Virginie Wiktor

Philippe Grosseau
email: grosseau@emse.fr

René Guyonnet

Eric Garcia-Diaz

Christine Lors

Rene Guyonnet

Eric Garcia- Diaz

Accelerated weathering of cementitious matrix for the development of an accelerated laboratory test of biodeterioration

Keywords: Biodeterioration, Cement paste, Carbonation, Leaching, Accelerated weathering, Microbial growth

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

I. Introduction

Micro-organisms-bacteria, cyanobacteria, fungi, algae, and lichens-are liable to grow on building materials. Biological activity contributes to deterioration of building material, and its interaction with physico-chemical mechanisms is considered central to understanding the long term deterioration [START_REF] Saiz-Jimenez | Biodeterioration vs biodegradation: the role of microorganisms in the removal of pollutants deposited on historic buildings[END_REF]. Physical, chemical, and biological agents act in co-association, ranging from synergistic to antagonistic, to deteriorate stone [START_REF] Warscheid | Biodeterioration of stone: a review[END_REF]. In Civil Engineering the most widely used material is concrete for different kind of application: bridge construction, sewer pipes, buildings and in some case during restoration of Cultural Heritage Monuments. Cement plays an essential role in concrete works behaviour, because it provides its mechanical resistance [START_REF] Guillon | Durabilité des matériaux cimentaires-Modélisation de l'influence des équilibres physico-chimiques sur la microstructure et les propriétés mécaniques résiduelles[END_REF]. Cement based materials are porous, may contain organic adjuvants, and thus possess an important primary bioreceptivity. Bioreceptivity, as defined by Guillitte [START_REF] Guillitte | Bioreceptivity: a new concept for building ecology studies[END_REF], is the totality of materials properties that contribute to the establishment, anchorage and development of fauna and/or flora. Primary bioreceptivity is the initial potential of colonisation [START_REF] Guillitte | Bioreceptivity: a new concept for building ecology studies[END_REF].

Fungi are among the most harmful organisms associated to biodeterioration of organic and inorganic materials [START_REF] Urzì | Recent advances in the molecular biology and ecophysiology of meristematic fungi[END_REF]. Their occurrence on the stones is reported to be combined not only with aesthetic spoiling of the monuments, due to colour changes and black spots, but also there is strong evidence that these organisms can colonize deeper cracks, cause crater shaped lesions, chipping and exfoliation of the rock surface combined with the loss of materials [START_REF] Urzì | Recent advances in the molecular biology and ecophysiology of meristematic fungi[END_REF][START_REF] Wollenzien | On the isolation of microcolonial fungi occurring on and in marble and other calcareous rocks[END_REF]. In general, two main groups of fungi are usually isolated from rock surfaces [START_REF] Leo | Fungal colonization on treated and untreated stone surfaces[END_REF]: (i) one group includes species of the genera of Hyphomycetes and Coelomycetes among which are included those that do not produce melanin, like Fusarium, Penicillium, Aspergillus, Phoma and those that are black pigmented like Alternaria, Ulocladium, Cladosporium melanin producers. They are fast growing fungi. (ii) The second group includes the so-called black yeasts and meristematic fungi. They are a wide and heterogeneous group of black pigmented fungi that share common characteristics such as the presence of melanins within the cells (swollen cells), hyphae and/or spores. The production of melanin and the meristematic development allow them to survive in stressed environmental conditions like low humidity and high sun irradiation [START_REF] De Hoog | Evolution of black yeasts: possible adaptation to human host[END_REF]. For this group of fungi is also used the term of rock-inhabiting fungi to underline the exclusive isolation of many of them from rock surface [START_REF] Leo | Fungal colonization on treated and untreated stone surfaces[END_REF]. The black fungi have the capacity to: (i) settle on the rocks surface, (ii) attach firmly to the surface and (iii) penetrate deeper into the rock. The major aesthetic damage, however, may occur when environmental conditions do not force fungi into the crevices, but due to favourable conditions for fungal growth they spread over the rock surface [START_REF] Diakumaku | Black fungi in marble and limestones-an aesthetical, chemical and physical problem for the conservation of monuments[END_REF]. To preserve constructions from fungal colonisation and to act efficiently against fungal biodeterioration, it is necessary to have a better understanding of biodeterioration mechanisms and its effects on materials properties. Synthetic stone such as concrete, brickwork and mortar show microbial colonisation once their initial highly alkaline pH (>12) falls after a period of weathering (<10) [START_REF] Allsopp | Introduction to biodeterioration[END_REF]. At the moment, tests to study biodeterioration of building materials exist. Some of them were developed without accelerated weathering of the matrix and so lead to experiment time ranging from 7 to 15 months [START_REF] Oshima | A study on growth of fungus and algae on mortar[END_REF][START_REF] Urzì | Evaluation of the efficiency of water-repellent and biocide compounds against microbial colonization of mortars[END_REF]. On the other hand, accelerated weathering of the matrix may reduce the duration of the experiment by increasing matrix bioreceptivity. Whereas accelerated weathering of the matrix to enhance its bioreceptivity is not truly innovative in biodeterioration study [START_REF] Barberousse | Etude de la diversité des algues et des cyanobactéries colonisant les revêtements de façade en France et recherche des facteurs favorisant leur implantation[END_REF][START_REF] Moraes Pinheiro | Alteration of concrete microstructure by biodeterioration mechanisms[END_REF][START_REF] De Muynck | Evaluation of strategies to prevent algal fouling on white architectural and cellular concrete[END_REF][START_REF] Dubosc | Etude du développement de salissures biologiques sur les parements en béton: mise au point d'essais accélérés de vieillissement[END_REF][START_REF] Escadeillas | Accelerated testing of biological stain growth on external concrete walls. Part 1: Development of the growth tests[END_REF][START_REF] Nielsen | Mould growth on building materials under low water activities. Influence of humidity and temperature on fungal growth and secondary metabolism[END_REF][START_REF] Shirakawa | The development of a method to evaluate bioreceptivity of indoor mortar plastering to fungal growth[END_REF], only very few studies [START_REF] Moraes Pinheiro | Alteration of concrete microstructure by biodeterioration mechanisms[END_REF][START_REF] Shirakawa | The development of a method to evaluate bioreceptivity of indoor mortar plastering to fungal growth[END_REF] focus on the accelerated test for fungal biodeterioration. An accelerated laboratory test which allows us to compare the growth of three fungal strains and the biodeterioration involved on a cementitious matrix was developed. We paid particular attention to accelerate the matrix weathering so as to not exceed 3 months of experiment. Three fungal strains were selected for the test in order to represent main kind of fungi involved in biodeterioration in natural environment [START_REF] De La Torre | Fungi in weathered sandstone from Salamanca cathedral, Spain[END_REF][START_REF] Garcia-Valles | Coloured mineral coatings surfaces as a result of biomineralization: the case of the Tarragona cathedral (Catalonia)[END_REF][START_REF] Sterflinger | Geomicrobiological investigations on the alteration of marble monuments by dematiaceous fungi (Sanctuary of Delos, Cyclades, Greece)[END_REF][START_REF] Urzì | Sampling with adhesive tape strips: an easy and rapid method to monitor microbial colonization on monument surfaces[END_REF][START_REF] Urzì | Colour changes of Notos calcareous sandstone as related to its colonisation by microorganisms[END_REF][START_REF] Warscheid | Biodeterioration of stone: a review[END_REF][START_REF] Wollenzien | On the isolation of microcolonial fungi occurring on and in marble and other calcareous rocks[END_REF]: Alternaria alternata to represent a melanin producer Hyphomycete, Exophiala sp. for yeast like fungi, and Coniosporium uncinatum (MC 557) as meristematic fungi. Aesthetical and physical biodeterioration was expected from the development of Alternaria alternata, Exophiala sp., Coniosporium uncinatum with hyphae penetration inside the matrix. The aim of this paper is to present the accelerated weathering operation which promotes fungal development on hardened cement paste: carbonation and leaching operations are performed to reduce matrix surface pH. The specimens (weathered and non-weathered) are first physico-chemically characterized, and then influence of the matrix weathering on the fungal development is studied.

II. Background

Once the cement based material has been made, leaching and ageing, and more specifically carbonation, start changing the original characteristics of the material. In nature, carbonation and leaching occur intermittently and alternately during the life cycle of the material [START_REF] Van Gerven | Effects of carbonation and leaching on porosity in cement-bound waste[END_REF].

I II I. .1 1. . C
Ca ar rb bo on na at ti io on n Carbonation is considered the most common chemical reaction influencing the performance of cement based materials in natural environment scenarios [START_REF] Macias | Impact of carbon dioxide on the immobilization potential of cemented wastes: chromium[END_REF]. The penetration of gaseous carbon dioxide within partially saturated concrete usually initiates a series of reactions with both ions dissolved in the pore solutions and the hydrated cement paste. The whole process can be summarized as a series of different steps [START_REF] Gervais | The effects of carbonation and drying during intermittent leaching on the release of inorganic constituents from a cement-based matrix[END_REF][START_REF] Glasser | Durability of concrete-degradation phenomena involving detrimental chemical reactions[END_REF][START_REF] Thiery | Investigation of the carbonation front shape on cementitious materials: effects of the chemical kinetics[END_REF]: Gaseous carbon dioxide first penetrates the material. It then dissolves in pore solution mainly as HCO 3 -and CO 3 2-. The CO 3 2-species then reacts with dissolved calcium to precipitate calcite, CaCO 3 , as well as other CO 2 -based solid phases. The pH drop associated with reactions leads to the dissolution of portlandite. Because carbonation products have higher molar volumes than the parent hydrates Ca(OH) 2 and C-S-H, a decrease of porosity is expected [START_REF] Gervais | The effects of carbonation and drying during intermittent leaching on the release of inorganic constituents from a cement-based matrix[END_REF][START_REF] Van Gerven | Effect of CO2 on leaching from a cement-stabilized MSWI fly ash[END_REF]. Progressive carbonation eventually leads to depletion of portlandite and subsequently decalcification of C-S-H to provide new portlandite for the carbonation reaction [START_REF] Gervais | The effects of carbonation and drying during intermittent leaching on the release of inorganic constituents from a cement-based matrix[END_REF]. This decomposition of C-S-H is reported to increase [START_REF] Gervais | The effects of carbonation and drying during intermittent leaching on the release of inorganic constituents from a cement-based matrix[END_REF] or decrease [START_REF] Ngala | Effects of carbonation on pore structure and diffusional properties of hydrated cement pastes[END_REF] porosity. Since carbonation process in natural environments may be very slow, laboratory experimentation has been conducted under a variety of carbon dioxide levels. Many of the studies used low levels of CO 2 (i.e. 4-5% CO 2 ) [START_REF] Anstice | The pore solution phase of carbonated cement pastes[END_REF][START_REF] Dewaele | Permeability and porosity changes associated with cement grout carbonation[END_REF]. Accelerated carbonation studies using 100% CO2 [START_REF] Anstice | The pore solution phase of carbonated cement pastes[END_REF][START_REF] Fattuhi | Concrete carbonation as influenced by curing regime[END_REF][START_REF] Klemm | Accelerated curing of cementitious systems by carbon dioxide: part I. Portland cement[END_REF][START_REF] Lange | Effect of carbonation on properties of blended and non-blended cement solidified waste forms[END_REF][START_REF] Valls | Accelerated carbonation of sewage sludge-cement-sand mortars and its environmental impact[END_REF]; allow the characterization of carbonation processes in much shorter times. I II I. .2 2. . L Le ea ac ch hi in ng g The leaching of ions (mainly calcium and hydroxide) from the pore solution to the external environment is responsible for the dissolution of portlandite and C-S-H [START_REF] Glasser | Durability of concrete-degradation phenomena involving detrimental chemical reactions[END_REF]. The leaching of calcium is a coupled dissolution/diffusion process. Leaching by deionized water induces calcium and hydroxide concentration gradients which continuously decrease from the sound zone to the exposed surface of the material. This causes the diffusion of calcium and hydroxide ions from the pore solution to the aggressive solution, and thus lowers the amount of calcium concentration in the pore solution [START_REF] Glasser | Durability of concrete-degradation phenomena involving detrimental chemical reactions[END_REF]. Loss of calcium leads to the dissolution of portlandite and secondary precipitations of AFm, ettringite and calcite [START_REF] Faucon | Behaviour of crystallised phases of Portland cement upon water attack[END_REF][START_REF] Faucon | Long-term behaviour of cement pastes used for nuclear waste disposal: review of physico-chemical mechanisms of water degradation[END_REF][START_REF] Glasser | Durability of concrete-degradation phenomena involving detrimental chemical reactions[END_REF]. The degraded zone induced by water exposure is characterized by a decalcification of C-S-H inducing a silicate polymerization. The Ca/Si ratio of the C-S-H gradually decreases between sound and leached zone [START_REF] Glasser | Durability of concrete-degradation phenomena involving detrimental chemical reactions[END_REF][START_REF] Hidalgo | Microstructural characterization of leaching effects in cement pastes due to neutralisation of their alkaline nature: part I: Portland cement pastes[END_REF][START_REF] Mollah | An FTIR and XPS investigations of the effects of carbonation on the solidification stabilization of cement-based systems-Portland type V with zinc[END_REF]. Other factors affecting the rate and amount of leaching include the type and amount of constituent investigated, the alkalinity of the matrix, and the chemical properties of the pore water and leachant (pH, ionic strength, etc,...) [START_REF] Van Gerven | Effects of carbonation and leaching on porosity in cement-bound waste[END_REF]. Alteration of the chemical behaviour of the matrix resulting from carbonation influences the release of species from the matrix during leaching [START_REF] Garrabrants | Changes in constituent equilibrium leaching and pore water characteristics of a Portland cement mortar as a result of carbonation[END_REF]. Carbonation and leaching are widely investigated together in studies for the use of hazardous wastes in cementitious matrix. Many studies have investigated the effect of carbonation on the leaching of heavy metals [START_REF] Garrabrants | Changes in constituent equilibrium leaching and pore water characteristics of a Portland cement mortar as a result of carbonation[END_REF][START_REF] Lange | Effect of carbonation on properties of blended and non-blended cement solidified waste forms[END_REF][START_REF] Sanchez | Leaching of inorganic contaminants from cement-based waste materials as a result of carbonation during intermittent wetting[END_REF][START_REF] Valls | Accelerated carbonation of sewage sludge-cement-sand mortars and its environmental impact[END_REF][START_REF] Van Gerven | Influence of carbonation and carbonation methods on leaching of metals from mortars[END_REF][START_REF] Van Gerven | Effects of carbonation and leaching on porosity in cement-bound waste[END_REF]. In these studies, the carbonation is mainly performed during the cement hydration. Few focuses on the leaching of carbonated specimens after the curing period, from microstructural point of view. With respect to the effect on pore size distribution, both leaching and carbonation decrease the amount of smaller pores [START_REF] Van Gerven | Effects of carbonation and leaching on porosity in cement-bound waste[END_REF]. The research presented here was carried out to evaluate the changes undergone by the cementitious matrix as a result of accelerated weathering. The specific objectives were (i) to characterize the cementitious matrix after each weathering step (carbonation and leaching), and (ii) to evaluate the influence of the matrix accelerated weathering on the fungal growth. The microstructure of the cementitious matrix is investigated by X-ray diffraction (DRX), Fourier Transform Infrared spectroscopy (FT-IR), Scanning Electron Microscopy with X-ray microanalyses (SEM/EDS).

III. Materials and methods

I II II I. .1 1. . M Ma at tr ri ix x p pr re ep pa ar ra at ti io on n The study was conducted with ordinary white Portland cement CEM I 52.5 R. The Water/Cement ratio determines the capillary porosity of the cement paste: the higher this ratio, the higher the capillary porosity, but also the higher is the bleeding effect. A higher porosity favours microbial development [START_REF] Guillitte | Bioreceptivity: a new concept for building ecology studies[END_REF]. The best compromise between bleeding effect and capillary porosity is obtained in our case with a W/C mass ratio of 0.55. Hardened cement paste samples are prisms, 1 × 2.5 × 8.5 cm. Samples were demolded 24 h after production and stored 28 days at 100% relative humidity, at room temperature. Accelerated weathering is performed with carbonation for 48 h, or carbonation (48 h) and leaching operation (28 days) as described by Wiktor et al. [START_REF] Wiktor | Biodétérioration d'une matrice cimentaire par les champignons: influence du vieillissement accéléré sur le développement fongique[END_REF]. Briefly, specimens are exposed to pure CO 2 flow under controlled relative humidity (65% HR) for carbonation. Then, the carbonated specimens are dived into deionized water which is totally renewed in 24 h. Specimens were exposed overnight to UV light for sterilization. I II II I. .2 2. . A An na al ly ys se es s III. [START_REF] Anstice | The pore solution phase of carbonated cement pastes[END_REF]

.1. X-ray diffraction (DRX)

The XRD analyses were carried out using Cu K α radiation (λ = 1.5406 Å) on Brukner D8 advance device. The XRD scans were recorded from 5 to 59° 2θ with 0.040 step width and 5 s counting time. The analyses were directly performed on the bulk specimen; hence about the first hundred micrometers were analyzed.

III.2.2. Fourier transform infrared spectroscopy (FT-IR)

FT-IR analyses were carried out with Biorad FTS 185 (digilab) equipment using Attenuated Total Reflexion (ATR) unit. The spectra were usually recorded in the range of 4,000-400 cm -1 with 2 cm -1 resolution, and 32 scans were collected each time. The ATR analyses require very small amount of sample (~1 mg), and furthermore no preparation or dilution of the sample is needed. The surface of our specimens was gently scraped and analysed. Hence FT-IR analyses provided information from the first 500 μm.

III.2.3. Scanning electron microscope equipped with X-ray microanalyses (SEM/EDX)

The fractured surface samples were obtained from the specimens non-weathered, carbonated then leached, and carbonated only. Specimens' cross-sections were also prepared. They were vacuum impregnated with epoxy resin, and polished sections were prepared in the Civil engineering department of Ecole des Mines de Douai. For the investigation by SEM and EDX, the samples were dried in a desiccator for 24 h, and subsequently gold/palladium coated. Observations were carried out with a JEOL 840 SEM/EDX. Figure 1 shows the different areas analyzed for the EDX analyses. Each analysis is performed on a surface about 20 × 200 μm. Calcium, silicon, and aluminium are assayed. I II II I. .3 3. . M Mi ic cr ro ob bi io ol lo og gi ic ca al l t te ec ch hn ni iq qu ue es s III.3.1. Fungal strains and cultural media Alternaria alternata and Exophiala sp. were isolated from a monument in India (unpublished data). Coniosporium uncinatum MC557 was isolated from a Carrara marble statue located in the court-yard of the Messina Museum, Italy [START_REF] Leo | Two Coniosporium species from rock surfaces[END_REF]. These isolated strains were kept in the collection of Department of Microbiological, Genetic and Molecular Sciences in Messina. Alternaria alternata was cultivated in solid medium (Potato Dextrose Agar-PDA, Oxoid) for 5 days at 26°C. Exophiala sp. and Coniosporium uncinatum were cultivated in liquid medium (Malt Extract Broth-MEB Oxoid) for 5 days at 26°C. Nutritive medium used for biodeterioration test is composed of 1× concentrated Yeast Nitrogen Broth (YNB) (Difco) plus the addition of Glucose 0.01%.

III.3.2. Fungal units' suspension

Fungal colonies growing in solid medium were scraped from the agar surface and conidia suspended in physiological solution (0.9% of NaCl in distilled water), while the cultures in liquid medium were directly transferred in physiological solution. Each suspension was centrifuged 15 min at 2,095g (3,000 rpm with a Beckman and Coulter Allegra X-12R centrifuge-4750 A rotor). Centrifugation pellets were put in suspension in 1 ml of physiological solution. The number of fungal units was determined through a direct microscopic count in a counting chamber (Bürker chamber, ProSciTech, Australia) and adjusted at a concentration of 8.7 × 10 5 fungal units/ml in YNB 1× + glucose 0.01% medium. I II II I. .4 4. . B Bi io od de et te er ri io or ra at ti io on n t te es st t III.4.1. Experimental set up Polyethylene boxes of 9.5 × 9.5 × 9.5 cm 3 were used. In order to keep the humidity inside the box, the bottom was covered by vermiculite; paper sheet is disposed on it to avoid the direct contact between specimens and vermiculite. Boxes are autoclaved 15 min at 120°C, then sterile water is added on vermiculite to wet it. Two specimens of each matrix were disposed in each box (Figure 2). III.4.2. Inoculation Each specimen was inoculated with 1.5 ml of fungal units' suspension, except controls (only 1.5 ml of sterile medium). Inoculation was performed in duplicate, thus for each strain, 6 specimens were inoculated (2 unweathered, 2 carbonated, 2 carbonated and leached), and placed in 3 different boxes. Boxes were incubated at 26°C. III.4.3. SEM observation After 4 weeks of incubation, one specimen of each box was taken and broken in small pieces for SEM observation (Figure 3). Fixation of samples was performed overnight at 4°C in buffered aldehyde fixative 2% (w/v) formaldehyde (P6148, Sigma). Samples were then washed 3 times in 0.01 M phosphate buffer (0.01 M NaH 2 PO 4 .H 2 O, 0.01 M Na 2 HPO 4 , pH 7.2) for 10 min each. Dehydration was made in graded ethanol series (70, 85, 95% (v/v) and in anhydrous ethanol (100%) for 15 min each). Samples were air dried and then coated with gold for SEM observation (JEOL 840 SEM).

IV. Results and discussion

I IV V. .1 1. . A
Ac cc ce el le er ra at te ed d w we ea at th he er ri in ng g IV. [START_REF] Allsopp | Introduction to biodeterioration[END_REF]

.1. XRD analyses

The assignments of the XRD peaks to phases were based on 2θ values and the corresponding d-spacing (Figure 4). The peaks corresponding to portlandite appear at 4.900, 3.112, 2.628, 1.927, 1.796, and 1.687 Å. The main peaks corresponding to ettringite are present at 9.671, 5.583, and 3.859 Å. Presence of calcite (CaCO 3 ) and gypsum can be observed but at a lower extent. Additional peaks are noticed on the diffractogram of the carbonated specimen compared to non-weathered one. They are attributed to the formation of calcium carbonate polymorphsaragonite and vaterite-as a result of accelerated carbonation. The decrease in relative intensity of portlandite peaks is due to the dissolution of portlandite to form calcium carbonate. The presence of portlandite can be due to the fact that the depth investigated is larger than the carbonation front. The carbonation operation was performed for 48 h, hence the specimen is only superficially carbonated. The diffractogram of the carbonated and leached specimen exhibits only calcite and aragonite peaks. The disappearance of portlandite and ettringite is a consequence of their dissolution as a result of leaching. Vaterite is the less stable calcium carbonate polymorph, and it is easily transformed into other forms.

IV.1.2. FT-IR analyses Usually band assignments reported in literature are deducted from spectra performed in diffuse reflectance. In the present work, spectra acquisition is mainly performed in ATR. Band assignments on ATR spectrum are deducted from spectrum performed first in diffuse reflectance (Table 1). A shift of about 10-20 wave number units is noted between both spectra (Figure 5). Nevertheless the band assignments for hydrated cement paste are in close agreement with those previously reported [START_REF] Farcas | La spectrométrie infrarouge à transformée de Fourier (IRTF): Une méthode intéressante pour la caractérisation des ciments[END_REF][START_REF] Mollah | Chemical and physical effects of sodium lignosulfonate superplasticizer on the hydration of Portland cement and solidification/stabilization consequences[END_REF][START_REF] Mollah | An X-ray diffraction (XRD) and Fourier transform infrared spectroscopic (FT-IR) investigation of the long-term effect on the solidification/stabilization (S/S) of arsenic(V) in Portland cement type-V[END_REF]. The band at 3,645 cm -1 is due to the OH band from Ca(OH) 2 [START_REF] Mollah | A Fourier transform infrared spectroscopic investigation of the early hydration of Portland cement and the influence of sodium lignosulfonate[END_REF]. The water bands appear at approximately 3,440 (stretching) and 1,660 (bending) cm-1 [START_REF] Mollah | An X-ray diffraction (XRD) and Fourier transform infrared spectroscopic (FT-IR) characterization of the speciation of arsenic (V) in Portland cement type-V[END_REF]. The bands at 1,480-1,430, 875 and 720 cm -1 are due to carbonates. The wide and deep band between 1,480 and 1,430 cm -1 is attributed to ν 3 of CO 3 2-and the sharp bands at 875 and 720 cm -1 are due to ν 2 and ν 4 vibrations, respectively [START_REF] Mollah | An X-ray diffraction (XRD) and Fourier transform infrared spectroscopic (FT-IR) characterization of the speciation of arsenic (V) in Portland cement type-V[END_REF]. The band at 975 cm-1 is due to the Si-O asymmetric stretching band (ν3) from C-S-H [START_REF] Mollah | Chemical and physical effects of sodium lignosulfonate superplasticizer on the hydration of Portland cement and solidification/stabilization consequences[END_REF]. The spectral data changes upon weathering and analyses of these changes can provide valuable information about the reactions involved. The FT-IR data for band assignments are presented in Table 2. The OH band at 3,636 cm -1 from portlandite disappears with carbonation. However, XRD analyses show the presence of portlandite on carbonated specimen, but in small amounts. The majority of portlandite is mainly transformed during carbonation operation, and totally during leaching. The small amounts present on the carbonated specimen are probably not detectable with FT-IR, certainly due to smaller penetration depth of analysis on FT-IR compared to XRD. The FT-IR spectra of weathered specimens (carbonated and leached or carbonated only) show a strong broad band at 1,414-1,440 cm -1 , which is broader than the calcite band, characteristic of aragonite. According to the intensity it can be assumed that the aragonite overlays the calcite band. New bands appear at 1,080, 858, 702 cm -1 and are assigned to aragonite formed as a result of accelerated carbonation. The band at 742 cm -1 belonging to vaterite, appears only in the carbonated specimen. Other characteristic bands at 860, 850, and 710 cm -1 may be overlaid by calcite bands. Vaterite is a less stable polymorph of calcium carbonate and thereby transforms into more stable forms calcite and aragonite with leaching operation. The Si-O stretching (ν 3 ) band now appears at approximately 970 and 980 cm -1 for the carbonated and carbonated then leached specimens respectively, while the same band in the non-weathered specimen appears at 959 cm -1 . Hence, this shifting to higher frequency is caused by the polymerization of the orthosilicate units (SiO 4 4-) during weathering process. The magnitude of this shift is indicative of the degree of polymerization [START_REF] Mollah | Chemical and physical effects of sodium lignosulfonate superplasticizer on the hydration of Portland cement and solidification/stabilization consequences[END_REF].

IV.1.3. SEM observations Figure 6 shows surface fractures. Platy crystals and fine needles are the distinguishing characteristic shapes of portlandite and ettringite respectively [START_REF] Diamond | Cement paste microstructure-an overview at several levels[END_REF]. A thin layer covers progressively the specimen surface as accelerated weathering progresses. This is consistent with the weathering performed: carbonation results in a decrease in the amount of portlandite and the leaching gives rise to the dissolution of portlandite and ettringite. A net decrease in porosity is also observed: pores are progressively filled by CaCO 3 precipitation and become smaller and smaller as the matrix is weathered. Now, if one examines the following images of polished sections, microstructure alteration is not observed between non-weathered and carbonated specimens (Figure 7a,b). Contrarily, numerous cracks are observed for the carbonated and leached specimen (Figure 7c). These cracks are distributed along the periphery (Figure 8). This phenomenon may be explained by one of two scenarios: (i) the cracks appear progressively during leaching operation, this could be confirmed by direct observation or (ii) the matrix structure is weakened by the leaching operation, and thereby the cracks are due to the sample preparation. We also notice for this sample the presence of a thin band which seems to be more porous than the rest of specimen (Figure 7c). Its thickness is estimated to be 70 μm.

IV. 1.4. SEM/EDX analyses Chemical change was expected from EDX analysis. Figure 9 presents EDX analysis performed on carbonated and leached specimens. Twenty punctual analyses are performed for each area; mean values are plotted on graphs.

We compare concentrations in calcium, silicon, aluminium between analyses performed on the side and in the core. We assume that accelerated weathering has a superficial impact, so the chemical composition in the core is representative of a non-weathered matrix. No change in chemical composition is observed, there is just a tendency of Ca enrichment and thereby Si depletion of specimen.

IV.1.5. Discussion Our results show that accelerated carbonation results in calcite, vaterite and aragonite formation (Figure 4; Table 2). This is consistent with literature. The presence of metastable forms (vaterite and aragonite) in the carbonated area seems to be a feature of accelerated carbonation tests [START_REF] Thiery | Investigation of the carbonation front shape on cementitious materials: effects of the chemical kinetics[END_REF]. According to Thiery [START_REF] Thiery | Modélisation de la carbonatation atmosphérique des matériaux cimentaires ; Prise en compte des effets cinétiques et des modifications microstructurales et hydriques[END_REF], the thermal stability of CaCO3 produced is lower as the carbonation level increases. This author also showed that the thermodynamically metastable forms of CaCO3 correspond to the carbonation of C-S-H. The shift of the ν 3 SiO 4 4- from the FT-IR spectrum (Table 2) to higher wave number units suggests also a polymerisation of C-S-H with the carbonation as well as with leaching operations [START_REF] Mollah | Chemical and physical effects of sodium lignosulfonate superplasticizer on the hydration of Portland cement and solidification/stabilization consequences[END_REF]. The carbonation reaction mainly has two effects on the cement matrix and its pore water solution [START_REF] Van Gerven | Effect of CO2 on leaching from a cement-stabilized MSWI fly ash[END_REF]: lowering of the pH of the pore water, and decreasing of the matrix porosity. This is because of the formation of calcite, which is less dense than portlandite, thereby can clog pores more easily. Evolution of total porosity with carbonation is dependent of w/c ratio and cement type. Generally, the leaching of cement paste gives rise to matrix decalcification due to the dissolution of portlandite and C-S-H decalcification. Haga et al. [START_REF] Haga | Effects of porosity on leaching of Ca from hardened ordinary Portland cement paste[END_REF] concluded from their work that the major leached constituents of hardened ordinary Portland cement are portlandite and C-S-H gel, and the large pore size, associated with the leaching of portlandite, will significantly affect the diffusion of leached constituents. In the present study leaching occurs after carbonation. The porosity decrease diminishes the leaching of matrix constituents, while the decrease in pH increases leaching. The effect of porosity, however, exceeds that of pH, resulting in a net decrease of leaching due to carbonation [START_REF] Van Gerven | Effects of carbonation and leaching on porosity in cement-bound waste[END_REF]. It can be assumed that the dissolution of portlandite and decalcification of C-S-H result in CaCO 3 precipitation in the pore volume. Gervais [START_REF] Gervais | Evaluation environnementale des perspectives de valorisation en BTP de scories de première fusion de plomb et de zinc[END_REF] found that carbonation prior to leaching operation results in a decrease of calcium release compared to the leaching of non-carbonated specimens. The mobility of Ca 2+ is reduced, and it may precipitate with CO 3 2-before, thus not reaching leachate. This could explain EDX analyses and lack of the matrix decalcification observed (Figure 9): most of the dissolved Ca 2+ precipitates into CaCO 3 before, hence not exiting the matrix with the outcome: enrichment in calcium near the specimen surface. I IV V. .2 2. . B Bi io od de et te er ri io or ra at ti io on n t te es st t IV.2.1. Observations of non-weathered specimens Figure 10 presents direct observations of the non-weathered specimens performed after 4 weeks of incubation. No microbial growth is noticed on all exposed specimen. Coniosporium uncinatum cells are dark pigmented and taller than those of other fungi inoculated. Hence, this results in the presence of black spots on the specimen. Moreover, when the suspension of cells is inoculated on the non-weathered specimens, it does not penetrate immediately into the matrix. Thus, cells can agglomerate on the surface, resulting in observation of black spots. This spots are, in this case, absolutely not due to the fungal development.

IV.2.2. Observations of carbonated specimens Figure 11 shows direct observations performed on the carbonated specimens just after inoculation (T0) and after 4 weeks of incubation (T4). No microbial growth is noted on the control and specimens inoculated with Exophiala sp. A fungal development is observed on one specimen inoculated with Coniosporium uncinatum. It appears after 3 weeks of incubation and looks like small dark area of about 1 cm diameter. Nevertheless, it doesn't seem to be characteristic of Coniosporium uncinatum development, but looks more like Alternaria alternata. While boxes are handled with lot of care, they remain very close during experiment, so a fungal contamination can easily occur. This should be confirmed with microscopic observations. Interesting results are obtained with the specimens inoculated with Alternaria alternata. Therefore, a fungal development is noticed after 1 week only of incubation for one specimen. For the second specimen inoculated, it appears after 3 weeks of incubation. The fungus grows until the fourth week. It appears as dark area, scarcely spread on the specimen surface. Hydrated cement paste are heterogeneous material, and there are likely to exist micro-regions that could be prone to preferential fungal colonisation due to e.g. differences in the carbonation rate. Differences in specimen porosity would lead to different rates of CO 2ingress, which in turn would produce varying degree of carbonation. This would result in differences in the pH values in the micro-niches, thus influencing fungal growth [START_REF] Shirakawa | The development of a method to evaluate bioreceptivity of indoor mortar plastering to fungal growth[END_REF]. The fungal spreading may also reflect the distribution of fungal cells on the specimen surface after inoculation.

IV.2.3. Observations of the carbonated then leached specimens Figure 12 presents direct observations performed on the carbonated then leached specimens. Microbial development is noticed on all the specimens after 1 week of incubation only. For the specimens inoculated with Exophiala sp. and surprisingly also controls, it appears due to pink coloured spots which seem to grow along the specimen cracks. It is certainly due a bacterial contamination. Moreover, the specimens were inoculated in two series with 1 day of interval: firstly inoculation with Exophiala sp., and controls were performed, then the day after with Alternaria alternata and Coniosporium uncinatum. The bacterial contamination occurred probably during the inoculation of the first series. Nevertheless, this contamination remains localized on restricted area and doesn't spread all over the surface. Exophiala sp. growth is noticed after the first week on both specimens inoculated. The bacterial contamination doesn't seem to prevent the fungal development. The growth is more pronounced on the periphery area of the specimen surface. No microbial element is observed on the SEM examination of the cross sections. Alternaria alternata development is observed since the first week of incubation. It is characterized first with mycelial development and then sporulation is observed on the surface. The growth appears more homogeneously than for the carbonated only specimens. SEM observations point out numerous hyphae on the surface (Figure 13a). Crystals, plates like, resulting probably from the sample preparation, are noted from observations of the specimen surface. The cross-section observations show also hyphae but in a more dispersed way (Figure 13b). It may be explained by the fact that fungi can penetrate inside the matrix only by the open porosity, provided the pore diameter is large enough, or through the existing cracks. Therefore, the space available for the fungal growth inside the matrix does not permit extensive development. Nevertheless, the resulting biodeterioration could be more intense inside the exposed material than on the surface. The development of Coniosporium uncinatum is observed since the first week of incubation. It is characterized by the increase of the number and the size of the black spots. Mycelial growth is noted as the incubation time increases. The development seems to occur all over the surface, rather homogeneously. SEM pictures of the surface exhibit the hyphae spread on all over the surface (Figure 13c). Moreover, hyphae are closely linked to the matrix, and even seem to penetrate inside it. Examinations of the cross-section (Figure 13d) clearly show hyphal penetration into the matrix through cracks. It points out the capacity of the fungi to develop not only superficially but also inside the matrix, via structural default, accentuating the physical deterioration.

IV.2.4. Discussion

The results point out that in the present study the microbial growth is promoted with the accelerated weathering of the matrix. It plays a major role. Decrease of surface pH increases considerably matrix bioreceptivity. The microbial colonisation is observed on some carbonated specimens and on all carbonated then leached specimens. Carbonation is the most common chemical reaction influencing cement-based materials in natural environmental scenarios [START_REF] Gervais | Evaluation environnementale des perspectives de valorisation en BTP de scories de première fusion de plomb et de zinc[END_REF][START_REF] Macias | Impact of carbon dioxide on the immobilization potential of cemented wastes: chromium[END_REF]. This is the reason why accelerated weathering of matrix is generally performed by carbonation [START_REF] Moraes Pinheiro | Alteration of concrete microstructure by biodeterioration mechanisms[END_REF][START_REF] Gervais | Evaluation environnementale des perspectives de valorisation en BTP de scories de première fusion de plomb et de zinc[END_REF][START_REF] Shirakawa | The development of a method to evaluate bioreceptivity of indoor mortar plastering to fungal growth[END_REF]. But cement based materials are also exposed to the elements (humidity, acidic rain, snow…) which leads to cement compounds leaching [START_REF] Barbieri-Albert | Altération de matrices cimentaires par des eaux de pluie et des eaux sulfatées: approche expérimentale et thermodynamique[END_REF]. In our case, carbonation is followed by leaching operation. This allows to obtain a surface pH about 8.8 after 30 days (data not shown), but also to have two ways of weathering involved in natural weathering of materials. To our knowledge, only Escadeillas et al. [START_REF] Escadeillas | Accelerated testing of biological stain growth on external concrete walls. Part 1: Development of the growth tests[END_REF] and de Muynck et al. [START_REF] De Muynck | Evaluation of strategies to prevent algal fouling on white architectural and cellular concrete[END_REF] performed these two ways of accelerated weathering of the matrix, but giving greater importance to the carbonation step. Physical weathering of a mineral substrate acts to enhance or accelerate rates of chemical, biomechanical and biochemical weathering and vice versa. The development of cracks, fissures and weathering rinds in rocks accelerates biological weathering by providing a niche that can easily be exploited by opportunistic micro-organisms [START_REF] Burford | Fungal involvement in bioweathering and biotransformation of rocks and minerals[END_REF]. The presence of Alternaria alternata and Coniosporium uncinatum, two melanin producers, was noted on the SEM observations of the cross-sections. Melanin pigmentation of rockinhabiting fungi confers extra-mechanical strength to the hyphae that are then better able to grow into crevices [START_REF] Th | Biodecay of cultural heritage as a space/time-related ecological situation-an evaluation of a series of studies[END_REF][START_REF] Gorbushina | Life on rocks[END_REF][START_REF] Sterflinger | Dematiaceous fungi as a major agent for biopitting Mediterranean marbles and limestones[END_REF]. Increased penetration augments contact of fungal hyphae and their metabolites with the rock, amplifying their geochemical influence on the mineral substrate. Expansion of hyphal growth into deeper rock layers also helps to protect the cells from the UV radiation [START_REF] Gorbushina | Life on rocks[END_REF]. One characteristic of meristematic fungi is the ability to form filamentous hyphae that develop from clump-like colonies to penetrate deep into rocks thus protecting themselves from environmental stresses. In this sense, the visible portion of melanized micro colonial fungi is like the tip-of-the-iceberg, because the hyphae can rapidly penetrate several mm to cm into hard rocks in search of more protected environments [START_REF] Gorbushina | Life on rocks[END_REF]. The fungi form explorative hyphae to seek nutrients on the surface, or mycelium during the growth. Under natural conditions, nutrient availability, even if small, enhances the chance of fungal units (single cells, conidia, or hyphae fragments) surviving and starting the colonization [START_REF] Urzì | Sampling with adhesive tape strips: an easy and rapid method to monitor microbial colonization on monument surfaces[END_REF][START_REF] Urzì | Evaluation of the efficiency of water-repellent and biocide compounds against microbial colonization of mortars[END_REF].

V. Conclusion

Results showed that there was no microbial development when the cementitious matrix was not weathered. Microbial growth was noted on and inside the carbonated then leached specimens. Relating to the matrix weathering; combined process led to the maximum microbial growth. This process was composed by a carbonation operation followed by the leaching of the matrix. This resulted in modification of the matrix microstructure and above all in the surface pH decrease. Carbonation of hardened cement paste led to a surface composition nearer of a limestone than that of a cementitious matrix. The accelerated laboratory test developed in this study permits to obtain a rapid fungal development on cement specimens. Three months of experiments only are needed to obtain first results, which is rather shorter than other test developed to date to study fungal biodeterioration. Results are mainly related to aesthetical biodeterioration. Moreover, fungal strains presenting various growth patterns were used, and so may lead to different biodeterioration mechanisms in long term exposition experiment. These results will be completed by the quantification and estimation of the extent of the fungal growth. This test could be used to compare the bioreceptivity of various cement based materials, leading to results in 3 months only.

F Fi ig gu ur re e 1 13 3: : S SE EM M o ob bs se er rv va at ti io on ns s o of f t th he e c ca ar rb bo on na at te ed d t th he en n l le ea ac ch he ed d s sp pe ec ci im me en ns s --S Su ur rf fa ac ce e ( (a a) ) a an nd d c cr ro os ss s--s se ec ct ti io on n ( (b b) ) o of f s sp pe ec ci im me en ns s i in no oc cu ul la at te ed d w wi it th h A Al lt te er rn na ar ri ia a a al lt te er rn na at ta a --S Su ur rf fa ac ce e ( (c c) ) a an nd d c cr ro os ss s--s se ec ct ti io on n ( (d d) ) o of f s sp pe ec ci im me en n i in no oc cu ul la at te ed d w wi it th h C Co on ni io os sp po or ri iu um m u

un nc ci in na at tu um m. .

Tables

T Ta ab bl le e 1 1: : F FT T--I IR R c ch ha ar ra ac ct te er ri iz za at ti io on n o of f h hy yd dr ra at te ed d c ce em me en

Figures 14 F

 14 Figures

Acknowledgments

Authors would like to thank Patrick Dégrugilliers (Ecole des Mines de Douai, Civil Engineering Department) for the preparation of thin section. Authors acknowledge Clara Urzì and Filomena De Leo (Messina University, Department of Microbiology, Genetic and Molecular Sciences) for their precious help of the microbiology part of the study.


Laetitia Patural

Patrice Porion

Henri Van Damme

Alexandre Govin
email: govin@emse.fr

Philippe Grosseau

Bertrand Ruot

Olivier Devès

A pulsed field gradient and NMR imaging investigations of the water retention mechanism by cellulose ethers in mortars

Keywords: Cellulose ether, Diffusion coefficient, Freshly-mixed, Mortar (E), Water retention

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

LAETITIA PATURAL (1) , PATRICE PORION (2) , HENRI VAN DAMME (3) , ALEXANDRE GOVIN (1) *, PHILIPPE GROSSEAU (1) , BERTRAND RUOT (4) , OLIVIER DEVÈS (4)

I. Introduction

Cellulose ethers (CE) are water-soluble polymers derived from cellulose, the most abundant polysaccharide in Nature. This polymer is built up from 1,4-anhydroglucose units linked through β-1,4 glycosidic bonds. It is insoluble in water because of the tight connection of the paired cellulose chains via hydrogen bridges. These bonds can be disrupted by partial or total etherification of the cellulose hydroxyl groups with various organic groups, then turning the material into water-soluble. Among these derivatives, hydroxypropyl methyl cellulose, HPMC (Figure 1a), hydroxyethyl methyl cellulose, HEMC (Figure 1b) and hydroxyethyl cellulose, HEC (Figure 1c) are extensively used in the formulation of various industrial products such as food, pharmaceuticals, construction products, etc., thanks to their non-toxic character and their interesting rheological and mechanical properties. In building materials, such as mortars, cellulose derived products are used as water-retention agents, thickeners, and film formers.

Water retention is the most important property imparted to CE when used in a cement based mortar. The water retention capacity of neat cementitious pastes submitted to a hydric stress (evaporation, filtration under pressure, or suction by the substrate) is poor and organic admixtures are generally added when strong water retention is required [START_REF] Mansoutre | Water retention and granular rheological behavior of fresh C 3 S paste as a function of concentration[END_REF]. Cellulose derivatives are the most suitable molecules used to improve this property. This specific behaviour of CE is particularly crucial when the mortar is applied in thin layers on highly absorbent substrates (that is, in conditions where water loss is occurring by evaporation and by suction). Cellulosic admixtures are added in order to retain the amount of water required for proper hydration and setting of the cement and to achieve sufficient mortar-substrate adhesion.

In spite of the success of CE in practical applications, little work has been devoted to the mechanism of their remarkable water retention capacity [START_REF] Peschard | Aspects physico-chimiques de l'interaction ciment-polysaccharides[END_REF]. Pourchez et al. characterized some HPMC, HEMC and HEC in terms of water retention [START_REF] Pourchez | Aspects physico-chimiques de l'interaction des éthers de cellulose avec la matrice cimentaire[END_REF], [START_REF] Pourchez | HPMC and HEMC influence on cement hydration[END_REF] and [START_REF] Pourchez | Some aspects of cellulose ethers influence on water transport and porous structure of cement-based materials[END_REF]. A significant influence of the polymer molecular parameters (i.e. molecular weight and substitution degrees) was evidenced, but no clear water retention mechanism could be identified. One hypothesis was put forward concerning the water bound to cellulose ethers. Nevertheless, cryo-differential scanning calorimetry was used to quantify the bound water with the results that water bound to CE was not responsible for strong water retention capacities [START_REF] Pourchez | Aspects physico-chimiques de l'interaction des éthers de cellulose avec la matrice cimentaire[END_REF]. The concept of water retention itself covers different properties, depending on whether one is interested in limiting water evaporation, or limiting water loss to a porous substrate, or both. During evaporation, no admixture loss is occurring, due to the very low vapour pressure of the admixture. Hence, as the water evaporates, the average admixture concentration increases.

According to the basic thermodynamics of polymer solutions, this leads to a decrease of the water vapour pressure, due to the decrease of the water chemical potential [START_REF] Kuhn | Principles of Physical Chemistry[END_REF].

Simultaneously, a decrease of the water mobility due to the jamming of the diffusion space by the polymer molecules is expected [START_REF] Penke | Proton diffusion and T1 relaxation in polyacrylamide gels: a unified approach using volume averaging[END_REF]. Both effects may be amplified by attractive polymerwater interactions. In addition, a polymer-enriched layer of low permeability (ultimately, a polymer film) may form at the mortar-air interface. As far as water loss to the substrate is concerned, the only dangerous situation is that of an unsaturated substrate. Indeed, with a fully water-saturated substrate, there is no driving force for water loss. Due to the higher osmotic pressure of the interstitial solution in the mortar, the driving force is towards pumping water from the substrate into the mortar. On the other hand, with an unsaturated substrate, a competition will set up between the capillary pressure, which drives the CE solution into the substrate, and the osmotic pressure, which drives the solvent (water) from the substrate to the mortar. In addition to the osmotic and capillary pressures, the competition is ruled by the solution and water transport properties (viscosity, diffusion coefficient).

In the present work, we choose to probe the influence of CE on the mobility of water in fresh cement pastes by Nuclear Magnetic Resonance (NMR). NMR can be used in a wide variety of modes. High resolution solid-state NMR spectroscopy has been extensively used to study the atomic structure of cement hydrates and the measure of T 1 or T 2 relaxation times is now a classical technique to follow the microstructural evolution during hydration and setting [START_REF] Colombet | Nuclear magnetic resonance spectroscopy of cement-based materials[END_REF].

Besides, the time evolution of the cement paste microstructure was characterized by nuclear magnetic relaxation methods at variable magnetic fields [START_REF] Korb | NMR and nuclear spin relaxation of cement and concrete materials[END_REF]. In this work, we characterized the diffusion properties of water molecules using Pulsed-Gradient Spin-Echo (PGSE) NMR on one hand and Magnetic Resonance Imaging (MRI) on the other hand. PGSE NMR is probably one of the most convenient and reliable methods to measure diffusion coefficient [START_REF] Callaghan | Principles of nuclear magnetic resonance microscopy[END_REF] and [START_REF] Stejskal | Spin diffusion measurements -spin echoes in presence of time-dependent field gradient[END_REF]. It is non-invasive, it does not involve the introduction of a chemical tracer and the system is kept at equilibrium. Moreover, in anisotropic materials, the self-diffusion tensor can be extracted from a set of measurements performed along well-chosen non-collinear directions [START_REF] Porion | Anisotropy of the solvent self-diffusion tensor as a probe of nematic ordering within dispersions of nanocomposites[END_REF] and [START_REF] Porion | 1 H and 7 Li NMR pulsed gradient spin echo measurements and multiscale modeling of the water and ionic mobility within aqueous dispersions of charged anisotropic nanoparticles[END_REF]. PGSE NMR has been widely applied to a variety of porous materials [START_REF] Busignies | Application of PGSTE-NMR technique to characterize the porous structure of pharmaceutical tablets[END_REF]. In cement-based materials, it has been used to monitor moisture and salt transport inside concrete [START_REF] Van Der Heijden | Moisture transport in heated concrete, as studied by NMR, and its consequences for fire spalling[END_REF] and water absorption in mortars [START_REF] Pel | Water absorption in mortar determined by NMR[END_REF].

In the second part of the paper, MRI was used to follow water diffusion in cement pastes at large length scale (a few millimeters). In comparison with PGSE NMR which gives dynamical information at pore scale (10 μm), the MRI length scale is about 1000× larger. MRI is extensively used as a medical diagnostic tool but it is also recognized as an important tool for non medical applications, especially in materials research and process engineering [START_REF] Callaghan | Principles of nuclear magnetic resonance microscopy[END_REF]. Many of these applications depict a fluid, usually water, in porous inorganic materials and highlight the dynamics of the internal water distribution during capillary flow [START_REF] Gummerson | Unsaturated water flow within porous materials observed by NMR imaging[END_REF]. Generally speaking, MRI is a suitable technique to follow the diffusion of a solvent in complex structures. For instance, in order to study water diffusion in colloidal gels by 1 H MRI, Duval et al. followed the time-dependence of the one-dimensional proton ( 1 H) profiles during water interdiffusion between a pieces of H 2 O-based Laponite gel and a piece D 2 O-based gels in close contact [START_REF] Duval | An NMR investigation of water self-diffusion and relaxation rates in controlled ionic strength laponite sols and gels[END_REF].

In spite of the complex structure and the high modulus of this kind of gel, the water selfdiffusion coefficient was only marginally smaller than that of bulk water.

Our investigation of the water retention mechanism and water mobility in CE-admixed mortars is organized as follows. First, water retention was measured for all CE-admixed mortars using standard methods of the mortar industry. Then, the effect of the CE on the selfdiffusion coefficient of water in polymer solution was assessed using PGSE NMR. Afterwards, the water diffusion between admixed cement pastes and a substrate was studied. Finally, interdiffusion imaging experiments between H 2 O-based and D 2 O-based cement pastes were performed. As will be demonstrated, very little influence of the CE on the water mobility in these various situations was detected, in spite of their strong water retention properties.

II. Materials and experimental methods

I II I. .1 1. . M
Mi in ne er ra al l a an nd d o or rg ga an ni ic c p pr ro od du uc ct ts s Cement pastes were prepared with white cement in order to minimize the Fe 3+ amount due to C 4 AF in the material. Indeed, ferromagnetic and paramagnetic constituents enhance the NMR relaxation kinetics, thus leading to very poor signals during NMR measurements. Chemical analysis of this cement was performed by X-ray fluorescence spectroscopy (XRF). The phase compositions were calculated using Bogue's formula with a correction on CaO due to sulfates [START_REF] Taylor | Cement Chemistry[END_REF]. The cement composition is given in Table 1. Each analysis was performed three times with the results averaged for the three measurements. For NMR experiments, cement pastes were prepared with a water/cement ratio equal to 0.7 in order to have an appropriate consistency. Indeed, preliminary tests with various water/cement ratios, including the formulation imposed by the CEReM (W/C = 1, Section II.2.) [START_REF] Ruot | Some aspects of cellulose ethers and latexes influence on the properties of cement-based materials -examples of results obtained within the CEReM[END_REF], provided inadequate consistencies. With such high water to cement ratio, bleeding problems appeared. The admixture amount was equal to 0.27% (regarding cement). The characteristics of the eight cellulose ethers selected for the study are summarized in Table 2. The number of substituted hydroxyl groups per anhydroglucose unit is expressed as degree of substitution (DS). Moreover the molar ratio of alkoxy groups in the side chains to cellulose is expressed as the average molecular substitution (MS) [START_REF] Crössmann | Wasserlösliche Celluloseäther im Spiegel der Anwendungstechnik, Collection of publication of a group of authors of Kalle[END_REF]. The cellulose ether weight-average molecular mass ( W M ) was determined using size exclusion chromatography [START_REF] Clasen | Determination of viscoelastic and rheo-optical material functions of water-soluble cellulose derivatives[END_REF].

In the MRI experiments, plaster of Paris (CaSO 4 , ½ H 2 O) was used as a substrate. It was characterized by mercury intrusion porosimetry. The total porosity of this material was 64.7% and the average pore size diameter was approximately 545 nm. I II I. .2 2. . W Wa at te er r r re et te en nt ti io on n For the water retention measurements, mortars were prepared according to the CEReM mixture proportions: 65% sand, 30% cement, 5% filler with a water/cement ratio equal to 1 [START_REF] Ruot | Some aspects of cellulose ethers and latexes influence on the properties of cement-based materials -examples of results obtained within the CEReM[END_REF]. Mixing procedure was in accordance with EN 196-1 [START_REF]Methods of testing cement -Part 1: determination of strength[END_REF]. Admixture amount (0.27%) was in addition to the total dry mixture (i.e. cement, sand and filler). In consequence, the CE concentration, for this formulation was 9 g L -1 . Siliceous sand (ref. DU 0.1/0.35) was used, in agreement with NF EN 13139 standard [START_REF]Granulats pour mortiers -Spécifications des caractéristiques des granulats et fillers utilisés dans les mortiers[END_REF]. The filler was calcium carbonate (CaCO 3 ). Grey Portland cement CEM I 52.5 R was used, according to EN 197-1 [START_REF]Cement. Part 1: Composition[END_REF]. The water retention capacity can be assessed using diverse testing methods. One of these methods used is described in the DIN 18 555-7 standard [START_REF]Testing of mortars containing mineral binders; part 7: determination of water retentivity of freshly mixed mortar by the filter plate method[END_REF]. With this method, a freshlymixed mortar is in contact with filter paper, thereby simulating the action of an absorptive substrate. DIN measurements have to be performed 5 min after mixing. The water retention capacity of a freshly-mixed mortar is characterized by the mass of water it retains after capillarity action of an absorbent substrate. The standardised apparatus is described in Figure 2. The mortar (3) was poured into the ring. Then, knowing that the water content of the tested mortar was equal to 23%, for CEReM formulation, the water retention was calculated by weight differences (Equation ( 1)).

100 100

fp m m WR x m   (1)
Where m fp is the quantity of water retained by the filter paper and m m is the quantity inside the studied mortar. I II I. .3 3. . N NM MR R e eq qu ui ip pm me en nt t All proton NMR measurements were performed using a Bruker DSX100 spectrometer operating at a proton ( 1 H) frequency of 100 MHz with a 2.35 T superconducting magnet. A microimaging probehead (Micro5 Bruker) with gradient coils in three perpendicular directions was used to generate magnetic field gradients along any arbitrary directions. All experiments were performed at room temperature (293 K). All samples were introduced into a 10 mm-diameter standard NMR tube, with an inner diameter of about 8.5 mm. I II I. . [START_REF] Pourchez | HPMC and HEMC influence on cement hydration[END_REF]

. L

Lo on ng gi it tu ud di in na al l r re el la ax xa at ti io on n t ti im me e ( (T T 1 1) ) m me ea as su ur re em me en nt ts s The longitudinal relaxation time T 1 is the characteristic time for re-establishing thermal equilibrium of the longitudinal component of the magnetization vector [START_REF] Abragam | The principles of nuclear magnetism[END_REF]. In other words, the T 1 value corresponds to the time required for the magnetization to recover 63% of its initial value after being flipped into the magnetic transverse plane by a 90° Radio Frequency pulse. To ensure that the magnetization relaxes to its thermal equilibrium before each pulses sequence, the repetition time was chosen up to 5 T 1 , i.e. 15 s (corresponding to a recovery of about 99.3% of the equilibrium state). The longitudinal relaxation times were measured in all samples using the standard inversion-recovery pulse sequence (180° -τ 90 -Acq.) [START_REF] Martin | Practical NMR spectroscopy[END_REF], with a pulse width of the 90° pulse equal to 7.75 μs and a delay τ varying between 250 ms and 15 s (8 steps). The T 1 values were then obtained by fitting the maximum amplitude of the NMR spectra versus the delay τ by the following recovery equation (Equation ( 2)):

0 1 1 2 exp( ) z M M x T           (2)
I II I. .5 5. . S Se el lf f--d di if ff fu us si io on n P PG GS SE E m me ea as su ur re em me en nt ts s Water self-diffusion coefficient measurements in CE solutions were performed by using the PGSE NMR method originally described by Stejkal and Tanner [START_REF] Stejskal | Spin diffusion measurements -spin echoes in presence of time-dependent field gradient[END_REF]. In this pulse sequence, depicted in Figure 3, the proton 1 H spins are labelled at time t = 0 with respect to their positions in a given direction (O i ) by applying a 90°-pulse of magnetic field gradient along this direction. After an evolution time Δ called diffusion time, a second 180°-pulse of magnetic field gradient is then applied. This second gradient pulse cancels the effect of the first one, unless the spins have moved during the time Δ along the same direction (O i ). If the spins have diffused along this chosen direction (O i ), the second gradient pulse does not refocus the entire spins signal and the echo intensity is less intense. So, this decrease of the echo amplitude is a function of the self-diffusion coefficient D, and the echo attenuation E(q,Δ) measured using this method is given by the following equation: ( , ) ( , ) exp 4 ( / 3) exp ( / 3) (0, )

I q E q q D g D I                        (3)
where 2 g q     , with γ the gyromagnetic ratio of the proton (γ = 2.675 × 10 8 rad s -1 T -1 ), g is the intensity of the pulsed magnetic field gradient, δ its duration, Δ the diffusion time and D the self-diffusion coefficient, I(q,Δ) and I(0,Δ) being the echo intensities measured with and without the magnetic field gradient g, respectively. This method, based on the T 2 spin-echo sequence [START_REF] Hahn | Spin echoes[END_REF], is limited by the value of the transverse relaxation time T 2 that dictates the maximal value of the diffusion time Δ and the magnetic field gradient duration δ. In our experiments, pulse sequence parameters were fixed at 20 ms for Δ and 1 ms for δ, whereas g was varied from 0 to 1.5 T m -1 (8 steps). By performing a series of experiments with variable gradient intensities g, from the slope of the echo attenuation ln[E(q,Δ)] versus the quantity -γ 2 g 2 δ 2 (Δ -δ / 3), the self-diffusion coefficient D is calculated. I II I. .6 6. . I Im ma ag gi in ng g e ex xp pe er ri im me en nt ts s a at t t th he e p pa as st te e/ /u un ns sa at tu ur ra at te ed d s su ub bs st tr ra at te e i in nt te er rf fa ac ce e A standard one-dimensional spin-echo sequence [START_REF] Callaghan | Principles of nuclear magnetic resonance microscopy[END_REF] was used to obtain 1D profiles with a repetition time (TR) of 1 s. Depending on the length of the sample, the field of view was taken varying between 20 mm and 25 mm leading to a spatial resolution of 39 μm and 49 μm with 512 pixels along the profile. From these 1D profiles, the water concentration can be determined inside the sample at a given time. To quantify the diffusion process, the migration of the water inside the different experiment configurations is detected by the analysis of a set of 1D profiles acquired at constant time intervals. The experimental procedure was the following: the fresh cement paste was introduced into the tube 15 min after mixing. Then, as soon as possible, a piece of hardened plaster of Paris was put into contact with the paste, as illustrated on Figure 4. The one-dimensional proton concentration profiles along the tube axis were then acquired according to a constant temporal interval (a few minutes typically). Several substrates were tested. Plaster of Paris was chosen as the most convenient substrate. Indeed, its affinity as regards to water and appropriate porosity lead to relatively short-time analysis. I II I. .7 7. . I

In nt te er rd di if ff fu us si io on n i im ma ag gi in ng g e ex xp pe er ri im me en nt ts s a at t t th he e p pa as st te e/ /p pa as st te e i in nt te er rf fa ac ce e In the last series of experiments, the influence of CE on water mobility was measured at the interface between two fresh cement pastes. These experiments were performed using two open pieces of glass tubes filled with fresh cement paste. They were then assembled in a larger NMR glass tube as displayed in Figure 5. One was prepared with water and the other with heavy water. The one-dimensional proton concentration profile along the cylinder axis was then followed as a function of time. Figure 5 shows the three studied configurations. In spite of the cement hydration delay in presence of D 2 O [START_REF] King | Hydration of tricalcium silicate with D 2 O[END_REF] and [START_REF] Thomas | The surface area of cement paste as measured by neutron scattering: evidence for two C-S-H morphologies[END_REF], experiments were not affected because they were performed during the dormant period. Besides, the results can be relatively compared because the conditions were the same for the three configurations and absolute values were not determined. The cellulose ether HPMC J2 (see Table 2) was chosen for this experiment, due to its strong water retention capacity to enhance the observed phenomenon. 3. The cellulose ethers improved water retention up to 92.3% for HEC H1, to 99.5% for HPMC C6. With respect to the water retention capacity of the non-admixed mortar (60.2%), this represents a very large increase. These figures are qualitatively in agreement with the results of Pourchez et al. [START_REF] Pourchez | HPMC and HEMC influence on cement hydration[END_REF] obtained with other HPMC and HEMC admixtures. I II II I. .2 2. . R Re el la ax xa at ti io on n a an nd d w wa at te er r p pr ro ot to on n s se el lf f--d di if ff fu us si io on n i in n C CE E s so ol lu ut ti io on n The first step of our investigation is to know whether the cellulose derivatives affect the T 1 longitudinal relaxation time and/or the proton self-diffusion coefficient D in an aqueous solution (prepared with distilled water). For this purpose, both parameters were measured in CE solutions for two CE solutions: HEC H1 and HEMC C4. These polymers were chosen because they provide to the mortar critical values of water retention (Section III.1.). Experiments were performed with two H1 concentrations: 1 g L -1 and 9 g L -1 , respectively, to reach the CEReM mortar concentration. The results for HEC are shown in Table 4. The selfdiffusion coefficient of water ranged from 2.02 × 10 -9 m² s -1 in distilled water to 1.98 × 10 -9 m² s -1 in the most concentrated H1 solution. Considering the fluctuations (D = 2.05 × 10 -9 m² s -1 at 1 g L -1 ), this cannot be considered as a significant decrease. The conclusion is that HEC H1 does not significantly modify the proton or water self-diffusion coefficient from the polymer solutions with concentrations ranging from 1 to 9 g L -1 . After the results were obtained, a larger CE concentration range, up to 60 g L -1 , was investigated with HEMC C4. The results are shown in Figure 6. In the concentration range from 10 to 60 g L -1 , the proton self-diffusion coefficient decreased from 1.96 × 10 -9 m² s -1 to 1.81 × 10 -9 m² s -1 . This represents a mere 10% reduction with respect to pure water. At 9 g L -1 , which is the CE concentration in the reference CEReM mortar, the decrease of D amounts to about 2%. In relative terms, this is vastly weaker than the increase of water retention measured on mortars at the same CE concentration (Section III.1.).

III. Results

I

In conclusion, at the concentrations used in mortars formulation, CE does not significantly modify the water self-diffusion coefficient in aqueous solutions. I II II I. .3 3. . W Wa at te er r m mo ob bi il li it ty y a at t t th he e c ce em me en nt t p pa as st te e/ /s su ub bs st tr ra at te e i in nt te er rf fa ac ce e Using MRI, one-dimensional proton concentration profiles between a fresh cement paste and a piece of (initially) dry plaster were recorded. Figure 7a and b displays the signal intensity profiles along the z-axis for a neat cement paste and a cement paste admixed with H1, respectively. The left parts of the curves represent the cement paste and the right ones are the porous substrate. Capillary invasion of the substrate starts as soon as contact is established. Note that t = 0, in Figure 7, corresponds to the time when the first profile record was completed, which was approximately 1 min after contact. This explains that at t = 0 significant suction of water by the substrate was already observed. For admixed cement paste, in approximately 30 min, the substrate was homogeneously saturated. A series of profiles were recorded for each cellulose derivative. To estimate the CE effect on the suction process, the profiles were integrated in order to determine when a steady state was achieved for the system. Then, the curves were normalized to 1. As illustrated in Figure 8a and b, the normalized integral value is increasing rapidly, reaching a plateau value after a few minutes for the neat cement and about 30 min for the admixed cement. The time needed to reach 99% of the final value will be noted Ts. Note that the increase of the profile integral does not mean that the overall quantity of water in the paste/substrate system is increasing. The evolution stems from the fact that the relaxation times of protons are different in the paste and the substrate. In other words, the one-dimensional proton concentration profiles are weighted by the transverse relaxation times T 2 of protons for each part of the sample. This leads to a different sensitivity in each medium and the profile intensities cannot be directly compared each other. For non admixed cement paste, Ts is equal to 4 min whereas for HEC H1, Ts is equal to 28 min. The values for the other CE are tabulated in Table 3. Ts characterizes the water retention capacity of the paste in contact with a porous substrate. Therefore, a strong correlation is expected with the water retention capacity determined by the standard DIN method (Section III.1.). In the later, the amount of water absorbed by a filter paper in contact with the mortar is measured after 5 min. Indeed, when the mortar water retention capacity is plotted versus the MRI Ts value, a good linear correlation is obtained (Figure 9; R² = 0.98). This validates the standard DIN method and the use of filter paper as a valuable method to evaluate water retention in real mortar/substrate configurations. I II II I. . [START_REF] Pourchez | HPMC and HEMC influence on cement hydration[END_REF] 10a illustrates the signal intensity profiles along the z-axis at initial time. The signal perturbations of the profiles present at the different interfaces of the sample are due to the presence of little drops of liquid (interface and top of the sample) which are generated when the two parts of the sample are put into contact. These drops of liquid also induce inhomogeneities of the magnetic susceptibility at these boundaries. These intensity profiles were normalized in order to be able to compare them to each other at later time. However, the proton amount cannot be determined using these profiles. Indeed, as already pointed out in Section III.3, the intensity profiles do not reflect the actual proton density. They are related only to the proton density weighted by the transverse relaxation time T 2 . Nevertheless, at initial time, for each type of experiment, the profiles were the same when both compartments were put into contact. As time went on, water molecules diffused from one side to the other. Remarkably, whatever the type of experiment and the time (25 min and 75 min), the profiles in the three types of experiments remained identical to each other (Figure 10). Thus, water diffusion at the paste/paste interface is not affected by the presence of CE. This conclusion is valid when the CE is introduced on both sides and when it is introduced on one side only.

IV. Discussion and conclusions

The standard DIN 18 555-7 method is used by the mortar industry to estimate the water retention capacity of admixed mortars, based on the water loss at the mortar/filter paper interface (Section III.1.). A first conclusion which can be safely drawn from the experiments reported in this paper is that, this method is reliably related to the actual water migration process occurring through the interface between a fresh cement paste and a dry porous substrate (Section III.3.). To our best knowledge, this is the first time that such a direct validation is made. A second important conclusion related to the water retention mechanism is that, at the concentrations used in practice and even beyond, CE does not significantly modify the water self-diffusion coefficient. This was established in CE solutions (Section III.2.) and in admixed cement pastes (Section III.4., comparison of type a with type b experiments). This is in agreement with previous studies on dilute or semi-dilute polymer and colloidal solutions or even gels [START_REF] Penke | Proton diffusion and T1 relaxation in polyacrylamide gels: a unified approach using volume averaging[END_REF] and [START_REF] Duval | An NMR investigation of water self-diffusion and relaxation rates in controlled ionic strength laponite sols and gels[END_REF]. In spite of their potentially very high viscosity and yield stress, these media are actually very "open" systems, with large and well connected solvent-filled voids within and between the polymer molecules coils, or between the colloidal particles. More surprising is the observation that water self-diffusion is not modified even when an osmotic gradient is counteracting the diffusion flux. This is shown by the results of type c experiments (with osmotic gradient) in Section III.4., undistinguishable from those of type b experiments (without osmotic gradient). Of course, this does not mean that osmotic effects would remain negligible at higher CE concentrations. What type c experiments do show (or, confirm) also is that CE do not introduce any water retention property when the admixed cement paste is applied on a water saturated substrate. Indeed, the D 2 O-saturated admixture-free cement paste in these experiments is nothing else than a saturated substrate. When taken together, our experiments and results lead to the conclusion that the only configuration where water mobility is strongly modified by small amounts of CE is that of a mortar/unsaturated substrate interface. This is a configuration where water loss is driven by capillary suction, not by diffusion. The flow induced by capillary suction is itself controlled by the substrate pore pressure, which depends on pore size and interface tension via Laplace's equation, and by the fluid viscosity [START_REF] Guéguen | Introduction to the physics of rocks[END_REF]. This suggests that the rheological properties of the aqueous CE solutions may be the key properties for water retention. However, this working hypothesis is against the common acceptance that other viscosity enhancing admixtures such as high molecular weight PEO (polyethylene oxides) and starches are less effective than CE. Alternatively, one may consider the formation of a thin CE-and fine particles-enriched filter cake at the mortar/substrate interface, similarly to what happens at the fluid/rock interface with polysaccharide-based drilling muds in oil well drilling [START_REF] Nelson | Well cementing[END_REF]. In any case, further work is necessary to solve the question. The main point established in this work is that an organic admixture may be a strong water retaining agent without modifying the mobility of water in either in solution or paste.

  II II I. .1 1. . W Wa at te er r r re et te en nt ti io on n o of f f fr re es sh hl ly y--m mi ix xe ed d m mo or rt ta ar rs s The water retention capacity of each admixed mortar was assessed using the standard DIN method. The results are presented in Table

Figures

  Figures

  4. . I In nt te er rd di if ff fu us si io on n a at t t th he e p pa as st te e/ /p pa as st te e i in nt te er rf fa ac ce e As explained in Section II, three types of H 2 O/D 2 O interdiffusion experiments at the paste/paste interface were performed. They all have in common to have H 2 O on one side and D 2 O on the other. In type (a) experiments, both sides contain CE at the same concentration. In type (b), both sides are CE-free. In type (c), the H 2 O side contains CE whereas the D 2 O side does not. Figure

Acknowledgements

The authors would like to acknowledge all the industrial and academic CEReM members (http://cerem.cstb.fr). The Bruker DSX100 NMR spectrometer and the Bruker Micro5 microimaging probehead used in this study were purchased thanks to grants from the CNRS and the Région Centre (France).