Liu Yonghua 劉永華, professeur à l'Université de Xiamen, propose une passionnante étude d'histoire religieuse et sociale portant sur les hybridations entre culture régionale et orthopraxie confucéenne au cours des derniers siècles de l'ère impériale (dynasties Ming et Qing). À partir de monographies de district, de généalogies patrilignagères et de manuscrits rituels collectés dans la région de Sibao 四堡 (district de Liancheng, à l'ouest de la province du Fujian), il analyse la réalité complexe et la diversité de la société de la Chine méridionale, ainsi que ses transformations dans la longue durée. Le livre se compose de cinq parties et comporte un dossier d'annexes très bien fourni, ainsi qu'une bibliographie détaillée et un index.

La première partie pose le cadre de l'étude. Le district de Sibao, situé dans des marges montagneuses, était essentiellement peuplé d'ethnies She 畲族 et Hakka 客家. Jusqu'à la dynastie des Ming, l'État a eu beaucoup de mal à contrôler cette région à l'écart de l'administration et des circuits marchands. À la fin du XVII e siècle, malgré le petit nombre de lettrés locaux ayant réussi aux examens mandarinaux, certains d'entre eux, entreprenants, développèrent une importante activité d'imprimerie et une grande foire du livre. Des ouvrages portant sur l'éducation et l'éthique confucéenne, les rituels familiaux, des fictions et des ouvrages de belles lettres, mais aussi des manuels de médecine, de géomancie et de divination, ainsi que des almanachs hémérologiques fabriqués, et parfois rédigés, à Sibao furent alors diffusés à grande échelle depuis ce site.

La deuxième partie porte sur la catégorie de spécialistes religieux locaux appelés lisheng 禮生, particulièrement présente en Chine méridionale à la fin de l'ère impériale. Après une étude des multiples rôles et statuts plus ou moins officiels des lisheng au cours de l'histoire, l'auteur se focalise sur l'utilisation du terme dans les sociétés locales du Fujian. Son analyse détaillée des compétences de ces ritualistes, souvent qualifiés de "confucéens" fournit un exemple édifiant du hiatus qui existe entre texte canonique et réalité sociale, en particulier dans une civilisation aussi marquée par la diversité religieuse. Souvent issus du milieu des lettrés sans poste administratif, les lisheng de Sibao intervenaient dans de nombreux rituels : passages des âges de la vie, rituels familiaux, funéraires et ancestraux, sacrifices à Confucius et aux dieux du sol, rituels de régulation des pluies. L'analyse révèle aussi l'important rôle économique et juridique des lisheng, au travers de la rédaction de contrats impliquant les ancêtres et les divinités dans la vie quotidienne (adoptions de fils, vente de terre, partenariats commerciaux). Les lisheng collaboraient aussi régulièrement avec des maîtres taoïstes et bouddhistes, notamment dans le cadre de et de grands rituels d'offrande aux divinités (jiao 醮 ). L'étude de leurs manuscrits montre enfin qu'ils utilisaient couramment des talismans et des incantations, ainsi que des méthodes exorcistes et géomantiques. Liu Yonghua en conclut : "Lisheng functioned as mediators not only between official and gentry culture and village world but also between Confucian rituals and the rituals of other traditions, in particular those of Daoism" (p. 102). Ainsi, les lisheng de Sibao apparaissent moins comme de "purs" représentants de l'orthopraxie confucéenne que comme des ritualistes typiques de la religion locale du Fujian, dépositaires de savoirs hybrides relevant de différentes traditions canoniques et régionales.

La troisième partie porte sur les transformations des rituels et de la propriété de la terre qui ont été centrales dans la construction de la société patrilignagère de la Chine du Sud à partir du XV e siècle. À travers l'étude de trois patrilignages de Sibao se reconnaissant tous dans la figure tutélaire de la divinité-ancêtre Zougong 鄒公, l'auteur distingue trois phases, au sein de ce très important processus de changement social que David Faure a qualifié de "révolution rituelle." 1 À la fin du XV e siècle, l'essor du patrilignage Ma a été le fruit de l'activisme de lettrésfonctionnaires visant à instaurer une conscience de groupe et une stabilité sociale sur un mode orthodoxe. Un siècle plus tard, la fondation du patrilignage Zou a plutôt été l'oeuvre de petits fonctionnaires, d'étudiants, de villageois sans titre officiel ayant des compétences de géomanciens, de propriétaires terriens et de commerçants. Au XVIII e siècle, la fondation du patrilignage Yan s'est opérée par une segmentation à partir du patrilignage Ma à travers la construction de sanctuaires ancestraux dotés de terres communautaires, ainsi que la compilation de généalogies et l'organisation de rituels ancestraux. La présence de nombreux noms d'initiation "exorcistes Hakka" (langming 郞名) est frappante dans ces généalogies. 2 Néanmoins, l'auteur insiste plutôt sur l'importance de la mise en commun de terres qui donnait un pouvoir accru aux doyens et qui a abouti au fait que, à la veille des réformes agraires communistes du début des années 1950, 51% des terres de Sibao étaient des "terres communautaires tournantes" (gongluntian 公輪田, p. 148). La quatrième partie s'intéresse à cette forme d'organisations locales, moins étudiées que les patrilignages et autres communautés de cultes, que sont les community compacts (xiangyue 鄉約). Ces organisations, mises en place par des lettrés dès la dynastie des Song, se sont renforcées et multipliées au Fujian à partir du XV e siècle. L'objectif originel de ces communautés semi-officielles, souvent tissées entre plusieurs villages, était de maintenir l'ordre social par l'orthopraxie rituelle : lecture des édits sacrés impériaux, de serments et de maximes sur la piété filiale, récitations de textes canoniques par des lisheng. Lors de la crise sociale du siècle suivant, concomitante avec le déclin du système d'administration directe par les lijia 里甲, ces xiangyue ont progressivement fait évoluer leurs prérogatives vers le maintien de la loi et de la milice, l'organisation des systèmes d'irrigation, des marchés, de l'aide aux démunis et de grands rituels d'offrande aux divinités. Ces organisations restaient donc des intermédiaires entre la société et l'État, mais davantage au moyen de l'administration et de la fiscalité que du rituel orthodoxe. Ainsi, des personnes compétentes en gestion des affaires publiques et en comptabilité étaient souvent amenées à diriger les activités de ces communautés.

La cinquième et dernière partie est composée de trois chapitres qui traitent du développement des principaux cultes se tenant à Sibao. Le cas de Zougong est particulièrement significatif du changement social lié à la "révolution rituelle," puisque ce maître exorciste local divinisé fut progressivement transformé en ancêtre fondateur patrilignager et en lettré-fonctionnaire célèbre à partir de la dynastie des Ming. Au même moment se multiplièrent de nombreux temples dédiés à la divinité de la Guerre (Guandi 關帝), dont le culte contribua à la structuration de la géomancie et des frontières villageoises après le déclin des lijia. À partir du XVIII e siècle, les marchands de Sibao bâtirent, sur les places des marchés, des temples dédiés à l'impératrice céleste (Tianhou 天后 ou Mazu 媽祖), divinité particulièrement populaire en Chine méridionale et femme maître ancestrale des guildes commerciales.

Selon l'auteur, la construction patrilignagère, les réseaux de différents cultes et la conjoncture économique ont ainsi participé au changement social, caractérisé moins par la sinisation ou la "gentrification" de la société que par des phénomènes de syncrétisme et d'hybridation impliquant l'agentivité de différents groupes locaux et formant de la sorte une "mosaïque culturelle" complexe.

Liu Yonghua nous livre un excellent travail d'histoire régionale, montrant une capacité exemplaire à explorer aussi bien les textes les plus officiels que ceux, plus "locaux" et de nature très diverse, dans lesquels les informations sont toujours clairsemées et parcellaires. L'analyse de telles sources implique de se distancier des catégories analytiques préétablies (religion, parenté, politique, économie…) pour se confronter à la complexité de la réalité sociale. Cet effort déjà colossal pourrait être prolongé par l'auteur s'il explorait plus avant les questions rituelles et identitaires liées à l'ethnicité, telles qu'elles sont par exemple étudiées par Chan Wing-Hoi, John Lagerwey 3 ou Wu Nengchang [START_REF] Nengchang | Rituels, divinités et société locale: Une étude sur la tradition des maîtres rituels du Lingying-tang à l'ouest du Fujian[END_REF] . Concernant la classification des ritualistes, même si l'auteur montre les multiples hybridations entre les pratiques locales et les différentes grandes traditions liturgiques, il persiste néanmoins à les définir en fonction de leur affiliation canonique (par exemple le "noyau identitaire" du fashi 法 師 reste taoïste ou celui du lisheng, confucianiste). Nous nous situons néanmoins à la frontière d'un point de basculement : dans quelle mesure le texte de référence (qui n'est sans doute appliqué à la lettre que dans l'acte d'écriture de son auteur) est-il un critère définitoire plus pertinent que les compétences concrètes qu'un ritualiste met en oeuvre quotidiennement dans un contexte sociologique spécifique ? Dans cette étude, la "religion locale" n'apparaît pas comme une catégorie située au même niveau que celui du "confucianisme" ou du "taoïsme," mais bien comme un moyen d'aborder le religieux et le social en Chine par une analyse rigoureuse des données émanant du terrain. La recherche de l'auteur se heurte aussi à la difficulté de s'émanciper des catégories et des divisions entre thématiques de recherche quand il étudie la figure cultuelle de Zougong. Bien qu'il l'analyse comme une arène d'interprétations multiples, discordantes et changeantes, il s'en tient à la distinction canonique entre "divinité" et "ancêtre," se privant ainsi de mettre en regard les études de la parenté et de la religion. Pourtant, alors que le système patrilignager repose sur des fondements rituels et cultuels, les communautés dites "religieuses" sont elles-mêmes structurées par des relations de parenté (spi)rituelle. Enfin, le titre de l'ouvrage révèle, par un détail semble-t-il anodin, un hiatus qui me semble important : jusqu'à quel point la date de 1949 est-elle pertinente pour construire une périodisation heuristique de l'histoire chinoise contemporaine et pour comprendre les changements qui s'opèrent dans cette société? La rupture de la "libération pacifique" de la Chine par le parti communiste tend à s'instaurer comme un repère porteur d'une ambivalence sacrée, entre horreur et fascination, qu'il nous incombe donc de transgresser intellectuellement. Sous le coup de l'émotion et de l'officialité de cet événement de grande échelle, "Tradition" et "Modernité" tendent à être essentialisées. L'étude conjointe des continuités et des ruptures du XX e siècle est en train de se faire, et l'auteur partage lui-même certaines observations faites sur le terrain depuis les années 1990. En tant qu'anthropologue, je ne peux que témoigner de mon vif intérêt pour ces matériaux de première main et souhaiter que l'historien rigoureux et passionnant qu'est Liu Yonghua ait d'autres occasions de tirer parti de son intime connaissance de sa région d'étude et de ses matériaux ethnographiques. L'ouvrage qu'il nous livre est un exemple de rigueur et de clarté académique. Il constitue une référence incontournable en langue occidentale sur l'histoire sociale de la Chine méridionale et un modèle méthodologique quant à l'utilisation de documents historiques qui nous permettent de descendre en échelle dans les structures sociales pour s'ancrer, autant que faire se peut, dans l'histoire vécue des populations.

Georges Favraud, Université Toulouse Jean Jaurès

2

  À propos des langming, voir Chan Wing-Hoi, "Ordination Names in Hakka Genealogies: A Religious Practice and its Decline,", in Down to Earth: The Territorial Bound in South China, éd. David Faure et Helen F. Siu (Stanford: Stanford Univ. Press, 1995), 65-82.

voir David Faure, "La solution lignagère. La révolution rituelle du XVI e siècle et l'État impérial chinois," Annales, Histoire et Sciences Sociales 61 (2006) : 1291-1316.


Valentina Musumeci

Paula Sanz Camacho

Ke Xu Cd

Paulo J M Monteiro

Jorge S Dolado Be

Cyril Aymonier

Sub-and supercritical hydrothermal route for the synthesis of xonotlite nanofibers for application to green concrete materials

Despite a wide range of applications, naturally occurring minerals suffer from some limitations for industrial use. Consequently, many research efforts have been conducted to develop a fast, optimized, and sustainable methodology to produce synthetic minerals. In the case of calcium silicate hydrates (CSH), the hydrothermal flow approach allows to mimic the environment at high temperature and pressure of the natural geological processes for the synthesis of xonotlite under sub-and supercritical conditions in only few seconds. The ultra-fast, flexible, and effective production of xonotlite particles reported in this work expands its use towards industrial requirements, especially for applications to cement-based materials. In this context, CSH nanominerals can be used to impart new functionality or to accelerate the hydration process of cement for developing green cement materials. This study sheds light on the acceleration effect of crystalline xonotlite seed, measured using isothermal calorimetry and synchrotron radiation based X-ray microtomography, as a means of lowering the cement content without compromising the performance of the paste.

Introduction

A natural mineral is defined as a solid with specific chemical composition and a highly ordered atomic arrangement that has been formed as a result of a geological process. From the beginning of civilization, natural minerals have played a key role as raw materials for the social, technological, and economic development of modern society [START_REF] Claverie | Continuous synthesis of nanominerals in supercritical water[END_REF]. Among the wide variety found in nature, calcium silicate hydrates are members of a family of minerals that feature layered crystalline structures with different calcium-to-silicon (Ca/Si) ratios. In nature, they form consequently to the geological hydrothermal reactions between basic igneous rocks and hyperalkaline fluids. In the last years, calcium silicate hydrates have been highlighted as attractive materials due to their potential applicability in numerous fields, such as orthopedic applications [START_REF] Li | A novel hydrothermal route to the synthesis of xonotlite nanofibers and investigation on their bioactivity[END_REF], insulating materials [START_REF] Wei | Thermal conductivities study on silica aerogel and its composite insulation materials[END_REF], stabilizing agents for trapping wastewater impurities, and as an addition for reinforced concrete [START_REF] Low | Mechanical properties and microstructure of cement binders reinforced with synthesized xonotlite micro-fibres[END_REF]. In the latter case, employing these mineral phases in cement relies on their high chemical compatibility with the calcium silicate hydrate (C-S-H) gel, the main hydration product of cement paste and the primarily responsible for its strength and performance. Nowadays, concrete is the most widely used construction material, and as a consequence of the great quantity of clinker produced, every year, the manufacturing industry of cement accounts for about 7-8% of the global CO2 anthropogenic emissions [START_REF]Concrete needs to lose its colossal carbon footprint[END_REF], [START_REF]Making Concrete Change: Innovation in Low-carbon Cement and Concrete[END_REF]. In this context, research for producing more ecological cement is a challenge of fundamental importance to reducing the environmental footprint. The sustainability of concrete can be improved by partially replacing the amount of clinker by different supplementary cementitious materials or enhancing the material performance, making the life cycle of cement structures more resilient and durable. In this context, a significant number of studies have been developed to improve the knowledge of the chemical admixture technology to reduce the cement content without compromising the concrete performance, or even to improve the material mechanical properties [START_REF] Low | Mechanical properties and microstructure of cement binders reinforced with synthesized xonotlite micro-fibres[END_REF], [START_REF] Li | Fibrillar calcium silicate hydrate seeds from hydrated tricalcium silicate lower cement demand[END_REF], [START_REF] Hernández-Cruz | Fiber reinforced mortar affected by alkali-silica reaction: a study by synchrotron microtomography[END_REF], [START_REF] Sharma | Effect of particle size of nanosilica on microstructure of C-S-H and its impact on mechanical strength[END_REF], [START_REF] Li | Enhanced performances of cement and powder silane based waterproof mortar modified by nucleation C-S-H seed[END_REF]. Furthermore, there is great interest in the development of new admixtures to impart new functionalities (i.e., photocatalytic or sensing capabilities) or to modify the hydration process [START_REF] Sobolev | How nanotechnology can change the concrete world[END_REF], [START_REF] Monteiro | Advances in characterizing and understanding the microstructure of cementitious materials[END_REF]. Several classes of admixtures exist and can be identified depending on their utility and effect on the concrete. Among them, there are water-reducing, setting timemodifiers, which are classified in retarding or accelerating admixtures, etc. An interesting category includes additions or chemical admixtures to accelerate the early hydration of cement. These additions play a relevant role in precast structures where speeding up the hydration kinetics allows the removal of the mold and finalizing the structure in shorter times, minimizing the cost of construction. Additionally, accelerators are particularly suitable in a cold environment to avoid the risk of frost damage in the concrete skeleton.

Recent studies have shown the effect of xonotlite, a calcium silicate hydrate mineral, as a seed to trigger the autocatalytic formation of the C-S-H gel [START_REF] Land | Controlling cement hydration with nanoparticles[END_REF], [START_REF] Diez-Garcia | Synthesis by supercritical fluids methods of advanced additions for cementitious materials[END_REF]. Xonotlite is usually described as having the following formula Ca6Si6O17(OH)2 [START_REF] Kudoh | Polytypism of xonotlite: (I) structure of an A-1 polytype[END_REF]. The basic elements of the xonotlite structure have been identified as CaOpolyhedral layer and Si6O17-double chains. The Ca-polyhedral layer is composed of three different calcium sites depending on the Ca coordination in the structure. Two of them are seven-fold coordinated by oxygen, whereas the third calcium atom has octahedral coordination. The other basic xonotlite structural elements are Si6O17-double chains bonded via sheets of edge-sharing Ca-polyhedra. Each Si6O17-double chains is formed by condensation of two wollastonite-like Si3O9-dreier-einfachketten chains [START_REF] Kudoh | Polytypism of xonotlite: (I) structure of an A-1 polytype[END_REF], [START_REF] Hejny | Polytypism in xonotlite Ca6Si6O17(OH)2[END_REF]. That single unit consists of repetitive silicon-oxygen tetrahedra with a periodicity of three SiO4. Among them, two are paired together to form Si2O7, and the third one corresponds to a single tetrahedron in a bridging position to connect the paired SiO4. Consequently, in xonotlite, the Si3O9 chains are linked together by sharing the apical oxygen atoms of two bridging tetrahedra, forming what is known as Si6O17-dreier-doppelketten double chain.

Despite a wide range of applications, the employment of natural minerals as xonotlite often suffers from impurities or contaminant phases and limited natural deposits. Therefore, the increase in demand stimulated the search for new and green approaches for sustainable and rapid production at the industrial scale. So far, the hydrothermal methodology represents the most common route for producing synthetic minerals. The conventional hydrothermal synthesis of xonotlite has been investigated, in the literature, in a wide range of temperatures, times, and precursors. The most common temperature ranges between 150 and 250 °C [START_REF] Spudulis | Influence of hydrothermal synthesis condition on xonotlite crystal morphology[END_REF]. However, in the case of low temperature, the synthesis time is very long (133 days at 150 °C) [START_REF] Speakman | The stability of tobermorite in the system CaO-SiO2-H2O at elevated temperatures and pressures[END_REF], and can be reduced to several hours (∼ 17 h to obtain crystalline single-phase material) at 250 °C [START_REF] Diez-Garcia | Synthesis by supercritical fluids methods of advanced additions for cementitious materials[END_REF]. It appears clear that the main disadvantage of this technique relies on the very long synthesis times for synthesizing high crystalline xonotlite particles. In this context, the innovative supercritical hydrothermal flow synthesis (SHFS) represents a breakthrough, allowing producing model nanominerals very close to the natural counterpart in the crystallinity and morphologies [START_REF] Dumas | Fastgeomimicking using chemistry in supercritical water[END_REF]. Supercritical water, exhibiting unique thermophysical properties that differ largely from water at ambient conditions, acts as a solvent for mimicking the environments at high temperature and pressure of the geological natural process [START_REF] Claverie | Continuous synthesis of nanominerals in supercritical water[END_REF], [START_REF] Aymonier | Review of supercritical fluids in inorganic materials science[END_REF], [START_REF] Aymonier | Playing with chemistry in supercritical solvents and the associated technologies for advanced materials by design[END_REF]. In 2018, the synthesis of xonotlite particles in supercritical water was reported by Diez-Garcia et al. [START_REF] Diez-Garcia | Supercritical hydrothermal flow synthesis of xonotlite nanofibers[END_REF]. However, in their work, beyond the proof of concept, the study of the structure and the applications of the mineral were limited by the amount of the produced material. Most calcium silicate hydrate minerals have fiber-like morphology, which results in clogging problems from an experimental point of view. Consequently, the first objective of this work is on the design and the development of a more suitable setup for higher and more effective mineral production. Furthermore, we have conducted a parametric study to identify the effect of temperature, in near-or supercritical conditions, on the crystallinity and structural properties of xonotlite, to then compare them with those reported in the literature so far.

After determining the optimal value parameters for producing pure single-phase xonotlite, we have focused on the employment of the as-produced particles into cement-based materials to partially replace the clinker content into the paste. In this context, our study could open the door to a more flexible and effective production for the industrial uses of xonotlite in several application fields.

Experimental

Materials

Xonotlite nanoparticles were synthesized by an SHFS starting from the following precursors: calcium nitrate tetrahydrate (Ca(NO3)2.4 H2O, purity ≥ 99%) and sodium metasilicate nonahydrate (Na2SiO3.9 H2O, purity ≥ 98%). The precursors were purchased from Sigma Aldrich and used as received.

Hydrothermal flow synthesis apparatus and procedure

The production of xonotlite nanoparticles was carried out in a continuous reactor. A process flow diagram and a picture of the entire experimental system are shown in Fig. 1. The set-up is composed of three sections: the injection of the chemical precursors, the reactor where the crystallization of the xonotlite nanoparticles occurs, and the product recovery.

To assess the impact of the experimental parameters on the material properties and then to be able to use it in several application fields, a work of process engineering was carried out to design, construct, develop, implement, and monitor the synthesis equipment. Firstly, to avoid any clogging problems, we have decided to scale up a conventional laboratory-scale reactor. The developed supercritical continuous system consists of homemade user-customized equipment composed of several task-specific units, some commercially available and others specially designed, and assembled together. The set-up is highly modular and offers the opportunity to configure the equipment with specific requirements easily.

The developed experimental apparatus produces ∼ 2.9 g/h of xonotlite. Behind the high modularity of the system, two more advantages rely on the fine control of the reaction kinetics through the chosen residence times and the possibility of using high molar concentration precursor solutions for effective production of materials without the risk of clogging.

For the synthesis, two precursors solutions were prepared by dissolving a stoichiometric amount (M = 0.1 mol/L) of Ca(NO3)2.4H2O and Na2SiO3.9H2O in pure water to obtain Ca/Si molar ratio = 1. Afterward, the precursor solutions were kept under an N2 flow using three outlets line equipping the experimental setup to prevent their carbonation. Initially, only pure water was injected by the two Jasco Isocratic HPLC pumps into the system, and the pressure was progressively increased by closing the BPR. At that time, the system's temperature was increased by setting the operating temperature value on the Eurotherm controllers. When the reactor reached the desired temperature and pressure conditions, the three-way junction valves were turned to switch to the precursor lines. The precursor solutions were pumped at 2.8 mL/min and fed to the heated and pressurized reactor. At the end of the synthesis time, the heating system was shut down, and the reactor was let cool down to room temperature. Finally, once the system returned to ambient conditions, the filter was disassembled from the reactor and opened by a 15-inch adjustable wrench to collect the synthesized particles. The produced materials were dried in an oven at 60 °C overnight before being finely grounded into a powder for the characterization analyses.

Cement paste preparation

All cement pastes were prepared using ASTM Type I cement at a water-to-binder ratio (w/b) of 0.5 at 25 °C. In the seeded pastes, a variable concentration of dried xonotlite nanoparticles, 0.5 wt% or 1 wt%, per weight of cement was employed. The seeds were dispersed in deionized water and then sonicated for 120 s. Subsequently, the obtained colloidal suspension was added to the cement powder and mixed for 90 s at 800 rpm using a vortex mixer. Lastly, after waiting for 60 s, the slurry was mixed at 1000 rpm for 90 s.

Materials characterization

P a g e | 5

Powder X-Ray diffraction (PXRD)

Powder X-ray diffraction patterns were recorded on a PANalytical X′Pert PRO MPD X-ray diffractometer in Bragg-Brentano θ-θ geometry. The employed radiation source, Cu Kα (Kα1 λ = 1.5418 Å, Kα2 λ = 1.5444 Å), operated at a generator voltage of 45 kV and a current of 40 mA. Continuous scans were collected by an X′Celerator multi-strip detector within the range 8.012-99.991° (2θ) with a minimum step size of 0.017° (2θ). Before measurement, the samples were finely ground and sieved at 100 µm. Afterward, the powder was placed and uniformly distributed into the sample holder ring to form a flat pellet.

Solid-state nuclear magnetic resonance spectroscopy (SSNMR)

Solid-state nuclear magnetic resonance spectra were acquired on a Bruker Avance 300 MHz spectrometer, equipped with a 7.05 T wide-bore magnet. The experiments were performed at room temperature using a standard 4-mm magic angle spinning (MAS) probe with MAS rates of 10 kHz. For 1 H solid-state NMR, a rotorsynchronized Hahn-echo sequence was employed with a π/2 pulse of 2 μs and a recycle delay of 5 s. The chemical shifts refer to ultrapure Milli-Q water (shift: + 4.7 ppm). While, for 29 Si solid-state NMR, the spectra were acquired either directly, using a short pulse length of 2.4 µs and a recycle delay of 10 s, or using cross polarization from 1 H ( 1 H- 29 Si CP/MAS NMR) with square contact pulse of 6 ms and a recycle delay of 3 s. In these cases, chemical shifts are referenced relative to silicon oil at -22 ppm.

Fourier transform infrared spectroscopy (FTIR)

Fourier transform infrared measurements were conducted using a Bruker Equinox 55 spectrophotometer. The spectra were recorded over 32 scans in the spectral absorption region of 400-4000 cm -1 , with a spectral resolution of 4 cm -1 . Before the analysis, the sample (1 wt%) was dispersed into an appropriate amount of KBr, which is optically transparent within the mid-IR region.

High-resolution Scanning electron microscopy (HR-SEM)

High-resolution field emission gun SEM-FEG HR (JEOL 6700 F) operating at a voltage of 5 kV, working distance of 6 mm and a magnification of 30,000 × or 50,000 × was employed to investigate the morphology of produced xonotlite particles. To carry out a SEM analysis, the dry powder samples (< 2 mg) were dispersed in ethanol, and then placed by drop-casting on an aluminium stub. Before imaging, being xonotlite a nonconductive material, a 1 nm layer of platinum was deposited on the surface through sputter coating.

High-resolution transmission electron microscopy (HR-TEM)

High-resolution transmission electron imaging was performed using a JEOL Jem 2200FS FEG high-resolution microscope, operating at 200 kV with a range of accessible magnification within 4000 × -600000 × and a resolution of 0.1 nm. Samples for HR-TEM analysis were prepared by drop-casting a suspension of nanoparticles (< 2 mg) in ethanol (∼ 1 mL) onto a carbon-coated copper grid.

Calorimetry analysis

The calorimetry analysis was employed to measure, over time, the heat released during the hydration process of cement. To carry out the experiments, 5 g of cement paste was placed into a 20 mL hermetic glass bottle and then loaded into TAM Air isothermal calorimeter within 6 min after the initial contact of clinker with water. The measurements were performed at 25 ± 0.001 °C for 48 h for each sample.

Synchrotron X-ray computed microtomography (μCT)

Synchrotron μCT was performed at beamline 8.3.2 at the Advanced Light Source (ALS) of the Lawrence Berkeley National Laboratory [START_REF] Macdowell | X-ray micro-tomography at the Advanced Light Source[END_REF]. Each of the as-prepared pastes was loaded into a cylindrical thin-wall polyimide tube (∼ 3 mm diameter) to perform the analysis. The cement pastes were tested at different setting times, 7 and 36 h, at ∼ 25 °C. During the measurement, the sample was irradiated by a focused X-ray beam of 40 mm × ∼ 4.6 mm size (at the sample) with an energy of 35 keV. The ring energy was 1.9 GeV to carry out the experiment, and the beam current was maintained constant at 500 mA. The sample was rotated around the axis perpendicular to the X-ray beam propagation at each scan. For each cement paste, a totality of 1969 projected radiographs with an angle step of 0.09° and an exposure time of 20 ms were acquired onto a 5x Mitutoyo magnification optical objective lens and a 2560px PCO.Edge Scientific CMOS camera. Under the above experimental settings, the field of view was 3.3 mm, and the pixel resolution was 1.3 µm/voxel for each scan. During the X-ray computed microtomography experiment, a tomogram with dimensions of 2560*2560*1969 voxels 3 (32-bit, 56.7 GB) was reconstructed using the filtered back projection algorithm [START_REF] Gürsoy | TomoPy: a framework for the analysis of synchrotron tomographic data[END_REF], [START_REF] Kanitpanyacharoen | A comparative study of X-ray tomographic microscopy on shales at different synchrotron facilities: ALS, APS and SLS[END_REF] for each sample.

Results and discussion

The very complex system identified as CaO-SiO2-H2O is composed of about thirty crystalline phases, characterised by different Ca/Si molar ratios. Most of the minerals belonging to this family can be obtained through hydrothermal route. The formation of one synthetic specie or another depends on the control of numerous parameters, such as the Ca/Si ratio, the time and the temperature of the reaction. Therefore, the synthesis of calcium silicate hydrate minerals is quite challenging due to the difficulty in manufacturing crystalline and pure single phase materials. Several studies in the literature have investigated the mechanism of the xonotlite phase formation via the hydrothermal route [START_REF] Zou | Structure, morphology and mechanism research on synthesizing xonotlite fiber from acid-extracting residues of coal fly ash and carbide slag[END_REF], [START_REF] Shaw | Hydrothermal formation of the calcium silicate hydrates, tobermorite (Ca5Si6O16(OH)2•4H2O) and xonotlite (Ca6Si6O17(OH)2): an in situ synchrotron study[END_REF], [START_REF] Kalousek | Xonotlite: cell parameters, thermogravimetry and analytical electron microscopy[END_REF].

On the basic of previous researches and the results obtained in this work, the synthesis of xonotlite under sub-and supercritical conditions results from two steps.

First, the dissolution of precursors gives rise to the formation of siliceous species (SiO3 2-) which go around positively charged calcium ions (Ca 2+ ) [START_REF] Zou | Structure, morphology and mechanism research on synthesizing xonotlite fiber from acid-extracting residues of coal fly ash and carbide slag[END_REF]. After rapid reaction, the precipitation at room temperature of the Ca-and Si-precursors in the T-mixing piece at the inlet of the reactor takes place to form a calcium silicate hydrate compound (C-S-H) in the form of gel. This poorly crystalline material is constituted by CaO layers and disordered silicate chains with a good periodicity along the ab plane but a low order along the [001] direction [START_REF] Shaw | Hydrothermal formation of the calcium silicate hydrates, tobermorite (Ca5Si6O16(OH)2•4H2O) and xonotlite (Ca6Si6O17(OH)2): an in situ synchrotron study[END_REF]. That C-S-H compound can be considered as the starting precursor for the synthesis of xonotlite under near-and supercritical conditions. In a second stage, inside the tubular reactor kept under near-or supercritical conditions, the CaO layers and silicate chains become more ordered with an increasing of the periodicity along the c-direction giving rise to the crystallization of xonotlite particles [START_REF] Shaw | Hydrothermal formation of the calcium silicate hydrates, tobermorite (Ca5Si6O16(OH)2•4H2O) and xonotlite (Ca6Si6O17(OH)2): an in situ synchrotron study[END_REF]. The chemical process that results in the formation of crystalline xonotlite starting from calcium nitrate tetrahydrate and sodium metasilicate nonahydrate is described by the following formula:

6 Na2SiO3.9H2O + 6 Ca(NO3)2.4H2O → Ca6Si6O17(OH)2 + 12 NaNO3 + 12H2O
To study the chemical process that results in the formation of crystalline xonotlite, syntheses at different temperatures were performed, as will be discussed in the following.

Crystallization of xonotlite: influence of the synthesis parameters

Influence of temperature

The samples were prepared via hydrothermal flow synthesis at 23.5 MPa and different temperatures: 250, 300, 350, and 400 °C. The syntheses were performed keeping the flow rate of each pump at 2.8 mL/min, which corresponds, depending on the temperature, to distinct residence time values. In the text, we refer to these samples with the labels T250, T300, T350, and T400. The list of parameters employed in these experiments is reported in Table 1.

Powder X-ray diffraction patterns of the as-synthesized samples are shown in Fig. 2. The majority of the peaks in all samples are coincident in positions and intensities with those of xonotlite, as reported in PDF file no. 00-023-0125 of monoclinic xonotlite (space group P2/a) in the International Center for Diffraction Data (ICDD) database. Furthermore, the most intense diffraction peak, centered at 28.9° (2θ), is related to the (320) reflection in all the diffraction patterns.

Comparing the diffraction patterns of T250, T300, T350, and T400 samples, the effect of temperature is clear: higher temperature, despite the shorter time that growing particles spend in the reactor, increases the material crystallinity. T350 and T400 exhibit sharper and less broadened diffraction peaks compared to samples synthesized at a lower temperature. Furthermore, it is possible to observe that in these conditions of low flow rate (high residence time), the increase of the temperature leads to the crystallization of wollastonite (principal diffraction peak centered around 30° (2θ)) and calcium carbonate polymorphs, as calcite and vaterite. The presence of vaterite and wollastonite is responsible for broadening the (202) peak in T250 and T300, which at higher temperatures (T300 and T400) clearly shows three different diffraction peaks. At the same time, the diffraction peaks at 43.4°, 44.3°, and 50.4° (2θ) came from the sample holder in stainless steel.

These findings represent the first proof of xonotlite synthesis under near critical conditions (250-350 °C) in only tens of seconds in a continuous process. Even if the reaction yields and the samples' crystallinity are reduced compared to the specimen synthesized at 400 °C, these results at lower temperatures are very promising. Suffice it to say that several hours are required for the conventional hydrothermal synthesis at 250 °C [START_REF] Diez-Garcia | Synthesis by supercritical fluids methods of advanced additions for cementitious materials[END_REF]. In this way, for some applications that are not needed an excellent degree of crystallinity, the methodology developed in this work can represent a real innovation for the production of xonotlite.

The effect of temperature on the formation of xonotlite was further analyzed by investigating the local environment of protons at the atomic nearest neighbor and next-nearest neighbor scale. Fig. 3 displays the 1 H MAS NMR spectra of samples T250, T300, T350, and T400.

All the spectra show two peaks at 1.9 and 2.3 ppm, attributed to the presence of Si-OH and Ca-OH groups, respectively. In contrast, the broad signal around 5 ppm can be assigned to molecular water adsorbed on the mineral particle surface. Although xonotlite's ideal crystal structure should contain only Ca-OH units like in portlandite, previous studies have shown deviations from the ideal model [START_REF] Diez-Garcia | Synthesis by supercritical fluids methods of advanced additions for cementitious materials[END_REF]. As a consequence, our results are in agreement with the existence of two different types of proton environments already detected in both natural and synthetic xonotlite produced under conventional hydrothermal conditions [START_REF] Kudoh | Polytypism of xonotlite: (I) structure of an A-1 polytype[END_REF], [29], [START_REF] Churakov | Structure of the hydrogen bonds and silica defects in the tetrahedral double chain of xonotlite[END_REF], [START_REF] Méducin | Calcium silicate hydrates investigated by solid-state high resolution 1H and 29Si nuclear magnetic resonance[END_REF].

Furthermore, the spectrum of the sample T250 shows two more intense peaks at 1.2 and 3.7 ppm, which can be assigned to the presence of unreacted C-S-H precursor due to the low synthesis temperature. Small contributions of these peaks are also observed in the sample T300. The inset in Fig. 3 shows that another peak at around 15 ppm appears by increasing the temperature. This signal can be assigned to strongly hydrogen-bonded protons involving Si-OH groups, as already reported for other silicate minerals [START_REF] Brunet | Application of 29Si homonuclear and 1H-29Si heteronuclear NMR correlation to structural studies of calcium silicate hydrates[END_REF]. From the deconvolution of the spectra, this contribution has been calculated to be equal to 5% in T400, and around 1-2% in T300 and T350, whereas it was not detected in T250.

More information can be obtained from the comparison between these spectra. First, by increasing the temperature, the quantity of xonotlite in the sample increases (especially, the amount of Ca-OH increases), NMR giving semi-quantitative information. That result agrees with the PXRD analyses of the previously reported samples. Second, the quantity of molecular water adsorbed on the mineral surface decreases. Third, the peak at around 15.2 ppm becomes more intense the higher the temperature is. These findings suggest that higher temperature and thus higher structure crystallinity increase the H-bond network strength and a lower wettability of the mineral surface.

The silicate structure and the local chemical environment of Si atoms in xonotlite samples, synthesized at different temperatures, were investigated by 29 Si MAS NMR. To interpret the results, we must recall the structure of this mineral phase. As shown in Fig. 4(a), three different silicon sites are present in the silicate chains of xonotlite. Among them, two are in bridging positions, Si(1) and Si [START_REF] Li | A novel hydrothermal route to the synthesis of xonotlite nanofibers and investigation on their bioactivity[END_REF], and the third one is located in a branching place, Si(3). Consequently, three contributions, namely Q 2 H, Q 2 L, and Q 3 , appear in the SSNMR xonotlite spectrum. The presence of two Q 2 components is because the bridging Si atoms have different Si-O bond lengths and Si-O-Si bond angles, in agreement with the model proposed by Kudoh and Takeuchi [START_REF] Kudoh | Polytypism of xonotlite: (I) structure of an A-1 polytype[END_REF]. In Fig. 4(b), the signal centered around - 86 ppm is attributed to the Q 2 Si sites, and the other at - 97 ppm to the presence of Q 3 silicon tetrahedra in branching position. We can observe that splitting the Q 2 peak into two components becomes much clearer at high temperatures; on the contrary, it is less detectable due to the broadening of the signals when decreasing the temperature. The sharpness of SSNMR peaks in T350 and T400 samples prove their higher crystallinity, in agreement with the PXRD results. Furthermore, another peak appears in T250 and T300 at around - 80 ppm due to the Q 1 contribution related to the end site of the silicate anion chains.

The presence of Q 1 Si sites is the direct evidence of defects and a more defective and low crystalline material. In this sense, it is interesting to observe that the Q 1 contribution decreases by increasing the temperature. On the other hand, the spectra of samples synthesized at higher temperatures, T350 and T400, show another signal at around - 88 ppm, which can be associated with the wollastonite presence in the samples. The proof that this contribution comes from wollastonite was obtained through the 1 H- 29 Si CP/MAS NMR experiment, which is sensitive only to Si in close proximity to H atoms. Being protons absent in wollastonite structure, 1 H- 29 Si CP/MAS NMR spectrum of T400 does not exhibit the peak a - 88 ppm, Fig. 4(c). In this sense, SSNMR confirms the formation of wollastonite in T350 and T400, already observed by powder X-ray diffraction.

The morphology of xonotlite crystals continuously synthesized at different process conditions was investigated by HR-SEM. As shown in Fig. 5, the mineral phase exhibits a fiber-like morphology. Especially, it is possible to observe that although fibers present a wide distribution in size, increasing the temperature from 250 °C to 400 °C, they tend to be longer and less wide. This result could be explained by taking into account the work of Zou et al. [START_REF] Zou | Structure, morphology and mechanism research on synthesizing xonotlite fiber from acid-extracting residues of coal fly ash and carbide slag[END_REF]. They observed that by performing xonotlite synthesis at 240 °C, extending the synthesis time from 6 to 48 h, the fibers split into thinner separated units. In our case, the ultra-fast kinetics of SHFS could favor the growth of the fiber along the silicates chains, resulting in longer particles that splitting into many units less wide.

Influence of residence time

To avoid as much as possible the crystallization of wollastonite and obtain single-phase xonotlite, the synthesis was performed at 400 °C, and 25 MPa, reducing the residence time down to 7 s. In Fig. 6, the PXRD pattern of this sample shows well-defined diffraction peaks corresponding, in positions and intensities, to those reported in PDF file no. 00-023-0125 of monoclinic xonotlite. Under these synthesis conditions, the presence of wollastonite is reduced to a neglectable amount.

Xonotlite synthesized under sub-or supercritical conditions mainly consist of tiny flat fibers [START_REF] Lin | A simple method to synthesize single-crystalline β-wollastonite nanowires[END_REF], [START_REF] Musumeci | A supercritical water-based technology for calcium silicate hydrate nanoparticles production[END_REF], [START_REF] Musumeci | Correlation between the dynamics of nanoconfined water and the local chemical environment in calcium silicate hydrate nanominerals[END_REF]. HR-TEM analysis was performed to get information on the preferential growing direction of the crystals. Fig. 7(a) and (b) show well crystalline xonotlite fibers, which confirm the PXRD results. It is possible to observe the (2,0,0) atomic planes (Fig. 7(b)) characteristic of the xonotlite structure, suggesting that the fiber grows along the direction of the silicate chains parallel to the b-axis (Fig. 7(d)). The interlayer distance between these planes was measured to be equal to 8.6 Å (Fig. 7(c)) in agreement with the (200) reflection observed in the PXRD pattern.

To study the effect of the residence time shortening (from 20 down to 7 s) on the silicate structure of xonotlite, 29 Si MAS NMR was performed, Fig. 8. The 29 Si MAS NMR spectrum of xonotlite synthesized in supercritical water in only 7 s shows three signals assigned to Q 2 H, Q 2 L, and Q 3 , agreeing with the silicate structure xonotlite reported in Fig. 4(a). The areas of Q 2 and Q 3 signals were calculated through the deconvolution of the peaks. The ratio between the areas, Q 2 :Q 3 was approximately 2:0.9. However, the ideal crystal structure of xonotlite is built by parallel silicate chains in "dreiereinfachketten" arrangement with two "paired" Q 2 tetrahedra (Si(1), Si( 2)) and one Q 3 site (Si(3)); consequently, Q 2 :Q 3 ratio should be equal to 2:1. That difference implies that in xonotlite synthesized under supercritical conditions, some Q 3 silicon sites are missing. Similar results were also found in other studies on synthetic and natural xonotlite [START_REF] Hiroaki | 29Si and 1H NMR of natural and synthetic xonotlite[END_REF]. Furthermore, the absence of wollastonite in this sample, as previously observed by PXRD, is confirmed by 29 Si MAS NMR, as the peak characteristic for the dehydrated phase at - 88 ppm is not anymore detectable if compared to Fig. 4

(b).

Another technique that gives detailed information on the chemical structure of a mineral is infrared spectroscopy. The characteristic IR spectrum of xonotlite is shown in Fig. 9. The spectrum, acquired in the range 1800-400 cm -1 , can be divided into two groups of bands related to vibrations of (i) water molecules, and (ii) octahedral Ca and tetrahedral Si, as commonly observed in calcium silicate hydrates [START_REF] Churakov | Structure of the hydrogen bonds and silica defects in the tetrahedral double chain of xonotlite[END_REF], [38].

The peak at 1632 cm -1 is attributed to the scissoring bending mode of water [38]. Instead, the bending mode vibration of CaO-H, typical of xonotlite structure, is observed at 631 cm -1 . The band around 1202 cm -1 is assigned to Si-O stretching vibrations involving Q 3 branching sites ((Si-O)Q 3 ) [38]. The spectral features of silicates, which appear around 978 and 930 cm -1 are attributed to Si-O stretching of Q 2 sites ((Si-O)Q 2 ).

It is worth mentioning that in the spectrum does not appear the vibration band (Si-O)Q 1 , commonly detected around 828 cm -1 [38], confirming the SSNMR results. The signal observed at 1088 cm -1 could be assigned to the asymmetric (νas(Si-O-Si)) vibration of Si-O-Si bridges; the respective symmetric stretching bands (νs(Si-O-Si)) appear at 672 and 610 cm -1 . Lastly, the band at 536 cm -1 is assigned to Si-O-Si bending mode (δs (Si-O-Si)); on the contrary, the O-Si-O bending mode (δs (O-Si-O)) appear at 456 cm -1 [38]. Although no peaks related to carbonates were observed in the diffraction pattern, in the IR spectrum of xonotlite, small contributions related to CO3 2-vibration bands were detected and marked with an asterisk. The presence of small carbonate traces in xonotlite is commonly reported in the literature [38].

So far, the results related to xonotlite crystallization via a near-and supercritical hydrothermal flow synthesis were presented. Our findings have shown that temperature, residence time, and pressure play a fundamental role in the synthesis process, highlighting the importance of controlling the reaction kinetics to avoid secondary phases. Xonotlite nanoparticles synthesized at 25 MPa, 400 °C in 7 s show fiber-like morphology and well-crystallized structure. After the investigation of the structure, the physicochemical properties, and the morphology, in the following, we will focus on the employment of xonotlite synthesized via an SHFS into the application of a cement-based material.

Application of the as-synthesized xonotlite particles

Calorimetric test

Within the frame of this work, calorimetry analysis was used as a sensitive tool for studying the exothermic hydration process of cement over time. The heat evolution is strongly correlated to the rate of the chemical reactions that take place during hydration, and chemical additions influence it. Consequently, the effect of xonotlite particles as seeds can be detected by monitoring the hydration curve changes. In Fig. 10(a), the first exothermic peak is associated with the precipitation of the main hydration products, C-S-H gel, and portlandite. Later, a second peak corresponds to the precipitation of ettringite and the dissolution of C3A [START_REF] Taylor | Cement Chemistry[END_REF].

To evaluate the accelerating behavior of xonotlite, the position of the heat flow maximum, tmax (in hours), at the end of the acceleration period was determined [START_REF][END_REF]. A heat flow maximum that occurs earlier than in the reference sample can be defined as an acceleration of the cement hydration. In Fig. 10(a), the reference sample shows the longest pre-induction period, while increasing xonotlite dosage gradually accelerates that period. Additionally, the induction period is essentially eliminated when 1 wt% of xonotlite is added into the paste. This phenomenon is due to the larger number of available nuclei for the hydration product growth from time zero. Furthermore, the seed addition promotes faster calcium, hydroxyl, and silicate species saturation, contributing to the achievement of the supersaturation state in the solution in a shorter period of time. In turn, this phenomenon leads to the earlier C-S-H precipitation, promoting the earlier starting of acceleration period compared to the case of the reference sample. In particular, the onset of C-S-H gel and portlandite formation from the silicate hydration appears after ∼ 70 and 80 min for the cement paste prepared with 0.5 wt% and 1 wt% of xonotlite addition, respectively.

At the same time, the onset of the silicate hydration of cement paste used as reference occurs at ∼ 100 min Fig. 10(a) shows that the gain in heat flow after the induction period is much faster when the particle dosage increases. The shoulder of silicate hydration occurs earlier in seeded cement pastes, at ∼ 4 h and ∼ 6.2 h with an addition of 0.5 and 1 wt% respectively, if compared with the reference paste where it appears after ∼ 7.5 h. Although, the heat flow maximum related to silicate hydration increases significantly in both seeded pastes of ∼ 2.5% and ∼ 9.5% in respect to the unseeded sample. This phenomenon is related to the higher number of hydration product regions that grow simultaneously. Additionally, the cement pastes prepared with xonotlite seeds have accelerated aluminate hydration.

The peak related to this process for cement paste containing 1 wt% of xonotlite is shifted to ∼ 2 h earlier, and its height increased by 8% if compared with the calorimetric curve of the reference sample. That acceleration is related to an earlier calcium sulfoaluminate hydrate (AFt) depletion and might be due to stronger calcium and hydroxide ion flows when seeds are added, as previously proposed by Nicoleau et al. [START_REF] Nicoleau | The acceleration of cement hydration by seeding: influence of the cement mineralogy ZKG Int[END_REF], [START_REF] Nicoleau | Oriented aggregation of calcium silicate hydrate platelets by the use of comb-like copolymers[END_REF]. It is worth mentioning that, despite this acceleration, the silicate peaks appear earlier than that of aluminate; contrarily, the over-accelerated hydration of aluminate would suppress that one of silicate, disturbing the strength gain of seeded cement at the early age [START_REF] Li | Fibrillar calcium silicate hydrate seeds from hydrated tricalcium silicate lower cement demand[END_REF], [START_REF] Marchon | 12 -Impact of Chemical Admixtures on Cement Hydration[END_REF].

The total heat released during the early nucleation and growth period, Fig. 10(b), is more significant in seeded pastes. The xonotlite additions allow the hydration product to nucleate in the pore space between the particles reducing the capillary porosity, as reported in the model by Thomas et al. [START_REF] Thomas | Influence of nucleation seeding on the hydration mechanisms of tricalcium silicate and cement[END_REF]. In particular, the total generated heat during 48 h hydration of 0.5 wt% and 1 wt% seeded pastes were ∼ 3.5% and 11% higher than the reference, respectively. This result is promising if compared with that of other semicrystalline C-S-H seeds. Wang et al. have reported that the acceleration effect related to 1 wt% of nanometric C-S-H particles (Ca/Si molar ratio = 1) was not sustained up to 48 h [START_REF] Wang | Synthesis and microwave dielectric properties of CaSiO3 nanopowder by the sol-gel process[END_REF]. With this material, they observed the same total heat release in the reference and seeded pastes after 48 h. These results imply that xonotlite synthesized by SHFS, besides influencing the C-S-H gel nucleation, continues affecting the hydration process after tens of hours.

X-ray computed microtomography analysis

Based on the calorimetry results, the X-ray computed microtomography measurements have been performed at two different hydration times: 7 and 36 h. In this way, the first analysis allows investigating the effect of xonotlite on the acceleration of the silicate hydration with respect to the reference sample. On the other hand, the second measurement verifies if the acceleration effect is sustained up to longer hydration times. The acceleration effect of xonotlite on cement hydration, already observed by calorimetry analysis, was further confirmed by X-ray computed microtomography. In Fig. 11, the seeded paste with 1 wt% of xonotlite exhibit a lower fraction of unreacted material (in orange), especially the large anhydrate cement particles compared to the reference specimen at the same hydration age, 7 h.

The accelerated kinetics of cement hydration induced by seed addition was further investigated by analyzing the evolution and depletion of the unhydrated phase over time. First, a 3D bilateral filter was applied on the reconstructed tomograms to enhance the phase contrast between unhydrated phase, hydration phase, and pore phase [START_REF] Ushizima | Statistical segmentation and porosity quantification of 3D x-ray microtomography[END_REF], [START_REF] Ushizima | Materials data science for microstructural characterization of archaeological concrete[END_REF]. Then, Ostu algorithm [START_REF] Li | Fibrillar calcium silicate hydrate seeds from hydrated tricalcium silicate lower cement demand[END_REF], [START_REF] Otsu | A threshold selection method from gray-level histograms[END_REF] was utilized to segment the unhydrated phase, and the segmented volumes of unhydrated phase were in good agreement with the observed unnhydrated particles in Fig. 10, which verified the accuracy of the phase segmentation. Further morphological measurements [START_REF] Xu | Microstructure and water absorption of ancient concrete from Pompeii: an integrated synchrotron microtomography and neutron radiography characterization[END_REF] of the volume and mean radius of segmented unhydrated particles, in this research, were conducted based on the segmented unhydrated phase. Table 2 gives the total volume and mean radius of unhydrated particles for reference and seeded pastes. These comparisons show that at the same hydration age, 7 h, the seeded pastes exhibit a lower content of unreacted particles. Such unhydrated particles also have a smaller mean radius in seeded sample than those in the reference sample, which verifies the acceleration of hydration in seeded sample.

Therefore, it can be stated that the introduction of xonotlite seeds influences the early hydration kinetics. Studying the hydration effect over time, we have observed that the seeds sustain the acceleration effect up to 36 h, as previously detected through calorimetric measurements. In fact, at that age of hydration, Table 3, the seeded paste exhibits a lower volume of unhydrated phase, confirming the earlier depletion of unhydrated particles to form the hydrated products.

To conclude, the difference between the mean diameters of unhydrated particles in the reference sample and the seeded paste is ∼ 4 µm when 1 wt% of xonotlite is added after 7 h. At 36 h of hydration, these differences between diameters decrease down to ∼ 3.2 µm. It is worth mentioning that the spatial resolution in measuring the diameters of unhydrated particles is 1.3 µm/pixel. In these experiments, the main factors that can cause errors in the results are: (i) the limitation of the µCT resolution and (ii) the image segmentation procedure.

Conclusions

This study has investigated the effect of several experimental parameters on the near-and supercritical hydrothermal flow synthesis of xonotlite. Our findings have shown that temperature, residence time, and pressure play a fundamental role in the synthesis process, highlighting the importance of controlling the reaction kinetics to avoid secondary phases or amorphous products. For the first time, we have shown that the formation of this mineral takes place in a wide range of temperatures, from 250° to 350°C at 23.5 MPa in only tens of seconds. Based on these findings, it appears clear the interest in using the SHFS as a fast, profitable, and scalable manufacturing route for producing synthetic xonotlite fibers to replace the very long conventional hydrothermal methodology. Furthermore, we have demonstrated that under supercritical conditions (400 °C and 25 MPa) single phase of high crystalline xonotlite can be produced (i.e. without the presence of secondary phases), just reducing the residence time down to 7 s. As a conclusion of these results, it can be stated that the supercritical hydrothermal flow synthesis allows producing perfect model nanominerals by mimicking natural environments at high temperatures and pressures.

Finally, we have explored the possible application of xonotlite additions in cement-based materials. Our results obtained through calorimetry and X-ray microtomography analyses have shown that xonotlite particles can act as nucleation points that trigger the quick and autocatalytic formation of C-S-H gel. The acceleration of cement hydration allows the development of faster, denser, and stronger cement paste, which from an application point of view permits to reduce the time and the costs of construction. Based on these findings, future works could investigate the effect of other crystalline CSH nanoparticles synthesized via SHFS with different Ca/Si ratios and specific surface areas.

Acknowledgments

This work was carried out under the umbrella of the Bordeaux-Euskampus Euro-regional Campus of International Excellence initiative and supported by the International Doctoral Program developed between the University of Bordeaux (UB) and the University of the Basque Country (UPV/EHU). This work was also performed in the frame of the LTC Green Concrete. Valentina Musumeci is grateful to the Initiative of Excellence (IdEX) of the University of Bordeaux for financial support. The authors also acknowledge CNRS, Bordeaux INP, and Région Nouvelle-Aquitaine for their financial support.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.