Publisher：Journal of the American Ceramic Society  

In modern Portland cement, calcium aluminum silicate hydrates (C-A-S-H) do not generally form crystalline Al-tobermorite. In this study, to identify the mechanism of Al-tobermorite formation, synchrotron micro X-ray diffraction and field-emission scanning electron microscopy-based energy-dispersive X-ray spectroscopy (EDS) investigations were performed on a slice of a thick concrete wall in a nuclear power plant, where the presence of Al-tobermorite was previously confirmed. Rietveld analysis of the diffraction diagrams obtained from the 2D diffraction patterns showed that Al-tobermorite was formed almost everywhere in the cement paste near sandstone aggregate and that Al-tobermorite platelets were distributed across most of the cement paste area in a random orientation. The basal spacing d-value of Al-tobermorite was identical and equal to 11.39 Å. The ratio plots from the EDS maps indicated that the chemical composition of the mixture of Al-tobermorite and C-A-S-H was almost homogeneous all around the cement paste. The Al-tobermorite in this concrete is probably formed via a dissolution–precipitation process. From the random orientation even next to the surface of aggregate, we infer that the Al-tobermorite formation does not need the surface of aggregate to precipitate.

DOI： 10.1111/jace.18624

Web of Science

Scopus

Language：Japanese   Publisher：Cement and Concrete Research  

This study aims to enhance moisture transport modeling by elucidating the so-called anomalous water diffusion in cementitious materials. Water desorption isotherms are measured from samples at various drying stages using techniques with different durations to quantify the impact of the drying duration on the water sorption ability. A single water sorption isotherm does not solely give a relationship between the water content and relative humidity but also represents the state of microstructure. The continuous evolution of a desorption isotherm due to drying-induced microstructural rearrangement is demonstrated. For numerical modeling, the microstructural alteration can be explicitly considered through a dynamic desorption isotherm model, which governs the local thermodynamic equilibrium at the capillary meniscus. This approach is implemented into a multiphase transport code, whose ability to predict drying is validated using literature data. Finally, the effect of prolonged drying on the colloidal nature of the calcium-silicate-hydrate gel is discussed.

DOI： 10.1016/j.cemconres.2021.106612

Web of Science

Scopus

Language：Japanese   Publisher：Journal of Advanced Concrete Technology  

For better understanding of irreversible shrinkage, nine hardened cement paste (hcp) samples with three different cement types and three different water to cement ratio were prepared. Four different relative humidity conditioning histories containing the first desorption, re-humidification and the second desorption are investigated for all the specimens to obtain the length change and water sorption isotherms. The irreversible shrinkage strain was developed when the specimen was dried up to less than 80% relative humidity (RH), while other previous experiments in literatures showed that the shrinkage strain between 40% RH and 11% RH is reversible. It is concluded that the irreversible shrinkage strain is developed between 80% RH and 40% RH, which is also supported by the change in water vapor BET surface area of hardened cement paste after long-term drying.

DOI： 10.3151/jact.19.585

Web of Science

Scopus

Language：Japanese   Publisher：Materials and Design  

The distribution of compressive strength in thick concrete members exposed to various environments in a nuclear power plant undergoing decommissioning is investigated. X-ray diffraction data, scanning electron microscopy analysis, and chemical analysis data reveal that rock-forming minerals in the aggregate had reacted and portlandite had been consumed to form calcium alumino silicate hydrates (C-A-S-H) when evaporable water content was sufficient. In addition, the study confirms Al-tobermorite formation in modern concrete after 16.5 years of elevated temperature conditions ranging from 40 to 55 °C. It is concluded that an appropriate aggregate and binder combination for the concrete enhances the compressive strength of thick concrete members thanks to the reaction of rock-forming minerals with portlandite, and also strengthens the chemical stability through the formation of Al-tobermorite under elevated temperature conditions and sufficient evaporable water content.

DOI： 10.1016/j.matdes.2020.109297

Web of Science

Scopus

Language：Japanese   Publisher：International Journal of Concrete Structures and Materials  

Mechanical properties and durability of cement-based materials are largely affected by pore structures. This paper provides an overview of several experimental techniques to characterize pore size distribution and specific surface area, with focus on pores in calcium silicate hydrates. The reviewed experimental techniques are nitrogen and water vapor sorption isotherm, proton nuclear magnetic resonance (1H-NMR) and small-angle scattering (SAS). Different pretreatment methods are compared for sorption measurements. Pore size distribution and specific surface area are analyzed using data from different methods to understand difference and consistency of these methods. It is found that pore size distribution calculated from sorption isotherm is very sensitive to adsorption model. Though specific surface areas from different techniques are quite different from each other, they are all able to detect the microstructural alteration due to long-term drying.

DOI： 10.1186/s40069-020-00431-y

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3151/jact.18.396

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.cemconres.2020.106036

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier {BV}  

DOI： 10.1016/j.cemconres.2019.105970

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Language：Japanese   Publisher：Proceedings of the fib Symposium 2020: Concrete Structures for Resilient Society  

The prediction of the delayed deformations of prestressed concrete structures like nuclear power plants (NPPs) is important in order to assess and/or extend the service life of such structures. It is also a safety concern because, in these structures, the prestressing is there to avoid tensile stresses and cracking in case of a severe accident (where an internal pressure is generated). In this paper, the relations proposed in MC2010 and in the future Eurocode 2 for the delayed strains (creep and shrinkage of concrete and relaxation of steel) are used to predict the behaviour of a NPP built 25 years ago. As proposed in the future Eurocode 2, the parameters of the relations giving the evolution of shrinkage and creep are fitted to measurements made on laboratory samples when the NPP was built. The influence of the temperature and of the relative humidity inside the containment are also taken into account. Finally, using the superposition principle in a biaxial context, the predicted delayed strains are compared with the measurements.

Scopus

Publisher：fib Symposium  

The prediction of the delayed deformations of prestressed concrete structures like nuclear power plants (NPPs) is important in order to assess and/or extend the service life of such structures. It is also a safety concern because, in these structures, the prestressing is there to avoid tensile stresses and cracking in case of a severe accident (where an internal pressure is generated). In this paper, the relations proposed in MC2010 and in the future Eurocode 2 for the delayed strains (creep and shrinkage of concrete and relaxation of steel) are used to predict the behaviour of a NPP built 25 years ago. As proposed in the future Eurocode 2, the parameters of the relations giving the evolution of shrinkage and creep are fitted to measurements made on laboratory samples when the NPP was built. The influence of the temperature and of the relative humidity inside the containment are also taken into account. Finally, using the superposition principle in a biaxial context, the predicted delayed strains are compared with the measurements.

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier {BV}  

DOI： 10.1016/j.cemconres.2018.02.023

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier {BV}  

DOI： 10.1016/j.cemconres.2016.09.014

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media {LLC}  

DOI： 10.1007/s11043-015-9277-5

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