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Yeelimite, Ca 4 [Al 6 O 12 ]SO 4 , is outstanding as an aluminate sodalite, being the framework of these type of materials flexible and dependent on ion sizes and anion ordering/disordering. On the other hand, yeelimite is also important from an applied perspective as it is the most important phase in calcium sulfoaluminate cements. However, its crystal structure is not well studied. Here, we characterize the room temperature crystal structure of stoichiometric yeelimite through joint Rietveld refinement using neutron and X-ray powder diffraction data coupled with chemical soft-constraints. Our structural study shows that yeelimite has a lower symmetry than that of the previously-reported tetragonal system, which we establish to likely be the acentric orthorhombic space group Pcc2, with a √2a×√2a×a superstructure based on the cubic sodalite structure. Final unit cell values were a=13.0356(7) Å, b=13.0350(7) Å, and c=9.1677(2) Å. We determine several structures using density functional theory calculations, with the lowest energy structure being Pcc2 in agreement with our experimental result. Yeelimite undergoes a reversible phase transition to a higher-symmetry phase which has been characterized to occur at 470ºC by thermodiffractometry. The higher-symmetry phase is likely cubic or pseudo-cubic possessing an incommensurate superstructure, as suggested by our theoretical calculations which show a phase transition from an orthorhombic to a tetragonal structure. Our theoretical study also predicts a pressure-induced phase transition to a cubic structure of space group I43m. Finally, we show that our reported crystal structure of yeelimite enables better mineralogical phase analysis of commercial calcium sulfoaluminate cements, as shown by R F values for this phase, 6.9% and 4.8% for the previously published orthorhombic structure and for the one reported in this study, respectively.

International audienceAn isotropic carbon fibre was surface-treated by microwave oxygen plasma at different conditions and characterised by scanning electron microscopy (SEM), scanning tunneling microscopy (STM), N2/CO2 adsorption, Raman spectrometry, X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). It is shown that the structure of the fibre suffers only limited alterations upon plasma treatment in such a way that the local disorder on its surface, which was already large in the fresh material, barely increases after the plasma exposure, as detected by Raman measurements. At the nanometre scale, STM images revealed a moderate increase in surface roughness. Evidence for chemical changes undergone by the fibre following the etching was provided by XPS and TPD, showing that stable oxygen functionalities were introduced by the plasma exposure, a result of practical importance for the application of this treatment not only to this type of carbon fibre, but to carbon materials in general. It was also observed that very gentle plasma exposures were generally sufficient to provide the fibre surface with a large amount of oxygen functional groups and that more intense treatments had a negative effect in this respect (i.e. they were not able to supply oxygen to the surface in larger amounts than the softer treatments did)

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1Ye'elimite is the main phase in calcium sulfoaluminate cements and also a key phase in sulfobelite 2 cements. However, its hydration mechanism is not well understood. Here we reported new data on 3 the hydration behaviour of ye'elimite using synchrotron and laboratory powder diffraction coupled 4 to the Rietveld methodology. Both internal and external standard methodologies have been used to 5 determine the overall amorphous contents. We have addressed the standard variables: water-to-6 ye'elimite ratio and additional sulfate sources of different solubility. Moreover, we report a deep 7 study of the role of the polymorphism of pure ye'elimites. The hydration behaviour of orthorhombic 8 stoichiometric and pseudo-cubic solid-solution ye'elimites is discussed. In the absence of additional 9 sulfate sources, stoichiometric-ye'elimite reacts slower than solid-solution-ye'elimite, and AFm-10 type phases are the main hydrated crystalline phases, as expected. Moreover, solid-solution-11 ye'elimite produces higher amounts of ettringite than stoichiometric-ye'elimite. However, in the 12 presence of additional sulfates, stoichiometric-ye'elimite reacts faster than solid-solution-ye'elimite. 13 14 15

Tricalcium silicate, the main constituent of Portland cement, hydrates to produce crystalline calcium hydroxide and calcium-silicate-hydrates (C-S-H) nanocrystalline gel. This hydration reaction is poorly understood at the nanoscale. The understanding of atomic arrangement in nanocrystalline phases is intrinsically complicated and this challenge is exacerbated by the presence of additional crystalline phase(s). Here, we use calorimetry and synchrotron X-ray powder diffraction to quantitatively follow tricalcium silicate hydration process: i) its dissolution, ii) portlandite crystallization and iii) C-S-H gel precipitation. Chiefly, synchrotron pair distribution function (PDF) allows to identify a defective clinotobermorite, Ca11Si9O28(OH)2.8.5H2O, as the nanocrystalline component of C-S-H. Furthermore, PDF analysis also indicates that C-S-H gel contains monolayer calcium hydroxide which is stretched as recently predicted by first principles calculations. These outcomes, plus additional laboratory characterization, yielded a multiscale picture for C-S-H nanocomposite gel which explains the observed densities and Ca/Si atomic ratios at the nano- and meso- scales.

Belite-rich cements hold promise for reduced energy consumption and CO 2 emissions, but their use is hindered by the slow hydration rates of ordinary belites. This drawback may be overcome by activation of belite by doping. Here, the doping mechanism of B and Na/B in belites is reported. For B-doping, three solid solutions have been tested: Ca 2-x/2  x/2 (SiO 4 ) 1-x (BO 3 ) x , Ca 2 (SiO 4 ) 1-x (BO 3 ) x O x/2 and Ca 2x B x (SiO 4 ) 1-x (BO 4 ) x . The experimental results support the substitution of silicate groups by tetrahedral borate groups with the concomitant substitution of calcium by boron for charge compensation, Ca 2x B x (SiO 4 ) 1-x (BO 4 ) x . Otherwise, the coupled Na/B-doping of belite has also been investigated and Ca 2-x Na x (SiO 4 ) 1-x (BO 3 ) x series is confirmed to exist for a large range of x values. Along this series, ' H -C 2 S is the main phase (for x0.10) and is single phase for x=0.25. Finally, a new structural description for borax doping in belite has been developed for ' H -Ca 1.85 Na 0.15 (SiO 4 ) 0.85 (BO 3 ) 0.15 , which fits better borax activated belite cements in Rietveld mineralogical analysis.