Mass Density and Water Content of Saturated Never-Dried Calcium Silicate Hydrates

Calcium silicate hydrates (C–S–H) are the most abundant hydration products in ordinary Portland cement paste. Yet, despite the critical role they play in determining mechanical and transport properties, there is still a debate about their density and exact composition. Here, the site-specific mass density and composition of C–S–H in hydrated cement paste are determined with nanoscale resolution in a nondestructive approach. We used ptychographic X-ray computed tomography in order to determine spatially resolved mass density and water content of the C–S–H within the microstructure of the cement paste. Our findings indicate that the C–S–H at the border of hydrated alite particles possibly have a higher density than the apparent inner-product C–S–H, which is contrary to the common expectations from previous works on hydrated cement paste

Chemistry and Mass Density of Aluminum Hydroxide Gel in Eco-Cements by Ptychographic X‑ray Computed Tomography

Eco-cements are a desirable alternative to ordinary Portland cements because of their lower CO₂ footprints. Some ye’elimite-based eco-cements are attracting a lot of interest. Understanding the reasons for the mechanical performances requires the characterization of features such as mass density of the hydrated component phases, including the amorphous gel, on the submicrometer scale, which is challenging. Here we use ptychographic X-ray computed tomography to provide 3D mass density and attenuation coefficient distributions of eco-cement pastes with an isotropic resolution close to 100 nm allowing to distinguish between component phases with very similar contrast. In combination with laboratory techniques such as the Rietveld method, ²⁷Al MAS NMR, and electron microscopies, we report compositions and densities of key components. The ettringite and gel volume distributions have been mapped out in the segmented tomograms. Moreover, we discriminate between an aluminum hydroxide gel and calcium aluminum monosulfate, which have close electron density values. Specifically, the composition and mass density of two aluminum hydroxide gel agglomerates have been determined: (CaO)_0.04Al­(OH)₃·2.3H₂O with 1.48(3) g·cm^–3 and (CaO)_0.12Al­(OH)₃ with 2.05(3) g·cm^–3, which was a long-standing challenge

Three-Dimensional Nanometer Features of Direct Current Electrical Trees in Low-Density Polyethylene

Electrical trees are one reason for the breakdown of insulating materials in electrical power systems. An understanding of the growth of electrical trees plays a crucial role in the development of reliable high voltage direct current (HVDC) power grid systems with transmission voltages up to 1 MV. A section that contained an electrical tree in low-density polyethylene (LDPE) has been visualized in three dimensions (3D) with a resolution of 92 nm by X-ray ptychographic tomography. The 3D imaging revealed prechannel-formations with a lower density with the width of a couple of hundred nanometers formed around the main branch of the electrical tree. The prechannel structures were partially connected with the main tree via paths through material with a lower density, proving that the tree had grown in a step-by-step manner via the prestep structures formed in front of the main channels. All the prechannel structures had a size well below the limit of the Paschen law and were thus not formed by partial discharges. Instead, it is suggested that the prechannel structures were formed by electro-mechanical stress and impact ionization, where the former was confirmed by simulations to be a potential explanation with electro-mechanical stress tensors being almost of the same order of magnitude as the short-term modulus of low-density polyethylene