Upscaling degradation of cementitious calcium (aluminate) silicate hydrate upon ultra-low temperature attack: A multiscale insight and a bottom-up enhancement route

Abstract

Low-carbon calcium (alumino)silicate hydrate (C-(A)-S-H) based composites (e.g., blended cement-based materials and alkali-activated materials) have broad application potential in ultra-low-temperature engineering areas. This study devotes to reveal the structural stability of C-(A)-S-H under ultra-low temperature attack (−170 °C) and evaluating the role of aluminum as a bottom-up enhancement strategy for C–S–H. The atomic structure, interlayer structure, internal basic building block structure, pore network, and micro-morphology were experimentally studied using 29Si MAS NMR, XRD, helium pycnometry, nitrogen adsorption, and SEM. These results indicate that an ultra-low temperature attack can deteriorate C-(A)-S-H structure, even though without the presence of bulk water, whereas the incorporation of aluminum positively stabilizes C-(A)-S-H structure. The silicate chains of Al-free C–S–H are ruptured with more defective vacant sites, which consequently initiates a decline in its d002 interlayer space and the volume collapse of basic building blocks, further resulting in the formation of microcracks. In contrast, aluminosilicate chains of C-A-S-H are more stable and even polymerized with longer chain length, stabilizing the micro/mesostructure. An upscaling degradation route from the atomic scale to mesoscale was proposed, providing a multiscale view to understand the degradation of concrete composites upon ultra-low temperature attack. We also confirm that incorporating aluminum in the silicate chains is an effective bottom-up strategy to enhance the cryogenic stability of C-(A)-S-H.