Abstract

Abstract

Hydrophobic treatment of cement pastes is most effective way to resist the penetration of water with aggressive ions to improve durability of cement-based materials. In this paper, the mechanism of carboxylic acid ammonium salt integral additionon hydrophobicity of cement-based materials is analyzed. The effects of carboxylic acid ammonium salt on hydrophobicity, moisture diffusion and hydration products are investigated by experimental methods.The results of water vapor sorption isotherm show that the addition of carboxylic acid ammonium salt increases the pore volume. The simplified analytical hydrodynamic model of cone type pore and Young’s relationship is proposed to analyze the relation between characteristic parameters of pore and the permeability of cementitious materials. It can be concluded that the hydrophobicity results from the change of pore structure and the hydrophobic surface of cone region. These results provide the guidance to design the durability and develop the new hydrophobic agent used in cementitious material.

Abstract

Abstract

Abstract

Ultra-high performance concrete (UHPC) is an innovative cement-based composite material characterized by extremely high durability and mechanical properties, which provides a potential possibility of using non-desalted sea-sand as aggregate. However, the long-term steel corrosion behavior in UHPC prepared by simulated sea-sand under different immersion environments remains unclear. In this study, the pore structure of sea-sand UHPC matrix was measured using nitrogen adsorption/desorption method. The corrosion behavior of rebar in sea-sand UHPC with different endogenous chloride ion contents soaking in pure water and NaCl solution was characterized by electrochemical workstation. The experimental results show that the initial UHPC pore structure is refined by incorporation ofsea-sand, and is similar to that of ordinary UHPC at the later hydration. Regardless of the endogenous chloride ion content and immersion environment, the corrosion rate evaluated by corrosion current density (icorr) and polarization resistance (Rp) of reinforcement in UHPC is extremely low. The measurement of EIS shows that UHPC group has higher matrix resistance (Rc) and charge transfer resistance (Rct), while lower electric double layer capacitance (CPEdl) of reinforcement, which indicates that the steel in UHPC made of sea-sand is still in a state ofpassivation. However, the steel bar in comparison specimen OPC soaking in NaCl solution is in the corrosion stage. Furthermore, sea-sand UHPC possesses of excellent ability of anti- pitting corrosion ofreinforcement after 180 days ofimmersion in NaCl solution.

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.