Abstract

Siliceous materials are commonly used in concrete with physical filling and higher pozzolanic reactivity, such as silica fume (SF) and nano-silica (NS). Investigations were conducted into the effects of SF, hydrophobic silica fume (HSF), and synthesized NS on the mechanical properties and microscopic characteristics of hardened cement pastes (HCPs). The surface of silica fume was hydrophobically modified using isooctyltriethoxysilane. Fourier infrared spectrometer (FTIR), X-ray diffractometer (XRD), and energy spectrometer (EDS) were used to evaluate the hydration, while mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM) were used to analyze the pore structure and microscopic morphology of HCPs. The results demonstrated that HSF has better dispersion in cement pore solution. The addition of HSF exhibits the higher pozzolanic reactivity, weakens CH diffraction peaks, generates higher levels Q3 C-S-H gels, and significantly lowers the Ca/Si ratio of C-S-H gels. Due to the hydrophobicity, HSF increases the volume of micro-pores in range of 10-100 nm, resulting in a decrease in strength. Compounding HSF and NS further weakens the CH diffraction peak, decreases the Ca/Si ratio of C-S-H gels, and reduces the micro-pore volume to refines the pore structure.