<p>The safety of foundations, bridge piers, and other works that have been in a corrosive environment rich in sulfate, chloride, and magnesium is worth considering. To investigate the deterioration mechanism of cast-in-situ concrete, magnesium sulfate solutions, composite solutions of magnesium sulfate and sodium chloride with different molar concentrations were prepared to simulate the corrosion process. The macroscopic properties of the samples cast-in-situ concrete were studied and evaluated from its appearance, mass change, compressive strength, and relative dynamic modulus of elasticity. XRD, SEM-EDS, TG, and FTIR were used, to analyze the evolution of microstructure and corrosion products. After 12&#xa0;months of corrosion, the results revealed a 47.9% loss of strength in cast-in-situ concrete exposed to 5% magnesium sulfate solution, whereas losses of 58.1% and 81.4% were observed in 5% and 10% composite solutions, respectively. A respective decrease of 4.6%, 15.2%, and 78.9% was observed in the relative dynamic modulus of elasticity. Microscopic test results show that M–S–H gel formation was detected at later stage of corrosion, which indicated that magnesium ions primarily concentrated on the surface, forming a brucite layer in the early stage, and subsequently penetrated the interior, causing corrosion in later stage. Friedel’s salt was observed at the early stage and was later decomposed to AFt by sulfate ions. This suggested that chloride ions exert a lesser impact on structural damage, but may exacerbate sulfate attack. The primary factor contributing to the failure of cast-in-situ concrete structures is corrosion by sulfate ions.</p>