Reduction of SO42− and Cl− migration rates and degradation of silica nanoparticles incorporated cement pastes exposed to co-existence of sulfate, chloride and electric fields

Abstract

The reinforced concrete structures in subway engineering are generally subjected to combined exposure of chloride, sulfate and electric fields. Thus, it is necessary to reveal a way of enhancing corrosion resistance of concrete in this complicated environment condition. Although improvement of concrete properties by addition of nano silica is well known, how it can lead to the reduced transport rates of aggressive ions in the cementitious systems has not been systematically confirmed. In this work, the strengthening mechanism of nano-SiO2 on the erosion resistance of cement pastes was investigated in terms of water absorption, distribution of sulfate concentration, free chloride concentration and bound chloride concentration, ionic diffusion coefficients, crystalline phase composition, thermal gravimetric and Vickers hardness. The results indicated that the sulfate concentration exhibited a decreased trend with increasing the depth from exposed surface, while both free chloride and bound chloride concentrations first increased and then decreased gradually. The modification of silica nanoparticles on the cement pastes was remarkable, and 3% nano-silica admixed binary mixtures exhibited a significant reduction of the ionic migration rates and the accompany degradation. Moreover, replacement of 1% nano silica to cement due to pozzolanic reaction and nucleation effect dramatically led to the effective chloride capture near the exposed surface as a result of the declined porosity, which can be substantiated by X-ray diffraction and thermal gravimetric results. The results of this work are beneficial for establishment of a finer transport model in further research and provide a reference for monitoring and evaluation of durability of reinforced concrete structures serving in subway engineering.