Microstructural Evolution of Concrete under the Attack of Chemical, Salt Crystallization, and Bending Stress

The present work investigates the microstructural evolution process of concrete under chemical attack by magnesium sulfate solution or a composite solution (sodium sulfate and magnesium chloride), salt crystallization attack through dry–wet cycles, and bending stress. The composition and microstructure are studied in detail using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results of the relative dynamic elastic modulus and visual inspection analysis indicate that damage and deterioration of concrete from the composite solution are more serious than those from magnesium sulfate. Moreover, damage and deterioration of concrete under chemical attack are remarkably accelerated by salt crystallization and bending stress. According to the microstructural analysis, crack initiation elements (a new concept) are present in the concrete, which are expected to lead to crack generation and expansion in the concrete. After the corrosion of concrete under chemical attack, salt crystallization attack and bending stress for 903 days, several crack initiation elements are found in the SEM photographs of the concrete, and a grouping of microcracks appears in the concrete. These microcracks form a network by diffusion and connection. The results show that the composite solution is a more severe corrosion medium than magnesium sulfate, according to a comparison of the crack width.