Sulfate-induced degradation of cast-in-situ concrete influenced by magnesium

•Effect of magnesium on sulfate induced attack to cast-in-situ concrete is studied.•Magnesium delays the strength development of concrete in the early stage.•Magnesium inhibits the sulfate induced attack in the late degradation stage.•Magnesium slows down the diffusion process of sulfate in cast-in-situ concrete. In this paper, the results of laboratory tests performed to investigate the effects of magnesium on concrete deterioration and sulfate diffusion in cast-in-situ concrete samples are reported. Concrete samples were prepared and then fully or partially immersed in two different corrosive solutions consisting of either sodium sulfate or magnesium sulfate. For a period of 12 months, the sample dimensions, mass, the sulfate concentration in the specimens and the compressive strength were continuously monitored. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), thermogravimetry-derivative thermogravimetry (TG/DTG) as well as differential scanning calorimetry (DSC) were used to analyze the microstructure and the complex mineral composition of the concrete after exposure to the corrosive environments. Results indicate that increasing the sulfate concentration in the immersion solutions accelerated the degradation of all tested concrete specimens. Partially immersed concrete samples underwent more severe attack, with both chemical attack and crystallization effects observed, when compared with fully immersed samples. The strength development of both the fully and partially immersed concrete samples in the early stages was delayed by the magnesium present in the sulfate solution. However, the presence of magnesium slowed down the degradation of concrete due to sulfate attack and inhibited the cracking and spalling of the concrete surface near the immersion level in the late degradation stage. Moreover, the existence of magnesium slowed down the sulfate diffusion process and consequently the sulfate attack.