Densification of the interlayer spacing governs the nanomechanical properties of calcium-silicate-hydrate

Calciuam-silicate-hydrate (C-S-H) is the principal binding phase in modern concrete. Molecular simulations imply that its nanoscale stiffness is 'defect-driven', i.e., dominated by crystallographic defects such as bridging site vacancies in its silicate chains. However, experimental validation of this result is difficult due to the hierarchically porous nature of C-S-H down to nanometers. Here, we integrate high pressure X-ray diffraction and atomistic simulations to correlate the anisotropic deformation of nanocrystalline C-S-H to its atomic-scale structure, which is changed by varying the Ca-to-Si molar ratio. Contrary to the 'defect-driven' hypothesis, we clearly observe stiffening of C-S-H with increasing Ca/Si in the range 0.8 ≤ Ca/Si ≤ 1.3, despite increasing numbers of vacancies in its silicate chains. The deformation of these chains along the b-axis occurs mainly through tilting of the Si-O-Si dihedral angle rather than shortening of the Si-O bond, and consequently there is no correlation between the incompressibilities of the a- and b-axes and the Ca/Si. On the contrary, the intrinsic stiffness of C-S-H solid is inversely correlated with the thickness of its interlayer space. This work provides direct experimental evidence to conduct more realistic modelling of C-S-H-based cementitious material

Transport properties evolution of cement model system under degradation - Incorporation of a pore-scale approach into reactive transport modelling

International audienceThis paper describes a multi-scale approach for the modelling of the degradation of model cement pastesusing reactive transport. It specifically aims at incorporating chemistry-transport feedback results from apore-scale approach into a continuum description. Starting from a numerical representative elementaryvolume of the model cement paste, which was built according to extensive experimental dedicatedchacarterizations, this paper provides three separate descriptions of two different degradations: leachingand carbonation. First, 2D pore-scale simulations are performed and predict degradation depths in verygood agreement with experiments. Second, 3D pore scale descriptions of how the microstructre evolvesprovides accurate description of the evolution of transport properties through degradation. Finally, thoselatter results are incorporated as a feedback law between porosity and effective diffusion coefficient intoa 1D continuum approach of reactive transport. This paper provides pore-scale explanations of whyreactive transport modelling has encountered mitigated success when applied to cementitious materials,especially during carbonation or degradations consisting of precipitation reactions. In addition to that,different degradation modellings are in very good agreement with experimental observations

Predicting the atmospheric carbonation of cementitious materials using fully coupled two-phase reactive transport modelling

International audienceThe durability assessment of cementitious materials and concrete subjected to atmospheric carbonation of concrete has been an extensive study of research. Experimental studies on the subject show, among other results, that the response depends strongly on the cement composition. This paper focuses on two model materials: an hydrated C3S paste and a low-pH paste, which exhibits a higher tendency to cracking. We show that a fully coupled reactive transport model can reproduce the measured experimental depths of carbonation without a need of fitting parameters. A sensitivity provides insights about the most relevant parameters to accurately model the atmospheric carbonation. Furthermore, results suggest that low-pH cement materials might be inherently less mechanically robust when subjected to atmospheric carbonation, due to a higher C-S-H decalcification rate. This implies that these materials are more likely to develop fractures, which could have implications in the framework of gas or radioactive waste disposal

Migrant-friendly hospitals: a paediatric perspective--improving hospital care for migrant children.

BACKGROUND: The European Union (EU) Migrant-Friendly Hospital (MFH) Initiative, introduced in 2002, promotes the adoption of care approaches adapted to meet the service needs of migrants. However, for paediatric hospitals, no specific recommendations have been offered for MFH care for children. Using the Swiss MFH project as a case study, this paper aims to identify hospital-based care needs of paediatric migrants (PMs) and good service approaches. METHODS: Semi-structured interviews were conducted with principal project leaders of five paediatric hospitals participating in the Swiss MFH project. A review of the international literature on non-clinical hospital service needs and service responses of paediatric MFHs was conducted. RESULTS: Paediatric care can be complex, usually involving both the patient and the patient's family. Key challenges include differing levels of acculturation between parents and children; language barriers; cultural differences between patient and provider; and time constraints. Current service and infrastructural responses include interpretation services for PMs and parents, translated information material, and special adaptations to ensure privacy, e.g., during breastfeeding. Clear standards for paediatric migrant-friendly hospitals (P-MFH) are lacking. CONCLUSIONS: International research on hospital care for migrant children is scarce. The needs of paediatric migrants and their families may differ from guidance for adults. Paediatric migrant needs should be systematically identified and used to inform paediatric hospital care approaches. Hospital processes from admission to discharge should be revised to ensure implementation of migrant-sensitive approaches suitable for children. Staff should receive adequate support, such as training, easily available interpreters and sufficient consultation time, to be able to provide migrant-friendly paediatric services. The involvement of migrant groups may be helpful. Improving the quality of care for PMs at both policy and service levels is an investment in the future that will benefit native and migrant families

eSanté et HealthCompass : Aspects relatifs aux droits du patient

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