Microstructure of hydrated Ca₃SiO₅ and interfacial transition zone characterized by X-ray computed nano-tomography

This study investigates the microstructure of hydrated Ca3SiO5 (C3S) and the interfacial transition zone (ITZ) between an aggregate and hydrated C3S, both of which critically influence the properties of concrete. Because cementitious materials are normally wet, it is difficult to observe their microstructure using scanning electron microscopy. Therefore, the microstructure was examined using X-ray computed nano-tomography at SPring-8 (with a spatial resolution limit of ∼100 nm), which enables the nondestructive imaging of internal structures in the atmosphere. X-ray computed nano-tomography allows observation of the microstructure in the wet state, where residual water is present and hydration products remain intact. The analyses revealed the formation of a calcium silicate hydrate, while Ca(OH)2 particles with diameters of several micrometers were not detected during the initial stage of hydration. At this stage, Ca(OH)2 likely dissolved in water in a supersaturated state with respect to Ca2+ ions, or alternatively, Ca(OH)2 nanoparticles (<100 nm) may be dispersed in the aqueous phase. In the hydrated mixture of C3S and SiO2 (aggregate), Ca(OH)2 particles with diameters of several micrometers were not present at the interface between C3S and SiO2 (ITZ) but were located around the hydrated C3S particles, a phenomenon that differs from previously reported observations. These findings contribute to the understanding of the formation behavior of the calcium silicate hydrate and microstructure of the ITZ.