Concrete Performance Attenuation of Mix Nano-SiO2 and Nano-CaCO3 under High Temperature: A Comprehensive Review

Fire and extreme heat environmental changes can have an impact on concrete performance, and as climate change increases, new concrete structures are being developed. Nano-silica and nano-calcium carbonate have shown excellent performances in modifying concrete due to their large specific surface areas. This review describes the changes in concrete modified with nano-silica (NS) and nano-calcium carbonate (NC), which accelerate the hydration reaction with the cementitious materials to produce more C-S-H, resulting in a denser microstructure and improved mechanical properties and durability of the concrete. The mechanical property decay and visualization of deformation of mixed NS and NC concrete were tested by exposure to high temperatures to investigate the practical application of mixed composite nanomaterials (NC+NS) to concrete. The nano-modified concrete had better overall properties and was heated at 200 °C, 400 °C, 600 °C and 800 °C to relatively improve the mechanical properties of the nano concrete structures. The review concluded that high temperatures of 800 °C to 1000 °C severely damaged the structure of the concrete, reducing the mechanical properties by around 60%, and the dense nano concrete structures were more susceptible to cracking and damage. The high temperature resistance of NS and NC-modified nano concrete was relatively higher than that of normal concrete, with NC concrete being more resistant to damage at high temperatures than the NS samples.

Unraveling the Rapid Performance Decay of Layered High-Energy Cathodes: From Nanoscale Degradation to Drastic Bulk Evolution

Lithium-rich layered oxides are promising cathode candidates because of their exceptional high capacity. The commercial application of these high-energy cathodes, however, is thwarted by the undesired rapid performance decay during cycling. Surface degradation has been widely considered to correlate with the performance decay of the cathodes, whereas, in this work, we demonstrate that the degradation of Li-rich high-energy Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ (HENMC) cathode material not only takes place at surfaces but also proceeds from its internal structure. In addition to demonstrating the surface reconstruction and the formation of a cathode-electrolyte interphase (CEI) layer of cycled HENMC cathode, this study uncovers the irreversible bulk phase transition from a Li-excess monoclinic ( C2/ m) solid solution into a conventional "layered" ( R3̅ m) phase, accompanied by complete loss of Li⁺ from the TM layers during cycling. Furthermore, the internal grains of HENMC bear lattice distortions, leading to the formation of "nano-defect" domains, which could limit the Li⁺ diffusion inside the grains. More prominently, the layered-to-spinel transition in the form of large spinel grains ( Fd3̅ m), hundreds of nanometers across, is discovered, and their detailed atomic arrangement is studied. The findings suggest that, instead of attributing the overall capacity fade to the surface degradation, these drastic bulk evolutions would be the main degradation mechanisms at the source of the rapid failure of Li-rich cathodes.