Water vapor sorption isotherm measurement is one of the promising techniques to understand the microstructure of hardened cement paste, because it always gives a higher surface area than sorption isotherm measurements performed with other adsorbents such as nitrogen and argon, which implies that water molecules can probe the widest range of the microstructure of hardened cement pastes. When, at 20°C, the water sorption measurement is conducted such as to last for a few days, a characteristic behavior-a sudden drop in adsorbed amount around a relative humidity of 0.35-is always observed on the desorption branch. Here, we prove that this sudden drop is caused by water cavitation, based on an analysis of experimental sorption isotherms acquired at various temperatures, scanning isotherms, and length-change isotherms. Cavitation in hardened cement paste is likely to occur in the C-S-H gel pores constricted by the C-S-H interlayer space.

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Short-term length-change isotherms and desorption isotherms of matured hardened cement paste were measured at different temperature conditions, namely, 20, 30, 40, 50, and 60°C. For the short-term length-change isotherms, higher temperature conditions resulted in a smaller drying shrinkage. This trend was largely controlled by the microstructural reorganization of the calcium-silicate-hydrates in the cement paste at elevated temperatures. The experimental results suggest that a portion of the evaporable water did not contribute to the shrinkage. In addition, based on the relationships between the incremental strain and the incremental evaporable water content, with a starting relative humidity of 5%, consistent trends were observed for the incremental strain and the incremental evaporable water when the incremental evaporable water content was less than 0.08 g/g-dried hcp. The microstructural reorganization pathways for elevated temperature and/or for drying were confirmed to be identical.

A variation of material properties with depth is observed in a massive, seismic-resistant internal wall of Unit 1 of the Hamaoka Nuclear Power Plant (NPP) 45 years after its construction. The maximum values are attained in the center of the structural element and gradually decrease towards the surface, while still well exceeding the required performance. It is found that this trend can be mostly attributed to the reaction between cement hydrates and rock-forming minerals of a feldspar group, which releases silica and alumina oxides into the pore solution, where they react with portlandite to form new hydrates. No evidence of expansive distress similar to the alkali-silica reaction (ASR) is found in the material, owing to this reaction. Possible implications for the aging management of existing concrete structures are discussed. 1

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