Mechanism of nano-silica to enhance the robustness and durability of concrete in low air pressure for sustainable civil infrastructures

Cement concrete is an extensively used construction material in buildings and civil infrastructures. As infrastructure moves into plateau areas, freeze-thaw failure is a main durability challenge for concrete due to the frequent freeze-thaw cycles and entrained air pore coarsening in high altitude and low air pressure (Lap). Recently, there has been a growing interest in the application of nano-silica (NS) due to its notable improvement in concrete durability in plateau areas as well as the consumption of industrial by-product, both improve the sustainability of civil infrastructures. Therefore, this paper studies the mechanism of NS for enhancing the robustness and durability of concrete in low air pressure, the results of which foster the application of NS concrete for designing and constructing sustainable civil infrastructures in plateau areas. Two different kinds of air-entraining agents (AEAs) were utilized to prepare air-entrained concrete. Following this, NS was employed to modify the micro and meso structure of the concrete, and then determine their influence on the performance of concrete. The influence of NS on the entrained air bubbles and air pores were analyzed via micro measurements including Fourier Transform infrared spectroscopy (FTIR), Energy-Dispersive X-ray Spectroscopy (EDS), and Scanning Electron Microscope (SEM). CABR-457 hardened concrete air pore structure analyzer was employed to study microstructure of air-entrained concrete with and without NS. The results demonstrate that the adsorption of NS at air bubble shell entrained by AEAs can densify the hydration products of the air pore wall and influence the pore size distribution, thereby improving the flexural strength and frost resistance of air-entrained concrete under low air pressure. The average air pore diameter of concrete with proper AEAs can be refined by 43.3% after adding NS. SEM, EDS and FTIR data analysis proves the adsorption of NS on the air bubble shell and differences in chemical composition of air bubble shell entrained by two types of AEAs. The differences in the composition of bubble shells result in diverse degree of NS optimized concrete performance. This study deepens our understanding into the fundamental mechanism of NS in concrete in Lap, which also prompts its application to create a sustainable built environment.