Macro Mechanical Properties and Microstructure Analysis of Nano-modified ECC at Low Temperature Available to Purchase

The building structure in cold areas has not been in a freeze–thaw cycle but in a low temperature and deep cold environment for years. In such environments, one-time freezing mode is required to analyze the low-temperature freezing problem. The purpose of this study is to optimize engineered cementitious composites (ECC) currently known to have good durability and prepare ratios suitable for low-temperature environments. By mixing nano silica (SiO₂) and SiO₂ fume in a certain proportion, the internal situation was optimized by using their physical filling effect and chemical activity. The macromechanical properties of four groups of ECC with different nano SiO₂ and SiO₂ fume content under normal temperature and low temperature were studied. Uniaxial compressive strength and full curves of tensile stress–strain at normal temperature and low temperature (−60°C) were established, whereas the optimum mechanical properties were obtained under low temperatures. It was found that the uniaxial compressive strength, tensile strength, and tensile strain capacity can reach 45 MPa, 4 Mpa, and 4 % at low temperature (−60°C) after 28 days of standard maintenance. Four groups of modified ECC were analyzed by computed tomography (CT), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). The microstructure, matrix, fiber–matrix interface, and fiber surface of ECC mixture with nano SiO₂ and SiO₂ fume were compared to evaluate the improvement of ductility. After determining the porosity by CT scanning, the microstructure was characterized. The characteristics of hydration products at the age of 7 and 28 days were analyzed by XRD. Matrix, fiber–matrix interface, and fiber surface characteristics at the age of 28 days were evaluated using SEM. The results prove that the combination of nano SiO₂ 0.5 % and SiO₂ fume 1.5 % can make the internal pore dense and uniform while optimizing the pore structure and fully exerting the bonding effect between matrix and fiber.