small off-diagonal terms (Table 5), and permeability along all three axis -→ x , -→ y and -→ z are very close, meaning that the GEOIL samples are isotropic.

Scenario 1 (water flow through the oil). Table 5 shows that water permeability through the oil emulsion is predicted at a value on the order of 1.1 to 4.2×10 -14 m 2 (GEOIL40-MK and GEOIL20-MK respectively). Multiplying the oil content by two does not change the order of magnitude of permeability. Moreover, this order of magnitude (10 -14 m 2 ) is usually encountered for highly permeable media, e.g. granular sand [START_REF] Peter | Reactive Transport in Porous Media[END_REF].

It means that water flow through the percolating oil emulsion should occur at relatively significant flowrates, on the order of a few cl/s (according to Eq. 1).

Scenario 2 (water flow through the GP mortar). For this calculation, no pore size is required, since it uses the local permeability value K mortar of the GP cement mortar of 1.4×10 -18 m 2 . Comparatively, should water flow occur through the GP cement mortar, water permeability is four orders of magnitude smaller than that through the oil emulsion, with an average of 0.8 to 1.2 ×10 -18 m 2 (for GEOIL40-MK and GEOIL20-MK respectively). With this Scenario 2, the expected water flowrate Q v will be five orders of magnitude smaller, i.e. on the order of 0.01 ml/h (according to Eq. 1). Such permeability and flowrate are more typical of high performance concretes [START_REF] Chen | Water retention and gas relative permeability of two industrial concretes[END_REF], which are known for their finely porous pore structure [START_REF] Brisard | Mesoscale pore structure of a high performance concrete by coupling FIB/SEM and SAXS[END_REF], and for their excellent durability.

Let us now compare these predictions with actual permeability measurements at the centimetric scale. For REF-MK, between the initial permeability value and the stabilized one, the decrease in K water represents a factor of 6.3. This decrease is attributed to interactions between GP mortar and the injected pore water, which tend to decrease water flow (possibly due to partial pore clogging, related to solids precipitation inside the pore system).

Experimental investigation of water permeability

For REF-MKS, during the first 3 days, the evolution of K water is similar to that of REF-MK, with a relatively stable value of 1.2×10 -18 m 2 ±0.5×10 -18 . This value is only slightly smaller than that of REF-MK, but, at this stage (after 3 days), it has not stabilized yet below 10% over 24h. As this cement matrix is made of a mix of MK and slag, leading to different hydrates (either sodium aluminium silicate hydrates N-A-S-H or calcium aluminium silicate hydrates C-A-S-H), with potentially different hydration kinetics, it is expected that water permeability may evolve differently than the other matrices. As a matter of comparison, for alkali-activated cements, Ismail et al. [START_REF] Ismail | Influence of fly ash on the water and chloride permeability of alkali-activated slag mortars and concretes[END_REF] show that the chemistry of the geopolymer binder controls mass and ionic transport, particularly in chloride-rich environments. Geopolymers comprising C-A-S-H have better resistance to fluid transport than GP made of sodium aluminium silicate hydrates (N-A-S-H).

In order to reach stability, the water permeability experiment on REF-MKS has been continued for 26 more days. Over time, K water decreases down to 1.9×10 This means that although having an initial better durability (i.e. smaller permeability) than REF-MK, the REF-MKS mortar degrades significantly after 10 days when emulsified with 20% oil. This result cannot be attributed to a leak of the apparatus, since no such result is observed with any of the other samples tested with the same apparatus and tightness devices. It is concluded that the REF-MKS mortar is not suitable for durable oil-GP mortar composites.

For REF-MK, the initial value of K water is decreased by the addition of 20% oil (1.4×10 -18 m 2 and 4.0×10 -19 m 2 respectively). This value is remarkably stable over the 40 days experiment, with an average K water = 4.4×10 -19 m 2 ±4.0 10 -19 . This means that emulsifying 20% oil inside a 100%MK GP mortar provides a steady and low water permeability, which is a positive indicator of the good durability of the GEOIL composite.

Validation of water flow Scenario 1 or 2

When assuming that water flows through the percolating oil emulsion, K water is predicted at a high value on the order of 10 -14 m 2 , which is 5 orders of magnitude larger than the measurements at the centimeter scale on GEOIL20-MK. Alternately, when water is assumed to flow through the GP mortar cement pores, the predicted value of K water is on the order of magnitude of 10 -18 m 2 , which is similar to the GP mortar without oil. None of these predictions is quantitatively accurate, although assuming fluid flow through the GP mortar provides the closest match to measured data. The capillary threshold barrier through the oil emulsion should also be overcome easily by the imposed 0.5 MPa water pressure gradient, but, owing to the experimental results, it is not sufficient to allow actual water flow through the oil.

In practice, whatever its amount, the presence of oil decreases the initial value of K water down to very similar values for all three GEOIL mortars tested, with 4.0×10 -19 m 2 (GEOIL20-MK), 3.5×10 -19 m 2 (GEOIL40-MK) and 2.3×10 -19 m 2 (GEOIL60-MK).

With the hypothesis made in Sub-section 3.3.2, numerical simulations do not yield a K water on the order of 10 -19 m 2 when mixing GP mortar and oil, despite using a realistic 3D representation of the oil emulsion and of the GP mortar. To understand the origin of this discrepancy, a water permeability experiment has been conducted with a GP mortar sample comprising only oil traces (REF-MKO), Fig. 10. For this GP mortar hardened without significant oil content, an initial K water of 4.4×10 -19 m 2 is measured, which is identical to the initial value of GEOIL20, and very close to those of GEOIL40 and GEOIL60. This value is significantly smaller than the initial value of the GP mortar, made without oil decantation.

It is inferred that either very small oil droplets, remaining in the GP mortar, or the oil additives, which are possibly also retained in the REF-MKO mortar, explain the low K water value, similar to that for 20, 40 or 60% oil. Indeed, no difference in the pore structure of REF-MK and REF-MKO is observed with MIP or nitrogen sorption-desorption, whereas their water permeabilities K water are significantly different.

The oil additive Bis(nonylphenyl)amine is a surfactant, with amphiphilic (i.e. hydrophobic and hydrophilic) properties. The molecule has a polar end (amine) and a non polar end (nonylphenyl). Owing to these properties, this surfactant is bound to impede water flow through the GP mortar. It is inferred that when oil is present (even as traces), thanks to this surfactant, water flows more easily through the GP mortar pores rather than through the oil emulsion. Should water interaction in the oil emulsion be promoted in presence of the amphiphilic compounds present in the oil, water would flow more easily through the composite and yield greater permeability than the measured values; moreover, organic matter would be released at the test cell outlet, which is neither the case. In other words, due to amphiphilic effects, water has more difficulty in flowing through the GP mortar pores when traces of oil are present.

It is concluded that Scenario 2 is the most probable, i.e. water flows preferentially through the GP mortar pores.

GEOIL composites with varying oil amount

Complementarily, permeability results for GEOIL20-MK, GEOIL40-MK and GEOIL60-MK are analyzed with time in Fig. 10, together with REF-MK and GP mortar with only oil traces (REF-MKO).

For GEOIL40-MK, a similar and stable K water is measured as for GEOIL20-MK, with a slight increase from 3.5×10 -19 m 2 up to 5.3×10 -19 m 2 after 9 days. As for GEOIL20-MK, K water of GEOIL40-MK is stable, and at a value smaller than REF-MK.

For GEOIL60-MK however, K water starts from an initial value of 2.3×10 -19 m 2 , and increases up to 2.0×10 -18 m 2 after 18 days, meaning that the GP matrix is not able to filter the fluid steadily over time. The GEOIL60-MK mortar degrades over time, with a regularly increasing K water . It is the only sample, for which products formed at the permeability cell could be retrieved and tested by X Ray fluorescence and diffraction (Fig. 11a-c). For all other GP and GEOIL mortars, the products formed at the cell outlet are mainly in solid form, but in too small amounts to be characterized. X Ray fluorescence shows that 87%wt of the formed solids at the cell outlet for GEOIL60 are made of organic matter, of 11.3%wt

Na 2 O and of traces (<1%) of SO 3 . Complementarily, with qualitative XRD (Fig. 11d Two potential water flow scenarii, corresponding to two distinct water flow mechanisms, are also investigated. In Scenario 1, water percolates through the oil emulsion; in Scenario 2, water flows through the GP mortar pores. In order to determine which is the actual water flow scenario, the 3D percolating oil emulsion is characterized by X Ray micro-CT for GEOIL20-MK and GEOIL40-MK (20 or 40%vol oil). The pore structure of GP mortar alone is also quantified. With these data, capillary threshold calculations and 3D numerical simulations are performed to predict K water through GEOIL20-MK and GEOIL40-MK. In both cases, simulations predict that water should flow more easily through the percolating oil emulsion rather than through the GP cement pores. K water is predicted bigger by four orders of magnitude if water flowed through the oil and not through the GP mortar. These predictions are contradicted experimentally.

The actual water flow scenario is validated using water permeability experiments at the centimetric scale. The initial value of K water is significantly smaller than all predictions, at 3.2×10 -19 m 2 ±0.8×10 -19 m 2 whatever the oil percentage (20 to 60% vol). This value is also obtained with a GP mortar where only oil traces remain. It is concluded that Scenario 2 is the most probable, i.e. water flows preferentially through the GP mortar pores. However, when the GEOIL 60%vol oil is tested over time, water and organic matter flow progressively out of the material. This oil percentage is not advised for encapsulation. Table 5: Permeability prediction with 3D numerical simulations of fluid flow either through (1) the oil system (quantified by 3D X Ray micro-computed tomography), or (2) the GP mortar (made with 100% MK). Simulations are performed on 1374×1395×500 voxels 3 (GEOIL20-MK) and 1254×889×500 voxels 3 (GEOIL40-MK). Each permeability tensor comprises nine scalar terms, namely K xx , K yx , K zx (first column), K xy , K yy , K zy , (second column) and K xz , K yz , K zz (third column). The diagonal terms (K xx , K yy , K zz , in bold)correspond to fluid permeability along each sample axis; the off-diagonal terms correspond to fluid flow couplings.

[ 37 ]

 37 , these indirect methods are chosen to characterize the pore structure of REF-MK, REF-MKO and REF-MKS. Prior to applying nitrogen sorption or MIP, each sample is cut into several parts of 0.5cm 3 size, and each one is freeze-dried. The experimental protocols and apparatus for nitrogen sorption-desorption and MIP are detailed in the Appendix.

3. 2 . 2

 22 Characterization of the GP mortar pore structure Nitrogen sorption-desorption isotherms are given in Fig. 7(a) for the three GP cement mortars REF-MK, REF-MKO (with oil traces) and REF-MKS (with slag substitution). REF-MK and REF-MKO display very similar sorption-desorption isotherms, meaning that the presence of oil traces has no significant effect on the pore structure. As expected from the literature [1, 37, 55], REF-MKS has a lower sorption-desoprtion ability than REF-MK, meaning that slag affects the pore structure. In particular, the BET specific surface area is 22.6 m 2 /g for REF-MKS, while it is of a significantly greater and comparable value of 42.0 and 41.4 m 2 /g for REF-MK and REF-MKO respectively.A differential pore entry size distribution is also provided in Fig.7busing the BJH model[START_REF] Barrett | The determination of pore volume and area distributions in porous substances. I -Computations from nitrogen isotherms[END_REF][START_REF] Baroghel-Bouny | Water vapour sorption experiments on hardened cementitious materials -Part I: Essential tool for analysis of hygral behaviour and its relation to pore structure[END_REF]. While REF-MKS displays two pore peaks of similar amplitude, at values of 3.2 and 12.9 nm, REF-MK and REF-MKO display an almost identical BJH differential curve, with the same peak at 5.6-6.2 nm.

  are given in Figs.8(a-b) for REF-MK, REF-MKO (with oil traces) and REF-MKS (with slag substitution). As with nitrogen isotherms, cumulative MIP intrusion curves are very close for REF-MK and REF-MKO (Fig. 8a). A smaller cumulated amount is measured for REF-MKS compared to REF-MK and REF-MKO. Indeed, compared to REF-MK and REF-MKO, REF-MKS displays a negligible pore amount above 10 microns. Sample porosity is assessed at 34.9% and 36.3% for REF-MK and REF-MKO respectively, and at a very close value of 32.5% for REF-MKS. With differential MIP curves (Fig. 8(b), a single pore peak is found for REF-MK and REF-MKO, at the same value of 5.2 nm, which is identical to that given by nitrogen sorption-desorption isotherms. For REF-MKS, two main pore peaks are found, at values of 3.0 and 10.0 nm; contrarily to nitrogen sorption-desorption, the peak with the highest amplitude is at 3.0 nm.

  sorption-desorption. For REF-MK, with φ=34.9% and d c =6 nm, the Katz-Thompson model leads to K = 5.6 × 10 -20 m 2 . Compared to actual water permeability measurements (with an average value of 1.4×10 -18 m 2 , see Section 3.4), this leads to an under-estimation of the actual water permeability of REF-MK mortar. For adequate permeability prediction by the Katz-Thompson model, a critical pore diameter d c of 30 nm is required. Therefore, in the following, for predicting water permeability of GEOIL composites, K mortar of the GP cement mortar alone is taken as the permeability measured experimentally, at an average value of 1.4×10 -18 m 2 .

3. 4 . 1

 41 Results analysis Reference materials. The first results in Fig. 9 (left) compare the reference GP mortars REF-MK and REF-MKS. Each experiment is conducted until water permeability is stable (with variations of less than 10% over 24h).

Fig. 9

 9 also compares both GP mortars without oil (REF-MK and REF-MKS) to the same mortars made with 20% oil (GEOIL20-MK and GEOIL20-MKS).The initial permeability of GEOIL20-MKS is similar to that of the same mortar without oil REF-MKS (2.4×10 -19 m 2 and 1.9×10 -19 m 2 respectively). However, after 16 days injection, K water of GEOIL20-MKS has increased by more than two orders of magnitude, starting from 2.4×10 -19 m 2 on Day 1, and going up to 2.6×10 -17 m 2 on Day 16. Destabilization lasts from Day 10 (K water =9.2×10 -20 m 2 ) to Day 16. After a decrease by a factor of 2.6 on the course of 9 days, K water increases by a factor of 283 (going from 9.2×10 -20 m 2 up to 2.6×10 -17 m 2 ) in the course of 6 days. This decrease in permeability is explained by the progressive clogging of the test cell outlet pipes; after a significant amount of solids has crystallized on the outlet pipe, it is removed and water is able to flow more significantly again, inducing a second increase in permeability up to 2.6×10 -17 m 2 . Visual inspection shows that the crystallized solids contain organic matter.

  -17 4.2×10 -14 1.0×10 -17 1.0×10 -17 1.0×10 -17 4.2×10 -14 Scenario (2): Water flows through the GP 1.2×10 -18 1.1×10 -21 1.0×10 -21 mortar pores -1.1×10 -21 1.2×10 -18 -1.5×10 -21 -0.9×10 -21 -1.5×10 -21 1.2×10 -18 Mortar name GEOIL40-MK Scenario (1): Water flows through the percolating 1.1×10 -14 6.1×10 -17 6.1×10 -17 oil emulsion 6.1×10 -17 1.1×10 -14 6.1×10 -17 6.1×10 -17 6.1×10 -17 1.1×10 -14 Scenario (2): Water flows through the GP 0.8×10 -18 1.3×10 -21 5.2×10 -21 mortar pores 1.1×10 -21 0.9×10 -18 0.8×10 -21 5.2×10 -21 0.9×10 -21 0.7×10 -18

Figure 1 :

 1 Figure 1: Principle of the two water flow scenarios investigated: water either flows through the percolating oil emulsion (Scenario 1) or through the GP cement mortar pores (Scenario 2).

Figure 2 :

 2 Figure 2: Fourier Transform InfraRed (FTIR) spectrum of the Shell Spirax oil used in this research.

Figure 3 :

 3 Figure 3: Principle of the accelerated leaching experiment. A pressure gradient of alkaline water is applied to a centimetric sample placed in a triaxial cell to avoid fluid leakage. Water carbonation is avoided using a small argon flux above the water reservoir.

Figure 4 :

 4 Figure 4: Average compressive strength results at 7 and 30 days for the two alkali-activated matrices used in this research (mortar samples with S/C=1, average on three 4x4x16cm 3 standard samples), Min/max intervals are also indicated as error bars.

Figure 5 :

 5 Figure 5: 3D X Ray micro-CT of GEOIL mortars at 20% oil (a, b and e) or 40% oil (c, d and f). Top two lines: example of a raw image (left), and the same image after segmentation (right). Third line provides the 3D representation of the 3D oil structures (top line: crop of 896×958×600 µm 3 , and bottom line: crop of 1254 ×889×621 µm 3 ). 47

Figure 6 :

 6 Figure 6: Oil droplet size distribution of GEOIL composites by image analysis of X-Ray micro-computed tomography: (1) mortar GEOIL20 S/C=1 (crop of 1374×1395×600 µm 3 ) and (2) mortar GEOIL40 S/C=1 (crop of 1254 ×889×621 µm 3 ). Both percolating and non percolating systems are accounted for.

Figure 7 :

 7 Figure 7: Nitrogen sorption-desorption isotherms (top) and BJH differential PSD or desorption (bottom) for the three main GP cement mortars REF-MK (100%MK), REF-MKO (100%MK and oil traces) and REF-MKS (with 20%wt slag substitution).

  -19 m 2 , i.e. by one order of magnitude. This decrease is larger than the one observed with REF-MK. It is inferred that the alkaline water flow through the slag-substituted mortar helps advancing the hydration of slag, by forming more C-A-S-H, owing to a better affinity with slag than with 100%MK. From a durability point of view, the REF-MKS mortar has better performance than REF-MK, with smaller K water , when tested with a synthetic Portland concrete porewater. This is well-known in the literature, see for instance Zhang et al.[START_REF] Zhang | Potential application of geopolymers as protection coatings for marine concrete: II. Microstructure and anticorrosion mechanism[END_REF].

	GEOIL at 20% oil.

  ), Trona (Na 3 H(CO 3 ) 2 .2H 2 O, PDF 00-029-1447) and Thermonatrite (Na 2 CO 3 .H 2 O, PDF 00-008-0448) are the two main phases explaining the diffractogram peaks. These two phases relate to the Na 2 O content in the formed solids (leached out of the samples and carbonated in the ambient air), whereas the organic matter is attributed to oil leaching from the GEOIL60 mortar. An amorphous content is also visible on the XRD diffractogram, as a significant bump between θ=13 and 27 o . It is attributed to the organic matter present in In the absence of oil, GP mortar REF-MKS also displays a significantly smaller water permeability K water than the REF-MK GP mortar, with values of 1.9×10 -19 m 2 and 1.4×10 -18 m 2 respectively, once equilibrium is reached. However, when mixed with 20%vol oil, the water permeability of REF-MK mortar remains stable with time, whereas GEOIL based on REF-MKS displays a significantly increased K water with time. This means that a GP made of 80%MK-20%slag does not provide durable GEOIL composites.

	In this contribution, water flow mechanisms through GEOIL (geopolymer-oil) composites
	are elucidated, and water permeability is quantified.
	Two distinct GP cements are compared, one made with 100%MK (REF-MK) and
	the other with 80%MK-20% slag (REF-MKS). Both display similar compressive strengths
	without oil, while the sample with 20% slag retains better strength at higher oil amounts
	up to 40%.
	the solids.
	20

Table 1 :

 1 The measured low permeability values are explained by the presence of additives (surfactants) in the oil. Surfactants hinder water flow through the GEOIL composites. They need to be more clearly identified, or analogues need to be selected, in order to obtain a significantly smaller K water for GEOIL mortars than GP mortars alone, whatever the oil emulsified in GP. Surfactant presence ensures improved long term durability of the GPoil composites. It is particularly useful when formulating peculiar oils or organic liquids, which do not incorporate these surfactants initially. four full rotations (4 × 360 o ), a detector frame rate of 4 to 3 images/s depending of the sample, and a number of averaged radiographies of 6. After the reconstruction procedure and acquisition artifacts removal, an output volume is computed, composed of 500 images in a (X, Y) plane (at 1.11-1.91 × 10 6 pixels 2 each) regularly spaced from one another by 1 micron. The corresponding voxel size is of 1 micron. The total volume of each sample corresponds to huge datasets of 557-958 x 10 6 voxels 3 , linearly scaled into a 16-bit format, corresponding to sample volumes of 0.557-0.958 mm 3 (Table4).Image analysis. Image analysis is performed with the ImageJ software, allowing to use the whole image datasets. Filtering and thresholding procedures are tuned in order to get as close as possible to the imposed %oil of 20 or 40% The greylevel histogram of the whole stack is calculated, and filtering aims at allowing the histogram to span the whole available grayscale (0-255), and to separate the existing peaks (one corresponds to darker pores and the other to lighter solids). While controlling the stack histogram at each step of the procedure, the contrast of the image stack is enhanced (with the Enhance Composition of the raw materials used to make the two GP cement pastes, either with 100% MK OR 80%MK, 20%GGBFS, obtained by X Ray fluorescence. H 2 O content is obtained by vaccum oven-drying prior to X Ray fluorescence of the solid residue.

	Contrast tool in ImageJ). Secondly, applying the Gaussian Blur filter with a 2 pixel radius
	removes high spatial frequencies (blurring the image but keeping all area edges), and helps
	separate the two main histogram peaks. The Window/Level Auto tool in ImageJ spans the
	stack histogram over the whole available range, and the Beat plugin/Background removal
	tool [67] removes any shading effect on the images. Finally, thresholding is performed at
	the greylevel corresponding to the local minimum (or, if there is no such minimum, to
	the inflexion point) between the two main peaks of the stack histogram, hence providing
	binary image stacks.
	From the binary image stack (pores in black and solids in white), porosity is derived and
	a pore size distribution is calculated with the Beat plugin/Pore Size Distribution (Discrete
	Method, 2D PSD -average of all slices).

Table 2 :

 2 Composition of GP mortars investigated in this research, potentially added with GGBFS or oil. Label ( * ) refers to a sodium silicate solution made with oil traces, see text for manufacturing details.

	Mortar	Oil	Particular	Duration of
	formulation	proportion	mixing	permeability
	name	(%)	conditions	test (days)
	REF-MK	0	-	7
	REF-MKO	0	presence of oil	9
			traces in the
			activating solution
	GEOIL20-MK	20	-	40
	GEOIL40-MK	40	-	8
	GEOIL60-MK	60	-	18
	REF-MKS	0	-	29
	GEOIL20-MKS	20	-	15

Table 3 :

 3 Summary of the GP mortar samples tested for water permeability, with total test duration.

		GEOIL20-MK GEOIL40-MK
		(S/C=1)	(S/C=1)
	Imposed oil percentage	20	40
	(% total vol)
	Image size	1374 x 1395	1254 x 889
	(pixels 3 )	x 500	x 500
	Analyzed sample size (mm 3 )	0.958	0.557
	Greylevel at threshold (0-255)	91	73
	Measured oil percentage	9.03	23.97
	above 2 µm (% total vol)
	Percolating oil percentage	6.70	23.54
	above 2 µm (% total vol)
	Oil droplet diameter d 50 (µm)	12	30

Table 4 :

 4 Main sample characteristics and results of the percolation analysis for the two GEOIL mortars, at imposed 20 or 40%vol oil, from 3D X Ray micro-computed tomography images (voxel size: 1 micron; oil droplet detection threshold: 2 microns and above).

days water injection (a crown of yellowish solids is clearly visible around the shaft) and (b) qualitative XRD results of the solid products formed, showing the identification of two main phases (see text for reference). . . . 53

Acknowledgments

We gratefully acknowledge the French Centre National de la Recherche Scientifique (CNRS) for funding the one year délégation of Prof. C. A. Davy at Sorbonne Université, Campus de Jussieu, laboratoire METIS, UMR 7619, Paris, France. Scientific input by Dr Christel Pierlot, UCCS, is also gratefully acknowledged.

Water permeability set-up

The sample is wrapped in a Vitton protective membrane and placed in the triaxial cell, which is filled with confining fluid at a given hydrostatic stress of 3 MPa (+/-0.2). This avoids any fluid leakage around the sample; fluid passes consistently through its pores, while being a low enough value not to damage the mortar (compared to the material compressive strength). The triaxial cell also allows to inject fluid at a given pressure P up on its upstream side, while the sample downstream side is at atmospheric pressure P down =P a . This provides an imposed pressure gradient ∆P = P up -P down of 0.5 MPa. This value is chosen in order to be higher than the expected capillary threshold pressure of the oil-water system. The injection pump (Gilson, USA) provides volumetric fluid flowrate Q v (in m 3 /s) versus time, at a rate of 1 data point every minute.

List of Tables