High-resolution nanoimaging reference study of a cement paste by near-field ptychographic X-ray computed tomography

Unlocking the finest details of Portland cement hydration is challenging researchers, as existing 3D imaging techniques struggled to capture the time-dependent processes with sufficient spatial resolution in relevant sample sizes. Here, we break new grounds by achieving very high spatial resolution for relatively large samples. It is demonstrated that near-field ptychographic X-ray computed tomography can image a 160 μm thick capillary sample with 92 nm spatial resolution for a reconstruction using 2400 angular projections. A reconstruction with 1200 projections gave a spatial resolution of 116 nm. This 3D study has allowed us to determine a good compromise between spatial resolution and acquisition time for future time-lapse nanoimaging studies. This investigation reveals hydration products in their natural state, including calcium-silicate-hydrate (C-S-H) gels of different natures, ettringite needle clusters within C-S-H gel shell and layered and bulk portlandite. Chiefly, quantitative electron densities allow to distinguish between C-S-H gel components. Inner product C-S-H gel has an average electron density of 0.66 e - Å -3 (mass density of 2.12 gcm -3 ) whereas outer product C-S-H gel has a 22% lower average electron density, 0.52 e - Å -3 or 1.66 gcm -3 . For the studied sample, the ettringite phase has a needle-like microstructure with an average length of 3(1) µm and an average thickness of 0.24(5) µm. This work provides nano-features and densities of hydration products and it will facilitate future 4D nanoimaging of cement hydration.