Charles-Antoine Guilloux
email: cguilloux@univ-brest.fr

Claudie Lamoureux
email: claudie.lamoureux@chu-brest.fr

Clémence Beauruelle
email: clemence.beauruelle@chu-brest.fr

Geneviève Héry- Arnaud
email: genevieve.hery-arnaud@univ-brest.fr

Porphyromonas: a neglected potential key genus in human microbiomes

Keywords: Porphyromonas, Porphyromonas catoniae, Porphyromonas pasteri, anaerobes, lung microbiome, skin microbiome, gastric microbiome, oral microbiome

Porphyromonas is a bacterial genus that belongs to the respiratory core microbiome

• Important species in the pulmonary niche are Porphyromonas other than P. gingivalis (POTG)

• POTG are understated and little studied

• The abundance of POTG is decreased whatever the respiratory disease

• POTG are also an interesting target of investigation in non-pulmonary microbiomes, particularly gastric, oral and cutaneous.

Introduction

In recent years, new mass sequencing technologies have enabled description of numerous communities of microorganisms, including unknown or overlooked microbial populations, in almost all niches of the human body. These microbiomes are now well studied and provide many keys to understand organ homeostasis and disease pathophysiology. An iconic example is the gut microbiota, and its implication in obesity, Crohn's disease, autism and many other diseases [START_REF] Mangiola | Gut microbiota in autism and mood disorders[END_REF][START_REF] Marchesi | The gut microbiota and host health: a new clinical frontier[END_REF].

Focusing on strict anaerobes in the various human niches provides much information, describing important genera or species as targets for scientific research. For instance, in deciphering the gut microbiome, two important strict anaerobes, Faecalibacterium prausnitzii and Akkermansia muciniphila, were discovered [START_REF] Miquel | Faecalibacterium prausnitzii and human intestinal health[END_REF][START_REF] Dao | Akkermansia muciniphila and improved metabolic health during a dietary intervention in obesity: relationship with gut microbiome richness and ecology[END_REF] and became the cornerstone in understanding the pathophysiology of chronic inflammatory diseases and obesity. In the lung microbiome, the importance of anaerobes is clear [START_REF] Lamoureux | Anaerobes in cystic fibrosis patients' airways[END_REF]. The lungs used to be considered a sterile environment [START_REF] Dickson | The lung microbiome: new principles for respiratory bacteriology in health and disease[END_REF], but it is now known that healthy lungs host anaerobes [START_REF] Lamoureux | Anaerobes in cystic fibrosis patients' airways[END_REF] and that this "anaerobiome" is resident in this ecological niche in healthy subjects [START_REF] Erb-Downward | Analysis of the lung microbiome in the "healthy" smoker and in COPD[END_REF][START_REF] Morris | Lung HIV Microbiome Project, Comparison of the respiratory microbiome in healthy nonsmokers and smokers[END_REF][START_REF] Park | Microbial communities in the upper respiratory tract of patients with asthma and chronic obstructive pulmonary disease[END_REF][START_REF] Bassis | Analysis of the upper respiratory tract microbiotas as the source of the lung and gastric microbiotas in healthy individuals[END_REF][START_REF] Lim | Analysis of the association between host genetics, smoking, and sputum microbiota in healthy humans[END_REF]. Several anaerobe genera are of particular interest, one of which is Porphyromonas. Many studies have focused on Porphyromonas gingivalis; in contrast, little is known of Porphyromonas other than gingivalis (POTG). Since the outset of microbiome studies, few data have been collected, most often fortuitously, and are dispersed in the scientific literature, often as "supplementary information".

The present review, based on our previous results showing a potential role of Porphyromonas catoniae in the lung microbiome of patients with cystic fibrosis (CF) [START_REF] Bernarde | Impact of the CFTR-potentiator ivacaftor on airway microbiota in cystic fibrosis patients carrying a G551D mutation[END_REF][START_REF] Keravec | Porphyromonas, a potential predictive biomarker of Pseudomonas aeruginosa pulmonary infection in cystic fibrosis[END_REF], provides an overview of anatomical sites where POTG have been reported, and describes their inherent potential roles in human health and disease.

Literature search and selection

A systematic search for relevant literature published before October 31, 2019 was conducted in PubMed, and Scopus. The following search terms were used to identify relevant articles:

("Porphyromonas catoniae" OR "Porphyromonas pasteri" OR "Porphyromonas" NOT "gingivalis"). One investigator (CAG) screened each abstract for inclusion, and two investigators (CAG and GHA) were independently reviewed full texts. The references of the included articles were studied to identify additional relevant articles. Any discrepancies between the authors were resolved with two other investigators (CL and CB).

After initial screening, 840 articles in Pubmed and 91 in Scopus met our inclusion criteria. All but 80 articles identified in Scopus were also identified in PubMed, leaving a total of 851 unique publications. There were 106 articles published before 2019 that were deemed relevant after abstract review, and 39 articles that were excluded after full text review. An additional 22 articles were identified in the references of the included articles bringing the total number of included articles published before 2019 to 89.

The genus Porphyromonas

Porphyromonas is a Gram-negative, obligatorily anaerobic, non-spore-forming, non-motile genus. In 2015, the number of validated species was 16 [START_REF] Summanen | Porphyromonas, in: Bergey's Manual of Systematics of Archaea and Bacteria[END_REF], but new species were more recently described or are under description [START_REF] Sato | Porphyromonas bronchialis sp. nov. isolated from intraoperative bronchial fluids of a patient with non-small Cell lung cancer[END_REF]. The type species among the genus is P. asaccharolytica [START_REF] Summanen | Porphyromonas, in: Bergey's Manual of Systematics of Archaea and Bacteria[END_REF], and two classes of species can be distinguished according to their natural reservoir. Some species were mainly isolated from human samples (P. asaccharolytica, P. endodontalis, P. gingivalis, P. catoniae, P. pasteri, P. somerae, and P. uenonis), and at least 10 others mostly from animal samples (P. cangingivalis, P. canoris, P. cansulsi, P. circumdentaria, P. crevioricanis, P. gingivicanis, P. salivosa, P. macacae, P. gulae, and P. levii) (Figure 1) [START_REF] Summanen | Porphyromonas, in: Bergey's Manual of Systematics of Archaea and Bacteria[END_REF][START_REF] Finegold | Recently described clinically important anaerobic bacteria: medical aspects[END_REF].

Most species are black pigmented due to production of protoheme, with the exception of P. catoniae, P. bronchialis, and P. pasteri. Major fermentation products comprise n-butyric acid and acetic acid; propionic, isovaleric, isobutyric and phenylacetic acid may also be produced.

Hemin and vitamin K1 are required to stimulate growth in most Porphyromonas species.

Porphyromonas is a slow-growing bacterium, as at least 48 hours' incubation at 37°C is necessary for colony detection, and 7 days' incubation at least is recommended to improve detection of all Porphyromonas species. Porphyromonas species are susceptible to most of the antimicrobial agents recommended for anaerobic infection treatment: amoxicillin-clavulanate, ampicillin-sulbactam, piperacillin-tazobactam, imipenem, cephalosporins and metronidazole. All Porphyromonas species are susceptible to vancomycin, except P. catoniae. Natural resistance to colistin, fosfomycin, polymyxin B and kanamycin was reported. Production of β-lactamase has been reported in P. asaccharolytica, P. catoniae, P. somerae and P. uenonis [START_REF] Summanen | Porphyromonas, in: Bergey's Manual of Systematics of Archaea and Bacteria[END_REF][START_REF]European committee on antimicrobial susceptibility testing, Breakpoint tables for interpretation of MICs and zone diameters[END_REF]. The genus was firstly, and is most often, reported in the oral cavity [START_REF] Könönen | Oral colonization by anaerobic bacteria during childhood: role in health and disease[END_REF][START_REF] Könönen | Anaerobes in the upper respiratory tract in infancy[END_REF]. One of the major species in this niche is P. gingivalis, which is a well-known and well-studied pathogen associated with periodontitis [START_REF] Yang | Occurrence of Porphyromonas gingivalis and Tannerella forsythensis in periodontally diseased and healthy subjects[END_REF]. P. catoniae, in contrast, was isolated mostly in control subjects [START_REF] Abusleme | The subgingival microbiome in health and periodontitis and its relationship with community biomass and inflammation[END_REF][START_REF] Camelo-Castillo | Subgingival microbiota in health compared to periodontitis and the influence of smoking[END_REF][START_REF] Payungporn | Identification of bacteria associated with a periodontal disease in Thai patients based on next-generation sequencing[END_REF]. The genus Porphyromonas has been described in various other body sites including: lungs [START_REF] Lim | Analysis of the association between host genetics, smoking, and sputum microbiota in healthy humans[END_REF][START_REF] Muhlebach | Anaerobic bacteria cultured from cystic fibrosis airways correlate to milder disease: a multisite study[END_REF], gut [START_REF] Madan | Serial analysis of the gut and respiratory microbiome in cystic fibrosis in infancy: interaction between intestinal and respiratory tracts and impact of nutritional exposures[END_REF][START_REF] Schubert | Microbiome data distinguish patients with Clostridium difficile infection and non-C. difficile-associated diarrhea from healthy controls[END_REF][START_REF] Hoen | Associations between gut microbial colonization in early life and respiratory outcomes in cystic fibrosis[END_REF], skin [START_REF] Ramadan | Skin microbiome differences in atopic dermatitis and healthy controls in Egyptian children and adults, and association with serum immunoglobulin E[END_REF][START_REF] Zheng | Alterations in the skin microbiome are associated with disease severity and treatment in the perioral zone of the skin of infants with atopic dermatitis[END_REF], stomach [START_REF] Bassis | Analysis of the upper respiratory tract microbiotas as the source of the lung and gastric microbiotas in healthy individuals[END_REF][START_REF] Aviles-Jimenez | Stomach microbiota composition varies between patients with non-atrophic gastritis and patients with intestinal type of gastric cancer[END_REF], vagina [START_REF] Hillier | The normal vaginal flora, H2O2producing Lactobacilli, and bacterial vaginosis in pregnant women[END_REF] and nose [START_REF] Bassis | Analysis of the upper respiratory tract microbiotas as the source of the lung and gastric microbiotas in healthy individuals[END_REF][START_REF] Hilty | Disordered microbial communities in asthmatic airways[END_REF].

Lung microbiota

The dogma of the sterile lung microbiome was discarded only recently [START_REF] Dickson | The lung microbiome: new principles for respiratory bacteriology in health and disease[END_REF], mainly thanks to the application of metagenomics to lung samples [START_REF] Hilty | Disordered microbial communities in asthmatic airways[END_REF]. Using independent culture methods, lungs were shown to shelter anaerobes [START_REF] Erb-Downward | Analysis of the lung microbiome in the "healthy" smoker and in COPD[END_REF][START_REF] Morris | Lung HIV Microbiome Project, Comparison of the respiratory microbiome in healthy nonsmokers and smokers[END_REF][START_REF] Park | Microbial communities in the upper respiratory tract of patients with asthma and chronic obstructive pulmonary disease[END_REF][START_REF] Bassis | Analysis of the upper respiratory tract microbiotas as the source of the lung and gastric microbiotas in healthy individuals[END_REF][START_REF] Lim | Analysis of the association between host genetics, smoking, and sputum microbiota in healthy humans[END_REF]. These findings were confirmed by extended culture-based methods [START_REF] Muhlebach | Anaerobic bacteria cultured from cystic fibrosis airways correlate to milder disease: a multisite study[END_REF]. In the past, lung detection of anaerobes was associated with oral contamination [START_REF] Jewes | The incidence of anaerobes in the sputum of patients with cystic fibrosis[END_REF], but this has been widely disputed in recent years. It is now clear that anaerobes are also lung residents [START_REF] Lamoureux | Anaerobes in cystic fibrosis patients' airways[END_REF][START_REF] Rogers | Use of 16S rRNA gene profiling by terminal restriction fragment length polymorphism analysis to compare bacterial communities in sputum and mouthwash samples from patients with cystic fibrosis[END_REF].

At first sight, the presence of anaerobes in the lungs may seem surprising, but can easily be explained by the presence of physiological gas gradients within the lung, independently of whether the lung is healthy or not, enabling onset of a partial pressure gradient in oxygen and carbon dioxide [START_REF] Man | The microbiota of the respiratory tract: gatekeeper to respiratory health[END_REF].

In this context, Porphyromonas was detected in the lung of healthy subjects, and described as a member of the core microbiota [START_REF] Erb-Downward | Analysis of the lung microbiome in the "healthy" smoker and in COPD[END_REF][START_REF] Morris | Lung HIV Microbiome Project, Comparison of the respiratory microbiome in healthy nonsmokers and smokers[END_REF][START_REF] Park | Microbial communities in the upper respiratory tract of patients with asthma and chronic obstructive pulmonary disease[END_REF][START_REF] Bassis | Analysis of the upper respiratory tract microbiotas as the source of the lung and gastric microbiotas in healthy individuals[END_REF][START_REF] Lim | Analysis of the association between host genetics, smoking, and sputum microbiota in healthy humans[END_REF]. It is one of the major anaerobic genera, along with Prevotella, Veillonella and Fusobacterium [START_REF] Guilloux | Les bactéries anaérobies, ces inconnues du microbiote pulmonaire[END_REF]. The genus Porphyromonas appears early in life and is detected in the lower airway microbiota before 1 year of age [START_REF] Pattaroni | Early-life formation of the microbial and immunological environment of the human airways[END_REF]. Pattaroni et al. showed that the lower airway microbiota of neonate patients is a key determinant for immune system maturation in the respiratory tract, and that the healthy pulmotype contains multiple bacterial species, including Porphyromonas [START_REF] Pattaroni | Early-life formation of the microbial and immunological environment of the human airways[END_REF]. Presence of Porphyromonas in the lungs was also noted in adults [START_REF] Morris | Lung HIV Microbiome Project, Comparison of the respiratory microbiome in healthy nonsmokers and smokers[END_REF][START_REF] Bassis | Analysis of the upper respiratory tract microbiotas as the source of the lung and gastric microbiotas in healthy individuals[END_REF]. No differences were observed between smokers and nonsmokers in terms of relative abundance of Porphyromonas in lung [START_REF] Morris | Lung HIV Microbiome Project, Comparison of the respiratory microbiome in healthy nonsmokers and smokers[END_REF][START_REF] Lim | Analysis of the association between host genetics, smoking, and sputum microbiota in healthy humans[END_REF]. Interestingly, based on a twin family cohort, Lim et al. [START_REF] Lim | Analysis of the association between host genetics, smoking, and sputum microbiota in healthy humans[END_REF] highlighted that Porphyromonas is one of the heritable taxa that are selected by genetic factors from the host. Interestingly, microbiological continuity has been described in the aerodigestive tract in healthy adults [START_REF] Bassis | Analysis of the upper respiratory tract microbiotas as the source of the lung and gastric microbiotas in healthy individuals[END_REF]. Microbial immigration from the oral cavity appears to be the primary source of the healthy lung microbiome. Bacterial communities in the healthy lung overlapped with those found in the mouth, but were found at lower concentrations, with different community composition and less diversity [START_REF] Bassis | Analysis of the upper respiratory tract microbiotas as the source of the lung and gastric microbiotas in healthy individuals[END_REF]. Among the bacterial species shared between the oral cavity and the lungs, many anaerobes were identified, including Porphyromonas.

Cystic Fibrosis (CF)

CF is an autosomal recessive genetic disease due to mutations on the cftr (cystic fibrosis transmembrane conductance regulator) gene. These mutations affect the CFTR protein and alter regulation of chloride ion and water transport across epithelial cells. One of the major consequences of these mutated proteins is the development of hyperviscous mucus which, combined with impaired immune response, weakens the endogenous lung microbiota to the benefits of pathogens such as Pseudomonas aeruginosa, Staphylococcus aureus or the Burkholderia cepacia complex [START_REF] Lamoureux | Anaerobes in cystic fibrosis patients' airways[END_REF].

Porphyromonas was first detected by Rogers et al. [START_REF] Rogers | characterization of bacterial community diversity in cystic fibrosis lung infections by use of 16s ribosomal DNA terminal restriction fragment length polymorphism profiling[END_REF], who used terminal restriction fragment length polymorphism profiling to characterize bacterial community diversity in CF lung infection. Since then, the genus was detected in many studies targeting CF populations, using culture-independent methods as well as culturomics (also called extended culture methods, based on the use of a high number of culture media and conditions) [START_REF] Muhlebach | Anaerobic bacteria cultured from cystic fibrosis airways correlate to milder disease: a multisite study[END_REF][START_REF] Van Der Gast | Partitioning core and satellite taxa from within cystic fibrosis lung bacterial communities[END_REF][START_REF] Cuthbertson | Respiratory microbiota resistance and resilience to pulmonary exacerbation and subsequent antimicrobial intervention[END_REF][START_REF] Laguna | Airway microbiota in bronchoalveolar lavage fluid from clinically well infants with cystic fibrosis[END_REF][START_REF] Carmody | Fluctuations in airway bacterial communities associated with clinical states and disease stages in cystic fibrosis[END_REF], and was classified as a member of the core CF lung microbiome [START_REF] Van Der Gast | Partitioning core and satellite taxa from within cystic fibrosis lung bacterial communities[END_REF]. Culturomics provided proof of viability of Porphyromonas isolated within the microbiota [START_REF] Lagier | Culturing the human microbiota and culturomics[END_REF]. The main species detected were P. catoniae, P. endodontalis, P. levii, P. pasteri and P. uenonis [START_REF] Keravec | Porphyromonas, a potential predictive biomarker of Pseudomonas aeruginosa pulmonary infection in cystic fibrosis[END_REF][START_REF] Rogers | characterization of bacterial community diversity in cystic fibrosis lung infections by use of 16s ribosomal DNA terminal restriction fragment length polymorphism profiling[END_REF][START_REF] Van Der Gast | Partitioning core and satellite taxa from within cystic fibrosis lung bacterial communities[END_REF][START_REF] Carmody | Fluctuations in airway bacterial communities associated with clinical states and disease stages in cystic fibrosis[END_REF][START_REF] Cuthbertson | The microbial community ecology of the cystic fibrosis, King's College[END_REF] and, in a lesser proportion, P. gingivalis [START_REF] Chouvarine | Filtration and normalization of sequencing read data in whole-metagenome shotgun samples[END_REF]. P. catoniae, P. uenonis and P. pasteri were revealed using bacterial extended culture of airway samples, unlike P. gingivalis which has so far only been revealed by metagenomics [START_REF] Muhlebach | Anaerobic bacteria cultured from cystic fibrosis airways correlate to milder disease: a multisite study[END_REF]. At this point, it is important to underline that P. gingivalis constitutes a minority of the Porphyromonas species within the pulmonary microbiome, in contrast to the oral microbiome.

In CF, children are rapidly colonized, before 1 year of age, by the genus Porphyromonas [START_REF] Frayman | Differences in the lower airway microbiota of infants with and without cystic fibrosis[END_REF], which is one of the top 10 taxa in terms of relative abundance in bronchoalveolar lavage (BAL) in clinically well CF infants [START_REF] Laguna | Airway microbiota in bronchoalveolar lavage fluid from clinically well infants with cystic fibrosis[END_REF]. Prevalence peaks at 10-60% at the age of 13-17 years [START_REF] Muhlebach | Anaerobic bacteria cultured from cystic fibrosis airways correlate to milder disease: a multisite study[END_REF][START_REF] Carmody | Fluctuations in airway bacterial communities associated with clinical states and disease stages in cystic fibrosis[END_REF] , then decreases with age [START_REF] Muhlebach | Anaerobic bacteria cultured from cystic fibrosis airways correlate to milder disease: a multisite study[END_REF][START_REF] Carmody | Fluctuations in airway bacterial communities associated with clinical states and disease stages in cystic fibrosis[END_REF]. After 45 years, another increase in several anaerobic genera, including Porphyromonas, is observed [START_REF] Carmody | Fluctuations in airway bacterial communities associated with clinical states and disease stages in cystic fibrosis[END_REF]; in this group of patients, with less severe disease phenotype (life expectancy of at least 45 years), Porphyromonas reaches almost the same prevalence as in the 6-10 year-old group. According to Carmody et al. [START_REF] Carmody | Fluctuations in airway bacterial communities associated with clinical states and disease stages in cystic fibrosis[END_REF], this second peak of anaerobic genera may be a survival effect linked with this extreme phenotype.

Porphyromonas lung colonization seems to be driven by different factors, and especially host characteristics. Firstly, Porphyromonas is one of the bacterial genera in which variance is explained by additive genetic effects, i.e two or more genes or alleles contributing to its presence in the lungs [START_REF] Lim | Analysis of the association between host genetics, smoking, and sputum microbiota in healthy humans[END_REF]. The underlying disease may also influence Porphyromonas level, which is greater in non-CF than CF children [START_REF] Frayman | Differences in the lower airway microbiota of infants with and without cystic fibrosis[END_REF]. In CF, there is a negative correlation between P. catoniae and F508del mutation (the major cftr mutation) [START_REF] Cox | Airway microbiota and pathogen abundance in age-stratified cystic fibrosis patients[END_REF]. Moreover, the clinical status of CF patients (e.g., P. aeruginosa colonization/infection) seems to influence Porphyromonas [START_REF] Keravec | Porphyromonas, a potential predictive biomarker of Pseudomonas aeruginosa pulmonary infection in cystic fibrosis[END_REF][START_REF] Quinn | Ecological networking of cystic fibrosis lung infections[END_REF]. Patient age is also a factor: a negative correlation was found between P. catoniae and patient age [START_REF] Frayman | Differences in the lower airway microbiota of infants with and without cystic fibrosis[END_REF][START_REF] Cox | Airway microbiota and pathogen abundance in age-stratified cystic fibrosis patients[END_REF]. Finally, environmental/exogenous factors may also have an impact on Porphyromonas colonization; Porphyromonas abundance was higher in the nasopharyngeal microbiota of a formula-fed than in a breast-fed group of CF infants [START_REF] Prevaes | Development of the nasopharyngeal microbiota in infants with cystic fibrosis[END_REF].

Focusing on the ecological networking of CF lung infection, an interesting dynamic between Porphyromonas and some important CF pathogens was established. Quinn et al. [START_REF] Quinn | Ecological networking of cystic fibrosis lung infections[END_REF] described a cluster of 17 operational taxonomic units (OTUs), including Prevotella, Parvimonas, Veillonella, Porphyromonas and Fusobacterium, with positive interactions. Contrarily, a negative correlation was observed between some commensal taxa (Streptococcus, Porphyromonas and Prevotella) and Pseudomonas [START_REF] Keravec | Porphyromonas, a potential predictive biomarker of Pseudomonas aeruginosa pulmonary infection in cystic fibrosis[END_REF][START_REF] Quinn | Ecological networking of cystic fibrosis lung infections[END_REF][START_REF] Zemanick | Assessment of airway microbiota and inflammation in cystic fibrosis using multiple sampling methods[END_REF]. The hypothesis is that fermentative anaerobes are able to create a micro-environment favorable to growth, while simultaneously making the environment less suitable for P. aeruginosa [START_REF] Quinn | Ecological networking of cystic fibrosis lung infections[END_REF]. The same trend was observed in oropharyngeal samples, where Porphyromonas was the fourth genus in terms of relative abundance to decrease over time prior to initial P. aeruginosa colonization [START_REF] Hoen | Associations between gut microbial colonization in early life and respiratory outcomes in cystic fibrosis[END_REF]. Quinn et al.

[48] also found that S. aureus interacts negatively and exclusively with Porphyromonas. A recent study combined methodological strategies, including the "change-point" method (association between changes in key taxa abundance and changes in patient clinical status over time), to identify key taxa during CF exacerbation [START_REF] Layeghifard | Microbiome networks and change-point analysis reveal key community changes associated with cystic fibrosis pulmonary exacerbations[END_REF]. Porphyromonas was the first genus in terms of relative frequency to be modified during exacerbation, and among the top 10 genera modified in the "unwell" clinical state, according to a method based on network interconnectedness. In another study, Porphyromonas was positively associated with lung function measured by forced expiratory volume (FEV-1), and increased after initiation of ivacaftor, a CFTR potentiator [START_REF] Bernarde | Impact of the CFTR-potentiator ivacaftor on airway microbiota in cystic fibrosis patients carrying a G551D mutation[END_REF].

These results favor the hypothesis of an association between Porphyromonas and improved or stable pulmonary state.

Looking more deeply into the taxonomic affiliation of Porphyromonas, P. catoniae appeared to be of particular interest. Cuthberston et al. [START_REF] Cuthbertson | Respiratory microbiota resistance and resilience to pulmonary exacerbation and subsequent antimicrobial intervention[END_REF] investigated the resistance and resilience of pulmonary microbiota to exacerbation and antimicrobial intervention. They found that P. catoniae provided the greatest contribution to dissimilarities between disease periods (baseline, exacerbation, treatment and recovery), with the exception of the comparison between the treatment and recovery periods, where Veillonella parvula provided the largest contribution [START_REF] Cuthbertson | Respiratory microbiota resistance and resilience to pulmonary exacerbation and subsequent antimicrobial intervention[END_REF]. Plotting P. catoniae's relative abundance during the clinical stages of CF disease, a drop between baseline and exacerbation and a significant drop during the treatment phase were observed. In contrast, an increase in P. catoniae abundance was observed between the treatment and recovery phases, with a final return to baseline. A recent study echoed this finding, showing an inverse dynamic between P. catoniae and P. aeruginosa in CF patients during early P. aeruginosa colonization [START_REF] Keravec | Porphyromonas, a potential predictive biomarker of Pseudomonas aeruginosa pulmonary infection in cystic fibrosis[END_REF]. Based on combined microbiome and targeted qPCR approaches, this study found that patients depleted in P. catoniae were more susceptible to colonization by P. aeruginosa. Moreover, the drop in P. catoniae abundance was found before P. aeruginosa colonization. This longitudinal study thus supported the idea of P. catoniae as a potential predictive biomarker of early P. aeruginosa colonization in CF patients [START_REF] Keravec | Porphyromonas, a potential predictive biomarker of Pseudomonas aeruginosa pulmonary infection in cystic fibrosis[END_REF].

The biological effects of the genus Porphyromonas and of P. catoniae in particular in the CF airway remain to be deciphered. The keys to understanding may reside in the study of Porphyromonas metabolites, including sphingolipids, fermentation-induced metabolites such as short-chain fatty acids, and metabolites of quorum-sensing [START_REF] Lamoureux | Anaerobes in cystic fibrosis patients' airways[END_REF].

Chronic Obstructive Pulmonary Disease (COPD)

Chronic obstructive pulmonary disease (COPD) is an inflammatory disorder with irreversible obstruction of the respiratory tract, and is usually classified in severity stages from mild to severe. The association of two factors causes this condition: reduced bronchiole caliber and destruction of the pulmonary alveoli. Porphyromonas was described as a member of the lung core microbiota in patients with COPD [START_REF] Erb-Downward | Analysis of the lung microbiome in the "healthy" smoker and in COPD[END_REF][START_REF] Diao | Characterization of throat microbial flora in smokers with or without COPD[END_REF], but in decreased abundance as in CF patients [START_REF] Wang | A Refined View of Airway Microbiome in Chronic Obstructive Pulmonary Disease at Species and Strain-levels[END_REF]. Some studies analyzed the airway microbiota to find markers of progression or better lung function. In 2017, Engel et al. [START_REF] Engel | Influence of lung CT changes in chronic obstructive pulmonary disease (COPD) on the human lung microbiome[END_REF] investigated the correlation between lung computed tomography (CT) changes and lung microbiome in patients with COPD, and found greater abundance of Porphyromonas in the mild/healthy group than in the severe group.

Porphyromonas was present in a co-occurrence network also composed of Tannerella and Prevotella. This study suggested that modifications in the microbial colonization of the lungs in COPD patients occur early, before significant impairment of health status, with the possibility of using bacterial biomarkers to characterize disease progression. Some studies assessed markers of disease progression in investigating the lung microbiota. Sze et al. [START_REF] Sze | Host response to the lung microbiome in chronic obstructive pulmonary disease[END_REF] investigated host response to the lung microbiome in COPD, and highlighted 10 OTUs able to discriminate controls from severe-stage COPD (GOLD 4). In these 10 OTUs, Porphyromonas was depleted in COPD patients compared to controls. A study of pharyngeal samples by Diao et al. [START_REF] Diao | Characterization of throat microbial flora in smokers with or without COPD[END_REF] found that P. endodontalis and P. catoniae levels correlated with FEV-1. The authors investigated the biomarker capacity of P. endodontalis (in combination with two other OTUs) in discriminating COPD patients from controls; sensitivity was 71.1% and specificity 75.0%. Surprisingly, Lee et al. [START_REF] Lee | Metagenome and metatranscriptome profiling of moderate and severe COPD sputum in taiwanese han males[END_REF] found contrasting results investigating the metagenome and metatranscriptome of moderate and severe COPD sputum samples, with higher Porphyromonas levels in severe than moderate COPD; however, the identified species were P. gingivalis and P. asacharolytica (supplementary data), known as pathogens, whose main niche seems not to be in the lungs, unlike for example P. catoniae. This observation highlighted the importance of taxonomic affiliation at species level, which could be an issue in 16S metagenomics-based studies. Clinical interpretation of bacterial genera frequently switches at species or even strain level, from a beneficial to a pathogenic species, modifying the study's conclusions.

Other pulmonary diseases

Lung cancer

Lung cancer is another major chronic pulmonary disease associated with Porphyromonas. Recent studies assessed the involvement of P. gingivalis in oral cancer and lung metastasis [START_REF] Woo | Oral cancer cells sustainedly infected with Porphyromonas gingivalis exhibit resistance to Taxol and have higher metastatic potential[END_REF]. Interestingly, studies that focused on the lower respiratory tract found a positive role of Porphyromonas. Exploring both saliva and BAL samples from patients with pulmonary bronchogenic carcinoma, Wang et al. [START_REF] Wang | A preliminary study of microbiota diversity in saliva and bronchoalveolar lavage fluid from patients with primary bronchogenic carcinoma[END_REF] found that patients with pulmonary cancer showed less microbial diversity than controls. The relative abundance of Porphyromonas was lower in BAL samples from lung cancer patients, and Porphyromonas relative abundance was higher in the microbiome of healthy subjects. These results are consistent with those of Yan et al. [START_REF] Yan | Discovery and validation of potential bacterial biomarkers for lung cancer[END_REF] for potential bacterial biomarkers of lung cancer; using saliva samples, they found that the relative abundance of Porphyromonas was significantly lower in patients with squamous cell carcinoma, and also (but non-significantly) lower in patients with adenocarcinoma. The recent study by Zhang et al. [START_REF] Zhang | Salivary microbial dysbiosis is associated with systemic inflammatory markers and predicted oral metabolites in non-small cell lung cancer patients[END_REF] also found significantly lower levels of anaerobes, including Porphyromonas, in the saliva of non-small-cell lung cancer patients compared to controls. Finally, a study by Han et al. [START_REF] Han | Potential screening and early diagnosis method for cancer: Tongue diagnosis[END_REF], comparing the tongue microbiota of healthy subjects versus lung, colorectal and gastric cancer patients, found higher relative abundance of Neisseria, Haemophilus, Fusobacterium, and Porphyromonas in the healthy controls. Although not all sample sites were in the lower airway microbiota, all these results agree on a decrease in Porphyromonas levels (presumed to be POTG) in lung cancer.

Tuberculosis

Porphyromonas was also reported to be involved in the course of tuberculosis (TB) [START_REF] Zhou | Correlation between either Cupriavidus or Porphyromonas and primary pulmonary tuberculosis found by analysing the microbiota in patients' bronchoalveolar lavage fluid[END_REF].

Comparing BAL samples between patients with TB and healthy subjects, the abundance of Mycobacteria and Porphyromonas was significantly higher in TB pulmonary lesions than in nonlesion areas in the contralateral lung; But comparing the top 30 genera in terms of abundance in each condition (healthy, TB lung without lesion, TB lung with lesion), Porphyromonas levels were lower in TB lungs without lesion than in controls, and no longer in the top 30 genera.

Porphyromonas in non-pulmonary pathologies

Rheumatoid arthritis

Porphyromonas was detected in BAL samples from rheumatoid arthritis (RA) patients [START_REF] Scher | The lung microbiota in early rheumatoid arthritis and autoimmunity[END_REF]. RA is a complex multifactorial disease, the etiology of which is not well explained. Recently, the role of periodontal disease, especially driven by P. gingivalis, was highlighted [START_REF] Scher | Periodontal disease, Porphyromonas gingivalis, and rheumatoid arthritis: what triggers autoimmunity and clinical disease?[END_REF]. In this context, Scher et al. investigated the lung microbiota in BAL samples in early RA and sarcoidosis [START_REF] Scher | The lung microbiota in early rheumatoid arthritis and autoimmunity[END_REF]. Contrary to their initial hypothesis, only 10% of patients with RA were colonized by Porphyromonas, versus 40% of controls. The relative abundance of both Porphyromonadaceae and Porphyromonas was significantly lower in patients with RA or sarcoidosis. Thus, despite the fact that P. gingivalis is an important pejorative actor in RA, the relative abundance of Porphyromonas in the lungs seems to be decreased in disease, again highlighting the fact that Porphyromonas lung species (which may be POTG) may not have the same action as P. gingivalis.

Sjögren's syndrome

A change in Porphyromonas abundance has also been observed in Sjögren's syndrome. A decrease in the relative abundance of Porphyromonas was observed in oral rinse fluid or saliva in patients with Sjögren's syndrome [START_REF] Siddiqui | Microbiological and bioinformatics analysis of primary Sjögren's syndrome patients with normal salivation[END_REF][START_REF] Zhou | Molecular analysis of oral microflora in patients with primary Sjögren's syndrome by using high-throughput sequencing[END_REF][START_REF] Rusthen | Dysbiotic salivary microbiota in dry mouth and primary Sjögren's syndrome patients[END_REF].

Looking at the species level, Rusthen et al. identified P. pasteri as the one with significantly lower prevalence in patients with primary Sjögren's syndrome than in controls (26.7% vs 80%) [START_REF] Rusthen | Dysbiotic salivary microbiota in dry mouth and primary Sjögren's syndrome patients[END_REF].

Oral diseases

Studies investigating the oral microbiome in periodontitis compared to healthy controls also found this contrast between P. gingivalis and POTG. P. catoniae was reported to be a member of the healthy oral microbiome, and P. gingivalis as a member of the core microbiome in case of periodontitis [START_REF] Yang | Occurrence of Porphyromonas gingivalis and Tannerella forsythensis in periodontally diseased and healthy subjects[END_REF][START_REF] Abusleme | The subgingival microbiome in health and periodontitis and its relationship with community biomass and inflammation[END_REF][START_REF] Camelo-Castillo | Subgingival microbiota in health compared to periodontitis and the influence of smoking[END_REF][START_REF] Payungporn | Identification of bacteria associated with a periodontal disease in Thai patients based on next-generation sequencing[END_REF].

In the context of oral squamous cell carcinoma, Porphyromonas quantified in oral rinsing has been identified as being decreased (with Streptococcus, Haemophilus and Actinomyces) during cancer progression [START_REF] Yang | Oral microbiota community dynamics associated with oral squamous cell carcinoma staging[END_REF]. This result has been proven by a single author and needs to be confirmed, as well as the species involved need to be specified. When analyzed up to the species level, P. pasteri was inversely associated with the progression of oral squamous cell carcinoma, and was selected by the authors as a member of a bacterial panel to distinguish patients with oral squamous cell carcinoma (stages 2, 3 and 4) from healthy individuals.

Skin diseases

Porphyromonas was described within the skin microbiota. Porphyromonas was detected on the skin of healthy subjects, and even in infants less than 1 year old, but also in patients with atopic dermatitis [START_REF] Ramadan | Skin microbiome differences in atopic dermatitis and healthy controls in Egyptian children and adults, and association with serum immunoglobulin E[END_REF][START_REF] Zheng | Alterations in the skin microbiome are associated with disease severity and treatment in the perioral zone of the skin of infants with atopic dermatitis[END_REF]. Within this microbiome as well, Porphyromonas showed an interesting contrast between healthy subjects and patients, and is one of the bacterial genera than could be relevant biomarkers to discriminate between healthy skin and atopic dermatitis and diagnose disease progression, either in combination with Neisseria, Paenibacillus, Granucatella and Enhydrobacter [START_REF] Ramadan | Skin microbiome differences in atopic dermatitis and healthy controls in Egyptian children and adults, and association with serum immunoglobulin E[END_REF], or with Veillonella, Prevotella, Leptotrichia and Agrobacterium [START_REF] Zheng | Alterations in the skin microbiome are associated with disease severity and treatment in the perioral zone of the skin of infants with atopic dermatitis[END_REF]. Porphyromonas levels were found to be decreased according to disease status (mild, moderate and severe disease) with a significant difference between the healthy and severe groups [START_REF] Zheng | Alterations in the skin microbiome are associated with disease severity and treatment in the perioral zone of the skin of infants with atopic dermatitis[END_REF]. After treatment of atopic dermatitis, Porphyromonas abundance increased. Conversely, P. aeruginosa abundance was increased in dermatitis [START_REF] Ramadan | Skin microbiome differences in atopic dermatitis and healthy controls in Egyptian children and adults, and association with serum immunoglobulin E[END_REF]; these findings echo those found in the CF lung microbiota, with a potential inverse correlation between Porphyromonas and P. aeruginosa [START_REF] Keravec | Porphyromonas, a potential predictive biomarker of Pseudomonas aeruginosa pulmonary infection in cystic fibrosis[END_REF]. A study of systemic antibiotic treatment for acne showed that antibiotic administration was associated with an initial decrease in bacterial diversity, followed by recovery. The same pattern was observed for Porphyromonas, with a significant drop in relative abundance at the start of antibiotic treatment and recovery after 12 weeks. In addition, a converse pattern was observed for Pseudomonas species, with a significant rise in relative abundance on initiation of treatment and a return to baseline after 12 weeks [START_REF] Chien | Association of systemic antibiotic treatment of acne with skin microbiota characteristics[END_REF].

Gastrointestinal diseases

Porphyromonas was reported in gastric cancer. Comparing gastric microbiota composition between non-atrophic gastritis and patients with intestinal gastric cancer, Aviles-Jimenez et al. [START_REF] Aviles-Jimenez | Stomach microbiota composition varies between patients with non-atrophic gastritis and patients with intestinal type of gastric cancer[END_REF] reported lower abundance of Porphyromonas in patients with cancer than in patients with gastritis. Two Porphyromonas OTUs were found, and were both significantly lower in the cancer group. An interesting point is the predominance of Pseudomonas in patients with gastric cancer, like in atopic dermatitis or CF lung disease. These results raise the question of a specific interaction between Porphyromonas species and Pseudomonas aeruginosa, whatever the niche. These findings were strengthened by a recent study by Kageyama et al. [START_REF] Kageyama | Characteristics of the salivary microbiota in patients with various digestive tract cancers[END_REF] investigating the salivary microbiome in patients with various forms of digestive tract cancer (tongue, pharynx, esophagus, stomach, and large intestine). They highlighted the importance of P. gingivalis, which was more abundant in the saliva of all groups of cancer patients compared to control subjects. The duality in Porphyromonas species was supported by a linear discriminant analysis (LDA), P. gingivalis being more abundant in cancer patients while three other Porphyromonas species, including P. pasteri (closely related to P. catoniae), were more abundant in control subjects.

Porphyromonas was also described in the digestive tract, represented by two main species: P. asaccharolytica [START_REF] Flynn | Spatial variation of the native colon microbiota in healthy adults[END_REF][START_REF] Dai | Multi-cohort analysis of colorectal cancer metagenome identified altered bacteria across populations and universal bacterial markers[END_REF] and P. gingivalis [START_REF] Purcell | Distinct gut microbiome patterns associate with consensus molecular subtypes of colorectal cancer[END_REF]. In healthy subjects, Porphyromonas was exclusively detected in the distal mucosa, with a high range of relative abundance (0.01%-16%), and consisting of the species P. asaccharolytica [START_REF] Flynn | Spatial variation of the native colon microbiota in healthy adults[END_REF]. Porphyromonas was elevated in the digestive tract of patients with colorectal cancer [START_REF] Dai | Multi-cohort analysis of colorectal cancer metagenome identified altered bacteria across populations and universal bacterial markers[END_REF][START_REF] Vogtmann | Colorectal cancer and the human gut microbiome: reproducibility with whole-genome shotgun sequencing[END_REF][START_REF] Rezasoltani | Applying simple linear combination, multiple logistic and factor analysis methods for candidate fecal bacteria as novel biomarkers for early detection of adenomatous polyps and colon cancer[END_REF][START_REF] Rezasoltani | The association between fecal microbiota and different types of colorectal polyp as precursors of colorectal cancer[END_REF][START_REF] Allali | Gut microbiome of Moroccan colorectal cancer patients[END_REF]. Conversely, several studies on oral microbiota-derived biomarkers for gastrointestinal cancers highlighted a higher abundance of Porphyromonas in controls' saliva and tongue coating compared to patients with colorectal cancer, gastric cancer or pancreatic cancer [START_REF] Torres | Characterization of the salivary microbiome in patients with pancreatic cancer[END_REF][START_REF] Hu | Variations of Tongue Coating Microbiota in Patients with Gastric Cancer[END_REF][START_REF] Wu | Tongue Coating Microbiota Community and Risk Effect on Gastric Cancer[END_REF][START_REF] Lu | Tongue coating microbiome data distinguish patients with pancreatic head cancer from healthy controls[END_REF]. A significant association was observed between P. gingivalis and polyp location, with elevated abundance in case of proximal polyp [START_REF] Rezasoltani | The association between fecal microbiota and different types of colorectal polyp as precursors of colorectal cancer[END_REF]. A significant positive association between polyp size and the absolute quantity of P. gingivalis was also described [START_REF] Rezasoltani | The association between fecal microbiota and different types of colorectal polyp as precursors of colorectal cancer[END_REF]. A set of several bacteria, including P. gingivalis and P. asaccharolytica, was significantly associated with colorectal cancer, and proposed as novel biomarkers for early detection of adenomatous polyps and colon cancer [START_REF] Rezasoltani | Applying simple linear combination, multiple logistic and factor analysis methods for candidate fecal bacteria as novel biomarkers for early detection of adenomatous polyps and colon cancer[END_REF][START_REF] Ai | Using decision tree aggregation with random forest model to identify gut microbes associated with colorectal cancer[END_REF].

Vaginal diseases

Porphyromonas was described as a member of the healthy vaginal microbiota [START_REF] Hill | Characterization of vaginal microflora of healthy, nonpregnant women by chaperonin-60 sequence-based methods[END_REF], but was mainly reported in a context of dysbiosis [START_REF] Hillier | The normal vaginal flora, H2O2producing Lactobacilli, and bacterial vaginosis in pregnant women[END_REF][START_REF] Smayevsky | Vaginal microflora associated with bacterial vaginosis in nonpregnant women: reliability of sialidase detection[END_REF][START_REF] Holst | Bacterial vaginosis and vaginal microorganisms in idiopathic premature labor and association with pregnancy outcome[END_REF][START_REF] Puapermpoonsiri | Vaginal microflora associated with bacterial vaginosis in Japanese and Thai pregnant women[END_REF]. In vaginosis, the incriminated species was P. asaccharolytica [START_REF] Hillier | The normal vaginal flora, H2O2producing Lactobacilli, and bacterial vaginosis in pregnant women[END_REF][START_REF] Smayevsky | Vaginal microflora associated with bacterial vaginosis in nonpregnant women: reliability of sialidase detection[END_REF][START_REF] Holst | Bacterial vaginosis and vaginal microorganisms in idiopathic premature labor and association with pregnancy outcome[END_REF][START_REF] Puapermpoonsiri | Vaginal microflora associated with bacterial vaginosis in Japanese and Thai pregnant women[END_REF], which was positively associated with vaginosis and preterm delivery [START_REF] Holst | Bacterial vaginosis and vaginal microorganisms in idiopathic premature labor and association with pregnancy outcome[END_REF]. P. levii [START_REF] Hillier | The normal vaginal flora, H2O2producing Lactobacilli, and bacterial vaginosis in pregnant women[END_REF], P. somerae [START_REF] Walther-António | Potential contribution of the uterine microbiome in the development of endometrial cancer[END_REF] and P. crevioricanis [START_REF] Walther-António | Potential contribution of the uterine microbiome in the development of endometrial cancer[END_REF] were also incriminated.

Higher prevalence of P. gingivalis was highlighted by molecular detection in chronic villous tissue of women with unexplained recurrent early miscarriages [START_REF] Ibrahim | Can Porphyromonas gingivalis be a novel aetiology for recurrent miscarriage?[END_REF]. Finally, P. somerae was positively associated with endometrial cancer (in combination with Atopobium vaginae), especially in case of vaginal pH >4.5 [START_REF] Walther-António | Potential contribution of the uterine microbiome in the development of endometrial cancer[END_REF].

Human immunodeficiency virus (HIV) infection

A study of Li et al. [START_REF] Li | HIV infection and microbial diversity in saliva[END_REF] was conducted in HIV seronegative and HIV seropositive patients and analyzed saliva microbiota from them using microarray. They highlighted that only seven genera were detected in HIV negative samples, and Porphyromonas was among them. Porphyromonas was detected in control subjects, not detected in HIV positive subjects before treatment, but detected in HIV positive patients after treatment. Among the genus Porphyromonas, detected species included P. catoniae, P. endodontalis and P. gingivalis.

Conclusions

In recent years, many studies of human microbiomes aimed to find potential probiotics and biomarkers related to specific diseases. The lung "anaerobiome" has increasingly been of interest and is now studied in CF and other chronic inflammatory lung diseases such as COPD.

Whatever the clinical context, disease and niche, Porphyromonas may become an important genus to analyze in order to discriminate disease stage/progression and to predict pathogen colonization. In line with this view of the implementation of the microbiome in precision medicine, identification to species level is mandatory as there is a strong duality in Porphyromonas between P. gingivalis, a pathogenic species, and some POTGs that are potentially beneficial. To perform this, new strategies can be used, such as long reads sequencing or even the use of extended culture, which is a reliable solution for the identification of species or strains. Thus, it is important to discriminate between species and to know exactly which are involved when discussing about Porphyromonas.

Few studies described POTG, but they all revealed a potential role for this group of species in both pulmonary and non-pulmonary diseases. Many studies have implicated P. gingivalis as a pathogen; However, it appeared that POTGs may have a different, even beneficial, role.

Unfortunately, only a few studies described Porphyromonas at species level in microbiomes; Consequently, the diversity of Porphyromonas species is understated. The development of high-throughput culture methods, including anaerobic cultures, may improve the description of this genus.

Porphyromonas should not be understated in a priori highly oxygenated microbiome, such as the pulmonary or cutaneous microbiome. Anaerobic culture should be much more systematically applied regardless of the site of origin of the sample.

In the future, POTGs could become new targets, to investigate their potential as biomarkers of disease progression or pathogen colonization: e.g., P. catoniae and P. aeruginosa in CF. Only prospective longitudinal cohort studies will answer these questions accurately.

(A) (H) (A) (H) (H) (A) (A) (A) (H) (H) (A) (A) (A) (H) (H) (H) (H) (H)

FigureFigure 1 .

 1 Figure

Acknowledgments

We thank Rosyne Lagrafeuille for careful reading of the manuscript, and Iain McGill for revision of the English manuscript.

List of abbreviations

Declaration of competing interest

All authors report no conflict of interest.


J Archez

A Gharzouni

I Sobrados

A Piancastelli

J F Caron

N Texier- Mandoki

X Bourbon

S Rossignol
email: sylvie.rossignol@unilim.fr

GEOPOLYMER LOCAL NETWORK EVOLUTION UNDER TIME AND TEMPERATURE

This study was undertaken in the context of Cigéo project to investigate the use of geopolymer for potential applications in high level waste cell. The aim was to evaluate the structural evolution of potassium metakaolin based geopolymer as a function of time and temperature for geological nuclear waste disposal applications. The evolution of mechanical properties, local structure, and porosity of geopolymer binder and composites stored at 0.5

2

and 20 months at 20 and 90 °C were studied. It was shown a decrease of density and stability of mechanical strength over time at 20 °C and a slight decrease at 90 °C. Moreover, it was evidenced the stability of the structure over time at 20 °C and the nucleation of a small amount of zeolite type phase at 90 °C. The mesoporosity was favored over time. However, an increase of porosity and pore size was evidenced at 90 °C due to pore coalescence and water removal. A model of aging was proposed in order to elucidate the structural changes over time and temperature. It has been shown that the aging of geopolymer at 20 and 90 °C induces changes of porosity and pore size distribution but seems not to degrade the structure and properties of the final material.

I. INTRODUCTION

Cigéo is the geological disposal facility project for radioactive waste managed by the French National Radioactive Waste Management Agency (Andra) for high-level and longlived intermediate-level radioactive wastes. If licensed, it will be built 500 m underground in a specific geological layer composed of the Callovo-Oxfordian (COx) argillites, in the East of France. Geopolymers are studied by Andra as potential new materials that can be used in Cigéo for example as grout at the boundary between the geological argillite and the lining of the high-level radioactive waste disposal cell [START_REF] Dupuy | Formulation of an alkali-activated grout based on Callovo-Oxfordian argillite for an application in geological radioactive waste disposal[END_REF], or as composite for the liner to reduce metallic use [START_REF] Archez | Adaptation of the geopolymer composite formulation binder to the shaping process[END_REF], or as fire-resistant materials [START_REF] Tognonvi | High-temperature resistant, argillite-based, alkali-activated materials with improved post-thermal treatment mechanical strength[END_REF]. Indeed, geopolymers are inorganic materials consolidated at low temperatures from the activation of an aluminosilicate source and an alkaline solution [START_REF] Dupuy | Thermal resistance of argillite-based alkaliactivated materials. Part 1: Effect of calcination processes and alkali cation[END_REF]. These materials are attractive because of their easy synthesis method, high working performance such as high mechanical strength, fire resistance, and wide range of applications [START_REF] Behzad | Geopolymer Technology, from Fundamentals to Advanced Applications: A Review[END_REF] and low environmental impact [START_REF] Liew | Investigating the possibility of utilization of kaolin and the potential of metakaolin to produce green cement for construction purposes[END_REF]. The use of geopolymer materials requires further comprehension of their evolution over time under different conditions such as temperature and relative humidity. To consider potential applications in high level waste cell, materials properties have to be determined in conditions representative of the operation (temperature between 20 and 90 °C, humidity, etc.).

The evolution of materials is mainly studied over time, in temperature, relative humidity, and under pressure [START_REF] Wan-Wendner | Aging concrete structures: a review of mechanics and concepts[END_REF]. For this, several data such as shrinkage, porosity, permeability, and mechanical data are collected over time and temperature [START_REF] Niveditha | Effect of durability properties on geopolymer concrete-a review[END_REF]. The effect of curing temperature during aging has been investigated by Cioffi et al. [START_REF] Cioffi | Optimization of geopolymer synthesis by calcination and polycondensation of a kaolinitic residue Resources[END_REF]. The decrease in strength and density of geopolymer at 60 °C was evidenced. Steins et al. [START_REF] Steins | Effect of Aging and Alkali Activator on the Porous Structure of a Geopolymer[END_REF] have investigated the porosity with aging. Densification of the solid network and a partial closure of the porosity at the nanometer scale (constriction of some narrow channel) was evidenced. Moreover, the durability can be improved by decreasing porosity and permeability [START_REF] Farhana | The relationship between water absorption and porosity for geopolymer paste[END_REF]. Greater compressive strength could also be reached with a lesser porosity sample [START_REF] Thokchom | Effect of water absorption, porosity and sorptivity on durability of geopolymer mortars[END_REF]. Other study made by Melar et al. [START_REF] Melar | The Porous Network and its Interface inside Geopolymers as a Function of Alkali Cation and Aging[END_REF] has evidenced that the potassium ions are responsible for the pore structure organization after 2 months and 5 years aging at 20 °C.

Other authors focused on the effect of relative humidity and temperature variation on the local network. For example, Kovalchuk et al. [START_REF] Kovalchuk | Alkali-activated fly ash : effect of thermal curing conditions on mechanical and microstructural development -Part II[END_REF] have studied alkali-activated material cured in dry condition (at 150 °C) and in steam condition (at 95 °C with 100 % RH). 29 Si NMR data evidenced a higher amount of Al-rich aluminosilicate gel (increase of Q 4 (4Al) species) when the relative humidity increases. It has also been shown that the mechanical resistance has been reduced with dry curing (30 MPa) compared with steam curing (72 MPa) due to the removal of water. The structural evolution of metakaolin-based geopolymers cured at different temperatures (50 and 110 °C) and relative humidity percentages (40 and 100 %) was also studied during 3 weeks and 3 years [START_REF] Lancellotti | Inorganic polymers from alkali activation of metakaolin : effect of setting and curing on structure[END_REF]. It was demonstrated that curing at 110 °C induces a semi-crystalline structure closer to zeolites. Moreover, curing at 40 % of relative humidity leads to a higher amorphous structure than 100 % RH. Moreover, a stable structure is evidenced after 3 years at room temperature.

This work aims to study the evolution of geopolymer composites formulation developed for the lining of the high-level radioactive waste disposal cell at ambient temperature and at 90 °C (which is the temperature that can be reached by the cell in operation). The evolution of mechanical properties, density, and porosity of geopolymer binder and composites stored at 0.5 and 20 months at 20 and 90 °C were studied. The structure of these samples was then analyzed with XRD, FTIR, and NMR measurements.

II. EXPERIMENTAL PART

Raw materials, samples preparation, and storage conditions

A commercial potassium solution provided by Woellner, with silicon to potassium (Si/K) molar ratio of 1.70 was modified by dissolving pellets of KOH by magnetic stirring for five minutes to reduce the Si/K molar ratio to 0.58. The geopolymer binder was synthesized by dissolving a commercial metakaolin (M1000) provided by Imerys in the silicate alkaline solution to obtain Si/Al molar ratio of 1.62. Based on a previous study [START_REF] Archez | Influence of the aluminum concentration, wollastonite and glass fibers on geopolymer composites workability and mechanical properties[END_REF], acicular particles (L. = 5-170 µm, D. = 3-15 µm) of wollastonite (W) provided by Imerys and/or alkaline resistant glass fibers (G) (L. = 6 mm, D. = 13-15 µm) provided by Owens Corning were also added to form a geopolymer composite. The resulting mixture was mixed until obtaining a homogenous mixture for a total time of ten minutes. Four formulations were studied: a geopolymer binder, named M1, and three geopolymer composites named M1W, M1G, and M1WG elaborated with the addition of a weight percentage (relative to the mass of the binder) of 18 % of wollastonite, 4,5 % of glass fibers, 9 % of wollastonite and 2.25 % of glass fibers. The reactive mixtures were then cast in polystyrene sealed mold. The samples were unmolded after 7 days and stored for 0.5 and 20 months in two storage conditions: (i) at 20 °C in a plastic sealed bag with a relative humidity of 100 % and (ii) at 90 °C in an oven with a relative humidity inferior to 5 %.

Characterization

The pH measurement was made at 20 °C on a consolidated geopolymer (solid) after a 7 days immersion with a pH-meter 3310 WTW as described in previous work [START_REF] Dupuy | Metakaolin-based geopolymer: formation of new phases influencing the setting time with the use of additives[END_REF].

Inductively coupled plasma optical emission spectrometry (ICPOES) was performed using a Perkin Elmer 8300 DV to determine the chemical composition of the solution after immersion of geopolymer. A sampling of 12 ml of solution (diluted 30-fold) was analyzed.

The pores size distribution was analyzed with a collaboration with ISTEC (Italy) on monolith sample by mercury porosimetry (Thermo Finnigan Pascal 140 and Thermo Finnigan Pascal 240) in the range 0.0078 -100 µm with a surface tension of 0.48 N/m, a contact angle equal to 140° and an experimental error of 4 % due to the accuracy of the instrument.

The microstructure was analyzed with a JEOL IT 300 LV scanning electron microscope at 10 kV. The samples (solid) were set on carbon paste and sprayed with a 10 nm layer of Pt before observations. Fourier-Transform Infrared (FTIR) spectra were obtained with a Vertex 70 apparatus made by Brucker. Spectra were acquired in transmission mode using polyethylene pressed discs (90 wt. % of polyethylene and 10 wt. % of geopolymer powders). The spectra were recorded between 50 to 800 cm -1 with a 4 cm -1 resolution and composed of 64 scans.

The XRD analysis was characterized using a Bruker-D8 Advance presenting a Bragg-Brentano geometry and Cu-Kα1α2 detector on powder samples. The analytical range was between 5 and 50° (2θ) with a step of 0.02° (2θ) and a dwell time of 1.5 s. The phase identification was performed with Powder Diffraction File (PDF) data from the International Center for Diffraction Data (ICDD).

High-resolution NMR experiments were performed on geopolymer powder at room temperature using a Bruker AVANCE-400 spectrometer, operating at 104.26 MHz for 27 Al signal and 79.49 MHz for 29 Si signal. A zirconia rotor (Ø = 4 mm) was used. The spinning rate was 10 kHz. The 27 Al (I = 5/2) MAS NMR spectra were recorded after p/8 pulse irradiation (1.5 μs) using a 1-MHz filter to improve the signal/noise ratio. The 29 Si (I = 1/2) MAS NMR spectra were recorded after a π/2-pulse irradiation (4 µs) using a 500 kHz filter to improve the signal/noise ratio. In each case, 400 scans were collected. The time between acquisitions was set to 5 and 10 s, respectively, to minimize saturation effects. A solution of AlCl3 and Tetramethylsilane (TMS) were used as references for 27 Al and 29 Si. The deconvolution of NMR spectra was performed with the Winfit program (Bruker). The estimated errors for chemical shifts and relative areas are 1 ppm and 2 % respectively.

The mechanical strength was measured by compressive tests using an Instron 5969 with a 50 kN load cell at a constant speed of 0.5 mm.min -1 , and was measured after seven days on six cylindrical samples for each parameter, with a 15 mm diameter and a 30 mm height [START_REF] Astm D | Standard Test Methods for Compressive Strength of Molded Soil-Cement Cylinders[END_REF]. The Young's modulus of the sample was determined by an ultrasonic contact echography technique detailed in previous work [START_REF] Dupuy | Formulation of an alkali-activated grout based on Callovo-Oxfordian argillite for an application in geological radioactive waste disposal[END_REF]. Samples with various thicknesses (15 to 20 mm) were placed between two couples of transducers (longitudinal and transverse) operating at 600 kHz.

III. RESULTS AND DISCUSSION

Influence of time and temperature on working properties

In order to elucidate the evolution and potential change of geopolymer over time and temperature, a geopolymer binder (M1) and three samples of geopolymer composites (M1W, M1G, M1WG) were demolded (7 days at 20 °C) then stored at 20 °C (RH=100 %) and 90 °C (RH < 5%) and studied over time (0.5, 3, 9 and 20 months).

For instance, the visual aspect and the microstructure of the two samples (M1 and M1WG samples) are reported in Table 1. Both formulations present the same evolution of their color and microstructure over time and temperature. In fact, at 20 °C during 0.5 month, the sample M1 has a brown color and an homogenous microstructure typical of metakaolin based geopolymer [START_REF] Gharzouni | Addition of low reactive clay into metakaolin-based geopolymer formulation: Synthesis, existence domains and properties[END_REF]. After aging (20 months at 20 °C), the color of the sample tending toward pale yellow is maybe due to some local reorganization of impurities in the M1 metakaolin [START_REF] Weber | Fading concrete block surfaces -Why do black, colored and gray concrete block surfaces appear lighter in the course of time?[END_REF][START_REF] Kong | Comparative performance of geopolymers made with metakaolin and fly ash after exposure to elevated temperatures[END_REF]. Storage at a higher temperature (at 90 °C during 0.5 or 20 months) induces the same behavior with pale yellow color and a less homogenous microstructure. This is due to the removal of physisorbed water leading to the formation of microstructural defects [START_REF] Kuenzel | Ambient temperature drying shrinkage and cracking in metakaolin-based geopolymers[END_REF]. The same phenomenon is observed after aging.

The values of density and maximum stress measured over 20 months for the formulations M1, M1W, M1G, and M1WG are reported in Figure 1 in order to evaluate the behavior of the several samples during these four conditions,. All the formulations present the same behavior in time and temperature. Indeed, a decrease of density (Figure 1-a) is observed over time and a more important decrease is measured at 90 °C. For instance, the M1WG sample presents a density of 1.663 g/cm 3 after a 0.5 month storage at 20 °C that decreases to 1.350 g/cm 3 after 20 months at 20 °C. It decreases even more at 90 °C during 20 months to reach 1.150 g/cm 3 . This suggests a transformation of structural entities forming the geopolymer network. The decrease of the density value with temperature and time can be also explained by a reinforcement of the structure consisting of a strengthening of network as in gel material [START_REF] Tognonvi | Physico-chimie de la gélification du silicate de sodium en milieu basique[END_REF].

Moreover, the compressive strength value of all samples (Figure 1 The specific stress is equal to 17 and 18 MPa.cm 3 g -1 , for a storage at 90 °C during 0.5 and 20 months, respectively which is similar to the specific stress at 20 °C (20 MPa.cm 3 g -1 ) due to the decrease of compressive stress and density.

Consequently, the binder and the composites display the same changes over time and temperature. In order to understand this behavior, further structural and physico-chemical measurements will be undertaken on the binder M1. Furthermore, peaks related to crystalline phases, such as quartz, muscovite and anatase from metakaolin, are detected. No significant differences can be observed between the samples in function of time and temperature. For further structural informations, the studied samples were characterized by 27 Al and Si MAS-NMR spectroscopy. The obtained 27 Al MAS-NMR and 29 Si MAS-NMR spectra are presented in Figure 3. The chemical shift and percentages of the curve area of the various contributions are given in Table 2. Typical spectra of geopolymer materials are obtained showing that aluminum is predominantly four coordinated with two contributions at 48 and 57.5 ppm and in a minor part six coordinated with a contribution at 1 ppm. At 20 °C, the aging from 0.5 to 20 months leads to a quite similar spectrum. The contribution areas are also quite similar. At 90 °C during 0.5 months, the spectrum is broader than 20 °C with an increase of the tetrahedral Al component at 49 ppm from 9.1 to 13.5 %. This contribution is not due to quadrupolar effect and can be attributed to a distinct tetrahedral Al species as demonstrated by Vuono et al., [START_REF] Vuono | Influence of NaOH and KOH on the synthesis of MCM-22 and MCM-49 zeolites[END_REF] in the case of zeolite. This result suggests the potential nucleation of a minor zeolite type phase at 90 °C.

A 20 months curing at 90 °C, slight changes can be observed such as the increase of the tetrahedral Al component area at 48 ppm in the detriment of the component at 57 ppm from 13.5 to 18.2 % and a slight increase of the octahedral Al component at 1 ppm from 2.8 to 3.2 %. Generally, broad 29 Si MAS-NMR spectra are detected. The deconvolution evidences six resonances: One at -79 ppm which can be attributed to unreacted or excess of silicate oligomers not bound to the gel and five components at -84,-89,-94,-100, -107 ppm assigned to Q 4 (n Al) with n varying from 4 to 0, respectively, revealing a three-dimensional network (Q 4 ) characteristic of geopolymer materials [START_REF] Duxson | 29 Si NMR Study of Structural Ordering in Aluminosilicate Geopolymer Gels[END_REF]. The broad peak at -107 ppm is due to quartz from metakaolin [START_REF] Lecomte | Synthesis and characterization of new inorganic polymeric composites based on kaolin or white clay and on ground-granulated blast furnace slag[END_REF]. As shown by 27 Al NMR-MAS data, at 20°C, no differences can be detected between 0.5 and 20 months. However, more changes can be observed at 90 °C with broader and more intense spectra revealing more disordered structure. Furthermore, it is noticed an increase of Q 4 (2Al) and Q 4 (1Al) to the detriment of Q 4 (3Al) and Q 4 (4Al) revealing a change in silicon and aluminum connectivity.

In order to understand the structural modifications occurring over time at 20 and 90 °C, the percentage ratio of Al IV /Al VI from 27 Al MAS-NMR, and Q 4 (4Al)/Q 4 (2Al) from 29 Si MAS-NMR were plotted as a function of storage conditions in Figure 4. At 20 °C, no change can be observed with increasing age either in Al IV /Al VI (23 and 22 for 0.5 and 20 months, respectively) nor in Q 4 (2Al)/Q 4 (4Al) percentage ratios (0.94 and 0.97 for 0.5 and 20 months, respectively), indicating a stable structure over time. However, at 90 °C for 0.5 months, an increase of Al IV /Al VI ratio to 35 is noticed. This fact is due to the increase of the tetrahedral Al contribution at 49 ppm as shown above which is attributed to zeolite nucleation. Similarly, the Q 4 (2Al)/Q 4 (4Al) percentage ratio increases to 1.33 revealing a change in Si and Al connectivity indicating a strengthening of iono-covalent bonds. Indeed, Si-rich species (Q 4 (2Al)) are more stable and with higher strength than Al-rich species Q 4 (4Al) [START_REF] Kovalchuk | Alkali-activated fly ash : effect of thermal curing conditions on mechanical and microstructural development -Part II[END_REF][START_REF] Palomo | Alkaline Activation of Fly Ashes: NMR Study of the Reaction Products[END_REF].

A slight decrease of the two percentage ratios Al IV /Al VI and Q 4 (2Al)/Q 4 (4Al) is observed over time between the 0.5 and 20 months samples stored at 90 °C (29 and 1.09, respectively) indicating a structural reorganization and maturation over time not favoring the formation of zeolite.

In order to verify these hypotheses, FTIR spectroscopy in the range of 650-100 cm -1 was performed (Figure 5). The band near 110 cm -1 is due to stretching K-O bond [START_REF] Zhang | Thermal behavior of vibrational phonons and hydroxyls of muscovite in dehydroxylation: In situ hightemperature infrared spectroscopic investigations[END_REF]. The range 650-400 cm -1 is characteristic of rings made from [SiO4] and [AlO4] tetrahedra, and therefore zeolite structure [START_REF] Krol | IR spectroscopy studies of zeolites in geopolymeric materials derived from kaolinite[END_REF], several broads and not well-resolved bands can be observed. A slight change can be observed in this region (especially at about 550 cm -1 ) in the spectrum of the sample stored during 0.5 months at 90 °C. However, the absence of a definite peak in the region of zeolites reveals that the system does not contain a significant quantity of zeolite nanocrystals [START_REF] Rees | Attenuated Total Reflectance Fourier Transform Infrared Analysis of Fly Ash Geopolymer Gel Aging[END_REF] which explains the very slight differences shown by XRD. pH value measurements were also undertaken. The sample stored 0.5 month at 20 °C has a pH value of 12.1 due to potassium ions that have not yet reacted [START_REF] Gharzouni | Effect of the reactivity of alkaline solution and metakaolin on geopolymer formation[END_REF]. This value decreases to 9.5 after 20 months of storage at 20 °C. This is significant in less unreacted species due to polycondensation reactions occurring over time. A 0.5 and 20 months storage leads respectively to a 9.8 and 9.4 pH value. The decrease of the pH values with the increase of temperature and time could be explained by either a lower amount of K + and OH -not bonded to the matrix [START_REF] Lancellotti | Inorganic polymers from alkali activation of metakaolin : effect of setting and curing on structure[END_REF] or encapsulated in the solid skeleton of geopolymer due to a strengthening of the network.

The porosity and the pore size distribution of the samples after the four storage conditions are reported in Figure 6. Different pore size and total porosity are observed depending on time and temperature. The geopolymer M1 stored 0.5 months at 20 °C presents a total porosity equal to 40 % and a wide pore size distribution varying between 0.0078 and 0.2 µm.

Two populations of pore size, 0.01-0.02 µm and 0.02-0.03 µm, can be distinguished which is typical of potassium solution and metakaolin-based geopolymer. A 20 months aging at the same temperature, leads to similar total porosity to 42 % and an increase of the 0.01-0.02 µm pore population. This fact is due to a densification of the network and maturation of the skeleton during the syneresis of the geopolymer gel [START_REF] Brinker | Sol-gel science, the physics and chemistry of sol-gel processing[END_REF]. After 0.5 months at 90 °C, most of the pore sizes are between 0.02 to 0.03 µm. The elimination of the small pore size appears with an increase of the total porosity (54 %). This phenomenon occurs because of the evacuation of physisorbed and poral water in dry condition and high temperature that leads to a pore coalescence [START_REF] Ye | Effect of elevated temperature on the properties of geopolymer synthesized from calcined ore-dressing tailing of bauxite and ground-granulated blast furnace slag[END_REF][START_REF] Perera | Influencing of curing schedule on the integrity of geopolymers[END_REF]. After 20 months at 90 °C, the pore size increases significantly (mainly 0.1 µm) due to the pore coalescence in dry conditions. In a comparison with a storage of 0.5 month at 90 °C, a small decrease of total porosity is observed (45 %). This is probably due to the strengthening of the skeleton over time and temperature.

Consequently, aging at 90 °C seems to have a more significant effect on the local structure and pore size distribution than aging at 20 °C.

According to the previously discussed data, a mechanism of structural evolution scheme as a function of storage time and temperature can be established in Figure 7. Initially, at 20 °C, The network of the geopolymer binder is composed of a solid framework with different pore sizes due to the poral water containing K + ions, which can diffuse within this framework. Indeed, these K + ions ensure electronic neutrality by counterbalancing the aluminosilicate charges and are encapsulated in water molecules in interaction with physisorbed water. The studied geopolymer is characterized by high porosity and low pore size due to the use of a relatively low reactive alkaline solution and metakaolin. The sample presents a Young modulus (determined by the ultrasonic contact echography technique) equal to 3.5 GPa which is similar of the geopolymer Young modulus values obtained in literature with mechanical test [START_REF] Duxson | Understanding the relationship between geopolymer composition, microstructure and mechanical properties[END_REF] or ultrasonic contact echography technique [START_REF] Duxson | Understanding the relationship between geopolymer composition, microstructure and mechanical properties[END_REF]. The increase in rigidity is due to the densification with the syneresis of the material over time and temperature [START_REF] Brinker | Sol-gel science, the physics and chemistry of sol-gel processing[END_REF].

During aging over time (20 months), the elimination of the physisorbed water leads to a contraction of the solid network leading to a consolidation of the bonds and a decrease in pore size. This is in accordance with an improvement of the young modulus from 3.5 to 4 GPa. This contraction of the network and the higher number of pores induce a decrease of the density. At 90 °C, the physisorbed water is removed very quickly, so there is a coalescence of the pore size. This induces a strengthening of the bonds (E= 4.2 GPa) and an increase in pore size manifested by a drastic decrease in density. This fact is accompanied by nucleation of metastable germs of zeolite as demonstrated by NMR and FTIR spectroscopies data. During aging at 90 °C, the phenomena are amplified and the Young's modulus increases (5.1 GPa) while the density decreases. This pore coalescence is responsible for the decrease in the mechanical strength value even if there is a reinforcement of the bonds. During the aging process, the mobility of K + ions is affected. At room temperature, the K + ions will be blocked in the pores, limiting their mobility and leading to a decrease of the pH value. Similarly, at 90 °C, the sudden removal of water results in a binding of these ions to the solid network causing the formation of Si-O -K + bonds detected by FTIR spectroscopy.

IV. CONCLUSION

This study aims to elucidate the evolution of geopolymer binder (potassium metakaolin based geopolymer) and composites (binder and/or wollastonite and glass fibers) in the function of storage conditions (time and temperature) to evaluate their suitability for radioactive waste geological disposal applications. These applications require long term stability at 90 °C. All samples (binder and composites) display the same behavior at 20 and 90 °C for 0.5 and 20 months i.e a decrease of density and stability of mechanical strength over time and a slight decrease at 90 °C. In order to further explain this behavior, a focus was made on the mechanical, structural and pore size distribution evolution of the binder in the function of the studied storage conditions (time and temperature). It has been evidenced that:

-At 20 °C, after 0.5 months, the studied geopolymer is characterized by high porosity and low pore size and a Young modulus of 3.5 GPa.

13

-The aging of the sample during 20 months at the same temperature 20 °C does not induce structural changes as evidenced by NMR and FTIR spectroscopies indicating similar local-order with a strengthening of the skeleton. However, small changes can be detected. In fact, the porosity slightly increases with favored mesoporosity due to the elimination of the physisorbed water and the contraction of the solid network.

-The increase of storage temperature to 90°C during 0.5 months seems to induce the beginning of crystallization of metastable germs of zeolite. In parallel, the release of physisorbed water leads to an increase of the porosity rate and the pore size (coalescence of the pores), a decrease of the pH value, and a slight decrease of the specific compressive strength.

-The aging of the studied geopolymer during 20 months at 90 °C seems not promoting more crystallization of zeolite. An increase of pore size (until 0.1 µm) and Young's modulus (5.1 GPa) and a decrease of density are noticed.

Consequently, the aging of geopolymer at 20 °C or 90 °C seems not to degrade the structure and the properties of the final materials. The major changes concern the porosity and the pore size distribution. Further investigation (at a longer time or under hydrothermal conditions) of these materials will be undertaken. 2. 27 Al and 29 Si NMR chemical shift and percentage of the curve area determined with a deconvolution of spectra for the M1 samples in differents conditions of storage Storage conditions (°C/ months) Chemical shift (ppm) 27 Al Area (%) 27

Al

Chemical shift (ppm) 29 Si Area (%) 29 Si 20 / 0.5

  -b) is similar over time at 20 °C. The decrease in compression values at 90 °C is due to the removal of physisorbed water. The water loss leads to the formation of larger pore sizes and a decrease in density. For instance, the M1WG sample presents a compressive strength equal to 29 and 26 MPa for a storage at 0.5 and 20 months at 20 °C. After a storage at a temperature of 90 °C, the compressive strength values decrease to 18 MPa at 0.5 month and to 12 MPa at 20 months.

2 .

 2 Influence of time and temperature structure and the porosity and of the geopolymer binder X-ray diffraction was performed. The XRD patterns are presented in Figure 2. For all samples, a broad reflection in the 2θ ≈ 28° related to amorphous phase is observed.

Figure 1 .Figure 2 .Figure 3 . 4 Figure 4 . 5 Figure 5 . 6 Figure 6 .

 123445566 Figure 1. Values of the (a) density (± 0.02 g/cm 3 ) and (b) compressive strength (± 3 MPa) as a function of time for the compositions ( ) M1, ( ) M1W, ( ) M1G, ( ) M1WG stored at ( , , , ) 20 °C and ( , , , ) 90 °C.

7 Figure 7 . 1 Table 1 .

 7711 Figure 7. Schematic image of the aging mechanisms of a geopolymer at 20 and 90 °C.

Acknowledgment

We thank the French National Radioactive Waste Management Agency (Andra) for the funding of this research and Nicolas Texier-Doyen for helping us measure the Young Modulus with the ultrasonic contact echography technique