Durability of concrete under sulfate attack exposed to freeze–thaw cycles

Properties of concrete (with additional 20% fly ash) subjected to freeze–thaw cycles in water and in sulfate solutions were investigated in this paper. The corrosion solutions include two types, namely, 5% sodium sulfate solution and 5% magnesium sulfate solution. Through the experiment, visual examination was conducted to evaluate the surface damage. The deterioration considering the weight loss, relative dynamic modulus of elastically (RDME) loss and compressive strength loss of concrete under the coupling effect were also investigated. To identify the products formed by sulfate attack, analytical techniques, including X-ray diffraction, scanning electron microscopy and thermal analysis were performed on the selected samples after freeze–thaw circulations. Test results show that freeze–thaw cycles and sulfate attack affected each other. On the one hand, a lower temperature during freeze–thaw cycles slows down the diffusion of sulfate ions in concrete. On the other hand, sulfate attack accelerates the formation of microcracks in concrete, which leads to more severe damage under freeze–thaw cycles. The rate of damage in concrete is significantly dependent on the types of sulfate solutions, and the concrete deterioration by magnesium sulfate covers the most aggressive corrosion subjected to freeze–thaw cycles. Furthermore, the compressive strength loss and RDME loss of concrete in sodium sulfate solution is less than that in water during the initial freeze–thaw cycles, but the damage is more severe in further test period.