High surface area and interconnected nanoporosity of clay-rich astromaterials.

Porosity affects key astromaterial processes from disruption in our atmosphere and impact with the ground, to the comminution of boulders by thermal and impact processes and slope mechanics on asteroid surfaces, to access and utilization of in-situ resources. Whereas the bulk porosity of clay-rich meteorites is well established, the magnitude of their surface area and nano-scale porosity is poorly known. Here we use N2 BET gas adsorption to measure the specific surface area and nanoscale pore distribution in several clay-rich meteorites. Two recent falls Tarda (C2-ung) and Aguas Zarcas (CM2) have specific surface areas of 72.5 and 16.5 m2/g, respectively. However, the specific surface area of Tarda ranges from 33.7 to 81.6 m2/g depending on outgassing conditions. The Tarda surface area is dominated by an interconnected network of ~ 3-nm-sized pores, whereas Aguas Zarcas shows a lower density of ~ 3 nm pores and broader size distribution around 40 nm. In contrast, Ivuna and Orgueil (CI1) have surface areas of ~ 15 to 18 m2/g: the low values compared to Tarda are likely due to the neoformation of pore-blocking minerals during atmospheric exposure. These data suggest that returned samples from asteroids Ryugu and Bennu, which are mineralogically and texturally similar to Tarda, also have interconnected nano-scale porosity with high surface area.

Mesoscale texture of cement hydrates.

Strength and other mechanical properties of cement and concrete rely upon the formation of calcium-silicate-hydrates (C-S-H) during cement hydration. Controlling structure and properties of the C-S-H phase is a challenge, due to the complexity of this hydration product and of the mechanisms that drive its precipitation from the ionic solution upon dissolution of cement grains in water. Departing from traditional models mostly focused on length scales above the micrometer, recent research addressed the molecular structure of C-S-H. However, small-angle neutron scattering, electron-microscopy imaging, and nanoindentation experiments suggest that its mesoscale organization, extending over hundreds of nanometers, may be more important. Here we unveil the C-S-H mesoscale texture, a crucial step to connect the fundamental scales to the macroscale of engineering properties. We use simulations that combine information of the nanoscale building units of C-S-H and their effective interactions, obtained from atomistic simulations and experiments, into a statistical physics framework for aggregating nanoparticles. We compute small-angle scattering intensities, pore size distributions, specific surface area, local densities, indentation modulus, and hardness of the material, providing quantitative understanding of different experimental investigations. Our results provide insight into how the heterogeneities developed during the early stages of hydration persist in the structure of C-S-H and impact the mechanical performance of the hardened cement paste. Unraveling such links in cement hydrates can be groundbreaking and controlling them can be the key to smarter mix designs of cementitious materials.