Fig. 6: Hydration time evolution of selected nano-features directly visualised by near field PXCT.
a Hollow-shell microstructure, also known as Hadley grain. The Hadley grains are fully hydrated small alite particles that contain a void within the original boundary of the anhydrous grain. The hollow regions are filled with water at 19 and 47 h (blue arrows) but dried at 93 h (red arrow) directly evidencing the porosity of the C-S-H shells. b Evolution of water porosity (dark-grey) to air porosity (black). It is noted that at 47 h, a tiny pore of about 1 μm size is already dried, but being very close to two larger water-filled pores, of sizes larger than 2 μm. This observation remarks the heterogeneity in cement hydration. It can be deduced that the relative humidity is not constant, at a given time, through the sample. This is due to a set of factors including the barriers to water diffusion and the crystallization/precipitation of different hydrates with quite different water contents, for instance ettringite and portlandite. c Evolution of water porosity inside a calcite grain, if connected to the surface. This calcite comes very likely from the limestone addition to the Portland cement as quantified in the anhydrous material, see Table S2. d Evolution of alite dissolution (hydration) which stops at the C4AF intergrown regions, highlighted by brown arrows. Moreover, alite hydration also stops as soon as air porosity (pore drying) develops on the surfaces of the anhydrous grains, see red rectangles. This panel also illustrates that (recently precipitated) hydrates can dissolve, see blue rectangles.
